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: stable/10/sys/netinet/tcp_syncache.c 324520 2017-10-11 06:28:46Z sephe $");
   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 #ifdef TCP_RFC7413
   82 #include <netinet/tcp_fastopen.h>
   83 #endif
   84 #include <netinet/tcp_fsm.h>
   85 #include <netinet/tcp_seq.h>
   86 #include <netinet/tcp_timer.h>
   87 #include <netinet/tcp_var.h>
   88 #include <netinet/tcp_syncache.h>
   89 #ifdef INET6
   90 #include <netinet6/tcp6_var.h>
   91 #endif
   92 #ifdef TCP_OFFLOAD
   93 #include <netinet/toecore.h>
   94 #endif
   95 
   96 #ifdef IPSEC
   97 #include <netipsec/ipsec.h>
   98 #ifdef INET6
   99 #include <netipsec/ipsec6.h>
  100 #endif
  101 #include <netipsec/key.h>
  102 #endif /*IPSEC*/
  103 
  104 #include <machine/in_cksum.h>
  105 
  106 #include <security/mac/mac_framework.h>
  107 
  108 static VNET_DEFINE(int, tcp_syncookies) = 1;
  109 #define V_tcp_syncookies                VNET(tcp_syncookies)
  110 SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, syncookies, CTLFLAG_RW,
  111     &VNET_NAME(tcp_syncookies), 0,
  112     "Use TCP SYN cookies if the syncache overflows");
  113 
  114 static VNET_DEFINE(int, tcp_syncookiesonly) = 0;
  115 #define V_tcp_syncookiesonly            VNET(tcp_syncookiesonly)
  116 SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, syncookies_only, CTLFLAG_RW,
  117     &VNET_NAME(tcp_syncookiesonly), 0,
  118     "Use only TCP SYN cookies");
  119 
  120 #ifdef TCP_OFFLOAD
  121 #define ADDED_BY_TOE(sc) ((sc)->sc_tod != NULL)
  122 #endif
  123 
  124 static void      syncache_drop(struct syncache *, struct syncache_head *);
  125 static void      syncache_free(struct syncache *);
  126 static void      syncache_insert(struct syncache *, struct syncache_head *);
  127 static int       syncache_respond(struct syncache *, const struct mbuf *);
  128 static struct    socket *syncache_socket(struct syncache *, struct socket *,
  129                     struct mbuf *m);
  130 static int       syncache_sysctl_count(SYSCTL_HANDLER_ARGS);
  131 static void      syncache_timeout(struct syncache *sc, struct syncache_head *sch,
  132                     int docallout);
  133 static void      syncache_timer(void *);
  134 
  135 static uint32_t  syncookie_mac(struct in_conninfo *, tcp_seq, uint8_t,
  136                     uint8_t *, uintptr_t);
  137 static tcp_seq   syncookie_generate(struct syncache_head *, struct syncache *);
  138 static struct syncache
  139                 *syncookie_lookup(struct in_conninfo *, struct syncache_head *,
  140                     struct syncache *, struct tcphdr *, struct tcpopt *,
  141                     struct socket *);
  142 static void      syncookie_reseed(void *);
  143 #ifdef INVARIANTS
  144 static int       syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
  145                     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
  146                     struct socket *lso);
  147 #endif
  148 
  149 /*
  150  * Transmit the SYN,ACK fewer times than TCP_MAXRXTSHIFT specifies.
  151  * 3 retransmits corresponds to a timeout of 3 * (1 + 2 + 4 + 8) == 45 seconds,
  152  * the odds are that the user has given up attempting to connect by then.
  153  */
  154 #define SYNCACHE_MAXREXMTS              3
  155 
  156 /* Arbitrary values */
  157 #define TCP_SYNCACHE_HASHSIZE           512
  158 #define TCP_SYNCACHE_BUCKETLIMIT        30
  159 
  160 static VNET_DEFINE(struct tcp_syncache, tcp_syncache);
  161 #define V_tcp_syncache                  VNET(tcp_syncache)
  162 
  163 static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, syncache, CTLFLAG_RW, 0,
  164     "TCP SYN cache");
  165 
  166 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, bucketlimit, CTLFLAG_RDTUN,
  167     &VNET_NAME(tcp_syncache.bucket_limit), 0,
  168     "Per-bucket hash limit for syncache");
  169 
  170 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, cachelimit, CTLFLAG_RDTUN,
  171     &VNET_NAME(tcp_syncache.cache_limit), 0,
  172     "Overall entry limit for syncache");
  173 
  174 SYSCTL_VNET_PROC(_net_inet_tcp_syncache, OID_AUTO, count, (CTLTYPE_UINT|CTLFLAG_RD),
  175     NULL, 0, &syncache_sysctl_count, "IU",
  176     "Current number of entries in syncache");
  177 
  178 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, hashsize, CTLFLAG_RDTUN,
  179     &VNET_NAME(tcp_syncache.hashsize), 0,
  180     "Size of TCP syncache hashtable");
  181 
  182 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, rexmtlimit, CTLFLAG_RW,
  183     &VNET_NAME(tcp_syncache.rexmt_limit), 0,
  184     "Limit on SYN/ACK retransmissions");
  185 
  186 VNET_DEFINE(int, tcp_sc_rst_sock_fail) = 1;
  187 SYSCTL_VNET_INT(_net_inet_tcp_syncache, OID_AUTO, rst_on_sock_fail,
  188     CTLFLAG_RW, &VNET_NAME(tcp_sc_rst_sock_fail), 0,
  189     "Send reset on socket allocation failure");
  190 
  191 static MALLOC_DEFINE(M_SYNCACHE, "syncache", "TCP syncache");
  192 
  193 #define SYNCACHE_HASH(inc, mask)                                        \
  194         ((V_tcp_syncache.hash_secret ^                                  \
  195           (inc)->inc_faddr.s_addr ^                                     \
  196           ((inc)->inc_faddr.s_addr >> 16) ^                             \
  197           (inc)->inc_fport ^ (inc)->inc_lport) & mask)
  198 
  199 #define SYNCACHE_HASH6(inc, mask)                                       \
  200         ((V_tcp_syncache.hash_secret ^                                  \
  201           (inc)->inc6_faddr.s6_addr32[0] ^                              \
  202           (inc)->inc6_faddr.s6_addr32[3] ^                              \
  203           (inc)->inc_fport ^ (inc)->inc_lport) & mask)
  204 
  205 #define ENDPTS_EQ(a, b) (                                               \
  206         (a)->ie_fport == (b)->ie_fport &&                               \
  207         (a)->ie_lport == (b)->ie_lport &&                               \
  208         (a)->ie_faddr.s_addr == (b)->ie_faddr.s_addr &&                 \
  209         (a)->ie_laddr.s_addr == (b)->ie_laddr.s_addr                    \
  210 )
  211 
  212 #define ENDPTS6_EQ(a, b) (memcmp(a, b, sizeof(*a)) == 0)
  213 
  214 #define SCH_LOCK(sch)           mtx_lock(&(sch)->sch_mtx)
  215 #define SCH_UNLOCK(sch)         mtx_unlock(&(sch)->sch_mtx)
  216 #define SCH_LOCK_ASSERT(sch)    mtx_assert(&(sch)->sch_mtx, MA_OWNED)
  217 
  218 /*
  219  * Requires the syncache entry to be already removed from the bucket list.
  220  */
  221 static void
  222 syncache_free(struct syncache *sc)
  223 {
  224 
  225         if (sc->sc_ipopts)
  226                 (void) m_free(sc->sc_ipopts);
  227         if (sc->sc_cred)
  228                 crfree(sc->sc_cred);
  229 #ifdef MAC
  230         mac_syncache_destroy(&sc->sc_label);
  231 #endif
  232 
  233         uma_zfree(V_tcp_syncache.zone, sc);
  234 }
  235 
  236 void
  237 syncache_init(void)
  238 {
  239         int i;
  240 
  241         V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
  242         V_tcp_syncache.bucket_limit = TCP_SYNCACHE_BUCKETLIMIT;
  243         V_tcp_syncache.rexmt_limit = SYNCACHE_MAXREXMTS;
  244         V_tcp_syncache.hash_secret = arc4random();
  245 
  246         TUNABLE_INT_FETCH("net.inet.tcp.syncache.hashsize",
  247             &V_tcp_syncache.hashsize);
  248         TUNABLE_INT_FETCH("net.inet.tcp.syncache.bucketlimit",
  249             &V_tcp_syncache.bucket_limit);
  250         if (!powerof2(V_tcp_syncache.hashsize) ||
  251             V_tcp_syncache.hashsize == 0) {
  252                 printf("WARNING: syncache hash size is not a power of 2.\n");
  253                 V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
  254         }
  255         V_tcp_syncache.hashmask = V_tcp_syncache.hashsize - 1;
  256 
  257         /* Set limits. */
  258         V_tcp_syncache.cache_limit =
  259             V_tcp_syncache.hashsize * V_tcp_syncache.bucket_limit;
  260         TUNABLE_INT_FETCH("net.inet.tcp.syncache.cachelimit",
  261             &V_tcp_syncache.cache_limit);
  262 
  263         /* Allocate the hash table. */
  264         V_tcp_syncache.hashbase = malloc(V_tcp_syncache.hashsize *
  265             sizeof(struct syncache_head), M_SYNCACHE, M_WAITOK | M_ZERO);
  266 
  267 #ifdef VIMAGE
  268         V_tcp_syncache.vnet = curvnet;
  269 #endif
  270 
  271         /* Initialize the hash buckets. */
  272         for (i = 0; i < V_tcp_syncache.hashsize; i++) {
  273                 TAILQ_INIT(&V_tcp_syncache.hashbase[i].sch_bucket);
  274                 mtx_init(&V_tcp_syncache.hashbase[i].sch_mtx, "tcp_sc_head",
  275                          NULL, MTX_DEF);
  276                 callout_init_mtx(&V_tcp_syncache.hashbase[i].sch_timer,
  277                          &V_tcp_syncache.hashbase[i].sch_mtx, 0);
  278                 V_tcp_syncache.hashbase[i].sch_length = 0;
  279                 V_tcp_syncache.hashbase[i].sch_sc = &V_tcp_syncache;
  280                 V_tcp_syncache.hashbase[i].sch_last_overflow =
  281                     -(SYNCOOKIE_LIFETIME + 1);
  282         }
  283 
  284         /* Create the syncache entry zone. */
  285         V_tcp_syncache.zone = uma_zcreate("syncache", sizeof(struct syncache),
  286             NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
  287         V_tcp_syncache.cache_limit = uma_zone_set_max(V_tcp_syncache.zone,
  288             V_tcp_syncache.cache_limit);
  289 
  290         /* Start the SYN cookie reseeder callout. */
  291         callout_init(&V_tcp_syncache.secret.reseed, 1);
  292         arc4rand(V_tcp_syncache.secret.key[0], SYNCOOKIE_SECRET_SIZE, 0);
  293         arc4rand(V_tcp_syncache.secret.key[1], SYNCOOKIE_SECRET_SIZE, 0);
  294         callout_reset(&V_tcp_syncache.secret.reseed, SYNCOOKIE_LIFETIME * hz,
  295             syncookie_reseed, &V_tcp_syncache);
  296 }
  297 
  298 #ifdef VIMAGE
  299 void
  300 syncache_destroy(void)
  301 {
  302         struct syncache_head *sch;
  303         struct syncache *sc, *nsc;
  304         int i;
  305 
  306         /* Cleanup hash buckets: stop timers, free entries, destroy locks. */
  307         for (i = 0; i < V_tcp_syncache.hashsize; i++) {
  308 
  309                 sch = &V_tcp_syncache.hashbase[i];
  310                 callout_drain(&sch->sch_timer);
  311 
  312                 SCH_LOCK(sch);
  313                 TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc)
  314                         syncache_drop(sc, sch);
  315                 SCH_UNLOCK(sch);
  316                 KASSERT(TAILQ_EMPTY(&sch->sch_bucket),
  317                     ("%s: sch->sch_bucket not empty", __func__));
  318                 KASSERT(sch->sch_length == 0, ("%s: sch->sch_length %d not 0",
  319                     __func__, sch->sch_length));
  320                 mtx_destroy(&sch->sch_mtx);
  321         }
  322 
  323         KASSERT(uma_zone_get_cur(V_tcp_syncache.zone) == 0,
  324             ("%s: cache_count not 0", __func__));
  325 
  326         /* Free the allocated global resources. */
  327         uma_zdestroy(V_tcp_syncache.zone);
  328         free(V_tcp_syncache.hashbase, M_SYNCACHE);
  329 
  330         callout_drain(&V_tcp_syncache.secret.reseed);
  331 }
  332 #endif
  333 
  334 static int
  335 syncache_sysctl_count(SYSCTL_HANDLER_ARGS)
  336 {
  337         int count;
  338 
  339         count = uma_zone_get_cur(V_tcp_syncache.zone);
  340         return (sysctl_handle_int(oidp, &count, 0, req));
  341 }
  342 
  343 /*
  344  * Inserts a syncache entry into the specified bucket row.
