The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: 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  * Copyright (c) 2001 McAfee, Inc.
    3  * Copyright (c) 2006,2013 Andre Oppermann, Internet Business Solutions AG
    4  * All rights reserved.
    5  *
    6  * This software was developed for the FreeBSD Project by Jonathan Lemon
    7  * and McAfee Research, the Security Research Division of McAfee, Inc. under
    8  * DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
    9  * DARPA CHATS research program. [2001 McAfee, Inc.]
   10  *
   11  * Redistribution and use in source and binary forms, with or without
   12  * modification, are permitted provided that the following conditions
   13  * are met:
   14  * 1. Redistributions of source code must retain the above copyright
   15  *    notice, this list of conditions and the following disclaimer.
   16  * 2. Redistributions in binary form must reproduce the above copyright
   17  *    notice, this list of conditions and the following disclaimer in the
   18  *    documentation and/or other materials provided with the distribution.
   19  *
   20  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   21  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   22  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   23  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   24  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   25  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   26  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   27  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   28  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   29  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   30  * SUCH DAMAGE.
   31  */
   32 
   33 #include <sys/cdefs.h>
   34 __FBSDID("$FreeBSD: releng/10.1/sys/netinet/tcp_syncache.c 270055 2014-08-16 14:03:00Z bz $");
   35 
   36 #include "opt_inet.h"
   37 #include "opt_inet6.h"
   38 #include "opt_ipsec.h"
   39 #include "opt_pcbgroup.h"
   40 
   41 #include <sys/param.h>
   42 #include <sys/systm.h>
   43 #include <sys/kernel.h>
   44 #include <sys/sysctl.h>
   45 #include <sys/limits.h>
   46 #include <sys/lock.h>
   47 #include <sys/mutex.h>
   48 #include <sys/malloc.h>
   49 #include <sys/mbuf.h>
   50 #include <sys/proc.h>           /* for proc0 declaration */
   51 #include <sys/random.h>
   52 #include <sys/socket.h>
   53 #include <sys/socketvar.h>
   54 #include <sys/syslog.h>
   55 #include <sys/ucred.h>
   56 
   57 #include <sys/md5.h>
   58 #include <crypto/siphash/siphash.h>
   59 
   60 #include <vm/uma.h>
   61 
   62 #include <net/if.h>
   63 #include <net/route.h>
   64 #include <net/vnet.h>
   65 
   66 #include <netinet/in.h>
   67 #include <netinet/in_systm.h>
   68 #include <netinet/ip.h>
   69 #include <netinet/in_var.h>
   70 #include <netinet/in_pcb.h>
   71 #include <netinet/ip_var.h>
   72 #include <netinet/ip_options.h>
   73 #ifdef INET6
   74 #include <netinet/ip6.h>
   75 #include <netinet/icmp6.h>
   76 #include <netinet6/nd6.h>
   77 #include <netinet6/ip6_var.h>
   78 #include <netinet6/in6_pcb.h>
   79 #endif
   80 #include <netinet/tcp.h>
   81 #include <netinet/tcp_fsm.h>
   82 #include <netinet/tcp_seq.h>
   83 #include <netinet/tcp_timer.h>
   84 #include <netinet/tcp_var.h>
   85 #include <netinet/tcp_syncache.h>
   86 #ifdef INET6
   87 #include <netinet6/tcp6_var.h>
   88 #endif
   89 #ifdef TCP_OFFLOAD
   90 #include <netinet/toecore.h>
   91 #endif
   92 
   93 #ifdef IPSEC
   94 #include <netipsec/ipsec.h>
   95 #ifdef INET6
   96 #include <netipsec/ipsec6.h>
   97 #endif
   98 #include <netipsec/key.h>
   99 #endif /*IPSEC*/
  100 
  101 #include <machine/in_cksum.h>
  102 
  103 #include <security/mac/mac_framework.h>
  104 
  105 static VNET_DEFINE(int, tcp_syncookies) = 1;
  106 #define V_tcp_syncookies                VNET(tcp_syncookies)
  107 SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, syncookies, CTLFLAG_RW,
  108     &VNET_NAME(tcp_syncookies), 0,
  109     "Use TCP SYN cookies if the syncache overflows");
  110 
  111 static VNET_DEFINE(int, tcp_syncookiesonly) = 0;
  112 #define V_tcp_syncookiesonly            VNET(tcp_syncookiesonly)
  113 SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, syncookies_only, CTLFLAG_RW,
  114     &VNET_NAME(tcp_syncookiesonly), 0,
  115     "Use only TCP SYN cookies");
  116 
  117 #ifdef TCP_OFFLOAD
  118 #define ADDED_BY_TOE(sc) ((sc)->sc_tod != NULL)
  119 #endif
  120 
  121 static void      syncache_drop(struct syncache *, struct syncache_head *);
  122 static void      syncache_free(struct syncache *);
  123 static void      syncache_insert(struct syncache *, struct syncache_head *);
  124 static int       syncache_respond(struct syncache *);
  125 static struct    socket *syncache_socket(struct syncache *, struct socket *,
  126                     struct mbuf *m);
  127 static int       syncache_sysctl_count(SYSCTL_HANDLER_ARGS);
  128 static void      syncache_timeout(struct syncache *sc, struct syncache_head *sch,
  129                     int docallout);
  130 static void      syncache_timer(void *);
  131 
  132 static uint32_t  syncookie_mac(struct in_conninfo *, tcp_seq, uint8_t,
  133                     uint8_t *, uintptr_t);
  134 static tcp_seq   syncookie_generate(struct syncache_head *, struct syncache *);
  135 static struct syncache
  136                 *syncookie_lookup(struct in_conninfo *, struct syncache_head *,
  137                     struct syncache *, struct tcphdr *, struct tcpopt *,
  138                     struct socket *);
  139 static void      syncookie_reseed(void *);
  140 #ifdef INVARIANTS
  141 static int       syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
  142                     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
  143                     struct socket *lso);
  144 #endif
  145 
  146 /*
  147  * Transmit the SYN,ACK fewer times than TCP_MAXRXTSHIFT specifies.
  148  * 3 retransmits corresponds to a timeout of 3 * (1 + 2 + 4 + 8) == 45 seconds,
  149  * the odds are that the user has given up attempting to connect by then.
  150  */
  151 #define SYNCACHE_MAXREXMTS              3
  152 
  153 /* Arbitrary values */
  154 #define TCP_SYNCACHE_HASHSIZE           512
  155 #define TCP_SYNCACHE_BUCKETLIMIT        30
  156 
  157 static VNET_DEFINE(struct tcp_syncache, tcp_syncache);
  158 #define V_tcp_syncache                  VNET(tcp_syncache)
  159 
  160 static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, syncache, CTLFLAG_RW, 0,
  161     "TCP SYN cache");
  162 
  163 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, bucketlimit, CTLFLAG_RDTUN,
  164     &VNET_NAME(tcp_syncache.bucket_limit), 0,
  165     "Per-bucket hash limit for syncache");
  166 
  167 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, cachelimit, CTLFLAG_RDTUN,
  168     &VNET_NAME(tcp_syncache.cache_limit), 0,
  169     "Overall entry limit for syncache");
  170 
  171 SYSCTL_VNET_PROC(_net_inet_tcp_syncache, OID_AUTO, count, (CTLTYPE_UINT|CTLFLAG_RD),
  172     NULL, 0, &syncache_sysctl_count, "IU",
  173     "Current number of entries in syncache");
  174 
  175 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, hashsize, CTLFLAG_RDTUN,
  176     &VNET_NAME(tcp_syncache.hashsize), 0,
  177     "Size of TCP syncache hashtable");
  178 
  179 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, rexmtlimit, CTLFLAG_RW,
  180     &VNET_NAME(tcp_syncache.rexmt_limit), 0,
  181     "Limit on SYN/ACK retransmissions");
  182 
  183 VNET_DEFINE(int, tcp_sc_rst_sock_fail) = 1;
  184 SYSCTL_VNET_INT(_net_inet_tcp_syncache, OID_AUTO, rst_on_sock_fail,
  185     CTLFLAG_RW, &VNET_NAME(tcp_sc_rst_sock_fail), 0,
  186     "Send reset on socket allocation failure");
  187 
  188 static MALLOC_DEFINE(M_SYNCACHE, "syncache", "TCP syncache");
  189 
  190 #define SYNCACHE_HASH(inc, mask)                                        \
  191         ((V_tcp_syncache.hash_secret ^                                  \
  192           (inc)->inc_faddr.s_addr ^                                     \
  193           ((inc)->inc_faddr.s_addr >> 16) ^                             \
  194           (inc)->inc_fport ^ (inc)->inc_lport) & mask)
  195 
  196 #define SYNCACHE_HASH6(inc, mask)                                       \
  197         ((V_tcp_syncache.hash_secret ^                                  \
  198           (inc)->inc6_faddr.s6_addr32[0] ^                              \
  199           (inc)->inc6_faddr.s6_addr32[3] ^                              \
  200           (inc)->inc_fport ^ (inc)->inc_lport) & mask)
  201 
  202 #define ENDPTS_EQ(a, b) (                                               \
  203         (a)->ie_fport == (b)->ie_fport &&                               \
  204         (a)->ie_lport == (b)->ie_lport &&                               \
  205         (a)->ie_faddr.s_addr == (b)->ie_faddr.s_addr &&                 \
  206         (a)->ie_laddr.s_addr == (b)->ie_laddr.s_addr                    \
  207 )
  208 
  209 #define ENDPTS6_EQ(a, b) (memcmp(a, b, sizeof(*a)) == 0)
  210 
  211 #define SCH_LOCK(sch)           mtx_lock(&(sch)->sch_mtx)
  212 #define SCH_UNLOCK(sch)         mtx_unlock(&(sch)->sch_mtx)
  213 #define SCH_LOCK_ASSERT(sch)    mtx_assert(&(sch)->sch_mtx, MA_OWNED)
  214 
  215 /*
  216  * Requires the syncache entry to be already removed from the bucket list.
