The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/netinet/tcp_syncache.c

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    1 /*-
    2  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
    3  *
    4  * Copyright (c) 2001 McAfee, Inc.
    5  * Copyright (c) 2006,2013 Andre Oppermann, Internet Business Solutions AG
    6  * All rights reserved.
    7  *
    8  * This software was developed for the FreeBSD Project by Jonathan Lemon
    9  * and McAfee Research, the Security Research Division of McAfee, Inc. under
   10  * DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
   11  * DARPA CHATS research program. [2001 McAfee, Inc.]
   12  *
   13  * Redistribution and use in source and binary forms, with or without
   14  * modification, are permitted provided that the following conditions
   15  * are met:
   16  * 1. Redistributions of source code must retain the above copyright
   17  *    notice, this list of conditions and the following disclaimer.
   18  * 2. Redistributions in binary form must reproduce the above copyright
   19  *    notice, this list of conditions and the following disclaimer in the
   20  *    documentation and/or other materials provided with the distribution.
   21  *
   22  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   23  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   24  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   25  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   26  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   27  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   28  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   29  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   30  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   31  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   32  * SUCH DAMAGE.
   33  */
   34 
   35 #include <sys/cdefs.h>
   36 __FBSDID("$FreeBSD$");
   37 
   38 #include "opt_inet.h"
   39 #include "opt_inet6.h"
   40 #include "opt_ipsec.h"
   41 
   42 #include <sys/param.h>
   43 #include <sys/systm.h>
   44 #include <sys/hash.h>
   45 #include <sys/refcount.h>
   46 #include <sys/kernel.h>
   47 #include <sys/sysctl.h>
   48 #include <sys/limits.h>
   49 #include <sys/lock.h>
   50 #include <sys/mutex.h>
   51 #include <sys/malloc.h>
   52 #include <sys/mbuf.h>
   53 #include <sys/proc.h>           /* for proc0 declaration */
   54 #include <sys/random.h>
   55 #include <sys/socket.h>
   56 #include <sys/socketvar.h>
   57 #include <sys/syslog.h>
   58 #include <sys/ucred.h>
   59 
   60 #include <sys/md5.h>
   61 #include <crypto/siphash/siphash.h>
   62 
   63 #include <vm/uma.h>
   64 
   65 #include <net/if.h>
   66 #include <net/if_var.h>
   67 #include <net/route.h>
   68 #include <net/vnet.h>
   69 
   70 #include <netinet/in.h>
   71 #include <netinet/in_kdtrace.h>
   72 #include <netinet/in_systm.h>
   73 #include <netinet/ip.h>
   74 #include <netinet/in_var.h>
   75 #include <netinet/in_pcb.h>
   76 #include <netinet/ip_var.h>
   77 #include <netinet/ip_options.h>
   78 #ifdef INET6
   79 #include <netinet/ip6.h>
   80 #include <netinet/icmp6.h>
   81 #include <netinet6/nd6.h>
   82 #include <netinet6/ip6_var.h>
   83 #include <netinet6/in6_pcb.h>
   84 #endif
   85 #include <netinet/tcp.h>
   86 #include <netinet/tcp_fastopen.h>
   87 #include <netinet/tcp_fsm.h>
   88 #include <netinet/tcp_seq.h>
   89 #include <netinet/tcp_timer.h>
   90 #include <netinet/tcp_var.h>
   91 #include <netinet/tcp_syncache.h>
   92 #include <netinet/tcp_ecn.h>
   93 #ifdef TCP_OFFLOAD
   94 #include <netinet/toecore.h>
   95 #endif
   96 #include <netinet/udp.h>
   97 
   98 #include <netipsec/ipsec_support.h>
   99 
  100 #include <machine/in_cksum.h>
  101 
  102 #include <security/mac/mac_framework.h>
  103 
  104 VNET_DEFINE_STATIC(int, tcp_syncookies) = 1;
  105 #define V_tcp_syncookies                VNET(tcp_syncookies)
  106 SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies, CTLFLAG_VNET | CTLFLAG_RW,
  107     &VNET_NAME(tcp_syncookies), 0,
  108     "Use TCP SYN cookies if the syncache overflows");
  109 
  110 VNET_DEFINE_STATIC(int, tcp_syncookiesonly) = 0;
  111 #define V_tcp_syncookiesonly            VNET(tcp_syncookiesonly)
  112 SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies_only, CTLFLAG_VNET | CTLFLAG_RW,
  113     &VNET_NAME(tcp_syncookiesonly), 0,
  114     "Use only TCP SYN cookies");
  115 
  116 VNET_DEFINE_STATIC(int, functions_inherit_listen_socket_stack) = 1;
  117 #define V_functions_inherit_listen_socket_stack \
  118     VNET(functions_inherit_listen_socket_stack)
  119 SYSCTL_INT(_net_inet_tcp, OID_AUTO, functions_inherit_listen_socket_stack,
  120     CTLFLAG_VNET | CTLFLAG_RW,
  121     &VNET_NAME(functions_inherit_listen_socket_stack), 0,
  122     "Inherit listen socket's stack");
  123 
  124 #ifdef TCP_OFFLOAD
  125 #define ADDED_BY_TOE(sc) ((sc)->sc_tod != NULL)
  126 #endif
  127 
  128 static void      syncache_drop(struct syncache *, struct syncache_head *);
  129 static void      syncache_free(struct syncache *);
  130 static void      syncache_insert(struct syncache *, struct syncache_head *);
  131 static int       syncache_respond(struct syncache *, const struct mbuf *, int);
  132 static struct    socket *syncache_socket(struct syncache *, struct socket *,
  133                     struct mbuf *m);
  134 static void      syncache_timeout(struct syncache *sc, struct syncache_head *sch,
  135                     int docallout);
  136 static void      syncache_timer(void *);
  137 
  138 static uint32_t  syncookie_mac(struct in_conninfo *, tcp_seq, uint8_t,
  139                     uint8_t *, uintptr_t);
  140 static tcp_seq   syncookie_generate(struct syncache_head *, struct syncache *);
  141 static struct syncache
  142                 *syncookie_lookup(struct in_conninfo *, struct syncache_head *,
  143                     struct syncache *, struct tcphdr *, struct tcpopt *,
  144                     struct socket *, uint16_t);
  145 static void     syncache_pause(struct in_conninfo *);
  146 static void     syncache_unpause(void *);
  147 static void      syncookie_reseed(void *);
  148 #ifdef INVARIANTS
  149 static int       syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
  150                     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
  151                     struct socket *lso, uint16_t port);
  152 #endif
  153 
  154 /*
  155  * Transmit the SYN,ACK fewer times than TCP_MAXRXTSHIFT specifies.
  156  * 3 retransmits corresponds to a timeout with default values of
  157  * tcp_rexmit_initial * (             1 +
  158  *                       tcp_backoff[1] +
  159  *                       tcp_backoff[2] +
  160  *                       tcp_backoff[3]) + 3 * tcp_rexmit_slop,
  161  * 1000 ms * (1 + 2 + 4 + 8) +  3 * 200 ms = 15600 ms,
  162  * the odds are that the user has given up attempting to connect by then.
  163  */
  164 #define SYNCACHE_MAXREXMTS              3
  165 
  166 /* Arbitrary values */
  167 #define TCP_SYNCACHE_HASHSIZE           512
  168 #define TCP_SYNCACHE_BUCKETLIMIT        30
  169 
  170 VNET_DEFINE_STATIC(struct tcp_syncache, tcp_syncache);
  171 #define V_tcp_syncache                  VNET(tcp_syncache)
  172 
  173 static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, syncache,
  174     CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
  175     "TCP SYN cache");
  176 
  177 SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, bucketlimit, CTLFLAG_VNET | CTLFLAG_RDTUN,
  178     &VNET_NAME(tcp_syncache.bucket_limit), 0,
  179     "Per-bucket hash limit for syncache");
  180 
  181 SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, cachelimit, CTLFLAG_VNET | CTLFLAG_RDTUN,
  182     &VNET_NAME(tcp_syncache.cache_limit), 0,
  183     "Overall entry limit for syncache");
  184 
  185 SYSCTL_UMA_CUR(_net_inet_tcp_syncache, OID_AUTO, count, CTLFLAG_VNET,
  186     &VNET_NAME(tcp_syncache.zone), "Current number of entries in syncache");
  187 
  188 SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, hashsize, CTLFLAG_VNET | CTLFLAG_RDTUN,
  189     &VNET_NAME(tcp_syncache.hashsize), 0,
  190     "Size of TCP syncache hashtable");
  191 
  192 SYSCTL_BOOL(_net_inet_tcp_syncache, OID_AUTO, see_other, CTLFLAG_VNET |
  193     CTLFLAG_RW, &VNET_NAME(tcp_syncache.see_other), 0,
  194     "All syncache(4) entries are visible, ignoring UID/GID, jail(2) "
  195     "and mac(4) checks");
  196 
  197 static int
  198 sysctl_net_inet_tcp_syncache_rexmtlimit_check(SYSCTL_HANDLER_ARGS)
  199 {
  200         int error;
  201         u_int new;
  202 
  203         new = V_tcp_syncache.rexmt_limit;
  204         error = sysctl_handle_int(oidp, &new, 0, req);
  205         if ((error == 0) && (req->newptr != NULL)) {
  206                 if (new > TCP_MAXRXTSHIFT)
  207                         error = EINVAL;
  208                 else
  209                         V_tcp_syncache.rexmt_limit = new;
  210         }
  211         return (error);
  212 }
  213 
  214 SYSCTL_PROC(_net_inet_tcp_syncache, OID_AUTO, rexmtlimit,
  215     CTLFLAG_VNET | CTLTYPE_UINT | CTLFLAG_RW | CTLFLAG_NEEDGIANT,
  216     &VNET_NAME(tcp_syncache.rexmt_limit), 0,
  217     sysctl_net_inet_tcp_syncache_rexmtlimit_check, "UI",
  218     "Limit on SYN/ACK retransmissions");
  219 
  220 VNET_DEFINE(int, tcp_sc_rst_sock_fail) = 1;
  221 SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, rst_on_sock_fail,
  222     CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(tcp_sc_rst_sock_fail), 0,
  223     "Send reset on socket allocation failure");
  224 
  225 static MALLOC_DEFINE(M_SYNCACHE, "syncache", "TCP syncache");
  226 
  227 #define SCH_LOCK(sch)           mtx_lock(&(sch)->sch_mtx)
  228 #define SCH_UNLOCK(sch)         mtx_unlock(&(sch)->sch_mtx)
  229 #define SCH_LOCK_ASSERT(sch)    mtx_assert(&(sch)->sch_mtx, MA_OWNED)
  230 
  231 /*
  232  * Requires the syncache entry to be already removed from the bucket list.
  233  */
  234 static void
  235 syncache_free(struct syncache *sc)
  236 {
  237 
  238         if (sc->sc_ipopts)
  239                 (void) m_free(sc->sc_ipopts);
  240         if (sc->sc_cred)
  241                 crfree(sc->sc_cred);
  242 #ifdef MAC
  243         mac_syncache_destroy(&sc->sc_label);
  244 #endif
  245 
  246         uma_zfree(V_tcp_syncache.zone, sc);
  247 }
  248 
  249 void
  250 syncache_init(void)
  251 {
  252         int i;
  253 
  254         V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
  255         V_tcp_syncache.bucket_limit = TCP_SYNCACHE_BUCKETLIMIT;
  256         V_tcp_syncache.rexmt_limit = SYNCACHE_MAXREXMTS;
  257         V_tcp_syncache.hash_secret = arc4random();
  258 
  259         TUNABLE_INT_FETCH("net.inet.tcp.syncache.hashsize",
  260             &V_tcp_syncache.hashsize);
  261         TUNABLE_INT_FETCH("net.inet.tcp.syncache.bucketlimit",
  262             &V_tcp_syncache.bucket_limit);
  263         if (!powerof2(V_tcp_syncache.hashsize) ||
  264             V_tcp_syncache.hashsize == 0) {
  265                 printf("WARNING: syncache hash size is not a power of 2.\n");
  266                 V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
  267         }
  268         V_tcp_syncache.hashmask = V_tcp_syncache.hashsize - 1;
  269 
  270         /* Set limits. */
  271         V_tcp_syncache.cache_limit =
  272             V_tcp_syncache.hashsize * V_tcp_syncache.bucket_limit;
  273         TUNABLE_INT_FETCH("net.inet.tcp.syncache.cachelimit",
  274             &V_tcp_syncache.cache_limit);
  275 
  276         /* Allocate the hash table. */
  277         V_tcp_syncache.hashbase = malloc(V_tcp_syncache.hashsize *
  278             sizeof(struct syncache_head), M_SYNCACHE, M_WAITOK | M_ZERO);
  279 
  280 #ifdef VIMAGE
  281         V_tcp_syncache.vnet = curvnet;
  282 #endif
  283 
  284         /* Initialize the hash buckets. */
  285         for (i = 0; i < V_tcp_syncache.hashsize; i++) {
  286                 TAILQ_INIT(&V_tcp_syncache.hashbase[i].sch_bucket);
  287                 mtx_init(&V_tcp_syncache.hashbase[i].sch_mtx, "tcp_sc_head",
  288                          NULL, MTX_DEF);
  289                 callout_init_mtx(&V_tcp_syncache.hashbase[i].sch_timer,
  290                          &V_tcp_syncache.hashbase[i].sch_mtx, 0);
  291                 V_tcp_syncache.hashbase[i].sch_length = 0;
  292                 V_tcp_syncache.hashbase[i].sch_sc = &V_tcp_syncache;
  293                 V_tcp_syncache.hashbase[i].sch_last_overflow =
  294                     -(SYNCOOKIE_LIFETIME + 1);
  295         }
  296 
  297         /* Create the syncache entry zone. */
  298         V_tcp_syncache.zone = uma_zcreate("syncache", sizeof(struct syncache),
  299             NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
  300         V_tcp_syncache.cache_limit = uma_zone_set_max(V_tcp_syncache.zone,
  301             V_tcp_syncache.cache_limit);
  302 
  303         /* Start the SYN cookie reseeder callout. */
  304         callout_init(&V_tcp_syncache.secret.reseed, 1);
  305         arc4rand(V_tcp_syncache.secret.key[0], SYNCOOKIE_SECRET_SIZE, 0);
  306         arc4rand(V_tcp_syncache.secret.key[1], SYNCOOKIE_SECRET_SIZE, 0);
  307         callout_reset(&V_tcp_syncache.secret.reseed, SYNCOOKIE_LIFETIME * hz,
  308             syncookie_reseed, &V_tcp_syncache);
  309 
  310         /* Initialize the pause machinery. */
  311         mtx_init(&V_tcp_syncache.pause_mtx, "tcp_sc_pause", NULL, MTX_DEF);
  312         callout_init_mtx(&V_tcp_syncache.pause_co, &V_tcp_syncache.pause_mtx,
  313             0);
  314         V_tcp_syncache.pause_until = time_uptime - TCP_SYNCACHE_PAUSE_TIME;
  315         V_tcp_syncache.pause_backoff = 0;
  316         V_tcp_syncache.paused = false;
  317 }
  318 
  319 #ifdef VIMAGE
  320 void
  321 syncache_destroy(void)
  322 {
  323         struct syncache_head *sch;
  324         struct syncache *sc, *nsc;
  325         int i;
  326 
  327         /*
  328          * Stop the re-seed timer before freeing resources.  No need to
  329          * possibly schedule it another time.