  345  * Locks and unlocks the syncache_head autonomously.
  346  */
  347 static void
  348 syncache_insert(struct syncache *sc, struct syncache_head *sch)
  349 {
  350         struct syncache *sc2;
  351 
  352         SCH_LOCK(sch);
  353 
  354         /*
  355          * Make sure that we don't overflow the per-bucket limit.
  356          * If the bucket is full, toss the oldest element.
  357          */
  358         if (sch->sch_length >= V_tcp_syncache.bucket_limit) {
  359                 KASSERT(!TAILQ_EMPTY(&sch->sch_bucket),
  360                         ("sch->sch_length incorrect"));
  361                 sc2 = TAILQ_LAST(&sch->sch_bucket, sch_head);
  362                 sch->sch_last_overflow = time_uptime;
  363                 syncache_drop(sc2, sch);
  364                 TCPSTAT_INC(tcps_sc_bucketoverflow);
  365         }
  366 
  367         /* Put it into the bucket. */
  368         TAILQ_INSERT_HEAD(&sch->sch_bucket, sc, sc_hash);
  369         sch->sch_length++;
  370 
  371 #ifdef TCP_OFFLOAD
  372         if (ADDED_BY_TOE(sc)) {
  373                 struct toedev *tod = sc->sc_tod;
  374 
  375                 tod->tod_syncache_added(tod, sc->sc_todctx);
  376         }
  377 #endif
  378 
  379         /* Reinitialize the bucket row's timer. */
  380         if (sch->sch_length == 1)
  381                 sch->sch_nextc = ticks + INT_MAX;
  382         syncache_timeout(sc, sch, 1);
  383 
  384         SCH_UNLOCK(sch);
  385 
  386         TCPSTAT_INC(tcps_sc_added);
  387 }
  388 
  389 /*
  390  * Remove and free entry from syncache bucket row.
  391  * Expects locked syncache head.
  392  */
  393 static void
  394 syncache_drop(struct syncache *sc, struct syncache_head *sch)
  395 {
  396 
  397         SCH_LOCK_ASSERT(sch);
  398 
  399         TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
  400         sch->sch_length--;
  401 
  402 #ifdef TCP_OFFLOAD
  403         if (ADDED_BY_TOE(sc)) {
  404                 struct toedev *tod = sc->sc_tod;
  405 
  406                 tod->tod_syncache_removed(tod, sc->sc_todctx);
  407         }
  408 #endif
  409 
  410         syncache_free(sc);
  411 }
  412 
  413 /*
  414  * Engage/reengage time on bucket row.
  415  */
  416 static void
  417 syncache_timeout(struct syncache *sc, struct syncache_head *sch, int docallout)
  418 {
  419         sc->sc_rxttime = ticks +
  420                 TCPTV_RTOBASE * (tcp_syn_backoff[sc->sc_rxmits]);
  421         sc->sc_rxmits++;
  422         if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc)) {
  423                 sch->sch_nextc = sc->sc_rxttime;
  424                 if (docallout)
  425                         callout_reset(&sch->sch_timer, sch->sch_nextc - ticks,
  426                             syncache_timer, (void *)sch);
  427         }
  428 }
  429 
  430 /*
  431  * Walk the timer queues, looking for SYN,ACKs that need to be retransmitted.
  432  * If we have retransmitted an entry the maximum number of times, expire it.
  433  * One separate timer for each bucket row.
  434  */
  435 static void
  436 syncache_timer(void *xsch)
  437 {
  438         struct syncache_head *sch = (struct syncache_head *)xsch;
  439         struct syncache *sc, *nsc;
  440         int tick = ticks;
  441         char *s;
  442 
  443         CURVNET_SET(sch->sch_sc->vnet);
  444 
  445         /* NB: syncache_head has already been locked by the callout. */
  446         SCH_LOCK_ASSERT(sch);
  447 
  448         /*
  449          * In the following cycle we may remove some entries and/or
  450          * advance some timeouts, so re-initialize the bucket timer.
  451          */
  452         sch->sch_nextc = tick + INT_MAX;
  453 
  454         TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc) {
  455                 /*
  456                  * We do not check if the listen socket still exists
  457                  * and accept the case where the listen socket may be
  458                  * gone by the time we resend the SYN/ACK.  We do
  459                  * not expect this to happens often. If it does,
  460                  * then the RST will be sent by the time the remote
  461                  * host does the SYN/ACK->ACK.
  462                  */
  463                 if (TSTMP_GT(sc->sc_rxttime, tick)) {
  464                         if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc))
  465                                 sch->sch_nextc = sc->sc_rxttime;
  466                         continue;
  467                 }
  468                 if (sc->sc_rxmits > V_tcp_syncache.rexmt_limit) {
  469                         if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  470                                 log(LOG_DEBUG, "%s; %s: Retransmits exhausted, "
  471                                     "giving up and removing syncache entry\n",
  472                                     s, __func__);
  473                                 free(s, M_TCPLOG);
  474                         }
  475                         syncache_drop(sc, sch);
  476                         TCPSTAT_INC(tcps_sc_stale);
  477                         continue;
  478                 }
  479                 if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  480                         log(LOG_DEBUG, "%s; %s: Response timeout, "
  481                             "retransmitting (%u) SYN|ACK\n",
  482                             s, __func__, sc->sc_rxmits);
  483                         free(s, M_TCPLOG);
  484                 }
  485 
  486                 (void) syncache_respond(sc, NULL);
  487                 TCPSTAT_INC(tcps_sc_retransmitted);
  488                 syncache_timeout(sc, sch, 0);
  489         }
  490         if (!TAILQ_EMPTY(&(sch)->sch_bucket))
  491                 callout_reset(&(sch)->sch_timer, (sch)->sch_nextc - tick,
  492                         syncache_timer, (void *)(sch));
  493         CURVNET_RESTORE();
  494 }
  495 
  496 /*
  497  * Find an entry in the syncache.
  498  * Returns always with locked syncache_head plus a matching entry or NULL.
  499  */
  500 static struct syncache *
  501 syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp)
  502 {
  503         struct syncache *sc;
  504         struct syncache_head *sch;
  505 
  506 #ifdef INET6
  507         if (inc->inc_flags & INC_ISIPV6) {
  508                 sch = &V_tcp_syncache.hashbase[
  509                     SYNCACHE_HASH6(inc, V_tcp_syncache.hashmask)];
  510                 *schp = sch;
  511 
  512                 SCH_LOCK(sch);
  513 
  514                 /* Circle through bucket row to find matching entry. */
  515                 TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
  516                         if (ENDPTS6_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie))
  517                                 return (sc);
  518                 }
  519         } else
  520 #endif
  521         {
  522                 sch = &V_tcp_syncache.hashbase[
  523                     SYNCACHE_HASH(inc, V_tcp_syncache.hashmask)];
  524                 *schp = sch;
  525 
  526                 SCH_LOCK(sch);
  527 
  528                 /* Circle through bucket row to find matching entry. */
  529                 TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
  530 #ifdef INET6
  531                         if (sc->sc_inc.inc_flags & INC_ISIPV6)
  532                                 continue;
  533 #endif
  534                         if (ENDPTS_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie))
  535                                 return (sc);
  536                 }
  537         }
  538         SCH_LOCK_ASSERT(*schp);
  539         return (NULL);                  /* always returns with locked sch */
  540 }
  541 
  542 /*
  543  * This function is called when we get a RST for a
  544  * non-existent connection, so that we can see if the
  545  * connection is in the syn cache.  If it is, zap it.
  546  */
  547 void
  548 syncache_chkrst(struct in_conninfo *inc, struct tcphdr *th)
  549 {
  550         struct syncache *sc;
  551         struct syncache_head *sch;
  552         char *s = NULL;
  553 
  554         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  555         SCH_LOCK_ASSERT(sch);
  556 
  557         /*
  558          * Any RST to our SYN|ACK must not carry ACK, SYN or FIN flags.