  217  */
  218 static void
  219 syncache_free(struct syncache *sc)
  220 {
  221 
  222         if (sc->sc_ipopts)
  223                 (void) m_free(sc->sc_ipopts);
  224         if (sc->sc_cred)
  225                 crfree(sc->sc_cred);
  226 #ifdef MAC
  227         mac_syncache_destroy(&sc->sc_label);
  228 #endif
  229 
  230         uma_zfree(V_tcp_syncache.zone, sc);
  231 }
  232 
  233 void
  234 syncache_init(void)
  235 {
  236         int i;
  237 
  238         V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
  239         V_tcp_syncache.bucket_limit = TCP_SYNCACHE_BUCKETLIMIT;
  240         V_tcp_syncache.rexmt_limit = SYNCACHE_MAXREXMTS;
  241         V_tcp_syncache.hash_secret = arc4random();
  242 
  243         TUNABLE_INT_FETCH("net.inet.tcp.syncache.hashsize",
  244             &V_tcp_syncache.hashsize);
  245         TUNABLE_INT_FETCH("net.inet.tcp.syncache.bucketlimit",
  246             &V_tcp_syncache.bucket_limit);
  247         if (!powerof2(V_tcp_syncache.hashsize) ||
  248             V_tcp_syncache.hashsize == 0) {
  249                 printf("WARNING: syncache hash size is not a power of 2.\n");
  250                 V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
  251         }
  252         V_tcp_syncache.hashmask = V_tcp_syncache.hashsize - 1;
  253 
  254         /* Set limits. */
  255         V_tcp_syncache.cache_limit =
  256             V_tcp_syncache.hashsize * V_tcp_syncache.bucket_limit;
  257         TUNABLE_INT_FETCH("net.inet.tcp.syncache.cachelimit",
  258             &V_tcp_syncache.cache_limit);
  259 
  260         /* Allocate the hash table. */
  261         V_tcp_syncache.hashbase = malloc(V_tcp_syncache.hashsize *
  262             sizeof(struct syncache_head), M_SYNCACHE, M_WAITOK | M_ZERO);
  263 
  264 #ifdef VIMAGE
  265         V_tcp_syncache.vnet = curvnet;
  266 #endif
  267 
  268         /* Initialize the hash buckets. */
  269         for (i = 0; i < V_tcp_syncache.hashsize; i++) {
  270                 TAILQ_INIT(&V_tcp_syncache.hashbase[i].sch_bucket);
  271                 mtx_init(&V_tcp_syncache.hashbase[i].sch_mtx, "tcp_sc_head",
  272                          NULL, MTX_DEF);
  273                 callout_init_mtx(&V_tcp_syncache.hashbase[i].sch_timer,
  274                          &V_tcp_syncache.hashbase[i].sch_mtx, 0);
  275                 V_tcp_syncache.hashbase[i].sch_length = 0;
  276                 V_tcp_syncache.hashbase[i].sch_sc = &V_tcp_syncache;
  277         }
  278 
  279         /* Create the syncache entry zone. */
  280         V_tcp_syncache.zone = uma_zcreate("syncache", sizeof(struct syncache),
  281             NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
  282         V_tcp_syncache.cache_limit = uma_zone_set_max(V_tcp_syncache.zone,
  283             V_tcp_syncache.cache_limit);
  284 
  285         /* Start the SYN cookie reseeder callout. */
  286         callout_init(&V_tcp_syncache.secret.reseed, 1);
  287         arc4rand(V_tcp_syncache.secret.key[0], SYNCOOKIE_SECRET_SIZE, 0);
  288         arc4rand(V_tcp_syncache.secret.key[1], SYNCOOKIE_SECRET_SIZE, 0);
  289         callout_reset(&V_tcp_syncache.secret.reseed, SYNCOOKIE_LIFETIME * hz,
  290             syncookie_reseed, &V_tcp_syncache);
  291 }
  292 
  293 #ifdef VIMAGE
  294 void
  295 syncache_destroy(void)
  296 {
  297         struct syncache_head *sch;
  298         struct syncache *sc, *nsc;
  299         int i;
  300 
  301         /* Cleanup hash buckets: stop timers, free entries, destroy locks. */
  302         for (i = 0; i < V_tcp_syncache.hashsize; i++) {
  303 
  304                 sch = &V_tcp_syncache.hashbase[i];
  305                 callout_drain(&sch->sch_timer);
  306 
  307                 SCH_LOCK(sch);
  308                 TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc)
  309                         syncache_drop(sc, sch);
  310                 SCH_UNLOCK(sch);
  311                 KASSERT(TAILQ_EMPTY(&sch->sch_bucket),
  312                     ("%s: sch->sch_bucket not empty", __func__));
  313                 KASSERT(sch->sch_length == 0, ("%s: sch->sch_length %d not 0",
  314                     __func__, sch->sch_length));
  315                 mtx_destroy(&sch->sch_mtx);
  316         }
  317 
  318         KASSERT(uma_zone_get_cur(V_tcp_syncache.zone) == 0,
  319             ("%s: cache_count not 0", __func__));
  320 
  321         /* Free the allocated global resources. */
  322         uma_zdestroy(V_tcp_syncache.zone);
  323         free(V_tcp_syncache.hashbase, M_SYNCACHE);
  324 
  325         callout_drain(&V_tcp_syncache.secret.reseed);
  326 }
  327 #endif
  328 
  329 static int
  330 syncache_sysctl_count(SYSCTL_HANDLER_ARGS)
  331 {
  332         int count;
  333 
  334         count = uma_zone_get_cur(V_tcp_syncache.zone);
  335         return (sysctl_handle_int(oidp, &count, 0, req));
  336 }
  337 
  338 /*
  339  * Inserts a syncache entry into the specified bucket row.
  340  * Locks and unlocks the syncache_head autonomously.
  341  */
  342 static void
  343 syncache_insert(struct syncache *sc, struct syncache_head *sch)
  344 {
  345         struct syncache *sc2;
  346 
  347         SCH_LOCK(sch);
  348 
  349         /*
  350          * Make sure that we don't overflow the per-bucket limit.
  351          * If the bucket is full, toss the oldest element.
  352          */
  353         if (sch->sch_length >= V_tcp_syncache.bucket_limit) {
  354                 KASSERT(!TAILQ_EMPTY(&sch->sch_bucket),
  355                         ("sch->sch_length incorrect"));
  356                 sc2 = TAILQ_LAST(&sch->sch_bucket, sch_head);
  357                 syncache_drop(sc2, sch);
  358                 TCPSTAT_INC(tcps_sc_bucketoverflow);
  359         }
  360 
  361         /* Put it into the bucket. */
  362         TAILQ_INSERT_HEAD(&sch->sch_bucket, sc, sc_hash);
  363         sch->sch_length++;
  364 
  365 #ifdef TCP_OFFLOAD
  366         if (ADDED_BY_TOE(sc)) {
  367                 struct toedev *tod = sc->sc_tod;
  368 
  369                 tod->tod_syncache_added(tod, sc->sc_todctx);
  370         }
  371 #endif
  372 
  373         /* Reinitialize the bucket row's timer. */
  374         if (sch->sch_length == 1)
  375                 sch->sch_nextc = ticks + INT_MAX;
  376         syncache_timeout(sc, sch, 1);
  377 
  378         SCH_UNLOCK(sch);
  379 
  380         TCPSTAT_INC(tcps_sc_added);
  381 }
  382 
  383 /*
  384  * Remove and free entry from syncache bucket row.
  385  * Expects locked syncache head.
  386  */
  387 static void
  388 syncache_drop(struct syncache *sc, struct syncache_head *sch)
  389 {
  390 
  391         SCH_LOCK_ASSERT(sch);
  392 
  393         TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
  394         sch->sch_length--;
  395 
  396 #ifdef TCP_OFFLOAD
  397         if (ADDED_BY_TOE(sc)) {
  398                 struct toedev *tod = sc->sc_tod;
  399 
  400                 tod->tod_syncache_removed(tod, sc->sc_todctx);
  401         }
  402 #endif
  403 
  404         syncache_free(sc);
  405 }
  406 
  407 /*
  408  * Engage/reengage time on bucket row.
  409  */
  410 static void
  411 syncache_timeout(struct syncache *sc, struct syncache_head *sch, int docallout)
  412 {
  413         sc->sc_rxttime = ticks +
  414                 TCPTV_RTOBASE * (tcp_syn_backoff[sc->sc_rxmits]);
  415         sc->sc_rxmits++;
  416         if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc)) {
  417                 sch->sch_nextc = sc->sc_rxttime;
  418                 if (docallout)
  419                         callout_reset(&sch->sch_timer, sch->sch_nextc - ticks,
  420                             syncache_timer, (void *)sch);
  421         }
  422 }
  423 
  424 /*
  425  * Walk the timer queues, looking for SYN,ACKs that need to be retransmitted.
  426  * If we have retransmitted an entry the maximum number of times, expire it.
  427  * One separate timer for each bucket row.
  428  */
  429 static void
  430 syncache_timer(void *xsch)
  431 {
  432         struct syncache_head *sch = (struct syncache_head *)xsch;
  433         struct syncache *sc, *nsc;
  434         int tick = ticks;
  435         char *s;
  436 
  437         CURVNET_SET(sch->sch_sc->vnet);
  438 
  439         /* NB: syncache_head has already been locked by the callout. */
  440         SCH_LOCK_ASSERT(sch);
  441 
  442         /*
  443          * In the following cycle we may remove some entries and/or
  444          * advance some timeouts, so re-initialize the bucket timer.
  445          */
  446         sch->sch_nextc = tick + INT_MAX;
  447 
  448         TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc) {
  449                 /*
  450                  * We do not check if the listen socket still exists
  451                  * and accept the case where the listen socket may be
  452                  * gone by the time we resend the SYN/ACK.  We do
  453                  * not expect this to happens often. If it does,
  454                  * then the RST will be sent by the time the remote
  455                  * host does the SYN/ACK->ACK.
  456                  */
  457                 if (TSTMP_GT(sc->sc_rxttime, tick)) {
  458                         if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc))
  459                                 sch->sch_nextc = sc->sc_rxttime;
  460                         continue;
  461                 }
  462                 if (sc->sc_rxmits > V_tcp_syncache.rexmt_limit) {
  463                         if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  464                                 log(LOG_DEBUG, "%s; %s: Retransmits exhausted, "
  465                                     "giving up and removing syncache entry\n",
  466                                     s, __func__);
  467                                 free(s, M_TCPLOG);
  468                         }
  469                         syncache_drop(sc, sch);
  470                         TCPSTAT_INC(tcps_sc_stale);
  471                         continue;
  472                 }
  473                 if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  474                         log(LOG_DEBUG, "%s; %s: Response timeout, "
  475                             "retransmitting (%u) SYN|ACK\n",
  476                             s, __func__, sc->sc_rxmits);
  477                         free(s, M_TCPLOG);
  478                 }
  479 
  480                 (void) syncache_respond(sc);
  481                 TCPSTAT_INC(tcps_sc_retransmitted);
  482                 syncache_timeout(sc, sch, 0);
  483         }
  484         if (!TAILQ_EMPTY(&(sch)->sch_bucket))
  485                 callout_reset(&(sch)->sch_timer, (sch)->sch_nextc - tick,
  486                         syncache_timer, (void *)(sch));
  487         CURVNET_RESTORE();
  488 }
  489 
  490 /*
  491  * Find an entry in the syncache.
  492  * Returns always with locked syncache_head plus a matching entry or NULL.