  330          */
  331         callout_drain(&V_tcp_syncache.secret.reseed);
  332 
  333         /* Stop the SYN cache pause callout. */
  334         mtx_lock(&V_tcp_syncache.pause_mtx);
  335         if (callout_stop(&V_tcp_syncache.pause_co) == 0) {
  336                 mtx_unlock(&V_tcp_syncache.pause_mtx);
  337                 callout_drain(&V_tcp_syncache.pause_co);
  338         } else
  339                 mtx_unlock(&V_tcp_syncache.pause_mtx);
  340 
  341         /* Cleanup hash buckets: stop timers, free entries, destroy locks. */
  342         for (i = 0; i < V_tcp_syncache.hashsize; i++) {
  343                 sch = &V_tcp_syncache.hashbase[i];
  344                 callout_drain(&sch->sch_timer);
  345 
  346                 SCH_LOCK(sch);
  347                 TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc)
  348                         syncache_drop(sc, sch);
  349                 SCH_UNLOCK(sch);
  350                 KASSERT(TAILQ_EMPTY(&sch->sch_bucket),
  351                     ("%s: sch->sch_bucket not empty", __func__));
  352                 KASSERT(sch->sch_length == 0, ("%s: sch->sch_length %d not 0",
  353                     __func__, sch->sch_length));
  354                 mtx_destroy(&sch->sch_mtx);
  355         }
  356 
  357         KASSERT(uma_zone_get_cur(V_tcp_syncache.zone) == 0,
  358             ("%s: cache_count not 0", __func__));
  359 
  360         /* Free the allocated global resources. */
  361         uma_zdestroy(V_tcp_syncache.zone);
  362         free(V_tcp_syncache.hashbase, M_SYNCACHE);
  363         mtx_destroy(&V_tcp_syncache.pause_mtx);
  364 }
  365 #endif
  366 
  367 /*
  368  * Inserts a syncache entry into the specified bucket row.
  369  * Locks and unlocks the syncache_head autonomously.
  370  */
  371 static void
  372 syncache_insert(struct syncache *sc, struct syncache_head *sch)
  373 {
  374         struct syncache *sc2;
  375 
  376         SCH_LOCK(sch);
  377 
  378         /*
  379          * Make sure that we don't overflow the per-bucket limit.
  380          * If the bucket is full, toss the oldest element.
  381          */
  382         if (sch->sch_length >= V_tcp_syncache.bucket_limit) {
  383                 KASSERT(!TAILQ_EMPTY(&sch->sch_bucket),
  384                         ("sch->sch_length incorrect"));
  385                 syncache_pause(&sc->sc_inc);
  386                 sc2 = TAILQ_LAST(&sch->sch_bucket, sch_head);
  387                 sch->sch_last_overflow = time_uptime;
  388                 syncache_drop(sc2, sch);
  389         }
  390 
  391         /* Put it into the bucket. */
  392         TAILQ_INSERT_HEAD(&sch->sch_bucket, sc, sc_hash);
  393         sch->sch_length++;
  394 
  395 #ifdef TCP_OFFLOAD
  396         if (ADDED_BY_TOE(sc)) {
  397                 struct toedev *tod = sc->sc_tod;
  398 
  399                 tod->tod_syncache_added(tod, sc->sc_todctx);
  400         }
  401 #endif
  402 
  403         /* Reinitialize the bucket row's timer. */
  404         if (sch->sch_length == 1)
  405                 sch->sch_nextc = ticks + INT_MAX;
  406         syncache_timeout(sc, sch, 1);
  407 
  408         SCH_UNLOCK(sch);
  409 
  410         TCPSTATES_INC(TCPS_SYN_RECEIVED);
  411         TCPSTAT_INC(tcps_sc_added);
  412 }
  413 
  414 /*
  415  * Remove and free entry from syncache bucket row.
  416  * Expects locked syncache head.
  417  */
  418 static void
  419 syncache_drop(struct syncache *sc, struct syncache_head *sch)
  420 {
  421 
  422         SCH_LOCK_ASSERT(sch);
  423 
  424         TCPSTATES_DEC(TCPS_SYN_RECEIVED);
  425         TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
  426         sch->sch_length--;
  427 
  428 #ifdef TCP_OFFLOAD
  429         if (ADDED_BY_TOE(sc)) {
  430                 struct toedev *tod = sc->sc_tod;
  431 
  432                 tod->tod_syncache_removed(tod, sc->sc_todctx);
  433         }
  434 #endif
  435 
  436         syncache_free(sc);
  437 }
  438 
  439 /*
  440  * Engage/reengage time on bucket row.
  441  */
  442 static void
  443 syncache_timeout(struct syncache *sc, struct syncache_head *sch, int docallout)
  444 {
  445         int rexmt;
  446 
  447         if (sc->sc_rxmits == 0)
  448                 rexmt = tcp_rexmit_initial;
  449         else
  450                 TCPT_RANGESET(rexmt,
  451                     tcp_rexmit_initial * tcp_backoff[sc->sc_rxmits],
  452                     tcp_rexmit_min, TCPTV_REXMTMAX);
  453         sc->sc_rxttime = ticks + rexmt;
  454         sc->sc_rxmits++;
  455         if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc)) {
  456                 sch->sch_nextc = sc->sc_rxttime;
  457                 if (docallout)
  458                         callout_reset(&sch->sch_timer, sch->sch_nextc - ticks,
  459                             syncache_timer, (void *)sch);
  460         }
  461 }
  462 
  463 /*
  464  * Walk the timer queues, looking for SYN,ACKs that need to be retransmitted.
  465  * If we have retransmitted an entry the maximum number of times, expire it.
  466  * One separate timer for each bucket row.
  467  */
  468 static void
  469 syncache_timer(void *xsch)
  470 {
  471         struct syncache_head *sch = (struct syncache_head *)xsch;
  472         struct syncache *sc, *nsc;
  473         struct epoch_tracker et;
  474         int tick = ticks;
  475         char *s;
  476         bool paused;
  477 
  478         CURVNET_SET(sch->sch_sc->vnet);
  479 
  480         /* NB: syncache_head has already been locked by the callout. */
  481         SCH_LOCK_ASSERT(sch);
  482 
  483         /*
  484          * In the following cycle we may remove some entries and/or
  485          * advance some timeouts, so re-initialize the bucket timer.
  486          */
  487         sch->sch_nextc = tick + INT_MAX;
  488 
  489         /*
  490          * If we have paused processing, unconditionally remove
  491          * all syncache entries.
  492          */
  493         mtx_lock(&V_tcp_syncache.pause_mtx);
  494         paused = V_tcp_syncache.paused;
  495         mtx_unlock(&V_tcp_syncache.pause_mtx);
  496 
  497         TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc) {
  498                 if (paused) {
  499                         syncache_drop(sc, sch);
  500                         continue;
  501                 }
  502                 /*
  503                  * We do not check if the listen socket still exists
  504                  * and accept the case where the listen socket may be
  505                  * gone by the time we resend the SYN/ACK.  We do
  506                  * not expect this to happens often. If it does,
  507                  * then the RST will be sent by the time the remote
  508                  * host does the SYN/ACK->ACK.
  509                  */
  510                 if (TSTMP_GT(sc->sc_rxttime, tick)) {
  511                         if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc))
  512                                 sch->sch_nextc = sc->sc_rxttime;
  513                         continue;
  514                 }
  515                 if (sc->sc_rxmits > V_tcp_ecn_maxretries) {
  516                         sc->sc_flags &= ~SCF_ECN_MASK;
  517                 }
  518                 if (sc->sc_rxmits > V_tcp_syncache.rexmt_limit) {
  519                         if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  520                                 log(LOG_DEBUG, "%s; %s: Retransmits exhausted, "
  521                                     "giving up and removing syncache entry\n",
  522                                     s, __func__);
  523                                 free(s, M_TCPLOG);
  524                         }
  525                         syncache_drop(sc, sch);
  526                         TCPSTAT_INC(tcps_sc_stale);
  527                         continue;
  528                 }
  529                 if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  530                         log(LOG_DEBUG, "%s; %s: Response timeout, "
  531                             "retransmitting (%u) SYN|ACK\n",
  532                             s, __func__, sc->sc_rxmits);
  533                         free(s, M_TCPLOG);
  534                 }
  535 
  536                 NET_EPOCH_ENTER(et);
  537                 syncache_respond(sc, NULL, TH_SYN|TH_ACK);
  538                 NET_EPOCH_EXIT(et);
  539                 TCPSTAT_INC(tcps_sc_retransmitted);
  540                 syncache_timeout(sc, sch, 0);
  541         }
  542         if (!TAILQ_EMPTY(&(sch)->sch_bucket))
  543                 callout_reset(&(sch)->sch_timer, (sch)->sch_nextc - tick,
  544                         syncache_timer, (void *)(sch));
  545         CURVNET_RESTORE();
  546 }
  547 
  548 /*
  549  * Returns true if the system is only using cookies at the moment.
  550  * This could be due to a sysadmin decision to only use cookies, or it
  551  * could be due to the system detecting an attack.
  552  */
  553 static inline bool
  554 syncache_cookiesonly(void)
  555 {
  556 
  557         return (V_tcp_syncookies && (V_tcp_syncache.paused ||
  558             V_tcp_syncookiesonly));
  559 }
  560 
  561 /*
  562  * Find the hash bucket for the given connection.
  563  */
  564 static struct syncache_head *
  565 syncache_hashbucket(struct in_conninfo *inc)
  566 {
  567         uint32_t hash;
  568 
  569         /*
  570          * The hash is built on foreign port + local port + foreign address.
  571          * We rely on the fact that struct in_conninfo starts with 16 bits
  572          * of foreign port, then 16 bits of local port then followed by 128
  573          * bits of foreign address.  In case of IPv4 address, the first 3
  574          * 32-bit words of the address always are zeroes.
  575          */
  576         hash = jenkins_hash32((uint32_t *)&inc->inc_ie, 5,
  577             V_tcp_syncache.hash_secret) & V_tcp_syncache.hashmask;
  578 
  579         return (&V_tcp_syncache.hashbase[hash]);
  580 }
  581 
  582 /*
  583  * Find an entry in the syncache.
  584  * Returns always with locked syncache_head plus a matching entry or NULL.
  585  */
  586 static struct syncache *
  587 syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp)
  588 {
  589         struct syncache *sc;
  590         struct syncache_head *sch;
  591 
  592         *schp = sch = syncache_hashbucket(inc);
  593         SCH_LOCK(sch);
  594 
  595         /* Circle through bucket row to find matching entry. */
  596         TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash)
  597                 if (bcmp(&inc->inc_ie, &sc->sc_inc.inc_ie,
  598                     sizeof(struct in_endpoints)) == 0)
  599                         break;
  600 
  601         return (sc);    /* Always returns with locked sch. */
  602 }
  603 
  604 /*
  605  * This function is called when we get a RST for a
  606  * non-existent connection, so that we can see if the
  607  * connection is in the syn cache.  If it is, zap it.
  608  * If required send a challenge ACK.
  609  */
  610 void
  611 syncache_chkrst(struct in_conninfo *inc, struct tcphdr *th, struct mbuf *m,
  612     uint16_t port)
  613 {
  614         struct syncache *sc;
  615         struct syncache_head *sch;
  616         char *s = NULL;
  617 
  618         if (syncache_cookiesonly())
  619                 return;
  620         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  621         SCH_LOCK_ASSERT(sch);
  622 
  623         /*
  624          * Any RST to our SYN|ACK must not carry ACK, SYN or FIN flags.
  625          * See RFC 793 page 65, section SEGMENT ARRIVES.
  626          */
  627         if (tcp_get_flags(th) & (TH_ACK|TH_SYN|TH_FIN)) {
  628                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  629                         log(LOG_DEBUG, "%s; %s: Spurious RST with ACK, SYN or "
  630                             "FIN flag set, segment ignored\n", s, __func__);
  631                 TCPSTAT_INC(tcps_badrst);
  632                 goto done;
  633         }
  634 
  635         /*
  636          * No corresponding connection was found in syncache.
  637          * If syncookies are enabled and possibly exclusively
  638          * used, or we are under memory pressure, a valid RST
  639          * may not find a syncache entry.  In that case we're
  640          * done and no SYN|ACK retransmissions will happen.
  641          * Otherwise the RST was misdirected or spoofed.
  642          */
  643         if (sc == NULL) {
  644                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  645                         log(LOG_DEBUG, "%s; %s: Spurious RST without matching "
  646                             "syncache entry (possibly syncookie only), "
  647                             "segment ignored\n", s, __func__);
  648                 TCPSTAT_INC(tcps_badrst);
  649                 goto done;
  650         }
  651 
  652         /* The remote UDP encaps port does not match. */
  653         if (sc->sc_port != port) {
  654                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  655                         log(LOG_DEBUG, "%s; %s: Spurious RST with matching "
  656                             "syncache entry but non-matching UDP encaps port, "
  657                             "segment ignored\n", s, __func__);
  658                 TCPSTAT_INC(tcps_badrst);
  659                 goto done;
  660         }
  661 
  662         /*
  663          * If the RST bit is set, check the sequence number to see
  664          * if this is a valid reset segment.
  665          *
  666          * RFC 793 page 37:
  667          *   In all states except SYN-SENT, all reset (RST) segments
  668          *   are validated by checking their SEQ-fields.  A reset is
  669          *   valid if its sequence number is in the window.