  559          * See RFC 793 page 65, section SEGMENT ARRIVES.
  560          */
  561         if (th->th_flags & (TH_ACK|TH_SYN|TH_FIN)) {
  562                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  563                         log(LOG_DEBUG, "%s; %s: Spurious RST with ACK, SYN or "
  564                             "FIN flag set, segment ignored\n", s, __func__);
  565                 TCPSTAT_INC(tcps_badrst);
  566                 goto done;
  567         }
  568 
  569         /*
  570          * No corresponding connection was found in syncache.
  571          * If syncookies are enabled and possibly exclusively
  572          * used, or we are under memory pressure, a valid RST
  573          * may not find a syncache entry.  In that case we're
  574          * done and no SYN|ACK retransmissions will happen.
  575          * Otherwise the RST was misdirected or spoofed.
  576          */
  577         if (sc == NULL) {
  578                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  579                         log(LOG_DEBUG, "%s; %s: Spurious RST without matching "
  580                             "syncache entry (possibly syncookie only), "
  581                             "segment ignored\n", s, __func__);
  582                 TCPSTAT_INC(tcps_badrst);
  583                 goto done;
  584         }
  585 
  586         /*
  587          * If the RST bit is set, check the sequence number to see
  588          * if this is a valid reset segment.
  589          * RFC 793 page 37:
  590          *   In all states except SYN-SENT, all reset (RST) segments
  591          *   are validated by checking their SEQ-fields.  A reset is
  592          *   valid if its sequence number is in the window.
  593          *
  594          *   The sequence number in the reset segment is normally an
  595          *   echo of our outgoing acknowlegement numbers, but some hosts
  596          *   send a reset with the sequence number at the rightmost edge
  597          *   of our receive window, and we have to handle this case.
  598          */
  599         if (SEQ_GEQ(th->th_seq, sc->sc_irs) &&
  600             SEQ_LEQ(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
  601                 syncache_drop(sc, sch);
  602                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  603                         log(LOG_DEBUG, "%s; %s: Our SYN|ACK was rejected, "
  604                             "connection attempt aborted by remote endpoint\n",
  605                             s, __func__);
  606                 TCPSTAT_INC(tcps_sc_reset);
  607         } else {
  608                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  609                         log(LOG_DEBUG, "%s; %s: RST with invalid SEQ %u != "
  610                             "IRS %u (+WND %u), segment ignored\n",
  611                             s, __func__, th->th_seq, sc->sc_irs, sc->sc_wnd);
  612                 TCPSTAT_INC(tcps_badrst);
  613         }
  614 
  615 done:
  616         if (s != NULL)
  617                 free(s, M_TCPLOG);
  618         SCH_UNLOCK(sch);
  619 }
  620 
  621 void
  622 syncache_badack(struct in_conninfo *inc)
  623 {
  624         struct syncache *sc;
  625         struct syncache_head *sch;
  626 
  627         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  628         SCH_LOCK_ASSERT(sch);
  629         if (sc != NULL) {
  630                 syncache_drop(sc, sch);
  631                 TCPSTAT_INC(tcps_sc_badack);
  632         }
  633         SCH_UNLOCK(sch);
  634 }
  635 
  636 void
  637 syncache_unreach(struct in_conninfo *inc, struct tcphdr *th)
  638 {
  639         struct syncache *sc;
  640         struct syncache_head *sch;
  641 
  642         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  643         SCH_LOCK_ASSERT(sch);
  644         if (sc == NULL)
  645                 goto done;
  646 
  647         /* If the sequence number != sc_iss, then it's a bogus ICMP msg */
  648         if (ntohl(th->th_seq) != sc->sc_iss)
  649                 goto done;
  650 
  651         /*
  652          * If we've rertransmitted 3 times and this is our second error,
  653          * we remove the entry.  Otherwise, we allow it to continue on.
  654          * This prevents us from incorrectly nuking an entry during a
  655          * spurious network outage.
  656          *
  657          * See tcp_notify().
  658          */
  659         if ((sc->sc_flags & SCF_UNREACH) == 0 || sc->sc_rxmits < 3 + 1) {
  660                 sc->sc_flags |= SCF_UNREACH;
  661                 goto done;
  662         }
  663         syncache_drop(sc, sch);
  664         TCPSTAT_INC(tcps_sc_unreach);
  665 done:
  666         SCH_UNLOCK(sch);
  667 }
  668 
  669 /*
  670  * Build a new TCP socket structure from a syncache entry.
  671  *
  672  * On success return the newly created socket with its underlying inp locked.
  673  */
  674 static struct socket *
  675 syncache_socket(struct syncache *sc, struct socket *lso, struct mbuf *m)
  676 {
  677         struct inpcb *inp = NULL;
  678         struct socket *so;
  679         struct tcpcb *tp;
  680         int error;
  681         char *s;
  682 
  683         INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
  684 
  685         /*
  686          * Ok, create the full blown connection, and set things up
  687          * as they would have been set up if we had created the
  688          * connection when the SYN arrived.  If we can't create
  689          * the connection, abort it.
  690          */
  691         so = sonewconn(lso, 0);
  692         if (so == NULL) {
  693                 /*
  694                  * Drop the connection; we will either send a RST or
  695                  * have the peer retransmit its SYN again after its
  696                  * RTO and try again.
  697                  */
  698                 TCPSTAT_INC(tcps_listendrop);
  699                 if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  700                         log(LOG_DEBUG, "%s; %s: Socket create failed "
  701                             "due to limits or memory shortage\n",
  702                             s, __func__);
  703                         free(s, M_TCPLOG);
  704                 }
  705                 goto abort2;
  706         }
  707 #ifdef MAC
  708         mac_socketpeer_set_from_mbuf(m, so);
  709 #endif
  710 
  711         inp = sotoinpcb(so);
  712         inp->inp_inc.inc_fibnum = so->so_fibnum;
  713         INP_WLOCK(inp);
  714         /*
  715          * Exclusive pcbinfo lock is not required in syncache socket case even
  716          * if two inpcb locks can be acquired simultaneously:
  717          *  - the inpcb in LISTEN state,
  718          *  - the newly created inp.
  719          *
  720          * In this case, an inp cannot be at same time in LISTEN state and
  721          * just created by an accept() call.
  722          */
  723         INP_HASH_WLOCK(&V_tcbinfo);
  724 
  725         /* Insert new socket into PCB hash list. */
  726         inp->inp_inc.inc_flags = sc->sc_inc.inc_flags;
  727 #ifdef INET6
  728         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
  729                 inp->in6p_laddr = sc->sc_inc.inc6_laddr;
  730         } else {
  731                 inp->inp_vflag &= ~INP_IPV6;
  732                 inp->inp_vflag |= INP_IPV4;
  733 #endif
  734                 inp->inp_laddr = sc->sc_inc.inc_laddr;
  735 #ifdef INET6
  736         }
  737 #endif
  738 
  739         /*
  740          * If there's an mbuf and it has a flowid, then let's initialise the
  741          * inp with that particular flowid.
  742          */
  743         if (m != NULL && M_HASHTYPE_GET(m) != M_HASHTYPE_NONE) {
  744                 inp->inp_flowid = m->m_pkthdr.flowid;
  745                 inp->inp_flowtype = M_HASHTYPE_GET(m);
  746         }
  747 
  748         /*
  749          * Install in the reservation hash table for now, but don't yet
  750          * install a connection group since the full 4-tuple isn't yet
  751          * configured.
  752          */
  753         inp->inp_lport = sc->sc_inc.inc_lport;
  754         if ((error = in_pcbinshash_nopcbgroup(inp)) != 0) {
  755                 /*
  756                  * Undo the assignments above if we failed to
  757                  * put the PCB on the hash lists.
  758                  */
  759 #ifdef INET6
  760                 if (sc->sc_inc.inc_flags & INC_ISIPV6)
  761                         inp->in6p_laddr = in6addr_any;
  762                 else
  763 #endif
  764                         inp->inp_laddr.s_addr = INADDR_ANY;
  765                 inp->inp_lport = 0;
  766                 if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  767                         log(LOG_DEBUG, "%s; %s: in_pcbinshash failed "
  768                             "with error %i\n",
  769                             s, __func__, error);
  770                         free(s, M_TCPLOG);
  771                 }
  772                 INP_HASH_WUNLOCK(&V_tcbinfo);
  773                 goto abort;
  774         }
  775 #ifdef IPSEC
  776         /* Copy old policy into new socket's. */
  777         if (ipsec_copy_policy(sotoinpcb(lso)->inp_sp, inp->inp_sp))
  778                 printf("syncache_socket: could not copy policy\n");
  779 #endif
  780 #ifdef INET6
  781         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
  782                 struct inpcb *oinp = sotoinpcb(lso);
  783                 struct in6_addr laddr6;
  784                 struct sockaddr_in6 sin6;
  785                 /*
  786                  * Inherit socket options from the listening socket.
  787                  * Note that in6p_inputopts are not (and should not be)
  788                  * copied, since it stores previously received options and is
  789                  * used to detect if each new option is different than the
  790                  * previous one and hence should be passed to a user.
  791                  * If we copied in6p_inputopts, a user would not be able to
  792                  * receive options just after calling the accept system call.