  493  */
  494 static struct syncache *
  495 syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp)
  496 {
  497         struct syncache *sc;
  498         struct syncache_head *sch;
  499 
  500 #ifdef INET6
  501         if (inc->inc_flags & INC_ISIPV6) {
  502                 sch = &V_tcp_syncache.hashbase[
  503                     SYNCACHE_HASH6(inc, V_tcp_syncache.hashmask)];
  504                 *schp = sch;
  505 
  506                 SCH_LOCK(sch);
  507 
  508                 /* Circle through bucket row to find matching entry. */
  509                 TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
  510                         if (ENDPTS6_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie))
  511                                 return (sc);
  512                 }
  513         } else
  514 #endif
  515         {
  516                 sch = &V_tcp_syncache.hashbase[
  517                     SYNCACHE_HASH(inc, V_tcp_syncache.hashmask)];
  518                 *schp = sch;
  519 
  520                 SCH_LOCK(sch);
  521 
  522                 /* Circle through bucket row to find matching entry. */
  523                 TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
  524 #ifdef INET6
  525                         if (sc->sc_inc.inc_flags & INC_ISIPV6)
  526                                 continue;
  527 #endif
  528                         if (ENDPTS_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie))
  529                                 return (sc);
  530                 }
  531         }
  532         SCH_LOCK_ASSERT(*schp);
  533         return (NULL);                  /* always returns with locked sch */
  534 }
  535 
  536 /*
  537  * This function is called when we get a RST for a
  538  * non-existent connection, so that we can see if the
  539  * connection is in the syn cache.  If it is, zap it.
  540  */
  541 void
  542 syncache_chkrst(struct in_conninfo *inc, struct tcphdr *th)
  543 {
  544         struct syncache *sc;
  545         struct syncache_head *sch;
  546         char *s = NULL;
  547 
  548         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  549         SCH_LOCK_ASSERT(sch);
  550 
  551         /*
  552          * Any RST to our SYN|ACK must not carry ACK, SYN or FIN flags.
  553          * See RFC 793 page 65, section SEGMENT ARRIVES.
  554          */
  555         if (th->th_flags & (TH_ACK|TH_SYN|TH_FIN)) {
  556                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  557                         log(LOG_DEBUG, "%s; %s: Spurious RST with ACK, SYN or "
  558                             "FIN flag set, segment ignored\n", s, __func__);
  559                 TCPSTAT_INC(tcps_badrst);
  560                 goto done;
  561         }
  562 
  563         /*
  564          * No corresponding connection was found in syncache.
  565          * If syncookies are enabled and possibly exclusively
  566          * used, or we are under memory pressure, a valid RST
  567          * may not find a syncache entry.  In that case we're
  568          * done and no SYN|ACK retransmissions will happen.
  569          * Otherwise the RST was misdirected or spoofed.
  570          */
  571         if (sc == NULL) {
  572                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  573                         log(LOG_DEBUG, "%s; %s: Spurious RST without matching "
  574                             "syncache entry (possibly syncookie only), "
  575                             "segment ignored\n", s, __func__);
  576                 TCPSTAT_INC(tcps_badrst);
  577                 goto done;
  578         }
  579 
  580         /*
  581          * If the RST bit is set, check the sequence number to see
  582          * if this is a valid reset segment.
  583          * RFC 793 page 37:
  584          *   In all states except SYN-SENT, all reset (RST) segments
  585          *   are validated by checking their SEQ-fields.  A reset is
  586          *   valid if its sequence number is in the window.
  587          *
  588          *   The sequence number in the reset segment is normally an
  589          *   echo of our outgoing acknowlegement numbers, but some hosts
  590          *   send a reset with the sequence number at the rightmost edge
  591          *   of our receive window, and we have to handle this case.
  592          */
  593         if (SEQ_GEQ(th->th_seq, sc->sc_irs) &&
  594             SEQ_LEQ(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
  595                 syncache_drop(sc, sch);
  596                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  597                         log(LOG_DEBUG, "%s; %s: Our SYN|ACK was rejected, "
  598                             "connection attempt aborted by remote endpoint\n",
  599                             s, __func__);
  600                 TCPSTAT_INC(tcps_sc_reset);
  601         } else {
  602                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  603                         log(LOG_DEBUG, "%s; %s: RST with invalid SEQ %u != "
  604                             "IRS %u (+WND %u), segment ignored\n",
  605                             s, __func__, th->th_seq, sc->sc_irs, sc->sc_wnd);
  606                 TCPSTAT_INC(tcps_badrst);
  607         }
  608 
  609 done:
  610         if (s != NULL)
  611                 free(s, M_TCPLOG);
  612         SCH_UNLOCK(sch);
  613 }
  614 
  615 void
  616 syncache_badack(struct in_conninfo *inc)
  617 {
  618         struct syncache *sc;
  619         struct syncache_head *sch;
  620 
  621         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  622         SCH_LOCK_ASSERT(sch);
  623         if (sc != NULL) {
  624                 syncache_drop(sc, sch);
  625                 TCPSTAT_INC(tcps_sc_badack);
  626         }
  627         SCH_UNLOCK(sch);
  628 }
  629 
  630 void
  631 syncache_unreach(struct in_conninfo *inc, struct tcphdr *th)
  632 {
  633         struct syncache *sc;
  634         struct syncache_head *sch;
  635 
  636         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  637         SCH_LOCK_ASSERT(sch);
  638         if (sc == NULL)
  639                 goto done;
  640 
  641         /* If the sequence number != sc_iss, then it's a bogus ICMP msg */
  642         if (ntohl(th->th_seq) != sc->sc_iss)
  643                 goto done;
  644 
  645         /*
  646          * If we've rertransmitted 3 times and this is our second error,
  647          * we remove the entry.  Otherwise, we allow it to continue on.
  648          * This prevents us from incorrectly nuking an entry during a
  649          * spurious network outage.
  650          *
  651          * See tcp_notify().
  652          */
  653         if ((sc->sc_flags & SCF_UNREACH) == 0 || sc->sc_rxmits < 3 + 1) {
  654                 sc->sc_flags |= SCF_UNREACH;
  655                 goto done;
  656         }
  657         syncache_drop(sc, sch);
  658         TCPSTAT_INC(tcps_sc_unreach);
  659 done:
  660         SCH_UNLOCK(sch);
  661 }
  662 
  663 /*
  664  * Build a new TCP socket structure from a syncache entry.
  665  */
  666 static struct socket *
  667 syncache_socket(struct syncache *sc, struct socket *lso, struct mbuf *m)
  668 {
  669         struct inpcb *inp = NULL;
  670         struct socket *so;
  671         struct tcpcb *tp;
  672         int error;
  673         char *s;
  674 
  675         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
  676 
  677         /*
  678          * Ok, create the full blown connection, and set things up
  679          * as they would have been set up if we had created the
  680          * connection when the SYN arrived.  If we can't create
  681          * the connection, abort it.
  682          */
  683         so = sonewconn(lso, SS_ISCONNECTED);
  684         if (so == NULL) {
  685                 /*
  686                  * Drop the connection; we will either send a RST or
  687                  * have the peer retransmit its SYN again after its
  688                  * RTO and try again.
  689                  */
  690                 TCPSTAT_INC(tcps_listendrop);
  691                 if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  692                         log(LOG_DEBUG, "%s; %s: Socket create failed "
  693                             "due to limits or memory shortage\n",
  694                             s, __func__);
  695                         free(s, M_TCPLOG);
  696                 }
  697                 goto abort2;
  698         }
  699 #ifdef MAC
  700         mac_socketpeer_set_from_mbuf(m, so);
  701 #endif
  702 
  703         inp = sotoinpcb(so);
  704         inp->inp_inc.inc_fibnum = so->so_fibnum;
  705         INP_WLOCK(inp);
  706         INP_HASH_WLOCK(&V_tcbinfo);
  707 
  708         /* Insert new socket into PCB hash list. */
  709         inp->inp_inc.inc_flags = sc->sc_inc.inc_flags;
  710 #ifdef INET6
  711         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
  712                 inp->in6p_laddr = sc->sc_inc.inc6_laddr;
  713         } else {
  714                 inp->inp_vflag &= ~INP_IPV6;
  715                 inp->inp_vflag |= INP_IPV4;
  716 #endif
  717                 inp->inp_laddr = sc->sc_inc.inc_laddr;
  718 #ifdef INET6
  719         }
  720 #endif
  721 
  722         /*
  723          * If there's an mbuf and it has a flowid, then let's initialise the
  724          * inp with that particular flowid.
  725          */
  726         if (m != NULL && m->m_flags & M_FLOWID) {
  727                 inp->inp_flags |= INP_HW_FLOWID;
  728                 inp->inp_flags &= ~INP_SW_FLOWID;
  729                 inp->inp_flowid = m->m_pkthdr.flowid;
  730         }
  731 
  732         /*
  733          * Install in the reservation hash table for now, but don't yet
  734          * install a connection group since the full 4-tuple isn't yet
  735          * configured.
  736          */
  737         inp->inp_lport = sc->sc_inc.inc_lport;
  738         if ((error = in_pcbinshash_nopcbgroup(inp)) != 0) {
  739                 /*
  740                  * Undo the assignments above if we failed to
  741                  * put the PCB on the hash lists.
  742                  */
  743 #ifdef INET6
  744                 if (sc->sc_inc.inc_flags & INC_ISIPV6)
  745                         inp->in6p_laddr = in6addr_any;
  746                 else
  747 #endif
  748                         inp->inp_laddr.s_addr = INADDR_ANY;
  749                 inp->inp_lport = 0;
  750                 if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  751                         log(LOG_DEBUG, "%s; %s: in_pcbinshash failed "
  752                             "with error %i\n",
  753                             s, __func__, error);
  754                         free(s, M_TCPLOG);
  755                 }
  756                 INP_HASH_WUNLOCK(&V_tcbinfo);
  757                 goto abort;
  758         }
  759 #ifdef IPSEC
  760         /* Copy old policy into new socket's. */
  761         if (ipsec_copy_policy(sotoinpcb(lso)->inp_sp, inp->inp_sp))
  762                 printf("syncache_socket: could not copy policy\n");
  763 #endif
  764 #ifdef INET6
  765         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
  766                 struct inpcb *oinp = sotoinpcb(lso);
  767                 struct in6_addr laddr6;
  768                 struct sockaddr_in6 sin6;
  769                 /*
  770                  * Inherit socket options from the listening socket.
  771                  * Note that in6p_inputopts are not (and should not be)
  772                  * copied, since it stores previously received options and is
  773                  * used to detect if each new option is different than the
  774                  * previous one and hence should be passed to a user.
  775                  * If we copied in6p_inputopts, a user would not be able to
  776                  * receive options just after calling the accept system call.