  670          *
  671          * RFC 793 page 69:
  672          *   There are four cases for the acceptability test for an incoming
  673          *   segment:
  674          *
  675          * Segment Receive  Test
  676          * Length  Window
  677          * ------- -------  -------------------------------------------
  678          *    0       0     SEG.SEQ = RCV.NXT
  679          *    0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
  680          *   >0       0     not acceptable
  681          *   >0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
  682          *               or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
  683          *
  684          * Note that when receiving a SYN segment in the LISTEN state,
  685          * IRS is set to SEG.SEQ and RCV.NXT is set to SEG.SEQ+1, as
  686          * described in RFC 793, page 66.
  687          */
  688         if ((SEQ_GEQ(th->th_seq, sc->sc_irs + 1) &&
  689             SEQ_LT(th->th_seq, sc->sc_irs + 1 + sc->sc_wnd)) ||
  690             (sc->sc_wnd == 0 && th->th_seq == sc->sc_irs + 1)) {
  691                 if (V_tcp_insecure_rst ||
  692                     th->th_seq == sc->sc_irs + 1) {
  693                         syncache_drop(sc, sch);
  694                         if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  695                                 log(LOG_DEBUG,
  696                                     "%s; %s: Our SYN|ACK was rejected, "
  697                                     "connection attempt aborted by remote "
  698                                     "endpoint\n",
  699                                     s, __func__);
  700                         TCPSTAT_INC(tcps_sc_reset);
  701                 } else {
  702                         TCPSTAT_INC(tcps_badrst);
  703                         /* Send challenge ACK. */
  704                         if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  705                                 log(LOG_DEBUG, "%s; %s: RST with invalid "
  706                                     " SEQ %u != NXT %u (+WND %u), "
  707                                     "sending challenge ACK\n",
  708                                     s, __func__,
  709                                     th->th_seq, sc->sc_irs + 1, sc->sc_wnd);
  710                         syncache_respond(sc, m, TH_ACK);
  711                 }
  712         } else {
  713                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  714                         log(LOG_DEBUG, "%s; %s: RST with invalid SEQ %u != "
  715                             "NXT %u (+WND %u), segment ignored\n",
  716                             s, __func__,
  717                             th->th_seq, sc->sc_irs + 1, sc->sc_wnd);
  718                 TCPSTAT_INC(tcps_badrst);
  719         }
  720 
  721 done:
  722         if (s != NULL)
  723                 free(s, M_TCPLOG);
  724         SCH_UNLOCK(sch);
  725 }
  726 
  727 void
  728 syncache_badack(struct in_conninfo *inc, uint16_t port)
  729 {
  730         struct syncache *sc;
  731         struct syncache_head *sch;
  732 
  733         if (syncache_cookiesonly())
  734                 return;
  735         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  736         SCH_LOCK_ASSERT(sch);
  737         if ((sc != NULL) && (sc->sc_port == port)) {
  738                 syncache_drop(sc, sch);
  739                 TCPSTAT_INC(tcps_sc_badack);
  740         }
  741         SCH_UNLOCK(sch);
  742 }
  743 
  744 void
  745 syncache_unreach(struct in_conninfo *inc, tcp_seq th_seq, uint16_t port)
  746 {
  747         struct syncache *sc;
  748         struct syncache_head *sch;
  749 
  750         if (syncache_cookiesonly())
  751                 return;
  752         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  753         SCH_LOCK_ASSERT(sch);
  754         if (sc == NULL)
  755                 goto done;
  756 
  757         /* If the port != sc_port, then it's a bogus ICMP msg */
  758         if (port != sc->sc_port)
  759                 goto done;
  760 
  761         /* If the sequence number != sc_iss, then it's a bogus ICMP msg */
  762         if (ntohl(th_seq) != sc->sc_iss)
  763                 goto done;
  764 
  765         /*
  766          * If we've rertransmitted 3 times and this is our second error,
  767          * we remove the entry.  Otherwise, we allow it to continue on.
  768          * This prevents us from incorrectly nuking an entry during a
  769          * spurious network outage.
  770          *
  771          * See tcp_notify().
  772          */
  773         if ((sc->sc_flags & SCF_UNREACH) == 0 || sc->sc_rxmits < 3 + 1) {
  774                 sc->sc_flags |= SCF_UNREACH;
  775                 goto done;
  776         }
  777         syncache_drop(sc, sch);
  778         TCPSTAT_INC(tcps_sc_unreach);
  779 done:
  780         SCH_UNLOCK(sch);
  781 }
  782 
  783 /*
  784  * Build a new TCP socket structure from a syncache entry.
  785  *
  786  * On success return the newly created socket with its underlying inp locked.
  787  */
  788 static struct socket *
  789 syncache_socket(struct syncache *sc, struct socket *lso, struct mbuf *m)
  790 {
  791         struct tcp_function_block *blk;
  792         struct inpcb *inp = NULL;
  793         struct socket *so;
  794         struct tcpcb *tp;
  795         int error;
  796         char *s;
  797 
  798         NET_EPOCH_ASSERT();
  799 
  800         /*
  801          * Ok, create the full blown connection, and set things up
  802          * as they would have been set up if we had created the
  803          * connection when the SYN arrived.
  804          */
  805         if ((so = solisten_clone(lso)) == NULL)
  806                 goto allocfail;
  807 #ifdef MAC
  808         mac_socketpeer_set_from_mbuf(m, so);
  809 #endif
  810         error = in_pcballoc(so, &V_tcbinfo);
  811         if (error) {
  812                 sodealloc(so);
  813                 goto allocfail;
  814         }
  815         inp = sotoinpcb(so);
  816         if ((tp = tcp_newtcpcb(inp)) == NULL) {
  817                 in_pcbdetach(inp);
  818                 in_pcbfree(inp);
  819                 sodealloc(so);
  820                 goto allocfail;
  821         }
  822         inp->inp_inc.inc_flags = sc->sc_inc.inc_flags;
  823 #ifdef INET6
  824         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
  825                 inp->inp_vflag &= ~INP_IPV4;
  826                 inp->inp_vflag |= INP_IPV6;
  827                 inp->in6p_laddr = sc->sc_inc.inc6_laddr;
  828         } else {
  829                 inp->inp_vflag &= ~INP_IPV6;
  830                 inp->inp_vflag |= INP_IPV4;
  831 #endif
  832                 inp->inp_ip_ttl = sc->sc_ip_ttl;
  833                 inp->inp_ip_tos = sc->sc_ip_tos;
  834                 inp->inp_laddr = sc->sc_inc.inc_laddr;
  835 #ifdef INET6
  836         }
  837 #endif
  838 
  839         /*
  840          * If there's an mbuf and it has a flowid, then let's initialise the
  841          * inp with that particular flowid.
  842          */
  843         if (m != NULL && M_HASHTYPE_GET(m) != M_HASHTYPE_NONE) {
  844                 inp->inp_flowid = m->m_pkthdr.flowid;
  845                 inp->inp_flowtype = M_HASHTYPE_GET(m);
  846 #ifdef NUMA
  847                 inp->inp_numa_domain = m->m_pkthdr.numa_domain;
  848 #endif
  849         }
  850 
  851         inp->inp_lport = sc->sc_inc.inc_lport;
  852 #ifdef INET6
  853         if (inp->inp_vflag & INP_IPV6PROTO) {
  854                 struct inpcb *oinp = sotoinpcb(lso);
  855 
  856                 /*
  857                  * Inherit socket options from the listening socket.
  858                  * Note that in6p_inputopts are not (and should not be)
  859                  * copied, since it stores previously received options and is
  860                  * used to detect if each new option is different than the
  861                  * previous one and hence should be passed to a user.
  862                  * If we copied in6p_inputopts, a user would not be able to
  863                  * receive options just after calling the accept system call.
  864                  */
  865                 inp->inp_flags |= oinp->inp_flags & INP_CONTROLOPTS;
  866                 if (oinp->in6p_outputopts)
  867                         inp->in6p_outputopts =
  868                             ip6_copypktopts(oinp->in6p_outputopts, M_NOWAIT);
  869                 inp->in6p_hops = oinp->in6p_hops;
  870         }
  871 
  872         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
  873                 struct in6_addr laddr6;
  874                 struct sockaddr_in6 sin6;
  875 
  876                 sin6.sin6_family = AF_INET6;
  877                 sin6.sin6_len = sizeof(sin6);
  878                 sin6.sin6_addr = sc->sc_inc.inc6_faddr;
  879                 sin6.sin6_port = sc->sc_inc.inc_fport;
  880                 sin6.sin6_flowinfo = sin6.sin6_scope_id = 0;
  881                 laddr6 = inp->in6p_laddr;
  882                 if (IN6_IS_ADDR_UNSPECIFIED(&inp->in6p_laddr))
  883                         inp->in6p_laddr = sc->sc_inc.inc6_laddr;
  884                 INP_HASH_WLOCK(&V_tcbinfo);
  885                 error = in6_pcbconnect_mbuf(inp, (struct sockaddr *)&sin6,
  886                     thread0.td_ucred, m, false);
  887                 INP_HASH_WUNLOCK(&V_tcbinfo);
  888                 if (error != 0) {
  889                         inp->in6p_laddr = laddr6;
  890                         goto abort;
  891                 }
  892                 /* Override flowlabel from in6_pcbconnect. */
  893                 inp->inp_flow &= ~IPV6_FLOWLABEL_MASK;
  894                 inp->inp_flow |= sc->sc_flowlabel;
  895         }
  896 #endif /* INET6 */
  897 #if defined(INET) && defined(INET6)
  898         else
  899 #endif
  900 #ifdef INET
  901         {
  902                 struct in_addr laddr;
  903                 struct sockaddr_in sin;
  904 
  905                 inp->inp_options = (m) ? ip_srcroute(m) : NULL;
  906 
  907                 if (inp->inp_options == NULL) {
  908                         inp->inp_options = sc->sc_ipopts;
  909                         sc->sc_ipopts = NULL;
  910                 }
  911 
  912                 sin.sin_family = AF_INET;
  913                 sin.sin_len = sizeof(sin);
  914                 sin.sin_addr = sc->sc_inc.inc_faddr;
  915                 sin.sin_port = sc->sc_inc.inc_fport;
  916                 bzero((caddr_t)sin.sin_zero, sizeof(sin.sin_zero));
  917                 laddr = inp->inp_laddr;
  918                 if (inp->inp_laddr.s_addr == INADDR_ANY)
  919                         inp->inp_laddr = sc->sc_inc.inc_laddr;
  920                 INP_HASH_WLOCK(&V_tcbinfo);
  921                 error = in_pcbconnect(inp, (struct sockaddr *)&sin,
  922                     thread0.td_ucred, false);
  923                 INP_HASH_WUNLOCK(&V_tcbinfo);
  924                 if (error != 0) {
  925                         inp->inp_laddr = laddr;
  926                         goto abort;
  927                 }
  928         }
  929 #endif /* INET */
  930 #if defined(IPSEC) || defined(IPSEC_SUPPORT)
  931         /* Copy old policy into new socket's. */
  932         if (ipsec_copy_pcbpolicy(sotoinpcb(lso), inp) != 0)
  933                 printf("syncache_socket: could not copy policy\n");
  934 #endif
  935         tp->t_state = TCPS_SYN_RECEIVED;
  936         tp->iss = sc->sc_iss;
  937         tp->irs = sc->sc_irs;
  938         tp->t_port = sc->sc_port;
  939         tcp_rcvseqinit(tp);
  940         tcp_sendseqinit(tp);
  941         blk = sototcpcb(lso)->t_fb;
  942         if (V_functions_inherit_listen_socket_stack && blk != tp->t_fb) {
  943                 /*
  944                  * Our parents t_fb was not the default,
  945                  * we need to release our ref on tp->t_fb and
  946                  * pickup one on the new entry.
  947                  */
  948                 struct tcp_function_block *rblk;
  949 
  950                 rblk = find_and_ref_tcp_fb(blk);
  951                 KASSERT(rblk != NULL,
  952                     ("cannot find blk %p out of syncache?", blk));
  953                 if (tp->t_fb->tfb_tcp_fb_fini)
  954                         (*tp->t_fb->tfb_tcp_fb_fini)(tp, 0);
  955                 refcount_release(&tp->t_fb->tfb_refcnt);
  956                 tp->t_fb = rblk;
  957                 /*
  958                  * XXXrrs this is quite dangerous, it is possible
  959                  * for the new function to fail to init. We also
  960                  * are not asking if the handoff_is_ok though at
  961                  * the very start thats probalbly ok.
  962                  */
  963                 if (tp->t_fb->tfb_tcp_fb_init) {
  964                         (*tp->t_fb->tfb_tcp_fb_init)(tp);
  965                 }
  966         }
  967         tp->snd_wl1 = sc->sc_irs;
  968         tp->snd_max = tp->iss + 1;
  969         tp->snd_nxt = tp->iss + 1;
  970         tp->rcv_up = sc->sc_irs + 1;
  971         tp->rcv_wnd = sc->sc_wnd;
  972         tp->rcv_adv += tp->rcv_wnd;
  973         tp->last_ack_sent = tp->rcv_nxt;
  974 
  975         tp->t_flags = sototcpcb(lso)->t_flags & (TF_NOPUSH|TF_NODELAY);
  976         if (sc->sc_flags & SCF_NOOPT)
  977                 tp->t_flags |= TF_NOOPT;
  978         else {
  979                 if (sc->sc_flags & SCF_WINSCALE) {
  980                         tp->t_flags |= TF_REQ_SCALE|TF_RCVD_SCALE;
  981                         tp->snd_scale = sc->sc_requested_s_scale;
  982                         tp->request_r_scale = sc->sc_requested_r_scale;
  983                 }
  984                 if (sc->sc_flags & SCF_TIMESTAMP) {
  985                         tp->t_flags |= TF_REQ_TSTMP|TF_RCVD_TSTMP;
  986                         tp->ts_recent = sc->sc_tsreflect;
  987                         tp->ts_recent_age = tcp_ts_getticks();
  988                         tp->ts_offset = sc->sc_tsoff;
  989                 }
  990 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
  991                 if (sc->sc_flags & SCF_SIGNATURE)
  992                         tp->t_flags |= TF_SIGNATURE;
  993 #endif
  994                 if (sc->sc_flags & SCF_SACK)
  995                         tp->t_flags |= TF_SACK_PERMIT;
  996         }
  997 
  998         tcp_ecn_syncache_socket(tp, sc);
  999 
 1000         /*
 1001          * Set up MSS and get cached values from tcp_hostcache.