  793                  */
  794                 inp->inp_flags |= oinp->inp_flags & INP_CONTROLOPTS;
  795                 if (oinp->in6p_outputopts)
  796                         inp->in6p_outputopts =
  797                             ip6_copypktopts(oinp->in6p_outputopts, M_NOWAIT);
  798 
  799                 sin6.sin6_family = AF_INET6;
  800                 sin6.sin6_len = sizeof(sin6);
  801                 sin6.sin6_addr = sc->sc_inc.inc6_faddr;
  802                 sin6.sin6_port = sc->sc_inc.inc_fport;
  803                 sin6.sin6_flowinfo = sin6.sin6_scope_id = 0;
  804                 laddr6 = inp->in6p_laddr;
  805                 if (IN6_IS_ADDR_UNSPECIFIED(&inp->in6p_laddr))
  806                         inp->in6p_laddr = sc->sc_inc.inc6_laddr;
  807                 if ((error = in6_pcbconnect_mbuf(inp, (struct sockaddr *)&sin6,
  808                     thread0.td_ucred, m)) != 0) {
  809                         inp->in6p_laddr = laddr6;
  810                         if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  811                                 log(LOG_DEBUG, "%s; %s: in6_pcbconnect failed "
  812                                     "with error %i\n",
  813                                     s, __func__, error);
  814                                 free(s, M_TCPLOG);
  815                         }
  816                         INP_HASH_WUNLOCK(&V_tcbinfo);
  817                         goto abort;
  818                 }
  819                 /* Override flowlabel from in6_pcbconnect. */
  820                 inp->inp_flow &= ~IPV6_FLOWLABEL_MASK;
  821                 inp->inp_flow |= sc->sc_flowlabel;
  822         }
  823 #endif /* INET6 */
  824 #if defined(INET) && defined(INET6)
  825         else
  826 #endif
  827 #ifdef INET
  828         {
  829                 struct in_addr laddr;
  830                 struct sockaddr_in sin;
  831 
  832                 inp->inp_options = (m) ? ip_srcroute(m) : NULL;
  833                 
  834                 if (inp->inp_options == NULL) {
  835                         inp->inp_options = sc->sc_ipopts;
  836                         sc->sc_ipopts = NULL;
  837                 }
  838 
  839                 sin.sin_family = AF_INET;
  840                 sin.sin_len = sizeof(sin);
  841                 sin.sin_addr = sc->sc_inc.inc_faddr;
  842                 sin.sin_port = sc->sc_inc.inc_fport;
  843                 bzero((caddr_t)sin.sin_zero, sizeof(sin.sin_zero));
  844                 laddr = inp->inp_laddr;
  845                 if (inp->inp_laddr.s_addr == INADDR_ANY)
  846                         inp->inp_laddr = sc->sc_inc.inc_laddr;
  847                 if ((error = in_pcbconnect_mbuf(inp, (struct sockaddr *)&sin,
  848                     thread0.td_ucred, m)) != 0) {
  849                         inp->inp_laddr = laddr;
  850                         if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  851                                 log(LOG_DEBUG, "%s; %s: in_pcbconnect failed "
  852                                     "with error %i\n",
  853                                     s, __func__, error);
  854                                 free(s, M_TCPLOG);
  855                         }
  856                         INP_HASH_WUNLOCK(&V_tcbinfo);
  857                         goto abort;
  858                 }
  859         }
  860 #endif /* INET */
  861         INP_HASH_WUNLOCK(&V_tcbinfo);
  862         tp = intotcpcb(inp);
  863         tcp_state_change(tp, TCPS_SYN_RECEIVED);
  864         tp->iss = sc->sc_iss;
  865         tp->irs = sc->sc_irs;
  866         tcp_rcvseqinit(tp);
  867         tcp_sendseqinit(tp);
  868         tp->snd_wl1 = sc->sc_irs;
  869         tp->snd_max = tp->iss + 1;
  870         tp->snd_nxt = tp->iss + 1;
  871         tp->rcv_up = sc->sc_irs + 1;
  872         tp->rcv_wnd = sc->sc_wnd;
  873         tp->rcv_adv += tp->rcv_wnd;
  874         tp->last_ack_sent = tp->rcv_nxt;
  875 
  876         tp->t_flags = sototcpcb(lso)->t_flags & (TF_NOPUSH|TF_NODELAY);
  877         if (sc->sc_flags & SCF_NOOPT)
  878                 tp->t_flags |= TF_NOOPT;
  879         else {
  880                 if (sc->sc_flags & SCF_WINSCALE) {
  881                         tp->t_flags |= TF_REQ_SCALE|TF_RCVD_SCALE;
  882                         tp->snd_scale = sc->sc_requested_s_scale;
  883                         tp->request_r_scale = sc->sc_requested_r_scale;
  884                 }
  885                 if (sc->sc_flags & SCF_TIMESTAMP) {
  886                         tp->t_flags |= TF_REQ_TSTMP|TF_RCVD_TSTMP;
  887                         tp->ts_recent = sc->sc_tsreflect;
  888                         tp->ts_recent_age = tcp_ts_getticks();
  889                         tp->ts_offset = sc->sc_tsoff;
  890                 }
  891 #ifdef TCP_SIGNATURE
  892                 if (sc->sc_flags & SCF_SIGNATURE)
  893                         tp->t_flags |= TF_SIGNATURE;
  894 #endif
  895                 if (sc->sc_flags & SCF_SACK)
  896                         tp->t_flags |= TF_SACK_PERMIT;
  897         }
  898 
  899         if (sc->sc_flags & SCF_ECN)
  900                 tp->t_flags |= TF_ECN_PERMIT;
  901 
  902         /*
  903          * Set up MSS and get cached values from tcp_hostcache.
  904          * This might overwrite some of the defaults we just set.
  905          */
  906         tcp_mss(tp, sc->sc_peer_mss);
  907 
  908         /*
  909          * If the SYN,ACK was retransmitted, indicate that CWND to be
  910          * limited to one segment in cc_conn_init().
  911          * NB: sc_rxmits counts all SYN,ACK transmits, not just retransmits.
  912          */
  913         if (sc->sc_rxmits > 1)
  914                 tp->snd_cwnd = 1;
  915 
  916 #ifdef TCP_OFFLOAD
  917         /*
  918          * Allow a TOE driver to install its hooks.  Note that we hold the
  919          * pcbinfo lock too and that prevents tcp_usr_accept from accepting a
  920          * new connection before the TOE driver has done its thing.
  921          */
  922         if (ADDED_BY_TOE(sc)) {
  923                 struct toedev *tod = sc->sc_tod;
  924 
  925                 tod->tod_offload_socket(tod, sc->sc_todctx, so);
  926         }
  927 #endif
  928         /*
  929          * Copy and activate timers.
  930          */
  931         tp->t_keepinit = sototcpcb(lso)->t_keepinit;
  932         tp->t_keepidle = sototcpcb(lso)->t_keepidle;
  933         tp->t_keepintvl = sototcpcb(lso)->t_keepintvl;
  934         tp->t_keepcnt = sototcpcb(lso)->t_keepcnt;
  935         tcp_timer_activate(tp, TT_KEEP, TP_KEEPINIT(tp));
  936 
  937         TCPSTAT_INC(tcps_accepts);
  938         return (so);
  939 
  940 abort:
  941         INP_WUNLOCK(inp);
  942 abort2:
  943         if (so != NULL)
  944                 soabort(so);
  945         return (NULL);
  946 }
  947 
  948 /*
  949  * This function gets called when we receive an ACK for a
  950  * socket in the LISTEN state.  We look up the connection
  951  * in the syncache, and if its there, we pull it out of
  952  * the cache and turn it into a full-blown connection in
  953  * the SYN-RECEIVED state.
  954  *
  955  * On syncache_socket() success the newly created socket
  956  * has its underlying inp locked.
  957  */
  958 int
  959 syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
  960     struct socket **lsop, struct mbuf *m)
  961 {
  962         struct syncache *sc;
  963         struct syncache_head *sch;
  964         struct syncache scs;
  965         char *s;
  966 
  967         /*
  968          * Global TCP locks are held because we manipulate the PCB lists
  969          * and create a new socket.
  970          */
  971         INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
  972         KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK,
  973             ("%s: can handle only ACK", __func__));
  974 
  975         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  976         SCH_LOCK_ASSERT(sch);
  977 
  978 #ifdef INVARIANTS
  979         /*
  980          * Test code for syncookies comparing the syncache stored
  981          * values with the reconstructed values from the cookie.
  982          */
  983         if (sc != NULL)
  984                 syncookie_cmp(inc, sch, sc, th, to, *lsop);
  985 #endif
  986 
  987         if (sc == NULL) {
  988                 /*
  989                  * There is no syncache entry, so see if this ACK is
  990                  * a returning syncookie.  To do this, first:
  991                  *  A. Check if syncookies are used in case of syncache
  992                  *     overflows
  993                  *  B. See if this socket has had a syncache entry dropped in
  994                  *     the recent past. We don't want to accept a bogus
  995                  *     syncookie if we've never received a SYN or accept it
  996                  *     twice.
  997                  *  C. check that the syncookie is valid.  If it is, then
  998                  *     cobble up a fake syncache entry, and return.
  999                  */
 1000                 if (!V_tcp_syncookies) {
 1001                         SCH_UNLOCK(sch);
 1002                         if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1003                                 log(LOG_DEBUG, "%s; %s: Spurious ACK, "
 1004                                     "segment rejected (syncookies disabled)\n",
 1005                                     s, __func__);
 1006                         goto failed;
 1007                 }
 1008                 if (!V_tcp_syncookiesonly &&
 1009                     sch->sch_last_overflow < time_uptime - SYNCOOKIE_LIFETIME) {
 1010                         SCH_UNLOCK(sch);
 1011                         if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1012                                 log(LOG_DEBUG, "%s; %s: Spurious ACK, "
 1013                                     "segment rejected (no syncache entry)\n",
 1014                                     s, __func__);
 1015                         goto failed;
 1016                 }
 1017                 bzero(&scs, sizeof(scs));
 1018                 sc = syncookie_lookup(inc, sch, &scs, th, to, *lsop);
 1019                 SCH_UNLOCK(sch);
 1020                 if (sc == NULL) {
 1021                         if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1022                                 log(LOG_DEBUG, "%s; %s: Segment failed "
 1023                                     "SYNCOOKIE authentication, segment rejected "
 1024                                     "(probably spoofed)\n", s, __func__);
 1025                         goto failed;
 1026                 }
 1027         } else {
 1028                 /* Pull out the entry to unlock the bucket row. */
 1029                 TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
 1030                 sch->sch_length--;
 1031 #ifdef TCP_OFFLOAD
 1032                 if (ADDED_BY_TOE(sc)) {
 1033                         struct toedev *tod = sc->sc_tod;
 1034 
 1035                         tod->tod_syncache_removed(tod, sc->sc_todctx);
 1036                 }
 1037 #endif
 1038                 SCH_UNLOCK(sch);
 1039         }
 1040 
 1041         /*
 1042          * Segment validation:
 1043          * ACK must match our initial sequence number + 1 (the SYN|ACK).
 1044          */
 1045         if (th->th_ack != sc->sc_iss + 1) {
 1046                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1047                         log(LOG_DEBUG, "%s; %s: ACK %u != ISS+1 %u, segment "
 1048                             "rejected\n", s, __func__, th->th_ack, sc->sc_iss);
 1049                 goto failed;
 1050         }
 1051 
 1052         /*
 1053          * The SEQ must fall in the window starting at the received
 1054          * initial receive sequence number + 1 (the SYN).