  777                  */
  778                 inp->inp_flags |= oinp->inp_flags & INP_CONTROLOPTS;
  779                 if (oinp->in6p_outputopts)
  780                         inp->in6p_outputopts =
  781                             ip6_copypktopts(oinp->in6p_outputopts, M_NOWAIT);
  782 
  783                 sin6.sin6_family = AF_INET6;
  784                 sin6.sin6_len = sizeof(sin6);
  785                 sin6.sin6_addr = sc->sc_inc.inc6_faddr;
  786                 sin6.sin6_port = sc->sc_inc.inc_fport;
  787                 sin6.sin6_flowinfo = sin6.sin6_scope_id = 0;
  788                 laddr6 = inp->in6p_laddr;
  789                 if (IN6_IS_ADDR_UNSPECIFIED(&inp->in6p_laddr))
  790                         inp->in6p_laddr = sc->sc_inc.inc6_laddr;
  791                 if ((error = in6_pcbconnect_mbuf(inp, (struct sockaddr *)&sin6,
  792                     thread0.td_ucred, m)) != 0) {
  793                         inp->in6p_laddr = laddr6;
  794                         if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  795                                 log(LOG_DEBUG, "%s; %s: in6_pcbconnect failed "
  796                                     "with error %i\n",
  797                                     s, __func__, error);
  798                                 free(s, M_TCPLOG);
  799                         }
  800                         INP_HASH_WUNLOCK(&V_tcbinfo);
  801                         goto abort;
  802                 }
  803                 /* Override flowlabel from in6_pcbconnect. */
  804                 inp->inp_flow &= ~IPV6_FLOWLABEL_MASK;
  805                 inp->inp_flow |= sc->sc_flowlabel;
  806         }
  807 #endif /* INET6 */
  808 #if defined(INET) && defined(INET6)
  809         else
  810 #endif
  811 #ifdef INET
  812         {
  813                 struct in_addr laddr;
  814                 struct sockaddr_in sin;
  815 
  816                 inp->inp_options = (m) ? ip_srcroute(m) : NULL;
  817                 
  818                 if (inp->inp_options == NULL) {
  819                         inp->inp_options = sc->sc_ipopts;
  820                         sc->sc_ipopts = NULL;
  821                 }
  822 
  823                 sin.sin_family = AF_INET;
  824                 sin.sin_len = sizeof(sin);
  825                 sin.sin_addr = sc->sc_inc.inc_faddr;
  826                 sin.sin_port = sc->sc_inc.inc_fport;
  827                 bzero((caddr_t)sin.sin_zero, sizeof(sin.sin_zero));
  828                 laddr = inp->inp_laddr;
  829                 if (inp->inp_laddr.s_addr == INADDR_ANY)
  830                         inp->inp_laddr = sc->sc_inc.inc_laddr;
  831                 if ((error = in_pcbconnect_mbuf(inp, (struct sockaddr *)&sin,
  832                     thread0.td_ucred, m)) != 0) {
  833                         inp->inp_laddr = laddr;
  834                         if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  835                                 log(LOG_DEBUG, "%s; %s: in_pcbconnect failed "
  836                                     "with error %i\n",
  837                                     s, __func__, error);
  838                                 free(s, M_TCPLOG);
  839                         }
  840                         INP_HASH_WUNLOCK(&V_tcbinfo);
  841                         goto abort;
  842                 }
  843         }
  844 #endif /* INET */
  845         INP_HASH_WUNLOCK(&V_tcbinfo);
  846         tp = intotcpcb(inp);
  847         tcp_state_change(tp, TCPS_SYN_RECEIVED);
  848         tp->iss = sc->sc_iss;
  849         tp->irs = sc->sc_irs;
  850         tcp_rcvseqinit(tp);
  851         tcp_sendseqinit(tp);
  852         tp->snd_wl1 = sc->sc_irs;
  853         tp->snd_max = tp->iss + 1;
  854         tp->snd_nxt = tp->iss + 1;
  855         tp->rcv_up = sc->sc_irs + 1;
  856         tp->rcv_wnd = sc->sc_wnd;
  857         tp->rcv_adv += tp->rcv_wnd;
  858         tp->last_ack_sent = tp->rcv_nxt;
  859 
  860         tp->t_flags = sototcpcb(lso)->t_flags & (TF_NOPUSH|TF_NODELAY);
  861         if (sc->sc_flags & SCF_NOOPT)
  862                 tp->t_flags |= TF_NOOPT;
  863         else {
  864                 if (sc->sc_flags & SCF_WINSCALE) {
  865                         tp->t_flags |= TF_REQ_SCALE|TF_RCVD_SCALE;
  866                         tp->snd_scale = sc->sc_requested_s_scale;
  867                         tp->request_r_scale = sc->sc_requested_r_scale;
  868                 }
  869                 if (sc->sc_flags & SCF_TIMESTAMP) {
  870                         tp->t_flags |= TF_REQ_TSTMP|TF_RCVD_TSTMP;
  871                         tp->ts_recent = sc->sc_tsreflect;
  872                         tp->ts_recent_age = tcp_ts_getticks();
  873                         tp->ts_offset = sc->sc_tsoff;
  874                 }
  875 #ifdef TCP_SIGNATURE
  876                 if (sc->sc_flags & SCF_SIGNATURE)
  877                         tp->t_flags |= TF_SIGNATURE;
  878 #endif
  879                 if (sc->sc_flags & SCF_SACK)
  880                         tp->t_flags |= TF_SACK_PERMIT;
  881         }
  882 
  883         if (sc->sc_flags & SCF_ECN)
  884                 tp->t_flags |= TF_ECN_PERMIT;
  885 
  886         /*
  887          * Set up MSS and get cached values from tcp_hostcache.
  888          * This might overwrite some of the defaults we just set.
  889          */
  890         tcp_mss(tp, sc->sc_peer_mss);
  891 
  892         /*
  893          * If the SYN,ACK was retransmitted, indicate that CWND to be
  894          * limited to one segment in cc_conn_init().
  895          * NB: sc_rxmits counts all SYN,ACK transmits, not just retransmits.
  896          */
  897         if (sc->sc_rxmits > 1)
  898                 tp->snd_cwnd = 1;
  899 
  900 #ifdef TCP_OFFLOAD
  901         /*
  902          * Allow a TOE driver to install its hooks.  Note that we hold the
  903          * pcbinfo lock too and that prevents tcp_usr_accept from accepting a
  904          * new connection before the TOE driver has done its thing.
  905          */
  906         if (ADDED_BY_TOE(sc)) {
  907                 struct toedev *tod = sc->sc_tod;
  908 
  909                 tod->tod_offload_socket(tod, sc->sc_todctx, so);
  910         }
  911 #endif
  912         /*
  913          * Copy and activate timers.
  914          */
  915         tp->t_keepinit = sototcpcb(lso)->t_keepinit;
  916         tp->t_keepidle = sototcpcb(lso)->t_keepidle;
  917         tp->t_keepintvl = sototcpcb(lso)->t_keepintvl;
  918         tp->t_keepcnt = sototcpcb(lso)->t_keepcnt;
  919         tcp_timer_activate(tp, TT_KEEP, TP_KEEPINIT(tp));
  920 
  921         INP_WUNLOCK(inp);
  922 
  923         TCPSTAT_INC(tcps_accepts);
  924         return (so);
  925 
  926 abort:
  927         INP_WUNLOCK(inp);
  928 abort2:
  929         if (so != NULL)
  930                 soabort(so);
  931         return (NULL);
  932 }
  933 
  934 /*
  935  * This function gets called when we receive an ACK for a
  936  * socket in the LISTEN state.  We look up the connection
  937  * in the syncache, and if its there, we pull it out of
  938  * the cache and turn it into a full-blown connection in
  939  * the SYN-RECEIVED state.
  940  */
  941 int
  942 syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
  943     struct socket **lsop, struct mbuf *m)
  944 {
  945         struct syncache *sc;
  946         struct syncache_head *sch;
  947         struct syncache scs;
  948         char *s;
  949 
  950         /*
  951          * Global TCP locks are held because we manipulate the PCB lists
  952          * and create a new socket.
  953          */
  954         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
  955         KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK,
  956             ("%s: can handle only ACK", __func__));
  957 
  958         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  959         SCH_LOCK_ASSERT(sch);
  960 
  961 #ifdef INVARIANTS
  962         /*
  963          * Test code for syncookies comparing the syncache stored
  964          * values with the reconstructed values from the cookie.
  965          */
  966         if (sc != NULL)
  967                 syncookie_cmp(inc, sch, sc, th, to, *lsop);
  968 #endif
  969 
  970         if (sc == NULL) {
  971                 /*
  972                  * There is no syncache entry, so see if this ACK is
  973                  * a returning syncookie.  To do this, first:
  974                  *  A. See if this socket has had a syncache entry dropped in
  975                  *     the past.  We don't want to accept a bogus syncookie
  976                  *     if we've never received a SYN.
  977                  *  B. check that the syncookie is valid.  If it is, then
  978                  *     cobble up a fake syncache entry, and return.
  979                  */
  980                 if (!V_tcp_syncookies) {
  981                         SCH_UNLOCK(sch);
  982                         if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  983                                 log(LOG_DEBUG, "%s; %s: Spurious ACK, "
  984                                     "segment rejected (syncookies disabled)\n",
  985                                     s, __func__);
  986                         goto failed;
  987                 }
  988                 bzero(&scs, sizeof(scs));
  989                 sc = syncookie_lookup(inc, sch, &scs, th, to, *lsop);
  990                 SCH_UNLOCK(sch);
  991                 if (sc == NULL) {
  992                         if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  993                                 log(LOG_DEBUG, "%s; %s: Segment failed "
  994                                     "SYNCOOKIE authentication, segment rejected "
  995                                     "(probably spoofed)\n", s, __func__);
  996                         goto failed;
  997                 }
  998         } else {
  999                 /* Pull out the entry to unlock the bucket row. */
 1000                 TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
 1001                 sch->sch_length--;
 1002 #ifdef TCP_OFFLOAD
 1003                 if (ADDED_BY_TOE(sc)) {
 1004                         struct toedev *tod = sc->sc_tod;
 1005 
 1006                         tod->tod_syncache_removed(tod, sc->sc_todctx);
 1007                 }
 1008 #endif
 1009                 SCH_UNLOCK(sch);
 1010         }
 1011 
 1012         /*
 1013          * Segment validation:
 1014          * ACK must match our initial sequence number + 1 (the SYN|ACK).
 1015          */
 1016         if (th->th_ack != sc->sc_iss + 1) {
 1017                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1018                         log(LOG_DEBUG, "%s; %s: ACK %u != ISS+1 %u, segment "
 1019                             "rejected\n", s, __func__, th->th_ack, sc->sc_iss);
 1020                 goto failed;
 1021         }
 1022 
 1023         /*
 1024          * The SEQ must fall in the window starting at the received
 1025          * initial receive sequence number + 1 (the SYN).
 1026          */
 1027         if (SEQ_LEQ(th->th_seq, sc->sc_irs) ||
 1028             SEQ_GT(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
 1029                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1030                         log(LOG_DEBUG, "%s; %s: SEQ %u != IRS+1 %u, segment "
 1031                             "rejected\n", s, __func__, th->th_seq, sc->sc_irs);
 1032                 goto failed;
 1033         }
 1034 
 1035         /*
 1036          * If timestamps were not negotiated during SYN/ACK they
 1037          * must not appear on any segment during this session.
 1038          */
 1039         if (!(sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS)) {
 1040                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1041                         log(LOG_DEBUG, "%s; %s: Timestamp not expected, "
 1042                             "segment rejected\n", s, __func__);
 1043                 goto failed;
 1044         }
 1045 
 1046         /*
 1047          * If timestamps were negotiated during SYN/ACK they should
 1048          * appear on every segment during this session.
 1049          * XXXAO: This is only informal as there have been unverified
 1050          * reports of non-compliants stacks.