 1002          * This might overwrite some of the defaults we just set.
 1003          */
 1004         tcp_mss(tp, sc->sc_peer_mss);
 1005 
 1006         /*
 1007          * If the SYN,ACK was retransmitted, indicate that CWND to be
 1008          * limited to one segment in cc_conn_init().
 1009          * NB: sc_rxmits counts all SYN,ACK transmits, not just retransmits.
 1010          */
 1011         if (sc->sc_rxmits > 1)
 1012                 tp->snd_cwnd = 1;
 1013 
 1014 #ifdef TCP_OFFLOAD
 1015         /*
 1016          * Allow a TOE driver to install its hooks.  Note that we hold the
 1017          * pcbinfo lock too and that prevents tcp_usr_accept from accepting a
 1018          * new connection before the TOE driver has done its thing.
 1019          */
 1020         if (ADDED_BY_TOE(sc)) {
 1021                 struct toedev *tod = sc->sc_tod;
 1022 
 1023                 tod->tod_offload_socket(tod, sc->sc_todctx, so);
 1024         }
 1025 #endif
 1026         /*
 1027          * Copy and activate timers.
 1028          */
 1029         tp->t_maxunacktime = sototcpcb(lso)->t_maxunacktime;
 1030         tp->t_keepinit = sototcpcb(lso)->t_keepinit;
 1031         tp->t_keepidle = sototcpcb(lso)->t_keepidle;
 1032         tp->t_keepintvl = sototcpcb(lso)->t_keepintvl;
 1033         tp->t_keepcnt = sototcpcb(lso)->t_keepcnt;
 1034         tcp_timer_activate(tp, TT_KEEP, TP_KEEPINIT(tp));
 1035 
 1036         TCPSTAT_INC(tcps_accepts);
 1037         TCP_PROBE6(state__change, NULL, tp, NULL, tp, NULL, TCPS_LISTEN);
 1038 
 1039         if (!solisten_enqueue(so, SS_ISCONNECTED))
 1040                 tp->t_flags |= TF_SONOTCONN;
 1041 
 1042         return (so);
 1043 
 1044 allocfail:
 1045         /*
 1046          * Drop the connection; we will either send a RST or have the peer
 1047          * retransmit its SYN again after its RTO and try again.
 1048          */
 1049         if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
 1050                 log(LOG_DEBUG, "%s; %s: Socket create failed "
 1051                     "due to limits or memory shortage\n",
 1052                     s, __func__);
 1053                 free(s, M_TCPLOG);
 1054         }
 1055         TCPSTAT_INC(tcps_listendrop);
 1056         return (NULL);
 1057 
 1058 abort:
 1059         in_pcbdetach(inp);
 1060         in_pcbfree(inp);
 1061         sodealloc(so);
 1062         if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
 1063                 log(LOG_DEBUG, "%s; %s: in%s_pcbconnect failed with error %i\n",
 1064                     s, __func__, (sc->sc_inc.inc_flags & INC_ISIPV6) ? "6" : "",
 1065                     error);
 1066                 free(s, M_TCPLOG);
 1067         }
 1068         TCPSTAT_INC(tcps_listendrop);
 1069         return (NULL);
 1070 }
 1071 
 1072 /*
 1073  * This function gets called when we receive an ACK for a
 1074  * socket in the LISTEN state.  We look up the connection
 1075  * in the syncache, and if its there, we pull it out of
 1076  * the cache and turn it into a full-blown connection in
 1077  * the SYN-RECEIVED state.
 1078  *
 1079  * On syncache_socket() success the newly created socket
 1080  * has its underlying inp locked.
 1081  */
 1082 int
 1083 syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
 1084     struct socket **lsop, struct mbuf *m, uint16_t port)
 1085 {
 1086         struct syncache *sc;
 1087         struct syncache_head *sch;
 1088         struct syncache scs;
 1089         char *s;
 1090         bool locked;
 1091 
 1092         NET_EPOCH_ASSERT();
 1093         KASSERT((tcp_get_flags(th) & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK,
 1094             ("%s: can handle only ACK", __func__));
 1095 
 1096         if (syncache_cookiesonly()) {
 1097                 sc = NULL;
 1098                 sch = syncache_hashbucket(inc);
 1099                 locked = false;
 1100         } else {
 1101                 sc = syncache_lookup(inc, &sch);        /* returns locked sch */
 1102                 locked = true;
 1103                 SCH_LOCK_ASSERT(sch);
 1104         }
 1105 
 1106 #ifdef INVARIANTS
 1107         /*
 1108          * Test code for syncookies comparing the syncache stored
 1109          * values with the reconstructed values from the cookie.
 1110          */
 1111         if (sc != NULL)
 1112                 syncookie_cmp(inc, sch, sc, th, to, *lsop, port);
 1113 #endif
 1114 
 1115         if (sc == NULL) {
 1116                 /*
 1117                  * There is no syncache entry, so see if this ACK is
 1118                  * a returning syncookie.  To do this, first:
 1119                  *  A. Check if syncookies are used in case of syncache
 1120                  *     overflows
 1121                  *  B. See if this socket has had a syncache entry dropped in
 1122                  *     the recent past. We don't want to accept a bogus
 1123                  *     syncookie if we've never received a SYN or accept it
 1124                  *     twice.
 1125                  *  C. check that the syncookie is valid.  If it is, then
 1126                  *     cobble up a fake syncache entry, and return.
 1127                  */
 1128                 if (locked && !V_tcp_syncookies) {
 1129                         SCH_UNLOCK(sch);
 1130                         if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1131                                 log(LOG_DEBUG, "%s; %s: Spurious ACK, "
 1132                                     "segment rejected (syncookies disabled)\n",
 1133                                     s, __func__);
 1134                         goto failed;
 1135                 }
 1136                 if (locked && !V_tcp_syncookiesonly &&
 1137                     sch->sch_last_overflow < time_uptime - SYNCOOKIE_LIFETIME) {
 1138                         SCH_UNLOCK(sch);
 1139                         if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1140                                 log(LOG_DEBUG, "%s; %s: Spurious ACK, "
 1141                                     "segment rejected (no syncache entry)\n",
 1142                                     s, __func__);
 1143                         goto failed;
 1144                 }
 1145                 bzero(&scs, sizeof(scs));
 1146                 sc = syncookie_lookup(inc, sch, &scs, th, to, *lsop, port);
 1147                 if (locked)
 1148                         SCH_UNLOCK(sch);
 1149                 if (sc == NULL) {
 1150                         if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1151                                 log(LOG_DEBUG, "%s; %s: Segment failed "
 1152                                     "SYNCOOKIE authentication, segment rejected "
 1153                                     "(probably spoofed)\n", s, __func__);
 1154                         goto failed;
 1155                 }
 1156 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
 1157                 /* If received ACK has MD5 signature, check it. */
 1158                 if ((to->to_flags & TOF_SIGNATURE) != 0 &&
 1159                     (!TCPMD5_ENABLED() ||
 1160                     TCPMD5_INPUT(m, th, to->to_signature) != 0)) {
 1161                         /* Drop the ACK. */
 1162                         if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1163                                 log(LOG_DEBUG, "%s; %s: Segment rejected, "
 1164                                     "MD5 signature doesn't match.\n",
 1165                                     s, __func__);
 1166                                 free(s, M_TCPLOG);
 1167                         }
 1168                         TCPSTAT_INC(tcps_sig_err_sigopt);
 1169                         return (-1); /* Do not send RST */
 1170                 }
 1171 #endif /* TCP_SIGNATURE */
 1172                 TCPSTATES_INC(TCPS_SYN_RECEIVED);
 1173         } else {
 1174                 if (sc->sc_port != port) {
 1175                         SCH_UNLOCK(sch);
 1176                         return (0);
 1177                 }
 1178 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
 1179                 /*
 1180                  * If listening socket requested TCP digests, check that
 1181                  * received ACK has signature and it is correct.
 1182                  * If not, drop the ACK and leave sc entry in th cache,
 1183                  * because SYN was received with correct signature.
 1184                  */
 1185                 if (sc->sc_flags & SCF_SIGNATURE) {
 1186                         if ((to->to_flags & TOF_SIGNATURE) == 0) {
 1187                                 /* No signature */
 1188                                 TCPSTAT_INC(tcps_sig_err_nosigopt);
 1189                                 SCH_UNLOCK(sch);
 1190                                 if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1191                                         log(LOG_DEBUG, "%s; %s: Segment "
 1192                                             "rejected, MD5 signature wasn't "
 1193                                             "provided.\n", s, __func__);
 1194                                         free(s, M_TCPLOG);
 1195                                 }
 1196                                 return (-1); /* Do not send RST */
 1197                         }
 1198                         if (!TCPMD5_ENABLED() ||
 1199                             TCPMD5_INPUT(m, th, to->to_signature) != 0) {
 1200                                 /* Doesn't match or no SA */
 1201                                 SCH_UNLOCK(sch);
 1202                                 if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1203                                         log(LOG_DEBUG, "%s; %s: Segment "
 1204                                             "rejected, MD5 signature doesn't "
 1205                                             "match.\n", s, __func__);
 1206                                         free(s, M_TCPLOG);
 1207                                 }
 1208                                 return (-1); /* Do not send RST */
 1209                         }
 1210                 }
 1211 #endif /* TCP_SIGNATURE */
 1212 
 1213                 /*
 1214                  * RFC 7323 PAWS: If we have a timestamp on this segment and
 1215                  * it's less than ts_recent, drop it.
 1216                  * XXXMT: RFC 7323 also requires to send an ACK.
 1217                  *        In tcp_input.c this is only done for TCP segments
 1218                  *        with user data, so be consistent here and just drop
 1219                  *        the segment.
 1220                  */
 1221                 if (sc->sc_flags & SCF_TIMESTAMP && to->to_flags & TOF_TS &&
 1222                     TSTMP_LT(to->to_tsval, sc->sc_tsreflect)) {
 1223                         SCH_UNLOCK(sch);
 1224                         if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1225                                 log(LOG_DEBUG,
 1226                                     "%s; %s: SEG.TSval %u < TS.Recent %u, "
 1227                                     "segment dropped\n", s, __func__,
 1228                                     to->to_tsval, sc->sc_tsreflect);
 1229                                 free(s, M_TCPLOG);
 1230                         }
 1231                         return (-1);  /* Do not send RST */
 1232                 }
 1233 
 1234                 /*
 1235                  * If timestamps were not negotiated during SYN/ACK and a
 1236                  * segment with a timestamp is received, ignore the
 1237                  * timestamp and process the packet normally.
 1238                  * See section 3.2 of RFC 7323.
 1239                  */
 1240                 if (!(sc->sc_flags & SCF_TIMESTAMP) &&
 1241                     (to->to_flags & TOF_TS)) {
 1242                         if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1243                                 log(LOG_DEBUG, "%s; %s: Timestamp not "
 1244                                     "expected, segment processed normally\n",
 1245                                     s, __func__);
 1246                                 free(s, M_TCPLOG);
 1247                                 s = NULL;
 1248                         }
 1249                 }
 1250 
 1251                 /*
 1252                  * If timestamps were negotiated during SYN/ACK and a
 1253                  * segment without a timestamp is received, silently drop
 1254                  * the segment, unless the missing timestamps are tolerated.
 1255                  * See section 3.2 of RFC 7323.
 1256                  */
 1257                 if ((sc->sc_flags & SCF_TIMESTAMP) &&
 1258                     !(to->to_flags & TOF_TS)) {
 1259                         if (V_tcp_tolerate_missing_ts) {
 1260                                 if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1261                                         log(LOG_DEBUG,
 1262                                             "%s; %s: Timestamp missing, "
 1263                                             "segment processed normally\n",
 1264                                             s, __func__);
 1265                                         free(s, M_TCPLOG);
 1266                                 }
 1267                         } else {
 1268                                 SCH_UNLOCK(sch);
 1269                                 if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1270                                         log(LOG_DEBUG,
 1271                                             "%s; %s: Timestamp missing, "
 1272                                             "segment silently dropped\n",
 1273                                             s, __func__);
 1274                                         free(s, M_TCPLOG);
 1275                                 }
 1276                                 return (-1);  /* Do not send RST */
 1277                         }
 1278                 }
 1279                 TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
 1280                 sch->sch_length--;
 1281 #ifdef TCP_OFFLOAD
 1282                 if (ADDED_BY_TOE(sc)) {
 1283                         struct toedev *tod = sc->sc_tod;
 1284 
 1285                         tod->tod_syncache_removed(tod, sc->sc_todctx);
 1286                 }
 1287 #endif
 1288                 SCH_UNLOCK(sch);
 1289         }
 1290 
 1291         /*
 1292          * Segment validation:
 1293          * ACK must match our initial sequence number + 1 (the SYN|ACK).
 1294          */
 1295         if (th->th_ack != sc->sc_iss + 1) {
 1296                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1297                         log(LOG_DEBUG, "%s; %s: ACK %u != ISS+1 %u, segment "
 1298                             "rejected\n", s, __func__, th->th_ack, sc->sc_iss);
 1299                 goto failed;
 1300         }
 1301 
 1302         /*
 1303          * The SEQ must fall in the window starting at the received
 1304          * initial receive sequence number + 1 (the SYN).