 1055          */
 1056         if (SEQ_LEQ(th->th_seq, sc->sc_irs) ||
 1057             SEQ_GT(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
 1058                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1059                         log(LOG_DEBUG, "%s; %s: SEQ %u != IRS+1 %u, segment "
 1060                             "rejected\n", s, __func__, th->th_seq, sc->sc_irs);
 1061                 goto failed;
 1062         }
 1063 
 1064         /*
 1065          * If timestamps were not negotiated during SYN/ACK they
 1066          * must not appear on any segment during this session.
 1067          */
 1068         if (!(sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS)) {
 1069                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1070                         log(LOG_DEBUG, "%s; %s: Timestamp not expected, "
 1071                             "segment rejected\n", s, __func__);
 1072                 goto failed;
 1073         }
 1074 
 1075         /*
 1076          * If timestamps were negotiated during SYN/ACK they should
 1077          * appear on every segment during this session.
 1078          * XXXAO: This is only informal as there have been unverified
 1079          * reports of non-compliants stacks.
 1080          */
 1081         if ((sc->sc_flags & SCF_TIMESTAMP) && !(to->to_flags & TOF_TS)) {
 1082                 if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1083                         log(LOG_DEBUG, "%s; %s: Timestamp missing, "
 1084                             "no action\n", s, __func__);
 1085                         free(s, M_TCPLOG);
 1086                         s = NULL;
 1087                 }
 1088         }
 1089 
 1090         /*
 1091          * If timestamps were negotiated the reflected timestamp
 1092          * must be equal to what we actually sent in the SYN|ACK.
 1093          */
 1094         if ((to->to_flags & TOF_TS) && to->to_tsecr != sc->sc_ts) {
 1095                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1096                         log(LOG_DEBUG, "%s; %s: TSECR %u != TS %u, "
 1097                             "segment rejected\n",
 1098                             s, __func__, to->to_tsecr, sc->sc_ts);
 1099                 goto failed;
 1100         }
 1101 
 1102         *lsop = syncache_socket(sc, *lsop, m);
 1103 
 1104         if (*lsop == NULL)
 1105                 TCPSTAT_INC(tcps_sc_aborted);
 1106         else
 1107                 TCPSTAT_INC(tcps_sc_completed);
 1108 
 1109 /* how do we find the inp for the new socket? */
 1110         if (sc != &scs)
 1111                 syncache_free(sc);
 1112         return (1);
 1113 failed:
 1114         if (sc != NULL && sc != &scs)
 1115                 syncache_free(sc);
 1116         if (s != NULL)
 1117                 free(s, M_TCPLOG);
 1118         *lsop = NULL;
 1119         return (0);
 1120 }
 1121 
 1122 #ifdef TCP_RFC7413
 1123 static void
 1124 syncache_tfo_expand(struct syncache *sc, struct socket **lsop, struct mbuf *m,
 1125     uint64_t response_cookie)
 1126 {
 1127         struct inpcb *inp;
 1128         struct tcpcb *tp;
 1129         unsigned int *pending_counter;
 1130 
 1131         /*
 1132          * Global TCP locks are held because we manipulate the PCB lists
 1133          * and create a new socket.
 1134          */
 1135         INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
 1136 
 1137         pending_counter = intotcpcb(sotoinpcb(*lsop))->t_tfo_pending;
 1138         *lsop = syncache_socket(sc, *lsop, m);
 1139         if (*lsop == NULL) {
 1140                 TCPSTAT_INC(tcps_sc_aborted);
 1141                 atomic_subtract_int(pending_counter, 1);
 1142         } else {
 1143                 inp = sotoinpcb(*lsop);
 1144                 tp = intotcpcb(inp);
 1145                 tp->t_flags |= TF_FASTOPEN;
 1146                 tp->t_tfo_cookie = response_cookie;
 1147                 tp->snd_max = tp->iss;
 1148                 tp->snd_nxt = tp->iss;
 1149                 tp->t_tfo_pending = pending_counter;
 1150                 TCPSTAT_INC(tcps_sc_completed);
 1151         }
 1152 }
 1153 #endif /* TCP_RFC7413 */
 1154 
 1155 /*
 1156  * Given a LISTEN socket and an inbound SYN request, add
 1157  * this to the syn cache, and send back a segment:
 1158  *      <SEQ=ISS><ACK=RCV_NXT><CTL=SYN,ACK>
 1159  * to the source.
 1160  *
 1161  * IMPORTANT NOTE: We do _NOT_ ACK data that might accompany the SYN.
 1162  * Doing so would require that we hold onto the data and deliver it
 1163  * to the application.  However, if we are the target of a SYN-flood
 1164  * DoS attack, an attacker could send data which would eventually
 1165  * consume all available buffer space if it were ACKed.  By not ACKing
 1166  * the data, we avoid this DoS scenario.
 1167  *
 1168  * The exception to the above is when a SYN with a valid TCP Fast Open (TFO)
 1169  * cookie is processed, V_tcp_fastopen_enabled set to true, and the
 1170  * TCP_FASTOPEN socket option is set.  In this case, a new socket is created
 1171  * and returned via lsop, the mbuf is not freed so that tcp_input() can
 1172  * queue its data to the socket, and 1 is returned to indicate the
 1173  * TFO-socket-creation path was taken.
 1174  */
 1175 int
 1176 syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
 1177     struct inpcb *inp, struct socket **lsop, struct mbuf *m, void *tod,
 1178     void *todctx)
 1179 {
 1180         struct tcpcb *tp;
 1181         struct socket *so;
 1182         struct syncache *sc = NULL;
 1183         struct syncache_head *sch;
 1184         struct mbuf *ipopts = NULL;
 1185         u_int ltflags;
 1186         int win, sb_hiwat, ip_ttl, ip_tos;
 1187         char *s;
 1188         int rv = 0;
 1189 #ifdef INET6
 1190         int autoflowlabel = 0;
 1191 #endif
 1192 #ifdef MAC
 1193         struct label *maclabel;
 1194 #endif
 1195         struct syncache scs;
 1196         struct ucred *cred;
 1197 #ifdef TCP_RFC7413
 1198         uint64_t tfo_response_cookie;
 1199         int tfo_cookie_valid = 0;
 1200         int tfo_response_cookie_valid = 0;
 1201 #endif
 1202 
 1203         INP_WLOCK_ASSERT(inp);                  /* listen socket */
 1204         KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_SYN,
 1205             ("%s: unexpected tcp flags", __func__));
 1206 
 1207         /*
 1208          * Combine all so/tp operations very early to drop the INP lock as
 1209          * soon as possible.
 1210          */
 1211         so = *lsop;
 1212         tp = sototcpcb(so);
 1213         cred = crhold(so->so_cred);
 1214 
 1215 #ifdef INET6
 1216         if ((inc->inc_flags & INC_ISIPV6) &&
 1217             (inp->inp_flags & IN6P_AUTOFLOWLABEL))
 1218                 autoflowlabel = 1;
 1219 #endif
 1220         ip_ttl = inp->inp_ip_ttl;
 1221         ip_tos = inp->inp_ip_tos;
 1222         win = sbspace(&so->so_rcv);
 1223         sb_hiwat = so->so_rcv.sb_hiwat;
 1224         ltflags = (tp->t_flags & (TF_NOOPT | TF_SIGNATURE));
 1225 
 1226 #ifdef TCP_RFC7413
 1227         if (V_tcp_fastopen_enabled && (tp->t_flags & TF_FASTOPEN) &&
 1228             (tp->t_tfo_pending != NULL) && (to->to_flags & TOF_FASTOPEN)) {
 1229                 /*
 1230                  * Limit the number of pending TFO connections to
 1231                  * approximately half of the queue limit.  This prevents TFO
 1232                  * SYN floods from starving the service by filling the
 1233                  * listen queue with bogus TFO connections.
 1234                  */
 1235                 if (atomic_fetchadd_int(tp->t_tfo_pending, 1) <=
 1236                     (so->so_qlimit / 2)) {
 1237                         int result;
 1238 
 1239                         result = tcp_fastopen_check_cookie(inc,
 1240                             to->to_tfo_cookie, to->to_tfo_len,
 1241                             &tfo_response_cookie);
 1242                         tfo_cookie_valid = (result > 0);
 1243                         tfo_response_cookie_valid = (result >= 0);
 1244                 } else
 1245                         atomic_subtract_int(tp->t_tfo_pending, 1);
 1246         }
 1247 #endif
 1248 
 1249         /* By the time we drop the lock these should no longer be used. */
 1250         so = NULL;
 1251         tp = NULL;
 1252 
 1253 #ifdef MAC
 1254         if (mac_syncache_init(&maclabel) != 0) {
 1255                 INP_WUNLOCK(inp);
 1256                 goto done;
 1257         } else
 1258                 mac_syncache_create(maclabel, inp);
 1259 #endif
 1260 #ifdef TCP_RFC7413
 1261         if (!tfo_cookie_valid)
 1262 #endif
 1263                 INP_WUNLOCK(inp);
 1264 
 1265         /*
 1266          * Remember the IP options, if any.
 1267          */
 1268 #ifdef INET6
 1269         if (!(inc->inc_flags & INC_ISIPV6))
 1270 #endif
 1271 #ifdef INET
 1272                 ipopts = (m) ? ip_srcroute(m) : NULL;
 1273 #else
 1274                 ipopts = NULL;
 1275 #endif
 1276 
 1277         /*
 1278          * See if we already have an entry for this connection.
 1279          * If we do, resend the SYN,ACK, and reset the retransmit timer.
 1280          *
 1281          * XXX: should the syncache be re-initialized with the contents
 1282          * of the new SYN here (which may have different options?)
 1283          *
 1284          * XXX: We do not check the sequence number to see if this is a
 1285          * real retransmit or a new connection attempt.  The question is
 1286          * how to handle such a case; either ignore it as spoofed, or
 1287          * drop the current entry and create a new one?
 1288          */
 1289         sc = syncache_lookup(inc, &sch);        /* returns locked entry */
 1290         SCH_LOCK_ASSERT(sch);
 1291         if (sc != NULL) {
 1292 #ifdef TCP_RFC7413
 1293                 if (tfo_cookie_valid)
 1294                         INP_WUNLOCK(inp);
 1295 #endif
 1296                 TCPSTAT_INC(tcps_sc_dupsyn);
 1297                 if (ipopts) {
 1298                         /*
 1299                          * If we were remembering a previous source route,
 1300                          * forget it and use the new one we've been given.