 1051          */
 1052         if ((sc->sc_flags & SCF_TIMESTAMP) && !(to->to_flags & TOF_TS)) {
 1053                 if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1054                         log(LOG_DEBUG, "%s; %s: Timestamp missing, "
 1055                             "no action\n", s, __func__);
 1056                         free(s, M_TCPLOG);
 1057                         s = NULL;
 1058                 }
 1059         }
 1060 
 1061         /*
 1062          * If timestamps were negotiated the reflected timestamp
 1063          * must be equal to what we actually sent in the SYN|ACK.
 1064          */
 1065         if ((to->to_flags & TOF_TS) && to->to_tsecr != sc->sc_ts) {
 1066                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1067                         log(LOG_DEBUG, "%s; %s: TSECR %u != TS %u, "
 1068                             "segment rejected\n",
 1069                             s, __func__, to->to_tsecr, sc->sc_ts);
 1070                 goto failed;
 1071         }
 1072 
 1073         *lsop = syncache_socket(sc, *lsop, m);
 1074 
 1075         if (*lsop == NULL)
 1076                 TCPSTAT_INC(tcps_sc_aborted);
 1077         else
 1078                 TCPSTAT_INC(tcps_sc_completed);
 1079 
 1080 /* how do we find the inp for the new socket? */
 1081         if (sc != &scs)
 1082                 syncache_free(sc);
 1083         return (1);
 1084 failed:
 1085         if (sc != NULL && sc != &scs)
 1086                 syncache_free(sc);
 1087         if (s != NULL)
 1088                 free(s, M_TCPLOG);
 1089         *lsop = NULL;
 1090         return (0);
 1091 }
 1092 
 1093 /*
 1094  * Given a LISTEN socket and an inbound SYN request, add
 1095  * this to the syn cache, and send back a segment:
 1096  *      <SEQ=ISS><ACK=RCV_NXT><CTL=SYN,ACK>
 1097  * to the source.
 1098  *
 1099  * IMPORTANT NOTE: We do _NOT_ ACK data that might accompany the SYN.
 1100  * Doing so would require that we hold onto the data and deliver it
 1101  * to the application.  However, if we are the target of a SYN-flood
 1102  * DoS attack, an attacker could send data which would eventually
 1103  * consume all available buffer space if it were ACKed.  By not ACKing
 1104  * the data, we avoid this DoS scenario.
 1105  */
 1106 void
 1107 syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
 1108     struct inpcb *inp, struct socket **lsop, struct mbuf *m, void *tod,
 1109     void *todctx)
 1110 {
 1111         struct tcpcb *tp;
 1112         struct socket *so;
 1113         struct syncache *sc = NULL;
 1114         struct syncache_head *sch;
 1115         struct mbuf *ipopts = NULL;
 1116         u_int ltflags;
 1117         int win, sb_hiwat, ip_ttl, ip_tos;
 1118         char *s;
 1119 #ifdef INET6
 1120         int autoflowlabel = 0;
 1121 #endif
 1122 #ifdef MAC
 1123         struct label *maclabel;
 1124 #endif
 1125         struct syncache scs;
 1126         struct ucred *cred;
 1127 
 1128         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 1129         INP_WLOCK_ASSERT(inp);                  /* listen socket */
 1130         KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_SYN,
 1131             ("%s: unexpected tcp flags", __func__));
 1132 
 1133         /*
 1134          * Combine all so/tp operations very early to drop the INP lock as
 1135          * soon as possible.
 1136          */
 1137         so = *lsop;
 1138         tp = sototcpcb(so);
 1139         cred = crhold(so->so_cred);
 1140 
 1141 #ifdef INET6
 1142         if ((inc->inc_flags & INC_ISIPV6) &&
 1143             (inp->inp_flags & IN6P_AUTOFLOWLABEL))
 1144                 autoflowlabel = 1;
 1145 #endif
 1146         ip_ttl = inp->inp_ip_ttl;
 1147         ip_tos = inp->inp_ip_tos;
 1148         win = sbspace(&so->so_rcv);
 1149         sb_hiwat = so->so_rcv.sb_hiwat;
 1150         ltflags = (tp->t_flags & (TF_NOOPT | TF_SIGNATURE));
 1151 
 1152         /* By the time we drop the lock these should no longer be used. */
 1153         so = NULL;
 1154         tp = NULL;
 1155 
 1156 #ifdef MAC
 1157         if (mac_syncache_init(&maclabel) != 0) {
 1158                 INP_WUNLOCK(inp);
 1159                 INP_INFO_WUNLOCK(&V_tcbinfo);
 1160                 goto done;
 1161         } else
 1162                 mac_syncache_create(maclabel, inp);
 1163 #endif
 1164         INP_WUNLOCK(inp);
 1165         INP_INFO_WUNLOCK(&V_tcbinfo);
 1166 
 1167         /*
 1168          * Remember the IP options, if any.
 1169          */
 1170 #ifdef INET6
 1171         if (!(inc->inc_flags & INC_ISIPV6))
 1172 #endif
 1173 #ifdef INET
 1174                 ipopts = (m) ? ip_srcroute(m) : NULL;
 1175 #else
 1176                 ipopts = NULL;
 1177 #endif
 1178 
 1179         /*
 1180          * See if we already have an entry for this connection.
 1181          * If we do, resend the SYN,ACK, and reset the retransmit timer.
 1182          *
 1183          * XXX: should the syncache be re-initialized with the contents
 1184          * of the new SYN here (which may have different options?)
 1185          *
 1186          * XXX: We do not check the sequence number to see if this is a
 1187          * real retransmit or a new connection attempt.  The question is
 1188          * how to handle such a case; either ignore it as spoofed, or
 1189          * drop the current entry and create a new one?
 1190          */
 1191         sc = syncache_lookup(inc, &sch);        /* returns locked entry */
 1192         SCH_LOCK_ASSERT(sch);
 1193         if (sc != NULL) {
 1194                 TCPSTAT_INC(tcps_sc_dupsyn);
 1195                 if (ipopts) {
 1196                         /*
 1197                          * If we were remembering a previous source route,
 1198                          * forget it and use the new one we've been given.
 1199                          */
 1200                         if (sc->sc_ipopts)
 1201                                 (void) m_free(sc->sc_ipopts);
 1202                         sc->sc_ipopts = ipopts;
 1203                 }
 1204                 /*
 1205                  * Update timestamp if present.
 1206                  */
 1207                 if ((sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS))
 1208                         sc->sc_tsreflect = to->to_tsval;
 1209                 else
 1210                         sc->sc_flags &= ~SCF_TIMESTAMP;
 1211 #ifdef MAC
 1212                 /*
 1213                  * Since we have already unconditionally allocated label
 1214                  * storage, free it up.  The syncache entry will already
 1215                  * have an initialized label we can use.
 1216                  */
 1217                 mac_syncache_destroy(&maclabel);
 1218 #endif
 1219                 /* Retransmit SYN|ACK and reset retransmit count. */
 1220                 if ((s = tcp_log_addrs(&sc->sc_inc, th, NULL, NULL))) {
 1221                         log(LOG_DEBUG, "%s; %s: Received duplicate SYN, "
 1222                             "resetting timer and retransmitting SYN|ACK\n",
 1223                             s, __func__);
 1224                         free(s, M_TCPLOG);
 1225                 }
 1226                 if (syncache_respond(sc) == 0) {
 1227                         sc->sc_rxmits = 0;
 1228                         syncache_timeout(sc, sch, 1);
 1229                         TCPSTAT_INC(tcps_sndacks);
 1230                         TCPSTAT_INC(tcps_sndtotal);
 1231                 }
 1232                 SCH_UNLOCK(sch);
 1233                 goto done;
 1234         }
 1235 
 1236         sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
 1237         if (sc == NULL) {
 1238                 /*
 1239                  * The zone allocator couldn't provide more entries.
 1240                  * Treat this as if the cache was full; drop the oldest
 1241                  * entry and insert the new one.
 1242                  */
 1243                 TCPSTAT_INC(tcps_sc_zonefail);
 1244                 if ((sc = TAILQ_LAST(&sch->sch_bucket, sch_head)) != NULL)
 1245                         syncache_drop(sc, sch);
 1246                 sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
 1247                 if (sc == NULL) {
 1248                         if (V_tcp_syncookies) {
 1249                                 bzero(&scs, sizeof(scs));
 1250                                 sc = &scs;
 1251                         } else {
 1252                                 SCH_UNLOCK(sch);
 1253                                 if (ipopts)
 1254                                         (void) m_free(ipopts);
 1255                                 goto done;
 1256                         }
 1257                 }
 1258         }
 1259         
 1260         /*
 1261          * Fill in the syncache values.
 1262          */
 1263 #ifdef MAC
 1264         sc->sc_label = maclabel;
 1265 #endif
 1266         sc->sc_cred = cred;
 1267         cred = NULL;
 1268         sc->sc_ipopts = ipopts;
 1269         bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
 1270 #ifdef INET6
 1271         if (!(inc->inc_flags & INC_ISIPV6))
 1272 #endif
 1273         {
 1274                 sc->sc_ip_tos = ip_tos;
 1275                 sc->sc_ip_ttl = ip_ttl;
 1276         }
 1277 #ifdef TCP_OFFLOAD
 1278         sc->sc_tod = tod;
 1279         sc->sc_todctx = todctx;
 1280 #endif
 1281         sc->sc_irs = th->th_seq;
 1282         sc->sc_iss = arc4random();
 1283         sc->sc_flags = 0;
 1284         sc->sc_flowlabel = 0;
 1285 
 1286         /*
 1287          * Initial receive window: clip sbspace to [0 .. TCP_MAXWIN].
 1288          * win was derived from socket earlier in the function.
 1289          */
 1290         win = imax(win, 0);
 1291         win = imin(win, TCP_MAXWIN);
 1292         sc->sc_wnd = win;
 1293 
 1294         if (V_tcp_do_rfc1323) {
 1295                 /*
 1296                  * A timestamp received in a SYN makes
 1297                  * it ok to send timestamp requests and replies.
 1298                  */
 1299                 if (to->to_flags & TOF_TS) {
 1300                         sc->sc_tsreflect = to->to_tsval;
 1301                         sc->sc_ts = tcp_ts_getticks();
 1302                         sc->sc_flags |= SCF_TIMESTAMP;
 1303                 }
 1304                 if (to->to_flags & TOF_SCALE) {
 1305                         int wscale = 0;
 1306 
 1307                         /*
 1308                          * Pick the smallest possible scaling factor that
 1309                          * will still allow us to scale up to sb_max, aka
 1310                          * kern.ipc.maxsockbuf.
 1311                          *
 1312                          * We do this because there are broken firewalls that
 1313                          * will corrupt the window scale option, leading to
 1314                          * the other endpoint believing that our advertised
 1315                          * window is unscaled.  At scale factors larger than
 1316                          * 5 the unscaled window will drop below 1500 bytes,
 1317                          * leading to serious problems when traversing these
 1318                          * broken firewalls.
 1319                          *
 1320                          * With the default maxsockbuf of 256K, a scale factor
 1321                          * of 3 will be chosen by this algorithm.  Those who
 1322                          * choose a larger maxsockbuf should watch out
 1323                          * for the compatiblity problems mentioned above.
 1324                          *
 1325                          * RFC1323: The Window field in a SYN (i.e., a <SYN>
 1326                          * or <SYN,ACK>) segment itself is never scaled.