 1305          */
 1306         if (SEQ_LEQ(th->th_seq, sc->sc_irs) ||
 1307             SEQ_GT(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
 1308                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1309                         log(LOG_DEBUG, "%s; %s: SEQ %u != IRS+1 %u, segment "
 1310                             "rejected\n", s, __func__, th->th_seq, sc->sc_irs);
 1311                 goto failed;
 1312         }
 1313 
 1314         *lsop = syncache_socket(sc, *lsop, m);
 1315 
 1316         if (__predict_false(*lsop == NULL)) {
 1317                 TCPSTAT_INC(tcps_sc_aborted);
 1318                 TCPSTATES_DEC(TCPS_SYN_RECEIVED);
 1319         } else
 1320                 TCPSTAT_INC(tcps_sc_completed);
 1321 
 1322 /* how do we find the inp for the new socket? */
 1323         if (sc != &scs)
 1324                 syncache_free(sc);
 1325         return (1);
 1326 failed:
 1327         if (sc != NULL) {
 1328                 TCPSTATES_DEC(TCPS_SYN_RECEIVED);
 1329                 if (sc != &scs)
 1330                         syncache_free(sc);
 1331         }
 1332         if (s != NULL)
 1333                 free(s, M_TCPLOG);
 1334         *lsop = NULL;
 1335         return (0);
 1336 }
 1337 
 1338 static struct socket *
 1339 syncache_tfo_expand(struct syncache *sc, struct socket *lso, struct mbuf *m,
 1340     uint64_t response_cookie)
 1341 {
 1342         struct inpcb *inp;
 1343         struct tcpcb *tp;
 1344         unsigned int *pending_counter;
 1345         struct socket *so;
 1346 
 1347         NET_EPOCH_ASSERT();
 1348 
 1349         pending_counter = intotcpcb(sotoinpcb(lso))->t_tfo_pending;
 1350         so = syncache_socket(sc, lso, m);
 1351         if (so == NULL) {
 1352                 TCPSTAT_INC(tcps_sc_aborted);
 1353                 atomic_subtract_int(pending_counter, 1);
 1354         } else {
 1355                 soisconnected(so);
 1356                 inp = sotoinpcb(so);
 1357                 tp = intotcpcb(inp);
 1358                 tp->t_flags |= TF_FASTOPEN;
 1359                 tp->t_tfo_cookie.server = response_cookie;
 1360                 tp->snd_max = tp->iss;
 1361                 tp->snd_nxt = tp->iss;
 1362                 tp->t_tfo_pending = pending_counter;
 1363                 TCPSTATES_INC(TCPS_SYN_RECEIVED);
 1364                 TCPSTAT_INC(tcps_sc_completed);
 1365         }
 1366 
 1367         return (so);
 1368 }
 1369 
 1370 /*
 1371  * Given a LISTEN socket and an inbound SYN request, add
 1372  * this to the syn cache, and send back a segment:
 1373  *      <SEQ=ISS><ACK=RCV_NXT><CTL=SYN,ACK>
 1374  * to the source.
 1375  *
 1376  * IMPORTANT NOTE: We do _NOT_ ACK data that might accompany the SYN.
 1377  * Doing so would require that we hold onto the data and deliver it
 1378  * to the application.  However, if we are the target of a SYN-flood
 1379  * DoS attack, an attacker could send data which would eventually
 1380  * consume all available buffer space if it were ACKed.  By not ACKing
 1381  * the data, we avoid this DoS scenario.
 1382  *
 1383  * The exception to the above is when a SYN with a valid TCP Fast Open (TFO)
 1384  * cookie is processed and a new socket is created.  In this case, any data
 1385  * accompanying the SYN will be queued to the socket by tcp_input() and will
 1386  * be ACKed either when the application sends response data or the delayed
 1387  * ACK timer expires, whichever comes first.
 1388  */
 1389 struct socket *
 1390 syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
 1391     struct inpcb *inp, struct socket *so, struct mbuf *m, void *tod,
 1392     void *todctx, uint8_t iptos, uint16_t port)
 1393 {
 1394         struct tcpcb *tp;
 1395         struct socket *rv = NULL;
 1396         struct syncache *sc = NULL;
 1397         struct syncache_head *sch;
 1398         struct mbuf *ipopts = NULL;
 1399         u_int ltflags;
 1400         int win, ip_ttl, ip_tos;
 1401         char *s;
 1402 #ifdef INET6
 1403         int autoflowlabel = 0;
 1404 #endif
 1405 #ifdef MAC
 1406         struct label *maclabel;
 1407 #endif
 1408         struct syncache scs;
 1409         struct ucred *cred;
 1410         uint64_t tfo_response_cookie;
 1411         unsigned int *tfo_pending = NULL;
 1412         int tfo_cookie_valid = 0;
 1413         int tfo_response_cookie_valid = 0;
 1414         bool locked;
 1415 
 1416         INP_RLOCK_ASSERT(inp);                  /* listen socket */
 1417         KASSERT((tcp_get_flags(th) & (TH_RST|TH_ACK|TH_SYN)) == TH_SYN,
 1418             ("%s: unexpected tcp flags", __func__));
 1419 
 1420         /*
 1421          * Combine all so/tp operations very early to drop the INP lock as
 1422          * soon as possible.
 1423          */
 1424         KASSERT(SOLISTENING(so), ("%s: %p not listening", __func__, so));
 1425         tp = sototcpcb(so);
 1426         cred = V_tcp_syncache.see_other ? NULL : crhold(so->so_cred);
 1427 
 1428 #ifdef INET6
 1429         if (inc->inc_flags & INC_ISIPV6) {
 1430                 if (inp->inp_flags & IN6P_AUTOFLOWLABEL) {
 1431                         autoflowlabel = 1;
 1432                 }
 1433                 ip_ttl = in6_selecthlim(inp, NULL);
 1434                 if ((inp->in6p_outputopts == NULL) ||
 1435                     (inp->in6p_outputopts->ip6po_tclass == -1)) {
 1436                         ip_tos = 0;
 1437                 } else {
 1438                         ip_tos = inp->in6p_outputopts->ip6po_tclass;
 1439                 }
 1440         }
 1441 #endif
 1442 #if defined(INET6) && defined(INET)
 1443         else
 1444 #endif
 1445 #ifdef INET
 1446         {
 1447                 ip_ttl = inp->inp_ip_ttl;
 1448                 ip_tos = inp->inp_ip_tos;
 1449         }
 1450 #endif
 1451         win = so->sol_sbrcv_hiwat;
 1452         ltflags = (tp->t_flags & (TF_NOOPT | TF_SIGNATURE));
 1453 
 1454         if (V_tcp_fastopen_server_enable && IS_FASTOPEN(tp->t_flags) &&
 1455             (tp->t_tfo_pending != NULL) &&
 1456             (to->to_flags & TOF_FASTOPEN)) {
 1457                 /*
 1458                  * Limit the number of pending TFO connections to
 1459                  * approximately half of the queue limit.  This prevents TFO
 1460                  * SYN floods from starving the service by filling the
 1461                  * listen queue with bogus TFO connections.
 1462                  */
 1463                 if (atomic_fetchadd_int(tp->t_tfo_pending, 1) <=
 1464                     (so->sol_qlimit / 2)) {
 1465                         int result;
 1466 
 1467                         result = tcp_fastopen_check_cookie(inc,
 1468                             to->to_tfo_cookie, to->to_tfo_len,
 1469                             &tfo_response_cookie);
 1470                         tfo_cookie_valid = (result > 0);
 1471                         tfo_response_cookie_valid = (result >= 0);
 1472                 }
 1473 
 1474                 /*
 1475                  * Remember the TFO pending counter as it will have to be
 1476                  * decremented below if we don't make it to syncache_tfo_expand().
 1477                  */
 1478                 tfo_pending = tp->t_tfo_pending;
 1479         }
 1480 
 1481 #ifdef MAC
 1482         if (mac_syncache_init(&maclabel) != 0) {
 1483                 INP_RUNLOCK(inp);
 1484                 goto done;
 1485         } else
 1486                 mac_syncache_create(maclabel, inp);
 1487 #endif
 1488         if (!tfo_cookie_valid)
 1489                 INP_RUNLOCK(inp);
 1490 
 1491         /*
 1492          * Remember the IP options, if any.
 1493          */
 1494 #ifdef INET6
 1495         if (!(inc->inc_flags & INC_ISIPV6))
 1496 #endif
 1497 #ifdef INET
 1498                 ipopts = (m) ? ip_srcroute(m) : NULL;
 1499 #else
 1500                 ipopts = NULL;
 1501 #endif
 1502 
 1503 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
 1504         /*
 1505          * When the socket is TCP-MD5 enabled check that,
 1506          *  - a signed packet is valid
 1507          *  - a non-signed packet does not have a security association
 1508          *
 1509          *  If a signed packet fails validation or a non-signed packet has a
 1510          *  security association, the packet will be dropped.
 1511          */
 1512         if (ltflags & TF_SIGNATURE) {
 1513                 if (to->to_flags & TOF_SIGNATURE) {
 1514                         if (!TCPMD5_ENABLED() ||
 1515                             TCPMD5_INPUT(m, th, to->to_signature) != 0)
 1516                                 goto done;
 1517                 } else {
 1518                         if (TCPMD5_ENABLED() &&
 1519                             TCPMD5_INPUT(m, NULL, NULL) != ENOENT)
 1520                                 goto done;
 1521                 }
 1522         } else if (to->to_flags & TOF_SIGNATURE)
 1523                 goto done;
 1524 #endif  /* TCP_SIGNATURE */
 1525         /*
 1526          * See if we already have an entry for this connection.
 1527          * If we do, resend the SYN,ACK, and reset the retransmit timer.
 1528          *
 1529          * XXX: should the syncache be re-initialized with the contents
 1530          * of the new SYN here (which may have different options?)
 1531          *
 1532          * XXX: We do not check the sequence number to see if this is a
 1533          * real retransmit or a new connection attempt.  The question is
 1534          * how to handle such a case; either ignore it as spoofed, or
 1535          * drop the current entry and create a new one?
 1536          */
 1537         if (syncache_cookiesonly()) {
 1538                 sc = NULL;
 1539                 sch = syncache_hashbucket(inc);
 1540                 locked = false;
 1541         } else {
 1542                 sc = syncache_lookup(inc, &sch);        /* returns locked sch */
 1543                 locked = true;
 1544                 SCH_LOCK_ASSERT(sch);
 1545         }
 1546         if (sc != NULL) {
 1547                 if (tfo_cookie_valid)
 1548                         INP_RUNLOCK(inp);
 1549                 TCPSTAT_INC(tcps_sc_dupsyn);
 1550                 if (ipopts) {
 1551                         /*
 1552                          * If we were remembering a previous source route,
 1553                          * forget it and use the new one we've been given.
 1554                          */
 1555                         if (sc->sc_ipopts)
 1556                                 (void) m_free(sc->sc_ipopts);
 1557                         sc->sc_ipopts = ipopts;
 1558                 }
 1559                 /*
 1560                  * Update timestamp if present.
 1561                  */
 1562                 if ((sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS))
 1563                         sc->sc_tsreflect = to->to_tsval;
 1564                 else
 1565                         sc->sc_flags &= ~SCF_TIMESTAMP;
 1566                 /*
 1567                  * Adjust ECN response if needed, e.g. different
 1568                  * IP ECN field, or a fallback by the remote host.
 1569                  */
 1570                 if (sc->sc_flags & SCF_ECN_MASK) {
 1571                         sc->sc_flags &= ~SCF_ECN_MASK;
 1572                         sc->sc_flags = tcp_ecn_syncache_add(tcp_get_flags(th), iptos);
 1573                 }
 1574 #ifdef MAC
 1575                 /*
 1576                  * Since we have already unconditionally allocated label
 1577                  * storage, free it up.  The syncache entry will already
 1578                  * have an initialized label we can use.
 1579                  */
 1580                 mac_syncache_destroy(&maclabel);
 1581 #endif
 1582                 TCP_PROBE5(receive, NULL, NULL, m, NULL, th);
 1583                 /* Retransmit SYN|ACK and reset retransmit count. */
 1584                 if ((s = tcp_log_addrs(&sc->sc_inc, th, NULL, NULL))) {
 1585                         log(LOG_DEBUG, "%s; %s: Received duplicate SYN, "
 1586                             "resetting timer and retransmitting SYN|ACK\n",
 1587                             s, __func__);
 1588                         free(s, M_TCPLOG);
 1589                 }
 1590                 if (syncache_respond(sc, m, TH_SYN|TH_ACK) == 0) {
 1591                         sc->sc_rxmits = 0;
 1592                         syncache_timeout(sc, sch, 1);
 1593                         TCPSTAT_INC(tcps_sndacks);
 1594                         TCPSTAT_INC(tcps_sndtotal);
 1595                 }
 1596                 SCH_UNLOCK(sch);
 1597                 goto donenoprobe;
 1598         }
 1599 
 1600         if (tfo_cookie_valid) {
 1601                 bzero(&scs, sizeof(scs));
 1602                 sc = &scs;
 1603                 goto skip_alloc;
 1604         }
 1605 
 1606         /*
 1607          * Skip allocating a syncache entry if we are just going to discard
 1608          * it later.
 1609          */
 1610         if (!locked) {
 1611                 bzero(&scs, sizeof(scs));
 1612                 sc = &scs;
 1613         } else
 1614                 sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
 1615         if (sc == NULL) {
 1616                 /*
 1617                  * The zone allocator couldn't provide more entries.
 1618                  * Treat this as if the cache was full; drop the oldest
 1619                  * entry and insert the new one.
 1620                  */
 1621                 TCPSTAT_INC(tcps_sc_zonefail);
 1622                 if ((sc = TAILQ_LAST(&sch->sch_bucket, sch_head)) != NULL) {
 1623                         sch->sch_last_overflow = time_uptime;
 1624                         syncache_drop(sc, sch);
 1625                         syncache_pause(inc);
 1626                 }
 1627                 sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
 1628                 if (sc == NULL) {
 1629                         if (V_tcp_syncookies) {
 1630                                 bzero(&scs, sizeof(scs));
 1631                                 sc = &scs;
 1632                         } else {
 1633                                 KASSERT(locked,
 1634                                     ("%s: bucket unexpectedly unlocked",
 1635                                     __func__));
 1636                                 SCH_UNLOCK(sch);
 1637                                 if (ipopts)
 1638                                         (void) m_free(ipopts);
 1639                                 goto done;
 1640                         }
 1641                 }
 1642         }
 1643 
 1644 skip_alloc:
 1645         if (!tfo_cookie_valid && tfo_response_cookie_valid)
 1646                 sc->sc_tfo_cookie = &tfo_response_cookie;
 1647 
 1648         /*
 1649          * Fill in the syncache values.
 1650          */
 1651 #ifdef MAC
 1652         sc->sc_label = maclabel;
 1653 #endif
 1654         sc->sc_cred = cred;
 1655         sc->sc_port = port;
 1656         cred = NULL;
 1657         sc->sc_ipopts = ipopts;
 1658         bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
 1659         sc->sc_ip_tos = ip_tos;
 1660         sc->sc_ip_ttl = ip_ttl;
 1661 #ifdef TCP_OFFLOAD
 1662         sc->sc_tod = tod;
 1663         sc->sc_todctx = todctx;
 1664 #endif
 1665         sc->sc_irs = th->th_seq;
 1666         sc->sc_flags = 0;
 1667         sc->sc_flowlabel = 0;
 1668 
 1669         /*
 1670          * Initial receive window: clip sbspace to [0 .. TCP_MAXWIN].