 1301                          */
 1302                         if (sc->sc_ipopts)
 1303                                 (void) m_free(sc->sc_ipopts);
 1304                         sc->sc_ipopts = ipopts;
 1305                 }
 1306                 /*
 1307                  * Update timestamp if present.
 1308                  */
 1309                 if ((sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS))
 1310                         sc->sc_tsreflect = to->to_tsval;
 1311                 else
 1312                         sc->sc_flags &= ~SCF_TIMESTAMP;
 1313 #ifdef MAC
 1314                 /*
 1315                  * Since we have already unconditionally allocated label
 1316                  * storage, free it up.  The syncache entry will already
 1317                  * have an initialized label we can use.
 1318                  */
 1319                 mac_syncache_destroy(&maclabel);
 1320 #endif
 1321                 /* Retransmit SYN|ACK and reset retransmit count. */
 1322                 if ((s = tcp_log_addrs(&sc->sc_inc, th, NULL, NULL))) {
 1323                         log(LOG_DEBUG, "%s; %s: Received duplicate SYN, "
 1324                             "resetting timer and retransmitting SYN|ACK\n",
 1325                             s, __func__);
 1326                         free(s, M_TCPLOG);
 1327                 }
 1328                 if (syncache_respond(sc, m) == 0) {
 1329                         sc->sc_rxmits = 0;
 1330                         syncache_timeout(sc, sch, 1);
 1331                         TCPSTAT_INC(tcps_sndacks);
 1332                         TCPSTAT_INC(tcps_sndtotal);
 1333                 }
 1334                 SCH_UNLOCK(sch);
 1335                 goto done;
 1336         }
 1337 
 1338 #ifdef TCP_RFC7413
 1339         if (tfo_cookie_valid) {
 1340                 bzero(&scs, sizeof(scs));
 1341                 sc = &scs;
 1342                 goto skip_alloc;
 1343         }
 1344 #endif
 1345 
 1346         sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
 1347         if (sc == NULL) {
 1348                 /*
 1349                  * The zone allocator couldn't provide more entries.
 1350                  * Treat this as if the cache was full; drop the oldest
 1351                  * entry and insert the new one.
 1352                  */
 1353                 TCPSTAT_INC(tcps_sc_zonefail);
 1354                 if ((sc = TAILQ_LAST(&sch->sch_bucket, sch_head)) != NULL) {
 1355                         sch->sch_last_overflow = time_uptime;
 1356                         syncache_drop(sc, sch);
 1357                 }
 1358                 sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
 1359                 if (sc == NULL) {
 1360                         if (V_tcp_syncookies) {
 1361                                 bzero(&scs, sizeof(scs));
 1362                                 sc = &scs;
 1363                         } else {
 1364                                 SCH_UNLOCK(sch);
 1365                                 if (ipopts)
 1366                                         (void) m_free(ipopts);
 1367                                 goto done;
 1368                         }
 1369                 }
 1370         }
 1371 
 1372 #ifdef TCP_RFC7413
 1373 skip_alloc:
 1374         if (!tfo_cookie_valid && tfo_response_cookie_valid)
 1375                 sc->sc_tfo_cookie = &tfo_response_cookie;
 1376 #endif
 1377 
 1378         /*
 1379          * Fill in the syncache values.
 1380          */
 1381 #ifdef MAC
 1382         sc->sc_label = maclabel;
 1383 #endif
 1384         sc->sc_cred = cred;
 1385         cred = NULL;
 1386         sc->sc_ipopts = ipopts;
 1387         bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
 1388 #ifdef INET6
 1389         if (!(inc->inc_flags & INC_ISIPV6))
 1390 #endif
 1391         {
 1392                 sc->sc_ip_tos = ip_tos;
 1393                 sc->sc_ip_ttl = ip_ttl;
 1394         }
 1395 #ifdef TCP_OFFLOAD
 1396         sc->sc_tod = tod;
 1397         sc->sc_todctx = todctx;
 1398 #endif
 1399         sc->sc_irs = th->th_seq;
 1400         sc->sc_iss = arc4random();
 1401         sc->sc_flags = 0;
 1402         sc->sc_flowlabel = 0;
 1403 
 1404         /*
 1405          * Initial receive window: clip sbspace to [0 .. TCP_MAXWIN].
 1406          * win was derived from socket earlier in the function.
 1407          */
 1408         win = imax(win, 0);
 1409         win = imin(win, TCP_MAXWIN);
 1410         sc->sc_wnd = win;
 1411 
 1412         if (V_tcp_do_rfc1323) {
 1413                 /*
 1414                  * A timestamp received in a SYN makes
 1415                  * it ok to send timestamp requests and replies.
 1416                  */
 1417                 if (to->to_flags & TOF_TS) {
 1418                         sc->sc_tsreflect = to->to_tsval;
 1419                         sc->sc_ts = tcp_ts_getticks();
 1420                         sc->sc_flags |= SCF_TIMESTAMP;
 1421                 }
 1422                 if (to->to_flags & TOF_SCALE) {
 1423                         int wscale = 0;
 1424 
 1425                         /*
 1426                          * Pick the smallest possible scaling factor that
 1427                          * will still allow us to scale up to sb_max, aka
 1428                          * kern.ipc.maxsockbuf.
 1429                          *
 1430                          * We do this because there are broken firewalls that
 1431                          * will corrupt the window scale option, leading to
 1432                          * the other endpoint believing that our advertised
 1433                          * window is unscaled.  At scale factors larger than
 1434                          * 5 the unscaled window will drop below 1500 bytes,
 1435                          * leading to serious problems when traversing these
 1436                          * broken firewalls.
 1437                          *
 1438                          * With the default maxsockbuf of 256K, a scale factor
 1439                          * of 3 will be chosen by this algorithm.  Those who
 1440                          * choose a larger maxsockbuf should watch out
 1441                          * for the compatiblity problems mentioned above.
 1442                          *
 1443                          * RFC1323: The Window field in a SYN (i.e., a <SYN>
 1444                          * or <SYN,ACK>) segment itself is never scaled.
 1445                          */
 1446                         while (wscale < TCP_MAX_WINSHIFT &&
 1447                             (TCP_MAXWIN << wscale) < sb_max)
 1448                                 wscale++;
 1449                         sc->sc_requested_r_scale = wscale;
 1450                         sc->sc_requested_s_scale = to->to_wscale;
 1451                         sc->sc_flags |= SCF_WINSCALE;
 1452                 }
 1453         }
 1454 #ifdef TCP_SIGNATURE
 1455         /*
 1456          * If listening socket requested TCP digests, and received SYN
 1457          * contains the option, flag this in the syncache so that
 1458          * syncache_respond() will do the right thing with the SYN+ACK.
 1459          * XXX: Currently we always record the option by default and will
 1460          * attempt to use it in syncache_respond().
 1461          */
 1462         if (to->to_flags & TOF_SIGNATURE || ltflags & TF_SIGNATURE)
 1463                 sc->sc_flags |= SCF_SIGNATURE;
 1464 #endif
 1465         if (to->to_flags & TOF_SACKPERM)
 1466                 sc->sc_flags |= SCF_SACK;
 1467         if (to->to_flags & TOF_MSS)
 1468                 sc->sc_peer_mss = to->to_mss;   /* peer mss may be zero */
 1469         if (ltflags & TF_NOOPT)
 1470                 sc->sc_flags |= SCF_NOOPT;
 1471         if ((th->th_flags & (TH_ECE|TH_CWR)) && V_tcp_do_ecn)
 1472                 sc->sc_flags |= SCF_ECN;
 1473 
 1474         if (V_tcp_syncookies)
 1475                 sc->sc_iss = syncookie_generate(sch, sc);
 1476 #ifdef INET6
 1477         if (autoflowlabel) {
 1478                 if (V_tcp_syncookies)
 1479                         sc->sc_flowlabel = sc->sc_iss;
 1480                 else
 1481                         sc->sc_flowlabel = ip6_randomflowlabel();
 1482                 sc->sc_flowlabel = htonl(sc->sc_flowlabel) & IPV6_FLOWLABEL_MASK;
 1483         }
 1484 #endif
 1485         SCH_UNLOCK(sch);
 1486 
 1487 #ifdef TCP_RFC7413
 1488         if (tfo_cookie_valid) {
 1489                 syncache_tfo_expand(sc, lsop, m, tfo_response_cookie);
 1490                 /* INP_WUNLOCK(inp) will be performed by the called */
 1491                 rv = 1;
 1492                 goto tfo_done;
 1493         }
 1494 #endif
 1495 
 1496         /*
 1497          * Do a standard 3-way handshake.
 1498          */
 1499         if (syncache_respond(sc, m) == 0) {
 1500                 if (V_tcp_syncookies && V_tcp_syncookiesonly && sc != &scs)
 1501                         syncache_free(sc);
 1502                 else if (sc != &scs)
 1503                         syncache_insert(sc, sch);   /* locks and unlocks sch */
 1504                 TCPSTAT_INC(tcps_sndacks);
 1505                 TCPSTAT_INC(tcps_sndtotal);
 1506         } else {
 1507                 if (sc != &scs)
 1508                         syncache_free(sc);
 1509                 TCPSTAT_INC(tcps_sc_dropped);
 1510         }
 1511 
 1512 done:
 1513         if (m) {
 1514                 *lsop = NULL;
 1515                 m_freem(m);
 1516         }
 1517 #ifdef TCP_RFC7413
 1518 tfo_done:
 1519 #endif
 1520         if (cred != NULL)
 1521                 crfree(cred);
 1522 #ifdef MAC
 1523         if (sc == &scs)
 1524                 mac_syncache_destroy(&maclabel);
 1525 #endif
 1526         return (rv);
 1527 }
 1528 
 1529 /*
 1530  * Send SYN|ACK to the peer.  Either in response to the peer's SYN,
 1531  * i.e. m0 != NULL, or upon 3WHS ACK timeout, i.e. m0 == NULL.