 1327                          */
 1328                         while (wscale < TCP_MAX_WINSHIFT &&
 1329                             (TCP_MAXWIN << wscale) < sb_max)
 1330                                 wscale++;
 1331                         sc->sc_requested_r_scale = wscale;
 1332                         sc->sc_requested_s_scale = to->to_wscale;
 1333                         sc->sc_flags |= SCF_WINSCALE;
 1334                 }
 1335         }
 1336 #ifdef TCP_SIGNATURE
 1337         /*
 1338          * If listening socket requested TCP digests, and received SYN
 1339          * contains the option, flag this in the syncache so that
 1340          * syncache_respond() will do the right thing with the SYN+ACK.
 1341          * XXX: Currently we always record the option by default and will
 1342          * attempt to use it in syncache_respond().
 1343          */
 1344         if (to->to_flags & TOF_SIGNATURE || ltflags & TF_SIGNATURE)
 1345                 sc->sc_flags |= SCF_SIGNATURE;
 1346 #endif
 1347         if (to->to_flags & TOF_SACKPERM)
 1348                 sc->sc_flags |= SCF_SACK;
 1349         if (to->to_flags & TOF_MSS)
 1350                 sc->sc_peer_mss = to->to_mss;   /* peer mss may be zero */
 1351         if (ltflags & TF_NOOPT)
 1352                 sc->sc_flags |= SCF_NOOPT;
 1353         if ((th->th_flags & (TH_ECE|TH_CWR)) && V_tcp_do_ecn)
 1354                 sc->sc_flags |= SCF_ECN;
 1355 
 1356         if (V_tcp_syncookies)
 1357                 sc->sc_iss = syncookie_generate(sch, sc);
 1358 #ifdef INET6
 1359         if (autoflowlabel) {
 1360                 if (V_tcp_syncookies)
 1361                         sc->sc_flowlabel = sc->sc_iss;
 1362                 else
 1363                         sc->sc_flowlabel = ip6_randomflowlabel();
 1364                 sc->sc_flowlabel = htonl(sc->sc_flowlabel) & IPV6_FLOWLABEL_MASK;
 1365         }
 1366 #endif
 1367         SCH_UNLOCK(sch);
 1368 
 1369         /*
 1370          * Do a standard 3-way handshake.
 1371          */
 1372         if (syncache_respond(sc) == 0) {
 1373                 if (V_tcp_syncookies && V_tcp_syncookiesonly && sc != &scs)
 1374                         syncache_free(sc);
 1375                 else if (sc != &scs)
 1376                         syncache_insert(sc, sch);   /* locks and unlocks sch */
 1377                 TCPSTAT_INC(tcps_sndacks);
 1378                 TCPSTAT_INC(tcps_sndtotal);
 1379         } else {
 1380                 if (sc != &scs)
 1381                         syncache_free(sc);
 1382                 TCPSTAT_INC(tcps_sc_dropped);
 1383         }
 1384 
 1385 done:
 1386         if (cred != NULL)
 1387                 crfree(cred);
 1388 #ifdef MAC
 1389         if (sc == &scs)
 1390                 mac_syncache_destroy(&maclabel);
 1391 #endif
 1392         if (m) {
 1393                 
 1394                 *lsop = NULL;
 1395                 m_freem(m);
 1396         }
 1397 }
 1398 
 1399 static int
 1400 syncache_respond(struct syncache *sc)
 1401 {
 1402         struct ip *ip = NULL;
 1403         struct mbuf *m;
 1404         struct tcphdr *th = NULL;
 1405         int optlen, error = 0;  /* Make compiler happy */
 1406         u_int16_t hlen, tlen, mssopt;
 1407         struct tcpopt to;
 1408 #ifdef INET6
 1409         struct ip6_hdr *ip6 = NULL;
 1410 #endif
 1411 
 1412         hlen =
 1413 #ifdef INET6
 1414                (sc->sc_inc.inc_flags & INC_ISIPV6) ? sizeof(struct ip6_hdr) :
 1415 #endif
 1416                 sizeof(struct ip);
 1417         tlen = hlen + sizeof(struct tcphdr);
 1418 
 1419         /* Determine MSS we advertize to other end of connection. */
 1420         mssopt = tcp_mssopt(&sc->sc_inc);
 1421         if (sc->sc_peer_mss)
 1422                 mssopt = max( min(sc->sc_peer_mss, mssopt), V_tcp_minmss);
 1423 
 1424         /* XXX: Assume that the entire packet will fit in a header mbuf. */
 1425         KASSERT(max_linkhdr + tlen + TCP_MAXOLEN <= MHLEN,
 1426             ("syncache: mbuf too small"));
 1427 
 1428         /* Create the IP+TCP header from scratch. */
 1429         m = m_gethdr(M_NOWAIT, MT_DATA);
 1430         if (m == NULL)
 1431                 return (ENOBUFS);
 1432 #ifdef MAC
 1433         mac_syncache_create_mbuf(sc->sc_label, m);
 1434 #endif
 1435         m->m_data += max_linkhdr;
 1436         m->m_len = tlen;
 1437         m->m_pkthdr.len = tlen;
 1438         m->m_pkthdr.rcvif = NULL;
 1439 
 1440 #ifdef INET6
 1441         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
 1442                 ip6 = mtod(m, struct ip6_hdr *);
 1443                 ip6->ip6_vfc = IPV6_VERSION;
 1444                 ip6->ip6_nxt = IPPROTO_TCP;
 1445                 ip6->ip6_src = sc->sc_inc.inc6_laddr;
 1446                 ip6->ip6_dst = sc->sc_inc.inc6_faddr;
 1447                 ip6->ip6_plen = htons(tlen - hlen);
 1448                 /* ip6_hlim is set after checksum */
 1449                 ip6->ip6_flow &= ~IPV6_FLOWLABEL_MASK;
 1450                 ip6->ip6_flow |= sc->sc_flowlabel;
 1451 
 1452                 th = (struct tcphdr *)(ip6 + 1);
 1453         }
 1454 #endif
 1455 #if defined(INET6) && defined(INET)
 1456         else
 1457 #endif
 1458 #ifdef INET
 1459         {
 1460                 ip = mtod(m, struct ip *);
 1461                 ip->ip_v = IPVERSION;
 1462                 ip->ip_hl = sizeof(struct ip) >> 2;
 1463                 ip->ip_len = htons(tlen);
 1464                 ip->ip_id = 0;
 1465                 ip->ip_off = 0;
 1466                 ip->ip_sum = 0;
 1467                 ip->ip_p = IPPROTO_TCP;
 1468                 ip->ip_src = sc->sc_inc.inc_laddr;
 1469                 ip->ip_dst = sc->sc_inc.inc_faddr;
 1470                 ip->ip_ttl = sc->sc_ip_ttl;
 1471                 ip->ip_tos = sc->sc_ip_tos;
 1472 
 1473                 /*
 1474                  * See if we should do MTU discovery.  Route lookups are
 1475                  * expensive, so we will only unset the DF bit if:
 1476                  *
 1477                  *      1) path_mtu_discovery is disabled
 1478                  *      2) the SCF_UNREACH flag has been set
 1479                  */
 1480                 if (V_path_mtu_discovery && ((sc->sc_flags & SCF_UNREACH) == 0))
 1481                        ip->ip_off |= htons(IP_DF);
 1482 
 1483                 th = (struct tcphdr *)(ip + 1);
 1484         }
 1485 #endif /* INET */
 1486         th->th_sport = sc->sc_inc.inc_lport;
 1487         th->th_dport = sc->sc_inc.inc_fport;
 1488 
 1489         th->th_seq = htonl(sc->sc_iss);
 1490         th->th_ack = htonl(sc->sc_irs + 1);
 1491         th->th_off = sizeof(struct tcphdr) >> 2;
 1492         th->th_x2 = 0;
 1493         th->th_flags = TH_SYN|TH_ACK;
 1494         th->th_win = htons(sc->sc_wnd);
 1495         th->th_urp = 0;
 1496 
 1497         if (sc->sc_flags & SCF_ECN) {
 1498                 th->th_flags |= TH_ECE;
 1499                 TCPSTAT_INC(tcps_ecn_shs);
 1500         }
 1501 
 1502         /* Tack on the TCP options. */
 1503         if ((sc->sc_flags & SCF_NOOPT) == 0) {
 1504                 to.to_flags = 0;
 1505 
 1506                 to.to_mss = mssopt;
 1507                 to.to_flags = TOF_MSS;
 1508                 if (sc->sc_flags & SCF_WINSCALE) {
 1509                         to.to_wscale = sc->sc_requested_r_scale;
 1510                         to.to_flags |= TOF_SCALE;
 1511                 }
 1512                 if (sc->sc_flags & SCF_TIMESTAMP) {
 1513                         /* Virgin timestamp or TCP cookie enhanced one. */
 1514                         to.to_tsval = sc->sc_ts;
 1515                         to.to_tsecr = sc->sc_tsreflect;
 1516                         to.to_flags |= TOF_TS;
 1517                 }
 1518                 if (sc->sc_flags & SCF_SACK)
 1519                         to.to_flags |= TOF_SACKPERM;
 1520 #ifdef TCP_SIGNATURE
 1521                 if (sc->sc_flags & SCF_SIGNATURE)
 1522                         to.to_flags |= TOF_SIGNATURE;
 1523 #endif
 1524                 optlen = tcp_addoptions(&to, (u_char *)(th + 1));
 1525 
 1526                 /* Adjust headers by option size. */
 1527                 th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
 1528                 m->m_len += optlen;
 1529                 m->m_pkthdr.len += optlen;
 1530 
 1531 #ifdef TCP_SIGNATURE
 1532                 if (sc->sc_flags & SCF_SIGNATURE)
 1533                         tcp_signature_compute(m, 0, 0, optlen,
 1534                             to.to_signature, IPSEC_DIR_OUTBOUND);
 1535 #endif
 1536 #ifdef INET6
 1537                 if (sc->sc_inc.inc_flags & INC_ISIPV6)
 1538                         ip6->ip6_plen = htons(ntohs(ip6->ip6_plen) + optlen);
 1539                 else
 1540 #endif
 1541                         ip->ip_len = htons(ntohs(ip->ip_len) + optlen);
 1542         } else
 1543                 optlen = 0;
 1544 
 1545         M_SETFIB(m, sc->sc_inc.inc_fibnum);
 1546         m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
 1547 #ifdef INET6
 1548         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
 1549                 m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
 1550                 th->th_sum = in6_cksum_pseudo(ip6, tlen + optlen - hlen,
 1551                     IPPROTO_TCP, 0);
 1552                 ip6->ip6_hlim = in6_selecthlim(NULL, NULL);
 1553 #ifdef TCP_OFFLOAD
 1554                 if (ADDED_BY_TOE(sc)) {
 1555                         struct toedev *tod = sc->sc_tod;
 1556 
 1557                         error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
 1558 
 1559                         return (error);
 1560                 }
 1561 #endif
 1562                 error = ip6_output(m, NULL, NULL, 0, NULL, NULL, NULL);
 1563         }
 1564 #endif
 1565 #if defined(INET6) && defined(INET)
 1566         else
 1567 #endif
 1568 #ifdef INET
 1569         {
 1570                 m->m_pkthdr.csum_flags = CSUM_TCP;
 1571                 th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
 1572                     htons(tlen + optlen - hlen + IPPROTO_TCP));
 1573 #ifdef TCP_OFFLOAD
 1574                 if (ADDED_BY_TOE(sc)) {
 1575                         struct toedev *tod = sc->sc_tod;
 1576 
 1577                         error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
 1578 
 1579                         return (error);
 1580                 }
 1581 #endif
 1582                 error = ip_output(m, sc->sc_ipopts, NULL, 0, NULL, NULL);
 1583         }
 1584 #endif
 1585         return (error);
 1586 }
 1587 
 1588 /*
 1589  * The purpose of syncookies is to handle spoofed SYN flooding DoS attacks
 1590  * that exceed the capacity of the syncache by avoiding the storage of any
 1591  * of the SYNs we receive.  Syncookies defend against blind SYN flooding
 1592  * attacks where the attacker does not have access to our responses.