 1671          * win was derived from socket earlier in the function.
 1672          */
 1673         win = imax(win, 0);
 1674         win = imin(win, TCP_MAXWIN);
 1675         sc->sc_wnd = win;
 1676 
 1677         if (V_tcp_do_rfc1323 &&
 1678             !(ltflags & TF_NOOPT)) {
 1679                 /*
 1680                  * A timestamp received in a SYN makes
 1681                  * it ok to send timestamp requests and replies.
 1682                  */
 1683                 if ((to->to_flags & TOF_TS) && (V_tcp_do_rfc1323 != 2)) {
 1684                         sc->sc_tsreflect = to->to_tsval;
 1685                         sc->sc_flags |= SCF_TIMESTAMP;
 1686                         sc->sc_tsoff = tcp_new_ts_offset(inc);
 1687                 }
 1688                 if ((to->to_flags & TOF_SCALE) && (V_tcp_do_rfc1323 != 3)) {
 1689                         int wscale = 0;
 1690 
 1691                         /*
 1692                          * Pick the smallest possible scaling factor that
 1693                          * will still allow us to scale up to sb_max, aka
 1694                          * kern.ipc.maxsockbuf.
 1695                          *
 1696                          * We do this because there are broken firewalls that
 1697                          * will corrupt the window scale option, leading to
 1698                          * the other endpoint believing that our advertised
 1699                          * window is unscaled.  At scale factors larger than
 1700                          * 5 the unscaled window will drop below 1500 bytes,
 1701                          * leading to serious problems when traversing these
 1702                          * broken firewalls.
 1703                          *
 1704                          * With the default maxsockbuf of 256K, a scale factor
 1705                          * of 3 will be chosen by this algorithm.  Those who
 1706                          * choose a larger maxsockbuf should watch out
 1707                          * for the compatibility problems mentioned above.
 1708                          *
 1709                          * RFC1323: The Window field in a SYN (i.e., a <SYN>
 1710                          * or <SYN,ACK>) segment itself is never scaled.
 1711                          */
 1712                         while (wscale < TCP_MAX_WINSHIFT &&
 1713                             (TCP_MAXWIN << wscale) < sb_max)
 1714                                 wscale++;
 1715                         sc->sc_requested_r_scale = wscale;
 1716                         sc->sc_requested_s_scale = to->to_wscale;
 1717                         sc->sc_flags |= SCF_WINSCALE;
 1718                 }
 1719         }
 1720 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
 1721         /*
 1722          * If incoming packet has an MD5 signature, flag this in the
 1723          * syncache so that syncache_respond() will do the right thing
 1724          * with the SYN+ACK.
 1725          */
 1726         if (to->to_flags & TOF_SIGNATURE)
 1727                 sc->sc_flags |= SCF_SIGNATURE;
 1728 #endif  /* TCP_SIGNATURE */
 1729         if (to->to_flags & TOF_SACKPERM)
 1730                 sc->sc_flags |= SCF_SACK;
 1731         if (to->to_flags & TOF_MSS)
 1732                 sc->sc_peer_mss = to->to_mss;   /* peer mss may be zero */
 1733         if (ltflags & TF_NOOPT)
 1734                 sc->sc_flags |= SCF_NOOPT;
 1735         /* ECN Handshake */
 1736         if (V_tcp_do_ecn)
 1737                 sc->sc_flags |= tcp_ecn_syncache_add(tcp_get_flags(th), iptos);
 1738 
 1739         if (V_tcp_syncookies)
 1740                 sc->sc_iss = syncookie_generate(sch, sc);
 1741         else
 1742                 sc->sc_iss = arc4random();
 1743 #ifdef INET6
 1744         if (autoflowlabel) {
 1745                 if (V_tcp_syncookies)
 1746                         sc->sc_flowlabel = sc->sc_iss;
 1747                 else
 1748                         sc->sc_flowlabel = ip6_randomflowlabel();
 1749                 sc->sc_flowlabel = htonl(sc->sc_flowlabel) & IPV6_FLOWLABEL_MASK;
 1750         }
 1751 #endif
 1752         if (locked)
 1753                 SCH_UNLOCK(sch);
 1754 
 1755         if (tfo_cookie_valid) {
 1756                 rv = syncache_tfo_expand(sc, so, m, tfo_response_cookie);
 1757                 /* INP_RUNLOCK(inp) will be performed by the caller */
 1758                 goto tfo_expanded;
 1759         }
 1760 
 1761         TCP_PROBE5(receive, NULL, NULL, m, NULL, th);
 1762         /*
 1763          * Do a standard 3-way handshake.
 1764          */
 1765         if (syncache_respond(sc, m, TH_SYN|TH_ACK) == 0) {
 1766                 if (V_tcp_syncookies && V_tcp_syncookiesonly && sc != &scs)
 1767                         syncache_free(sc);
 1768                 else if (sc != &scs)
 1769                         syncache_insert(sc, sch);   /* locks and unlocks sch */
 1770                 TCPSTAT_INC(tcps_sndacks);
 1771                 TCPSTAT_INC(tcps_sndtotal);
 1772         } else {
 1773                 if (sc != &scs)
 1774                         syncache_free(sc);
 1775                 TCPSTAT_INC(tcps_sc_dropped);
 1776         }
 1777         goto donenoprobe;
 1778 
 1779 done:
 1780         TCP_PROBE5(receive, NULL, NULL, m, NULL, th);
 1781 donenoprobe:
 1782         if (m)
 1783                 m_freem(m);
 1784         /*
 1785          * If tfo_pending is not NULL here, then a TFO SYN that did not
 1786          * result in a new socket was processed and the associated pending
 1787          * counter has not yet been decremented.  All such TFO processing paths
 1788          * transit this point.
 1789          */
 1790         if (tfo_pending != NULL)
 1791                 tcp_fastopen_decrement_counter(tfo_pending);
 1792 
 1793 tfo_expanded:
 1794         if (cred != NULL)
 1795                 crfree(cred);
 1796 #ifdef MAC
 1797         if (sc == &scs)
 1798                 mac_syncache_destroy(&maclabel);
 1799 #endif
 1800         return (rv);
 1801 }
 1802 
 1803 /*
 1804  * Send SYN|ACK or ACK to the peer.  Either in response to a peer's segment,
 1805  * i.e. m0 != NULL, or upon 3WHS ACK timeout, i.e. m0 == NULL.
 1806  */
 1807 static int
 1808 syncache_respond(struct syncache *sc, const struct mbuf *m0, int flags)
 1809 {
 1810         struct ip *ip = NULL;
 1811         struct mbuf *m;
 1812         struct tcphdr *th = NULL;
 1813         struct udphdr *udp = NULL;
 1814         int optlen, error = 0;  /* Make compiler happy */
 1815         u_int16_t hlen, tlen, mssopt, ulen;
 1816         struct tcpopt to;
 1817 #ifdef INET6
 1818         struct ip6_hdr *ip6 = NULL;
 1819 #endif
 1820 
 1821         NET_EPOCH_ASSERT();
 1822 
 1823         hlen =
 1824 #ifdef INET6
 1825                (sc->sc_inc.inc_flags & INC_ISIPV6) ? sizeof(struct ip6_hdr) :
 1826 #endif
 1827                 sizeof(struct ip);
 1828         tlen = hlen + sizeof(struct tcphdr);
 1829         if (sc->sc_port) {
 1830                 tlen += sizeof(struct udphdr);
 1831         }
 1832         /* Determine MSS we advertize to other end of connection. */
 1833         mssopt = tcp_mssopt(&sc->sc_inc);
 1834         if (sc->sc_port)
 1835                 mssopt -= V_tcp_udp_tunneling_overhead;
 1836         mssopt = max(mssopt, V_tcp_minmss);
 1837 
 1838         /* XXX: Assume that the entire packet will fit in a header mbuf. */
 1839         KASSERT(max_linkhdr + tlen + TCP_MAXOLEN <= MHLEN,
 1840             ("syncache: mbuf too small: hlen %u, sc_port %u, max_linkhdr %d + "
 1841             "tlen %d + TCP_MAXOLEN %ju <= MHLEN %d", hlen, sc->sc_port,
 1842             max_linkhdr, tlen, (uintmax_t)TCP_MAXOLEN, MHLEN));
 1843 
 1844         /* Create the IP+TCP header from scratch. */
 1845         m = m_gethdr(M_NOWAIT, MT_DATA);
 1846         if (m == NULL)
 1847                 return (ENOBUFS);
 1848 #ifdef MAC
 1849         mac_syncache_create_mbuf(sc->sc_label, m);
 1850 #endif
 1851         m->m_data += max_linkhdr;
 1852         m->m_len = tlen;
 1853         m->m_pkthdr.len = tlen;
 1854         m->m_pkthdr.rcvif = NULL;
 1855 
 1856 #ifdef INET6
 1857         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
 1858                 ip6 = mtod(m, struct ip6_hdr *);
 1859                 ip6->ip6_vfc = IPV6_VERSION;
 1860                 ip6->ip6_src = sc->sc_inc.inc6_laddr;
 1861                 ip6->ip6_dst = sc->sc_inc.inc6_faddr;
 1862                 ip6->ip6_plen = htons(tlen - hlen);
 1863                 /* ip6_hlim is set after checksum */
 1864                 /* Zero out traffic class and flow label. */
 1865                 ip6->ip6_flow &= ~IPV6_FLOWINFO_MASK;
 1866                 ip6->ip6_flow |= sc->sc_flowlabel;
 1867                 if (sc->sc_port != 0) {
 1868                         ip6->ip6_nxt = IPPROTO_UDP;
 1869                         udp = (struct udphdr *)(ip6 + 1);
 1870                         udp->uh_sport = htons(V_tcp_udp_tunneling_port);
 1871                         udp->uh_dport = sc->sc_port;
 1872                         ulen = (tlen - sizeof(struct ip6_hdr));
 1873                         th = (struct tcphdr *)(udp + 1);
 1874                 } else {
 1875                         ip6->ip6_nxt = IPPROTO_TCP;
 1876                         th = (struct tcphdr *)(ip6 + 1);
 1877                 }
 1878                 ip6->ip6_flow |= htonl(sc->sc_ip_tos << 20);
 1879         }
 1880 #endif
 1881 #if defined(INET6) && defined(INET)
 1882         else
 1883 #endif
 1884 #ifdef INET
 1885         {
 1886                 ip = mtod(m, struct ip *);
 1887                 ip->ip_v = IPVERSION;
 1888                 ip->ip_hl = sizeof(struct ip) >> 2;
 1889                 ip->ip_len = htons(tlen);
 1890                 ip->ip_id = 0;
 1891                 ip->ip_off = 0;
 1892                 ip->ip_sum = 0;
 1893                 ip->ip_src = sc->sc_inc.inc_laddr;
 1894                 ip->ip_dst = sc->sc_inc.inc_faddr;
 1895                 ip->ip_ttl = sc->sc_ip_ttl;
 1896                 ip->ip_tos = sc->sc_ip_tos;
 1897 
 1898                 /*
 1899                  * See if we should do MTU discovery.  Route lookups are
 1900                  * expensive, so we will only unset the DF bit if:
 1901                  *
 1902                  *      1) path_mtu_discovery is disabled
 1903                  *      2) the SCF_UNREACH flag has been set
 1904                  */
 1905                 if (V_path_mtu_discovery && ((sc->sc_flags & SCF_UNREACH) == 0))
 1906                        ip->ip_off |= htons(IP_DF);
 1907                 if (sc->sc_port == 0) {
 1908                         ip->ip_p = IPPROTO_TCP;
 1909                         th = (struct tcphdr *)(ip + 1);
 1910                 } else {
 1911                         ip->ip_p = IPPROTO_UDP;
 1912                         udp = (struct udphdr *)(ip + 1);
 1913                         udp->uh_sport = htons(V_tcp_udp_tunneling_port);
 1914                         udp->uh_dport = sc->sc_port;
 1915                         ulen = (tlen - sizeof(struct ip));
 1916                         th = (struct tcphdr *)(udp + 1);
 1917                 }
 1918         }
 1919 #endif /* INET */
 1920         th->th_sport = sc->sc_inc.inc_lport;
 1921         th->th_dport = sc->sc_inc.inc_fport;
 1922 
 1923         if (flags & TH_SYN)
 1924                 th->th_seq = htonl(sc->sc_iss);
 1925         else
 1926                 th->th_seq = htonl(sc->sc_iss + 1);
 1927         th->th_ack = htonl(sc->sc_irs + 1);
 1928         th->th_off = sizeof(struct tcphdr) >> 2;
 1929         th->th_win = htons(sc->sc_wnd);
 1930         th->th_urp = 0;
 1931 
 1932         flags = tcp_ecn_syncache_respond(flags, sc);
 1933         tcp_set_flags(th, flags);
 1934 
 1935         /* Tack on the TCP options. */
 1936         if ((sc->sc_flags & SCF_NOOPT) == 0) {
 1937                 to.to_flags = 0;
 1938 
 1939                 if (flags & TH_SYN) {
 1940                         to.to_mss = mssopt;
 1941                         to.to_flags = TOF_MSS;
 1942                         if (sc->sc_flags & SCF_WINSCALE) {
 1943                                 to.to_wscale = sc->sc_requested_r_scale;
 1944                                 to.to_flags |= TOF_SCALE;
 1945                         }
 1946                         if (sc->sc_flags & SCF_SACK)
 1947                                 to.to_flags |= TOF_SACKPERM;
 1948 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
 1949                         if (sc->sc_flags & SCF_SIGNATURE)
 1950                                 to.to_flags |= TOF_SIGNATURE;
 1951 #endif
 1952                         if (sc->sc_tfo_cookie) {
 1953                                 to.to_flags |= TOF_FASTOPEN;
 1954                                 to.to_tfo_len = TCP_FASTOPEN_COOKIE_LEN;
 1955                                 to.to_tfo_cookie = sc->sc_tfo_cookie;
 1956                                 /* don't send cookie again when retransmitting response */
 1957                                 sc->sc_tfo_cookie = NULL;
 1958                         }
 1959                 }
 1960                 if (sc->sc_flags & SCF_TIMESTAMP) {
 1961                         to.to_tsval = sc->sc_tsoff + tcp_ts_getticks();
 1962                         to.to_tsecr = sc->sc_tsreflect;
 1963                         to.to_flags |= TOF_TS;
 1964                 }
 1965                 optlen = tcp_addoptions(&to, (u_char *)(th + 1));
 1966 
 1967                 /* Adjust headers by option size. */
 1968                 th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
 1969                 m->m_len += optlen;
 1970                 m->m_pkthdr.len += optlen;
 1971 #ifdef INET6
 1972                 if (sc->sc_inc.inc_flags & INC_ISIPV6)
 1973                         ip6->ip6_plen = htons(ntohs(ip6->ip6_plen) + optlen);
 1974                 else
 1975 #endif
 1976                         ip->ip_len = htons(ntohs(ip->ip_len) + optlen);
 1977 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
 1978                 if (sc->sc_flags & SCF_SIGNATURE) {
 1979                         KASSERT(to.to_flags & TOF_SIGNATURE,
 1980                             ("tcp_addoptions() didn't set tcp_signature"));
 1981 
 1982                         /* NOTE: to.to_signature is inside of mbuf */
 1983                         if (!TCPMD5_ENABLED() ||
 1984                             TCPMD5_OUTPUT(m, th, to.to_signature) != 0) {
 1985                                 m_freem(m);
 1986                                 return (EACCES);
 1987                         }
 1988                 }
 1989 #endif
 1990         } else
 1991                 optlen = 0;
 1992 
 1993         if (udp) {
 1994                 ulen += optlen;
 1995                 udp->uh_ulen = htons(ulen);
 1996         }
 1997         M_SETFIB(m, sc->sc_inc.inc_fibnum);
 1998         /*
 1999          * If we have peer's SYN and it has a flowid, then let's assign it to
 2000          * our SYN|ACK.  ip6_output() and ip_output() will not assign flowid
 2001          * to SYN|ACK due to lack of inp here.