 1532  */
 1533 static int
 1534 syncache_respond(struct syncache *sc, const struct mbuf *m0)
 1535 {
 1536         struct ip *ip = NULL;
 1537         struct mbuf *m;
 1538         struct tcphdr *th = NULL;
 1539         int optlen, error = 0;  /* Make compiler happy */
 1540         u_int16_t hlen, tlen, mssopt;
 1541         struct tcpopt to;
 1542 #ifdef INET6
 1543         struct ip6_hdr *ip6 = NULL;
 1544 #endif
 1545 
 1546         hlen =
 1547 #ifdef INET6
 1548                (sc->sc_inc.inc_flags & INC_ISIPV6) ? sizeof(struct ip6_hdr) :
 1549 #endif
 1550                 sizeof(struct ip);
 1551         tlen = hlen + sizeof(struct tcphdr);
 1552 
 1553         /* Determine MSS we advertize to other end of connection. */
 1554         mssopt = max(tcp_mssopt(&sc->sc_inc), V_tcp_minmss);
 1555 
 1556         /* XXX: Assume that the entire packet will fit in a header mbuf. */
 1557         KASSERT(max_linkhdr + tlen + TCP_MAXOLEN <= MHLEN,
 1558             ("syncache: mbuf too small"));
 1559 
 1560         /* Create the IP+TCP header from scratch. */
 1561         m = m_gethdr(M_NOWAIT, MT_DATA);
 1562         if (m == NULL)
 1563                 return (ENOBUFS);
 1564 #ifdef MAC
 1565         mac_syncache_create_mbuf(sc->sc_label, m);
 1566 #endif
 1567         m->m_data += max_linkhdr;
 1568         m->m_len = tlen;
 1569         m->m_pkthdr.len = tlen;
 1570         m->m_pkthdr.rcvif = NULL;
 1571 
 1572 #ifdef INET6
 1573         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
 1574                 ip6 = mtod(m, struct ip6_hdr *);
 1575                 ip6->ip6_vfc = IPV6_VERSION;
 1576                 ip6->ip6_nxt = IPPROTO_TCP;
 1577                 ip6->ip6_src = sc->sc_inc.inc6_laddr;
 1578                 ip6->ip6_dst = sc->sc_inc.inc6_faddr;
 1579                 ip6->ip6_plen = htons(tlen - hlen);
 1580                 /* ip6_hlim is set after checksum */
 1581                 ip6->ip6_flow &= ~IPV6_FLOWLABEL_MASK;
 1582                 ip6->ip6_flow |= sc->sc_flowlabel;
 1583 
 1584                 th = (struct tcphdr *)(ip6 + 1);
 1585         }
 1586 #endif
 1587 #if defined(INET6) && defined(INET)
 1588         else
 1589 #endif
 1590 #ifdef INET
 1591         {
 1592                 ip = mtod(m, struct ip *);
 1593                 ip->ip_v = IPVERSION;
 1594                 ip->ip_hl = sizeof(struct ip) >> 2;
 1595                 ip->ip_len = htons(tlen);
 1596                 ip->ip_id = 0;
 1597                 ip->ip_off = 0;
 1598                 ip->ip_sum = 0;
 1599                 ip->ip_p = IPPROTO_TCP;
 1600                 ip->ip_src = sc->sc_inc.inc_laddr;
 1601                 ip->ip_dst = sc->sc_inc.inc_faddr;
 1602                 ip->ip_ttl = sc->sc_ip_ttl;
 1603                 ip->ip_tos = sc->sc_ip_tos;
 1604 
 1605                 /*
 1606                  * See if we should do MTU discovery.  Route lookups are
 1607                  * expensive, so we will only unset the DF bit if:
 1608                  *
 1609                  *      1) path_mtu_discovery is disabled
 1610                  *      2) the SCF_UNREACH flag has been set
 1611                  */
 1612                 if (V_path_mtu_discovery && ((sc->sc_flags & SCF_UNREACH) == 0))
 1613                        ip->ip_off |= htons(IP_DF);
 1614 
 1615                 th = (struct tcphdr *)(ip + 1);
 1616         }
 1617 #endif /* INET */
 1618         th->th_sport = sc->sc_inc.inc_lport;
 1619         th->th_dport = sc->sc_inc.inc_fport;
 1620 
 1621         th->th_seq = htonl(sc->sc_iss);
 1622         th->th_ack = htonl(sc->sc_irs + 1);
 1623         th->th_off = sizeof(struct tcphdr) >> 2;
 1624         th->th_x2 = 0;
 1625         th->th_flags = TH_SYN|TH_ACK;
 1626         th->th_win = htons(sc->sc_wnd);
 1627         th->th_urp = 0;
 1628 
 1629         if (sc->sc_flags & SCF_ECN) {
 1630                 th->th_flags |= TH_ECE;
 1631                 TCPSTAT_INC(tcps_ecn_shs);
 1632         }
 1633 
 1634         /* Tack on the TCP options. */
 1635         if ((sc->sc_flags & SCF_NOOPT) == 0) {
 1636                 to.to_flags = 0;
 1637 
 1638                 to.to_mss = mssopt;
 1639                 to.to_flags = TOF_MSS;
 1640                 if (sc->sc_flags & SCF_WINSCALE) {
 1641                         to.to_wscale = sc->sc_requested_r_scale;
 1642                         to.to_flags |= TOF_SCALE;
 1643                 }
 1644                 if (sc->sc_flags & SCF_TIMESTAMP) {
 1645                         /* Virgin timestamp or TCP cookie enhanced one. */
 1646                         to.to_tsval = sc->sc_ts;
 1647                         to.to_tsecr = sc->sc_tsreflect;
 1648                         to.to_flags |= TOF_TS;
 1649                 }
 1650                 if (sc->sc_flags & SCF_SACK)
 1651                         to.to_flags |= TOF_SACKPERM;
 1652 #ifdef TCP_SIGNATURE
 1653                 if (sc->sc_flags & SCF_SIGNATURE)
 1654                         to.to_flags |= TOF_SIGNATURE;
 1655 #endif
 1656 
 1657 #ifdef TCP_RFC7413
 1658                 if (sc->sc_tfo_cookie) {
 1659                         to.to_flags |= TOF_FASTOPEN;
 1660                         to.to_tfo_len = TCP_FASTOPEN_COOKIE_LEN;
 1661                         to.to_tfo_cookie = sc->sc_tfo_cookie;
 1662                         /* don't send cookie again when retransmitting response */
 1663                         sc->sc_tfo_cookie = NULL;
 1664                 }
 1665 #endif
 1666                 optlen = tcp_addoptions(&to, (u_char *)(th + 1));
 1667 
 1668                 /* Adjust headers by option size. */
 1669                 th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
 1670                 m->m_len += optlen;
 1671                 m->m_pkthdr.len += optlen;
 1672 
 1673 #ifdef TCP_SIGNATURE
 1674                 if (sc->sc_flags & SCF_SIGNATURE)
 1675                         tcp_signature_compute(m, 0, 0, optlen,
 1676                             to.to_signature, IPSEC_DIR_OUTBOUND);
 1677 #endif
 1678 #ifdef INET6
 1679                 if (sc->sc_inc.inc_flags & INC_ISIPV6)
 1680                         ip6->ip6_plen = htons(ntohs(ip6->ip6_plen) + optlen);
 1681                 else
 1682 #endif
 1683                         ip->ip_len = htons(ntohs(ip->ip_len) + optlen);
 1684         } else
 1685                 optlen = 0;
 1686 
 1687         M_SETFIB(m, sc->sc_inc.inc_fibnum);
 1688         m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
 1689         /*
 1690          * If we have peer's SYN and it has a flowid, then let's assign it to
 1691          * our SYN|ACK.  ip6_output() and ip_output() will not assign flowid
 1692          * to SYN|ACK due to lack of inp here.
 1693          */
 1694         if (m0 != NULL && M_HASHTYPE_GET(m0) != M_HASHTYPE_NONE) {
 1695                 m->m_pkthdr.flowid = m0->m_pkthdr.flowid;
 1696                 M_HASHTYPE_SET(m, M_HASHTYPE_GET(m0));
 1697         }
 1698 #ifdef INET6
 1699         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
 1700                 m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
 1701                 th->th_sum = in6_cksum_pseudo(ip6, tlen + optlen - hlen,
 1702                     IPPROTO_TCP, 0);
 1703                 ip6->ip6_hlim = in6_selecthlim(NULL, NULL);
 1704 #ifdef TCP_OFFLOAD
 1705                 if (ADDED_BY_TOE(sc)) {
 1706                         struct toedev *tod = sc->sc_tod;
 1707 
 1708                         error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
 1709 
 1710                         return (error);
 1711                 }
 1712 #endif
 1713                 error = ip6_output(m, NULL, NULL, 0, NULL, NULL, NULL);
 1714         }
 1715 #endif
 1716 #if defined(INET6) && defined(INET)
 1717         else
 1718 #endif
 1719 #ifdef INET
 1720         {
 1721                 m->m_pkthdr.csum_flags = CSUM_TCP;
 1722                 th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
 1723                     htons(tlen + optlen - hlen + IPPROTO_TCP));
 1724 #ifdef TCP_OFFLOAD
 1725                 if (ADDED_BY_TOE(sc)) {
 1726                         struct toedev *tod = sc->sc_tod;
 1727 
 1728                         error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
 1729 
 1730                         return (error);
 1731                 }
 1732 #endif
 1733                 error = ip_output(m, sc->sc_ipopts, NULL, 0, NULL, NULL);
 1734         }
 1735 #endif
 1736         return (error);
 1737 }
 1738 
 1739 /*
 1740  * The purpose of syncookies is to handle spoofed SYN flooding DoS attacks
 1741  * that exceed the capacity of the syncache by avoiding the storage of any
 1742  * of the SYNs we receive.  Syncookies defend against blind SYN flooding
 1743  * attacks where the attacker does not have access to our responses.
 1744  *
 1745  * Syncookies encode and include all necessary information about the
 1746  * connection setup within the SYN|ACK that we send back.  That way we
 1747  * can avoid keeping any local state until the ACK to our SYN|ACK returns
 1748  * (if ever).  Normally the syncache and syncookies are running in parallel
 1749  * with the latter taking over when the former is exhausted.  When matching
 1750  * syncache entry is found the syncookie is ignored.
 1751  *
 1752  * The only reliable information persisting the 3WHS is our inital sequence
 1753  * number ISS of 32 bits.  Syncookies embed a cryptographically sufficient
 1754  * strong hash (MAC) value and a few bits of TCP SYN options in the ISS
 1755  * of our SYN|ACK.  The MAC can be recomputed when the ACK to our SYN|ACK
 1756  * returns and signifies a legitimate connection if it matches the ACK.
 1757  *
 1758  * The available space of 32 bits to store the hash and to encode the SYN
 1759  * option information is very tight and we should have at least 24 bits for
 1760  * the MAC to keep the number of guesses by blind spoofing reasonably high.