 1593  *
 1594  * Syncookies encode and include all necessary information about the
 1595  * connection setup within the SYN|ACK that we send back.  That way we
 1596  * can avoid keeping any local state until the ACK to our SYN|ACK returns
 1597  * (if ever).  Normally the syncache and syncookies are running in parallel
 1598  * with the latter taking over when the former is exhausted.  When matching
 1599  * syncache entry is found the syncookie is ignored.
 1600  *
 1601  * The only reliable information persisting the 3WHS is our inital sequence
 1602  * number ISS of 32 bits.  Syncookies embed a cryptographically sufficient
 1603  * strong hash (MAC) value and a few bits of TCP SYN options in the ISS
 1604  * of our SYN|ACK.  The MAC can be recomputed when the ACK to our SYN|ACK
 1605  * returns and signifies a legitimate connection if it matches the ACK.
 1606  *
 1607  * The available space of 32 bits to store the hash and to encode the SYN
 1608  * option information is very tight and we should have at least 24 bits for
 1609  * the MAC to keep the number of guesses by blind spoofing reasonably high.
 1610  *
 1611  * SYN option information we have to encode to fully restore a connection:
 1612  * MSS: is imporant to chose an optimal segment size to avoid IP level
 1613  *   fragmentation along the path.  The common MSS values can be encoded
 1614  *   in a 3-bit table.  Uncommon values are captured by the next lower value
 1615  *   in the table leading to a slight increase in packetization overhead.
 1616  * WSCALE: is necessary to allow large windows to be used for high delay-
 1617  *   bandwidth product links.  Not scaling the window when it was initially
 1618  *   negotiated is bad for performance as lack of scaling further decreases
 1619  *   the apparent available send window.  We only need to encode the WSCALE
 1620  *   we received from the remote end.  Our end can be recalculated at any
 1621  *   time.  The common WSCALE values can be encoded in a 3-bit table.
 1622  *   Uncommon values are captured by the next lower value in the table
 1623  *   making us under-estimate the available window size halving our
 1624  *   theoretically possible maximum throughput for that connection.
 1625  * SACK: Greatly assists in packet loss recovery and requires 1 bit.
 1626  * TIMESTAMP and SIGNATURE is not encoded because they are permanent options
 1627  *   that are included in all segments on a connection.  We enable them when
 1628  *   the ACK has them.
 1629  *
 1630  * Security of syncookies and attack vectors:
 1631  *
 1632  * The MAC is computed over (faddr||laddr||fport||lport||irs||flags||secmod)
 1633  * together with the gloabl secret to make it unique per connection attempt.
 1634  * Thus any change of any of those parameters results in a different MAC output
 1635  * in an unpredictable way unless a collision is encountered.  24 bits of the
 1636  * MAC are embedded into the ISS.
 1637  *
 1638  * To prevent replay attacks two rotating global secrets are updated with a
 1639  * new random value every 15 seconds.  The life-time of a syncookie is thus
 1640  * 15-30 seconds.
 1641  *
 1642  * Vector 1: Attacking the secret.  This requires finding a weakness in the
 1643  * MAC itself or the way it is used here.  The attacker can do a chosen plain
 1644  * text attack by varying and testing the all parameters under his control.
 1645  * The strength depends on the size and randomness of the secret, and the
 1646  * cryptographic security of the MAC function.  Due to the constant updating
 1647  * of the secret the attacker has at most 29.999 seconds to find the secret
 1648  * and launch spoofed connections.  After that he has to start all over again.
 1649  *
 1650  * Vector 2: Collision attack on the MAC of a single ACK.  With a 24 bit MAC
 1651  * size an average of 4,823 attempts are required for a 50% chance of success
 1652  * to spoof a single syncookie (birthday collision paradox).  However the
 1653  * attacker is blind and doesn't know if one of his attempts succeeded unless
 1654  * he has a side channel to interfere success from.  A single connection setup
 1655  * success average of 90% requires 8,790 packets, 99.99% requires 17,578 packets.
 1656  * This many attempts are required for each one blind spoofed connection.  For
 1657  * every additional spoofed connection he has to launch another N attempts.
 1658  * Thus for a sustained rate 100 spoofed connections per second approximately
 1659  * 1,800,000 packets per second would have to be sent.
 1660  *
 1661  * NB: The MAC function should be fast so that it doesn't become a CPU
 1662  * exhaustion attack vector itself.
 1663  *
 1664  * References:
 1665  *  RFC4987 TCP SYN Flooding Attacks and Common Mitigations
 1666  *  SYN cookies were first proposed by cryptographer Dan J. Bernstein in 1996
 1667  *   http://cr.yp.to/syncookies.html    (overview)
 1668  *   http://cr.yp.to/syncookies/archive (details)
 1669  *
 1670  *
 1671  * Schematic construction of a syncookie enabled Initial Sequence Number:
 1672  *  0        1         2         3
 1673  *  12345678901234567890123456789012
 1674  * |xxxxxxxxxxxxxxxxxxxxxxxxWWWMMMSP|
 1675  *
 1676  *  x 24 MAC (truncated)
 1677  *  W  3 Send Window Scale index
 1678  *  M  3 MSS index
 1679  *  S  1 SACK permitted
 1680  *  P  1 Odd/even secret
 1681  */
 1682 
 1683 /*
 1684  * Distribution and probability of certain MSS values.  Those in between are
 1685  * rounded down to the next lower one.
 1686  * [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011]
 1687  *                            .2%  .3%   5%    7%    7%    20%   15%   45%
 1688  */
 1689 static int tcp_sc_msstab[] = { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };
 1690 
 1691 /*
 1692  * Distribution and probability of certain WSCALE values.  We have to map the
 1693  * (send) window scale (shift) option with a range of 0-14 from 4 bits into 3
 1694  * bits based on prevalence of certain values.  Where we don't have an exact
 1695  * match for are rounded down to the next lower one letting us under-estimate
 1696  * the true available window.  At the moment this would happen only for the
 1697  * very uncommon values 3, 5 and those above 8 (more than 16MB socket buffer
 1698  * and window size).  The absence of the WSCALE option (no scaling in either
 1699  * direction) is encoded with index zero.
 1700  * [WSCALE values histograms, Allman, 2012]
 1701  *                            X 10 10 35  5  6 14 10%   by host
 1702  *                            X 11  4  5  5 18 49  3%   by connections
 1703  */
 1704 static int tcp_sc_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };
 1705 
 1706 /*
 1707  * Compute the MAC for the SYN cookie.  SIPHASH-2-4 is chosen for its speed
 1708  * and good cryptographic properties.
 1709  */
 1710 static uint32_t
 1711 syncookie_mac(struct in_conninfo *inc, tcp_seq irs, uint8_t flags,
 1712     uint8_t *secbits, uintptr_t secmod)
 1713 {
 1714         SIPHASH_CTX ctx;
 1715         uint32_t siphash[2];
 1716 
 1717         SipHash24_Init(&ctx);
 1718         SipHash_SetKey(&ctx, secbits);
 1719         switch (inc->inc_flags & INC_ISIPV6) {
 1720 #ifdef INET
 1721         case 0:
 1722                 SipHash_Update(&ctx, &inc->inc_faddr, sizeof(inc->inc_faddr));
 1723                 SipHash_Update(&ctx, &inc->inc_laddr, sizeof(inc->inc_laddr));
 1724                 break;
 1725 #endif
 1726 #ifdef INET6
 1727         case INC_ISIPV6:
 1728                 SipHash_Update(&ctx, &inc->inc6_faddr, sizeof(inc->inc6_faddr));
 1729                 SipHash_Update(&ctx, &inc->inc6_laddr, sizeof(inc->inc6_laddr));
 1730                 break;
 1731 #endif
 1732         }
 1733         SipHash_Update(&ctx, &inc->inc_fport, sizeof(inc->inc_fport));
 1734         SipHash_Update(&ctx, &inc->inc_lport, sizeof(inc->inc_lport));
 1735         SipHash_Update(&ctx, &flags, sizeof(flags));
 1736         SipHash_Update(&ctx, &secmod, sizeof(secmod));
 1737         SipHash_Final((u_int8_t *)&siphash, &ctx);
 1738 
 1739         return (siphash[0] ^ siphash[1]);
 1740 }
 1741 
 1742 static tcp_seq
 1743 syncookie_generate(struct syncache_head *sch, struct syncache *sc)
 1744 {
 1745         u_int i, mss, secbit, wscale;
 1746         uint32_t iss, hash;
 1747         uint8_t *secbits;
 1748         union syncookie cookie;
 1749 
 1750         SCH_LOCK_ASSERT(sch);
 1751 
 1752         cookie.cookie = 0;
 1753 
 1754         /* Map our computed MSS into the 3-bit index. */
 1755         mss = min(tcp_mssopt(&sc->sc_inc), max(sc->sc_peer_mss, V_tcp_minmss));
 1756         for (i = sizeof(tcp_sc_msstab) / sizeof(*tcp_sc_msstab) - 1;
 1757              tcp_sc_msstab[i] > mss && i > 0;
 1758              i--)
 1759                 ;
 1760         cookie.flags.mss_idx = i;
 1761 
 1762         /*
 1763          * Map the send window scale into the 3-bit index but only if
 1764          * the wscale option was received.
 1765          */
 1766         if (sc->sc_flags & SCF_WINSCALE) {
 1767                 wscale = sc->sc_requested_s_scale;
 1768                 for (i = sizeof(tcp_sc_wstab) / sizeof(*tcp_sc_wstab) - 1;
 1769                      tcp_sc_wstab[i] > wscale && i > 0;
 1770                      i--)
 1771                         ;
 1772                 cookie.flags.wscale_idx = i;
 1773         }
 1774 
 1775         /* Can we do SACK? */
 1776         if (sc->sc_flags & SCF_SACK)
 1777                 cookie.flags.sack_ok = 1;
 1778 
 1779         /* Which of the two secrets to use. */
 1780         secbit = sch->sch_sc->secret.oddeven & 0x1;
 1781         cookie.flags.odd_even = secbit;
 1782 
 1783         secbits = sch->sch_sc->secret.key[secbit];
 1784         hash = syncookie_mac(&sc->sc_inc, sc->sc_irs, cookie.cookie, secbits,
 1785             (uintptr_t)sch);
 1786 
 1787         /*
 1788          * Put the flags into the hash and XOR them to get better ISS number
 1789          * variance.  This doesn't enhance the cryptographic strength and is
 1790          * done to prevent the 8 cookie bits from showing up directly on the
 1791          * wire.