 2002          */
 2003         if (m0 != NULL && M_HASHTYPE_GET(m0) != M_HASHTYPE_NONE) {
 2004                 m->m_pkthdr.flowid = m0->m_pkthdr.flowid;
 2005                 M_HASHTYPE_SET(m, M_HASHTYPE_GET(m0));
 2006         }
 2007 #ifdef INET6
 2008         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
 2009                 if (sc->sc_port) {
 2010                         m->m_pkthdr.csum_flags = CSUM_UDP_IPV6;
 2011                         m->m_pkthdr.csum_data = offsetof(struct udphdr, uh_sum);
 2012                         udp->uh_sum = in6_cksum_pseudo(ip6, ulen,
 2013                               IPPROTO_UDP, 0);
 2014                         th->th_sum = htons(0);
 2015                 } else {
 2016                         m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
 2017                         m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
 2018                         th->th_sum = in6_cksum_pseudo(ip6, tlen + optlen - hlen,
 2019                             IPPROTO_TCP, 0);
 2020                 }
 2021                 ip6->ip6_hlim = sc->sc_ip_ttl;
 2022 #ifdef TCP_OFFLOAD
 2023                 if (ADDED_BY_TOE(sc)) {
 2024                         struct toedev *tod = sc->sc_tod;
 2025 
 2026                         error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
 2027 
 2028                         return (error);
 2029                 }
 2030 #endif
 2031                 TCP_PROBE5(send, NULL, NULL, ip6, NULL, th);
 2032                 error = ip6_output(m, NULL, NULL, 0, NULL, NULL, NULL);
 2033         }
 2034 #endif
 2035 #if defined(INET6) && defined(INET)
 2036         else
 2037 #endif
 2038 #ifdef INET
 2039         {
 2040                 if (sc->sc_port) {
 2041                         m->m_pkthdr.csum_flags = CSUM_UDP;
 2042                         m->m_pkthdr.csum_data = offsetof(struct udphdr, uh_sum);
 2043                         udp->uh_sum = in_pseudo(ip->ip_src.s_addr,
 2044                               ip->ip_dst.s_addr, htons(ulen + IPPROTO_UDP));
 2045                         th->th_sum = htons(0);
 2046                 } else {
 2047                         m->m_pkthdr.csum_flags = CSUM_TCP;
 2048                         m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
 2049                         th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
 2050                             htons(tlen + optlen - hlen + IPPROTO_TCP));
 2051                 }
 2052 #ifdef TCP_OFFLOAD
 2053                 if (ADDED_BY_TOE(sc)) {
 2054                         struct toedev *tod = sc->sc_tod;
 2055 
 2056                         error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
 2057 
 2058                         return (error);
 2059                 }
 2060 #endif
 2061                 TCP_PROBE5(send, NULL, NULL, ip, NULL, th);
 2062                 error = ip_output(m, sc->sc_ipopts, NULL, 0, NULL, NULL);
 2063         }
 2064 #endif
 2065         return (error);
 2066 }
 2067 
 2068 /*
 2069  * The purpose of syncookies is to handle spoofed SYN flooding DoS attacks
 2070  * that exceed the capacity of the syncache by avoiding the storage of any
 2071  * of the SYNs we receive.  Syncookies defend against blind SYN flooding
 2072  * attacks where the attacker does not have access to our responses.
 2073  *
 2074  * Syncookies encode and include all necessary information about the
 2075  * connection setup within the SYN|ACK that we send back.  That way we
 2076  * can avoid keeping any local state until the ACK to our SYN|ACK returns
 2077  * (if ever).  Normally the syncache and syncookies are running in parallel
 2078  * with the latter taking over when the former is exhausted.  When matching
 2079  * syncache entry is found the syncookie is ignored.
 2080  *
 2081  * The only reliable information persisting the 3WHS is our initial sequence
 2082  * number ISS of 32 bits.  Syncookies embed a cryptographically sufficient
 2083  * strong hash (MAC) value and a few bits of TCP SYN options in the ISS
 2084  * of our SYN|ACK.  The MAC can be recomputed when the ACK to our SYN|ACK
 2085  * returns and signifies a legitimate connection if it matches the ACK.
 2086  *
 2087  * The available space of 32 bits to store the hash and to encode the SYN
 2088  * option information is very tight and we should have at least 24 bits for
 2089  * the MAC to keep the number of guesses by blind spoofing reasonably high.
 2090  *
 2091  * SYN option information we have to encode to fully restore a connection:
 2092  * MSS: is imporant to chose an optimal segment size to avoid IP level
 2093  *   fragmentation along the path.  The common MSS values can be encoded
 2094  *   in a 3-bit table.  Uncommon values are captured by the next lower value
 2095  *   in the table leading to a slight increase in packetization overhead.
 2096  * WSCALE: is necessary to allow large windows to be used for high delay-
 2097  *   bandwidth product links.  Not scaling the window when it was initially
 2098  *   negotiated is bad for performance as lack of scaling further decreases
 2099  *   the apparent available send window.  We only need to encode the WSCALE
 2100  *   we received from the remote end.  Our end can be recalculated at any
 2101  *   time.  The common WSCALE values can be encoded in a 3-bit table.
 2102  *   Uncommon values are captured by the next lower value in the table
 2103  *   making us under-estimate the available window size halving our
 2104  *   theoretically possible maximum throughput for that connection.
 2105  * SACK: Greatly assists in packet loss recovery and requires 1 bit.
 2106  * TIMESTAMP and SIGNATURE is not encoded because they are permanent options
 2107  *   that are included in all segments on a connection.  We enable them when
 2108  *   the ACK has them.
 2109  *
 2110  * Security of syncookies and attack vectors:
 2111  *
 2112  * The MAC is computed over (faddr||laddr||fport||lport||irs||flags||secmod)
 2113  * together with the gloabl secret to make it unique per connection attempt.
 2114  * Thus any change of any of those parameters results in a different MAC output
 2115  * in an unpredictable way unless a collision is encountered.  24 bits of the
 2116  * MAC are embedded into the ISS.
 2117  *
 2118  * To prevent replay attacks two rotating global secrets are updated with a
 2119  * new random value every 15 seconds.  The life-time of a syncookie is thus
 2120  * 15-30 seconds.
 2121  *
 2122  * Vector 1: Attacking the secret.  This requires finding a weakness in the
 2123  * MAC itself or the way it is used here.  The attacker can do a chosen plain
 2124  * text attack by varying and testing the all parameters under his control.
 2125  * The strength depends on the size and randomness of the secret, and the
 2126  * cryptographic security of the MAC function.  Due to the constant updating
 2127  * of the secret the attacker has at most 29.999 seconds to find the secret
 2128  * and launch spoofed connections.  After that he has to start all over again.
 2129  *
 2130  * Vector 2: Collision attack on the MAC of a single ACK.  With a 24 bit MAC
 2131  * size an average of 4,823 attempts are required for a 50% chance of success
 2132  * to spoof a single syncookie (birthday collision paradox).  However the
 2133  * attacker is blind and doesn't know if one of his attempts succeeded unless
 2134  * he has a side channel to interfere success from.  A single connection setup
 2135  * success average of 90% requires 8,790 packets, 99.99% requires 17,578 packets.
 2136  * This many attempts are required for each one blind spoofed connection.  For
 2137  * every additional spoofed connection he has to launch another N attempts.
 2138  * Thus for a sustained rate 100 spoofed connections per second approximately
 2139  * 1,800,000 packets per second would have to be sent.
 2140  *
 2141  * NB: The MAC function should be fast so that it doesn't become a CPU
 2142  * exhaustion attack vector itself.
 2143  *
 2144  * References:
 2145  *  RFC4987 TCP SYN Flooding Attacks and Common Mitigations
 2146  *  SYN cookies were first proposed by cryptographer Dan J. Bernstein in 1996
 2147  *   http://cr.yp.to/syncookies.html    (overview)
 2148  *   http://cr.yp.to/syncookies/archive (details)
 2149  *
 2150  *
 2151  * Schematic construction of a syncookie enabled Initial Sequence Number:
 2152  *  0        1         2         3
 2153  *  12345678901234567890123456789012
 2154  * |xxxxxxxxxxxxxxxxxxxxxxxxWWWMMMSP|
 2155  *
 2156  *  x 24 MAC (truncated)
 2157  *  W  3 Send Window Scale index
 2158  *  M  3 MSS index
 2159  *  S  1 SACK permitted
 2160  *  P  1 Odd/even secret
 2161  */
 2162 
 2163 /*
 2164  * Distribution and probability of certain MSS values.  Those in between are
 2165  * rounded down to the next lower one.
 2166  * [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011]
 2167  *                            .2%  .3%   5%    7%    7%    20%   15%   45%
 2168  */
 2169 static int tcp_sc_msstab[] = { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };
 2170 
 2171 /*
 2172  * Distribution and probability of certain WSCALE values.  We have to map the
 2173  * (send) window scale (shift) option with a range of 0-14 from 4 bits into 3
 2174  * bits based on prevalence of certain values.  Where we don't have an exact
 2175  * match for are rounded down to the next lower one letting us under-estimate
 2176  * the true available window.  At the moment this would happen only for the
 2177  * very uncommon values 3, 5 and those above 8 (more than 16MB socket buffer
 2178  * and window size).  The absence of the WSCALE option (no scaling in either
 2179  * direction) is encoded with index zero.
 2180  * [WSCALE values histograms, Allman, 2012]
 2181  *                            X 10 10 35  5  6 14 10%   by host
 2182  *                            X 11  4  5  5 18 49  3%   by connections
 2183  */
 2184 static int tcp_sc_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };
 2185 
 2186 /*
 2187  * Compute the MAC for the SYN cookie.  SIPHASH-2-4 is chosen for its speed
 2188  * and good cryptographic properties.
 2189  */
 2190 static uint32_t
 2191 syncookie_mac(struct in_conninfo *inc, tcp_seq irs, uint8_t flags,
 2192     uint8_t *secbits, uintptr_t secmod)
 2193 {
 2194         SIPHASH_CTX ctx;
 2195         uint32_t siphash[2];
 2196 
 2197         SipHash24_Init(&ctx);
 2198         SipHash_SetKey(&ctx, secbits);
 2199         switch (inc->inc_flags & INC_ISIPV6) {
 2200 #ifdef INET
 2201         case 0:
 2202                 SipHash_Update(&ctx, &inc->inc_faddr, sizeof(inc->inc_faddr));
 2203                 SipHash_Update(&ctx, &inc->inc_laddr, sizeof(inc->inc_laddr));
 2204                 break;
 2205 #endif
 2206 #ifdef INET6
 2207         case INC_ISIPV6:
 2208                 SipHash_Update(&ctx, &inc->inc6_faddr, sizeof(inc->inc6_faddr));
 2209                 SipHash_Update(&ctx, &inc->inc6_laddr, sizeof(inc->inc6_laddr));
 2210                 break;
 2211 #endif
 2212         }
 2213         SipHash_Update(&ctx, &inc->inc_fport, sizeof(inc->inc_fport));
 2214         SipHash_Update(&ctx, &inc->inc_lport, sizeof(inc->inc_lport));
 2215         SipHash_Update(&ctx, &irs, sizeof(irs));
 2216         SipHash_Update(&ctx, &flags, sizeof(flags));
 2217         SipHash_Update(&ctx, &secmod, sizeof(secmod));
 2218         SipHash_Final((u_int8_t *)&siphash, &ctx);
 2219 
 2220         return (siphash[0] ^ siphash[1]);
 2221 }
 2222 
 2223 static tcp_seq
 2224 syncookie_generate(struct syncache_head *sch, struct syncache *sc)
 2225 {
 2226         u_int i, secbit, wscale;
 2227         uint32_t iss, hash;
 2228         uint8_t *secbits;
 2229         union syncookie cookie;
 2230 
 2231         cookie.cookie = 0;
 2232 
 2233         /* Map our computed MSS into the 3-bit index. */
 2234         for (i = nitems(tcp_sc_msstab) - 1;
 2235              tcp_sc_msstab[i] > sc->sc_peer_mss && i > 0;
 2236              i--)
 2237                 ;
 2238         cookie.flags.mss_idx = i;
 2239 
 2240         /*
 2241          * Map the send window scale into the 3-bit index but only if
 2242          * the wscale option was received.
 2243          */
 2244         if (sc->sc_flags & SCF_WINSCALE) {
 2245                 wscale = sc->sc_requested_s_scale;
 2246                 for (i = nitems(tcp_sc_wstab) - 1;
 2247                     tcp_sc_wstab[i] > wscale && i > 0;
 2248                      i--)
 2249                         ;
 2250                 cookie.flags.wscale_idx = i;
 2251         }
 2252 
 2253         /* Can we do SACK? */
 2254         if (sc->sc_flags & SCF_SACK)
 2255                 cookie.flags.sack_ok = 1;
 2256 
 2257         /* Which of the two secrets to use. */
 2258         secbit = V_tcp_syncache.secret.oddeven & 0x1;
 2259         cookie.flags.odd_even = secbit;
 2260 
 2261         secbits = V_tcp_syncache.secret.key[secbit];
 2262         hash = syncookie_mac(&sc->sc_inc, sc->sc_irs, cookie.cookie, secbits,
 2263             (uintptr_t)sch);
 2264 
 2265         /*
 2266          * Put the flags into the hash and XOR them to get better ISS number
 2267          * variance.  This doesn't enhance the cryptographic strength and is
 2268          * done to prevent the 8 cookie bits from showing up directly on the
 2269          * wire.
 2270          */
 2271         iss = hash & ~0xff;
 2272         iss |= cookie.cookie ^ (hash >> 24);
 2273 
 2274         TCPSTAT_INC(tcps_sc_sendcookie);
 2275         return (iss);
 2276 }
 2277 
 2278 static struct syncache *
 2279 syncookie_lookup(struct in_conninfo *inc, struct syncache_head *sch,
 2280     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
 2281     struct socket *lso, uint16_t port)
 2282 {
 2283         uint32_t hash;
 2284         uint8_t *secbits;
 2285         tcp_seq ack, seq;
 2286         int wnd, wscale = 0;
 2287         union syncookie cookie;
 2288 
 2289         /*
 2290          * Pull information out of SYN-ACK/ACK and revert sequence number
 2291          * advances.
 2292          */
 2293         ack = th->th_ack - 1;
 2294         seq = th->th_seq - 1;
 2295 
 2296         /*
 2297          * Unpack the flags containing enough information to restore the
 2298          * connection.
 2299          */
 2300         cookie.cookie = (ack & 0xff) ^ (ack >> 24);
 2301 
 2302         /* Which of the two secrets to use. */
 2303         secbits = V_tcp_syncache.secret.key[cookie.flags.odd_even];
 2304 
 2305         hash = syncookie_mac(inc, seq, cookie.cookie, secbits, (uintptr_t)sch);
 2306 
 2307         /* The recomputed hash matches the ACK if this was a genuine cookie. */
 2308         if ((ack & ~0xff) != (hash & ~0xff))
 2309                 return (NULL);
 2310 
 2311         /* Fill in the syncache values. */
 2312         sc->sc_flags = 0;
 2313         bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
 2314         sc->sc_ipopts = NULL;
 2315 
 2316         sc->sc_irs = seq;
 2317         sc->sc_iss = ack;
 2318 
 2319         switch (inc->inc_flags & INC_ISIPV6) {
 2320 #ifdef INET
 2321         case 0:
 2322                 sc->sc_ip_ttl = sotoinpcb(lso)->inp_ip_ttl;
 2323                 sc->sc_ip_tos = sotoinpcb(lso)->inp_ip_tos;
 2324                 break;
 2325 #endif
 2326 #ifdef INET6
 2327         case INC_ISIPV6:
 2328                 if (sotoinpcb(lso)->inp_flags & IN6P_AUTOFLOWLABEL)
 2329                         sc->sc_flowlabel =
 2330                             htonl(sc->sc_iss) & IPV6_FLOWLABEL_MASK;
 2331                 break;
 2332 #endif
 2333         }
 2334 
 2335         sc->sc_peer_mss = tcp_sc_msstab[cookie.flags.mss_idx];
 2336 
 2337         /* We can simply recompute receive window scale we sent earlier. */
 2338         while (wscale < TCP_MAX_WINSHIFT && (TCP_MAXWIN << wscale) < sb_max)
 2339                 wscale++;
 2340 
 2341         /* Only use wscale if it was enabled in the orignal SYN. */
 2342         if (cookie.flags.wscale_idx > 0) {
 2343                 sc->sc_requested_r_scale = wscale;
 2344                 sc->sc_requested_s_scale = tcp_sc_wstab[cookie.flags.wscale_idx];
 2345                 sc->sc_flags |= SCF_WINSCALE;
 2346         }
 2347 
 2348         wnd = lso->sol_sbrcv_hiwat;
 2349         wnd = imax(wnd, 0);
 2350         wnd = imin(wnd, TCP_MAXWIN);
 2351         sc->sc_wnd = wnd;
 2352 
 2353         if (cookie.flags.sack_ok)
 2354                 sc->sc_flags |= SCF_SACK;
 2355 
 2356         if (to->to_flags & TOF_TS) {
 2357                 sc->sc_flags |= SCF_TIMESTAMP;
 2358                 sc->sc_tsreflect = to->to_tsval;
 2359                 sc->sc_tsoff = tcp_new_ts_offset(inc);
 2360         }
 2361 
 2362         if (to->to_flags & TOF_SIGNATURE)
 2363                 sc->sc_flags |= SCF_SIGNATURE;
 2364 
 2365         sc->sc_rxmits = 0;
 2366 
 2367         sc->sc_port = port;
 2368 
 2369         TCPSTAT_INC(tcps_sc_recvcookie);
 2370         return (sc);
 2371 }
 2372 
 2373 #ifdef INVARIANTS
 2374 static int
 2375 syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
 2376     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
 2377     struct socket *lso, uint16_t port)
 2378 {
 2379         struct syncache scs, *scx;
 2380         char *s;
 2381 
 2382         bzero(&scs, sizeof(scs));
 2383         scx = syncookie_lookup(inc, sch, &scs, th, to, lso, port);
 2384 
 2385         if ((s = tcp_log_addrs(inc, th, NULL, NULL)) == NULL)
 2386                 return (0);
 2387 
 2388         if (scx != NULL) {
 2389                 if (sc->sc_peer_mss != scx->sc_peer_mss)
 2390                         log(LOG_DEBUG, "%s; %s: mss different %i vs %i\n",
 2391                             s, __func__, sc->sc_peer_mss, scx->sc_peer_mss);
 2392 
 2393                 if (sc->sc_requested_r_scale != scx->sc_requested_r_scale)
 2394                         log(LOG_DEBUG, "%s; %s: rwscale different %i vs %i\n",
 2395                             s, __func__, sc->sc_requested_r_scale,
 2396                             scx->sc_requested_r_scale);
 2397 
 2398                 if (sc->sc_requested_s_scale != scx->sc_requested_s_scale)
 2399                         log(LOG_DEBUG, "%s; %s: swscale different %i vs %i\n",
 2400                             s, __func__, sc->sc_requested_s_scale,
 2401                             scx->sc_requested_s_scale);
 2402 
 2403                 if ((sc->sc_flags & SCF_SACK) != (scx->sc_flags & SCF_SACK))
 2404                         log(LOG_DEBUG, "%s; %s: SACK different\n", s, __func__);
 2405         }
 2406 
 2407         if (s != NULL)
 2408                 free(s, M_TCPLOG);
 2409         return (0);
 2410 }
 2411 #endif /* INVARIANTS */
 2412 
 2413 static void
 2414 syncookie_reseed(void *arg)
 2415 {
 2416         struct tcp_syncache *sc = arg;
 2417         uint8_t *secbits;
 2418         int secbit;
 2419 
 2420         /*
 2421          * Reseeding the secret doesn't have to be protected by a lock.
 2422          * It only must be ensured that the new random values are visible
 2423          * to all CPUs in a SMP environment.  The atomic with release
 2424          * semantics ensures that.
 2425          */
 2426         secbit = (sc->secret.oddeven & 0x1) ? 0 : 1;
 2427         secbits = sc->secret.key[secbit];
 2428         arc4rand(secbits, SYNCOOKIE_SECRET_SIZE, 0);
 2429         atomic_add_rel_int(&sc->secret.oddeven, 1);
 2430 
 2431         /* Reschedule ourself. */
 2432         callout_schedule(&sc->secret.reseed, SYNCOOKIE_LIFETIME * hz);
 2433 }
 2434 
 2435 /*
 2436  * We have overflowed a bucket. Let's pause dealing with the syncache.
 2437  * This function will increment the bucketoverflow statistics appropriately
 2438  * (once per pause when pausing is enabled; otherwise, once per overflow).
 2439  */
 2440 static void
 2441 syncache_pause(struct in_conninfo *inc)
 2442 {
 2443         time_t delta;
 2444         const char *s;
 2445 
 2446         /* XXX:
 2447          * 2. Add sysctl read here so we don't get the benefit of this
 2448          * change without the new sysctl.
 2449          */
 2450 
 2451         /*
 2452          * Try an unlocked read. If we already know that another thread
 2453          * has activated the feature, there is no need to proceed.
 2454          */
 2455         if (V_tcp_syncache.paused)
 2456                 return;
 2457 
 2458         /* Are cookied enabled? If not, we can't pause. */
 2459         if (!V_tcp_syncookies) {
 2460                 TCPSTAT_INC(tcps_sc_bucketoverflow);
 2461                 return;
 2462         }
 2463 
 2464         /*
 2465          * We may be the first thread to find an overflow. Get the lock
 2466          * and evaluate if we need to take action.
 2467          */
 2468         mtx_lock(&V_tcp_syncache.pause_mtx);
 2469         if (V_tcp_syncache.paused) {
 2470                 mtx_unlock(&V_tcp_syncache.pause_mtx);
 2471                 return;
 2472         }
 2473 
 2474         /* Activate protection. */
 2475         V_tcp_syncache.paused = true;
 2476         TCPSTAT_INC(tcps_sc_bucketoverflow);
 2477 
 2478         /*
 2479          * Determine the last backoff time. If we are seeing a re-newed
 2480          * attack within that same time after last reactivating the syncache,
 2481          * consider it an extension of the same attack.
 2482          */
 2483         delta = TCP_SYNCACHE_PAUSE_TIME << V_tcp_syncache.pause_backoff;
 2484         if (V_tcp_syncache.pause_until + delta - time_uptime > 0) {
 2485                 if (V_tcp_syncache.pause_backoff < TCP_SYNCACHE_MAX_BACKOFF) {
 2486                         delta <<= 1;
 2487                         V_tcp_syncache.pause_backoff++;
 2488                 }
 2489         } else {
 2490                 delta = TCP_SYNCACHE_PAUSE_TIME;
 2491                 V_tcp_syncache.pause_backoff = 0;
 2492         }
 2493 
 2494         /* Log a warning, including IP addresses, if able. */
 2495         if (inc != NULL)
 2496                 s = tcp_log_addrs(inc, NULL, NULL, NULL);
 2497         else
 2498                 s = (const char *)NULL;
 2499         log(LOG_WARNING, "TCP syncache overflow detected; using syncookies for "
 2500             "the next %lld seconds%s%s%s\n", (long long)delta,
 2501             (s != NULL) ? " (last SYN: " : "", (s != NULL) ? s : "",
 2502             (s != NULL) ? ")" : "");
 2503         free(__DECONST(void *, s), M_TCPLOG);
 2504 
 2505         /* Use the calculated delta to set a new pause time. */
 2506         V_tcp_syncache.pause_until = time_uptime + delta;
 2507         callout_reset(&V_tcp_syncache.pause_co, delta * hz, syncache_unpause,
 2508             &V_tcp_syncache);
 2509         mtx_unlock(&V_tcp_syncache.pause_mtx);
 2510 }
 2511 
 2512 /* Evaluate whether we need to unpause. */
 2513 static void
 2514 syncache_unpause(void *arg)
 2515 {
 2516         struct tcp_syncache *sc;
 2517         time_t delta;
 2518 
 2519         sc = arg;
 2520         mtx_assert(&sc->pause_mtx, MA_OWNED | MA_NOTRECURSED);
 2521         callout_deactivate(&sc->pause_co);
 2522 
 2523         /*
 2524          * Check to make sure we are not running early. If the pause
 2525          * time has expired, then deactivate the protection.
 2526          */
 2527         if ((delta = sc->pause_until - time_uptime) > 0)
 2528                 callout_schedule(&sc->pause_co, delta * hz);
 2529         else
 2530                 sc->paused = false;
 2531 }
 2532 
 2533 /*
 2534  * Exports the syncache entries to userland so that netstat can display
 2535  * them alongside the other sockets.  This function is intended to be
 2536  * called only from tcp_pcblist.
 2537  *
 2538  * Due to concurrency on an active system, the number of pcbs exported
 2539  * may have no relation to max_pcbs.  max_pcbs merely indicates the
 2540  * amount of space the caller allocated for this function to use.
 2541  */
 2542 int
 2543 syncache_pcblist(struct sysctl_req *req)
 2544 {
 2545         struct xtcpcb xt;
 2546         struct syncache *sc;
 2547         struct syncache_head *sch;
 2548         int error, i;
 2549 
 2550         bzero(&xt, sizeof(xt));
 2551         xt.xt_len = sizeof(xt);
 2552         xt.t_state = TCPS_SYN_RECEIVED;
 2553         xt.xt_inp.xi_socket.xso_protocol = IPPROTO_TCP;
 2554         xt.xt_inp.xi_socket.xso_len = sizeof (struct xsocket);
 2555         xt.xt_inp.xi_socket.so_type = SOCK_STREAM;
 2556         xt.xt_inp.xi_socket.so_state = SS_ISCONNECTING;
 2557 
 2558         for (i = 0; i < V_tcp_syncache.hashsize; i++) {
 2559                 sch = &V_tcp_syncache.hashbase[i];
 2560                 SCH_LOCK(sch);
 2561                 TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
 2562                         if (sc->sc_cred != NULL &&
 2563                             cr_cansee(req->td->td_ucred, sc->sc_cred) != 0)
 2564                                 continue;
 2565                         if (sc->sc_inc.inc_flags & INC_ISIPV6)
 2566                                 xt.xt_inp.inp_vflag = INP_IPV6;
 2567                         else
 2568                                 xt.xt_inp.inp_vflag = INP_IPV4;
 2569                         xt.xt_encaps_port = sc->sc_port;
 2570                         bcopy(&sc->sc_inc, &xt.xt_inp.inp_inc,
 2571                             sizeof (struct in_conninfo));
 2572                         error = SYSCTL_OUT(req, &xt, sizeof xt);
 2573                         if (error) {
 2574                                 SCH_UNLOCK(sch);
 2575                                 return (0);
 2576                         }
 2577                 }
 2578                 SCH_UNLOCK(sch);
 2579         }
 2580 
 2581         return (0);
 2582 }

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