 1792          */
 1793         iss = hash & ~0xff;
 1794         iss |= cookie.cookie ^ (hash >> 24);
 1795 
 1796         /* Randomize the timestamp. */
 1797         if (sc->sc_flags & SCF_TIMESTAMP) {
 1798                 sc->sc_ts = arc4random();
 1799                 sc->sc_tsoff = sc->sc_ts - tcp_ts_getticks();
 1800         }
 1801 
 1802         TCPSTAT_INC(tcps_sc_sendcookie);
 1803         return (iss);
 1804 }
 1805 
 1806 static struct syncache *
 1807 syncookie_lookup(struct in_conninfo *inc, struct syncache_head *sch, 
 1808     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
 1809     struct socket *lso)
 1810 {
 1811         uint32_t hash;
 1812         uint8_t *secbits;
 1813         tcp_seq ack, seq;
 1814         int wnd, wscale = 0;
 1815         union syncookie cookie;
 1816 
 1817         SCH_LOCK_ASSERT(sch);
 1818 
 1819         /*
 1820          * Pull information out of SYN-ACK/ACK and revert sequence number
 1821          * advances.
 1822          */
 1823         ack = th->th_ack - 1;
 1824         seq = th->th_seq - 1;
 1825 
 1826         /*
 1827          * Unpack the flags containing enough information to restore the
 1828          * connection.
 1829          */
 1830         cookie.cookie = (ack & 0xff) ^ (ack >> 24);
 1831 
 1832         /* Which of the two secrets to use. */
 1833         secbits = sch->sch_sc->secret.key[cookie.flags.odd_even];
 1834 
 1835         hash = syncookie_mac(inc, seq, cookie.cookie, secbits, (uintptr_t)sch);
 1836 
 1837         /* The recomputed hash matches the ACK if this was a genuine cookie. */
 1838         if ((ack & ~0xff) != (hash & ~0xff))
 1839                 return (NULL);
 1840 
 1841         /* Fill in the syncache values. */
 1842         sc->sc_flags = 0;
 1843         bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
 1844         sc->sc_ipopts = NULL;
 1845         
 1846         sc->sc_irs = seq;
 1847         sc->sc_iss = ack;
 1848 
 1849         switch (inc->inc_flags & INC_ISIPV6) {
 1850 #ifdef INET
 1851         case 0:
 1852                 sc->sc_ip_ttl = sotoinpcb(lso)->inp_ip_ttl;
 1853                 sc->sc_ip_tos = sotoinpcb(lso)->inp_ip_tos;
 1854                 break;
 1855 #endif
 1856 #ifdef INET6
 1857         case INC_ISIPV6:
 1858                 if (sotoinpcb(lso)->inp_flags & IN6P_AUTOFLOWLABEL)
 1859                         sc->sc_flowlabel = sc->sc_iss & IPV6_FLOWLABEL_MASK;
 1860                 break;
 1861 #endif
 1862         }
 1863 
 1864         sc->sc_peer_mss = tcp_sc_msstab[cookie.flags.mss_idx];
 1865 
 1866         /* We can simply recompute receive window scale we sent earlier. */
 1867         while (wscale < TCP_MAX_WINSHIFT && (TCP_MAXWIN << wscale) < sb_max)
 1868                 wscale++;
 1869 
 1870         /* Only use wscale if it was enabled in the orignal SYN. */
 1871         if (cookie.flags.wscale_idx > 0) {
 1872                 sc->sc_requested_r_scale = wscale;
 1873                 sc->sc_requested_s_scale = tcp_sc_wstab[cookie.flags.wscale_idx];
 1874                 sc->sc_flags |= SCF_WINSCALE;
 1875         }
 1876 
 1877         wnd = sbspace(&lso->so_rcv);
 1878         wnd = imax(wnd, 0);
 1879         wnd = imin(wnd, TCP_MAXWIN);
 1880         sc->sc_wnd = wnd;
 1881 
 1882         if (cookie.flags.sack_ok)
 1883                 sc->sc_flags |= SCF_SACK;
 1884 
 1885         if (to->to_flags & TOF_TS) {
 1886                 sc->sc_flags |= SCF_TIMESTAMP;
 1887                 sc->sc_tsreflect = to->to_tsval;
 1888                 sc->sc_ts = to->to_tsecr;
 1889                 sc->sc_tsoff = to->to_tsecr - tcp_ts_getticks();
 1890         }
 1891 
 1892         if (to->to_flags & TOF_SIGNATURE)
 1893                 sc->sc_flags |= SCF_SIGNATURE;
 1894 
 1895         sc->sc_rxmits = 0;
 1896 
 1897         TCPSTAT_INC(tcps_sc_recvcookie);
 1898         return (sc);
 1899 }
 1900 
 1901 #ifdef INVARIANTS
 1902 static int
 1903 syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
 1904     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
 1905     struct socket *lso)
 1906 {
 1907         struct syncache scs, *scx;
 1908         char *s;
 1909 
 1910         bzero(&scs, sizeof(scs));
 1911         scx = syncookie_lookup(inc, sch, &scs, th, to, lso);
 1912 
 1913         if ((s = tcp_log_addrs(inc, th, NULL, NULL)) == NULL)
 1914                 return (0);
 1915 
 1916         if (scx != NULL) {
 1917                 if (sc->sc_peer_mss != scx->sc_peer_mss)
 1918                         log(LOG_DEBUG, "%s; %s: mss different %i vs %i\n",
 1919                             s, __func__, sc->sc_peer_mss, scx->sc_peer_mss);
 1920 
 1921                 if (sc->sc_requested_r_scale != scx->sc_requested_r_scale)
 1922                         log(LOG_DEBUG, "%s; %s: rwscale different %i vs %i\n",
 1923                             s, __func__, sc->sc_requested_r_scale,
 1924                             scx->sc_requested_r_scale);
 1925 
 1926                 if (sc->sc_requested_s_scale != scx->sc_requested_s_scale)
 1927                         log(LOG_DEBUG, "%s; %s: swscale different %i vs %i\n",
 1928                             s, __func__, sc->sc_requested_s_scale,
 1929                             scx->sc_requested_s_scale);
 1930 
 1931                 if ((sc->sc_flags & SCF_SACK) != (scx->sc_flags & SCF_SACK))
 1932                         log(LOG_DEBUG, "%s; %s: SACK different\n", s, __func__);
 1933         }
 1934 
 1935         if (s != NULL)
 1936                 free(s, M_TCPLOG);
 1937         return (0);
 1938 }
 1939 #endif /* INVARIANTS */
 1940 
 1941 static void
 1942 syncookie_reseed(void *arg)
 1943 {
 1944         struct tcp_syncache *sc = arg;
 1945         uint8_t *secbits;
 1946         int secbit;
 1947 
 1948         /*
 1949          * Reseeding the secret doesn't have to be protected by a lock.
 1950          * It only must be ensured that the new random values are visible
 1951          * to all CPUs in a SMP environment.  The atomic with release
 1952          * semantics ensures that.
 1953          */
 1954         secbit = (sc->secret.oddeven & 0x1) ? 0 : 1;
 1955         secbits = sc->secret.key[secbit];
 1956         arc4rand(secbits, SYNCOOKIE_SECRET_SIZE, 0);
 1957         atomic_add_rel_int(&sc->secret.oddeven, 1);
 1958 
 1959         /* Reschedule ourself. */
 1960         callout_schedule(&sc->secret.reseed, SYNCOOKIE_LIFETIME * hz);
 1961 }
 1962 
 1963 /*
 1964  * Returns the current number of syncache entries.  This number
 1965  * will probably change before you get around to calling 
 1966  * syncache_pcblist.
 1967  */
 1968 int
 1969 syncache_pcbcount(void)
 1970 {
 1971         struct syncache_head *sch;
 1972         int count, i;
 1973 
 1974         for (count = 0, i = 0; i < V_tcp_syncache.hashsize; i++) {
 1975                 /* No need to lock for a read. */
 1976                 sch = &V_tcp_syncache.hashbase[i];
 1977                 count += sch->sch_length;
 1978         }
 1979         return count;
 1980 }
 1981 
 1982 /*
 1983  * Exports the syncache entries to userland so that netstat can display
 1984  * them alongside the other sockets.  This function is intended to be
 1985  * called only from tcp_pcblist.
 1986  *
 1987  * Due to concurrency on an active system, the number of pcbs exported
 1988  * may have no relation to max_pcbs.  max_pcbs merely indicates the
 1989  * amount of space the caller allocated for this function to use.
 1990  */
 1991 int
 1992 syncache_pcblist(struct sysctl_req *req, int max_pcbs, int *pcbs_exported)
 1993 {
 1994         struct xtcpcb xt;
 1995         struct syncache *sc;
 1996         struct syncache_head *sch;
 1997         int count, error, i;
 1998 
 1999         for (count = 0, error = 0, i = 0; i < V_tcp_syncache.hashsize; i++) {
 2000                 sch = &V_tcp_syncache.hashbase[i];
 2001                 SCH_LOCK(sch);
 2002                 TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
 2003                         if (count >= max_pcbs) {
 2004                                 SCH_UNLOCK(sch);
 2005                                 goto exit;
 2006                         }
 2007                         if (cr_cansee(req->td->td_ucred, sc->sc_cred) != 0)
 2008                                 continue;
 2009                         bzero(&xt, sizeof(xt));
 2010                         xt.xt_len = sizeof(xt);
 2011                         if (sc->sc_inc.inc_flags & INC_ISIPV6)
 2012                                 xt.xt_inp.inp_vflag = INP_IPV6;
 2013                         else
 2014                                 xt.xt_inp.inp_vflag = INP_IPV4;
 2015                         bcopy(&sc->sc_inc, &xt.xt_inp.inp_inc, sizeof (struct in_conninfo));
 2016                         xt.xt_tp.t_inpcb = &xt.xt_inp;
 2017                         xt.xt_tp.t_state = TCPS_SYN_RECEIVED;
 2018                         xt.xt_socket.xso_protocol = IPPROTO_TCP;
 2019                         xt.xt_socket.xso_len = sizeof (struct xsocket);
 2020                         xt.xt_socket.so_type = SOCK_STREAM;
 2021                         xt.xt_socket.so_state = SS_ISCONNECTING;
 2022                         error = SYSCTL_OUT(req, &xt, sizeof xt);
 2023                         if (error) {
 2024                                 SCH_UNLOCK(sch);
 2025                                 goto exit;
 2026                         }
 2027                         count++;
 2028                 }
 2029                 SCH_UNLOCK(sch);
 2030         }
 2031 exit:
 2032         *pcbs_exported = count;
 2033         return error;
 2034 }

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