 1761  *
 1762  * SYN option information we have to encode to fully restore a connection:
 1763  * MSS: is imporant to chose an optimal segment size to avoid IP level
 1764  *   fragmentation along the path.  The common MSS values can be encoded
 1765  *   in a 3-bit table.  Uncommon values are captured by the next lower value
 1766  *   in the table leading to a slight increase in packetization overhead.
 1767  * WSCALE: is necessary to allow large windows to be used for high delay-
 1768  *   bandwidth product links.  Not scaling the window when it was initially
 1769  *   negotiated is bad for performance as lack of scaling further decreases
 1770  *   the apparent available send window.  We only need to encode the WSCALE
 1771  *   we received from the remote end.  Our end can be recalculated at any
 1772  *   time.  The common WSCALE values can be encoded in a 3-bit table.
 1773  *   Uncommon values are captured by the next lower value in the table
 1774  *   making us under-estimate the available window size halving our
 1775  *   theoretically possible maximum throughput for that connection.
 1776  * SACK: Greatly assists in packet loss recovery and requires 1 bit.
 1777  * TIMESTAMP and SIGNATURE is not encoded because they are permanent options
 1778  *   that are included in all segments on a connection.  We enable them when
 1779  *   the ACK has them.
 1780  *
 1781  * Security of syncookies and attack vectors:
 1782  *
 1783  * The MAC is computed over (faddr||laddr||fport||lport||irs||flags||secmod)
 1784  * together with the gloabl secret to make it unique per connection attempt.
 1785  * Thus any change of any of those parameters results in a different MAC output
 1786  * in an unpredictable way unless a collision is encountered.  24 bits of the
 1787  * MAC are embedded into the ISS.
 1788  *
 1789  * To prevent replay attacks two rotating global secrets are updated with a
 1790  * new random value every 15 seconds.  The life-time of a syncookie is thus
 1791  * 15-30 seconds.
 1792  *
 1793  * Vector 1: Attacking the secret.  This requires finding a weakness in the
 1794  * MAC itself or the way it is used here.  The attacker can do a chosen plain
 1795  * text attack by varying and testing the all parameters under his control.
 1796  * The strength depends on the size and randomness of the secret, and the
 1797  * cryptographic security of the MAC function.  Due to the constant updating
 1798  * of the secret the attacker has at most 29.999 seconds to find the secret
 1799  * and launch spoofed connections.  After that he has to start all over again.
 1800  *
 1801  * Vector 2: Collision attack on the MAC of a single ACK.  With a 24 bit MAC
 1802  * size an average of 4,823 attempts are required for a 50% chance of success
 1803  * to spoof a single syncookie (birthday collision paradox).  However the
 1804  * attacker is blind and doesn't know if one of his attempts succeeded unless
 1805  * he has a side channel to interfere success from.  A single connection setup
 1806  * success average of 90% requires 8,790 packets, 99.99% requires 17,578 packets.
 1807  * This many attempts are required for each one blind spoofed connection.  For
 1808  * every additional spoofed connection he has to launch another N attempts.
 1809  * Thus for a sustained rate 100 spoofed connections per second approximately
 1810  * 1,800,000 packets per second would have to be sent.
 1811  *
 1812  * NB: The MAC function should be fast so that it doesn't become a CPU
 1813  * exhaustion attack vector itself.
 1814  *
 1815  * References:
 1816  *  RFC4987 TCP SYN Flooding Attacks and Common Mitigations
 1817  *  SYN cookies were first proposed by cryptographer Dan J. Bernstein in 1996
 1818  *   http://cr.yp.to/syncookies.html    (overview)
 1819  *   http://cr.yp.to/syncookies/archive (details)
 1820  *
 1821  *
 1822  * Schematic construction of a syncookie enabled Initial Sequence Number:
 1823  *  0        1         2         3
 1824  *  12345678901234567890123456789012
 1825  * |xxxxxxxxxxxxxxxxxxxxxxxxWWWMMMSP|
 1826  *
 1827  *  x 24 MAC (truncated)
 1828  *  W  3 Send Window Scale index
 1829  *  M  3 MSS index
 1830  *  S  1 SACK permitted
 1831  *  P  1 Odd/even secret
 1832  */
 1833 
 1834 /*
 1835  * Distribution and probability of certain MSS values.  Those in between are
 1836  * rounded down to the next lower one.
 1837  * [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011]
 1838  *                            .2%  .3%   5%    7%    7%    20%   15%   45%
 1839  */
 1840 static int tcp_sc_msstab[] = { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };
 1841 
 1842 /*
 1843  * Distribution and probability of certain WSCALE values.  We have to map the
 1844  * (send) window scale (shift) option with a range of 0-14 from 4 bits into 3
 1845  * bits based on prevalence of certain values.  Where we don't have an exact
 1846  * match for are rounded down to the next lower one letting us under-estimate
 1847  * the true available window.  At the moment this would happen only for the
 1848  * very uncommon values 3, 5 and those above 8 (more than 16MB socket buffer
 1849  * and window size).  The absence of the WSCALE option (no scaling in either
 1850  * direction) is encoded with index zero.
 1851  * [WSCALE values histograms, Allman, 2012]
 1852  *                            X 10 10 35  5  6 14 10%   by host
 1853  *                            X 11  4  5  5 18 49  3%   by connections
 1854  */
 1855 static int tcp_sc_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };
 1856 
 1857 /*
 1858  * Compute the MAC for the SYN cookie.  SIPHASH-2-4 is chosen for its speed
 1859  * and good cryptographic properties.
 1860  */
 1861 static uint32_t
 1862 syncookie_mac(struct in_conninfo *inc, tcp_seq irs, uint8_t flags,
 1863     uint8_t *secbits, uintptr_t secmod)
 1864 {
 1865         SIPHASH_CTX ctx;
 1866         uint32_t siphash[2];
 1867 
 1868         SipHash24_Init(&ctx);
 1869         SipHash_SetKey(&ctx, secbits);
 1870         switch (inc->inc_flags & INC_ISIPV6) {
 1871 #ifdef INET
 1872         case 0:
 1873                 SipHash_Update(&ctx, &inc->inc_faddr, sizeof(inc->inc_faddr));
 1874                 SipHash_Update(&ctx, &inc->inc_laddr, sizeof(inc->inc_laddr));
 1875                 break;
 1876 #endif
 1877 #ifdef INET6
 1878         case INC_ISIPV6:
 1879                 SipHash_Update(&ctx, &inc->inc6_faddr, sizeof(inc->inc6_faddr));
 1880                 SipHash_Update(&ctx, &inc->inc6_laddr, sizeof(inc->inc6_laddr));
 1881                 break;
 1882 #endif
 1883         }
 1884         SipHash_Update(&ctx, &inc->inc_fport, sizeof(inc->inc_fport));
 1885         SipHash_Update(&ctx, &inc->inc_lport, sizeof(inc->inc_lport));
 1886         SipHash_Update(&ctx, &irs, sizeof(irs));
 1887         SipHash_Update(&ctx, &flags, sizeof(flags));
 1888         SipHash_Update(&ctx, &secmod, sizeof(secmod));
 1889         SipHash_Final((u_int8_t *)&siphash, &ctx);
 1890 
 1891         return (siphash[0] ^ siphash[1]);
 1892 }
 1893 
 1894 static tcp_seq
 1895 syncookie_generate(struct syncache_head *sch, struct syncache *sc)
 1896 {
 1897         u_int i, secbit, wscale;
 1898         uint32_t iss, hash;
 1899         uint8_t *secbits;
 1900         union syncookie cookie;
 1901 
 1902         SCH_LOCK_ASSERT(sch);
 1903 
 1904         cookie.cookie = 0;
 1905 
 1906         /* Map our computed MSS into the 3-bit index. */
 1907         for (i = sizeof(tcp_sc_msstab) / sizeof(*tcp_sc_msstab) - 1;
 1908              tcp_sc_msstab[i] > sc->sc_peer_mss && i > 0;
 1909              i--)
 1910                 ;
 1911         cookie.flags.mss_idx = i;
 1912 
 1913         /*
 1914          * Map the send window scale into the 3-bit index but only if
 1915          * the wscale option was received.
 1916          */
 1917         if (sc->sc_flags & SCF_WINSCALE) {
 1918                 wscale = sc->sc_requested_s_scale;
 1919                 for (i = sizeof(tcp_sc_wstab) / sizeof(*tcp_sc_wstab) - 1;
 1920                      tcp_sc_wstab[i] > wscale && i > 0;
 1921                      i--)
 1922                         ;
 1923                 cookie.flags.wscale_idx = i;
 1924         }
 1925 
 1926         /* Can we do SACK? */
 1927         if (sc->sc_flags & SCF_SACK)
 1928                 cookie.flags.sack_ok = 1;
 1929 
 1930         /* Which of the two secrets to use. */
 1931         secbit = sch->sch_sc->secret.oddeven & 0x1;
 1932         cookie.flags.odd_even = secbit;
 1933 
 1934         secbits = sch->sch_sc->secret.key[secbit];
 1935         hash = syncookie_mac(&sc->sc_inc, sc->sc_irs, cookie.cookie, secbits,
 1936             (uintptr_t)sch);
 1937 
 1938         /*
 1939          * Put the flags into the hash and XOR them to get better ISS number
 1940          * variance.  This doesn't enhance the cryptographic strength and is
 1941          * done to prevent the 8 cookie bits from showing up directly on the
 1942          * wire.
 1943          */
 1944         iss = hash & ~0xff;
 1945         iss |= cookie.cookie ^ (hash >> 24);
 1946 
 1947         /* Randomize the timestamp. */
 1948         if (sc->sc_flags & SCF_TIMESTAMP) {
 1949                 sc->sc_ts = arc4random();
 1950                 sc->sc_tsoff = sc->sc_ts - tcp_ts_getticks();
 1951         }
 1952 
 1953         TCPSTAT_INC(tcps_sc_sendcookie);
 1954         return (iss);
 1955 }
 1956 
 1957 static struct syncache *
 1958 syncookie_lookup(struct in_conninfo *inc, struct syncache_head *sch, 
 1959     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
 1960     struct socket *lso)
 1961 {
 1962         uint32_t hash;
 1963         uint8_t *secbits;
 1964         tcp_seq ack, seq;
 1965         int wnd, wscale = 0;
 1966         union syncookie cookie;
 1967 
 1968         SCH_LOCK_ASSERT(sch);
 1969 
 1970         /*
 1971          * Pull information out of SYN-ACK/ACK and revert sequence number
 1972          * advances.
 1973          */
 1974         ack = th->th_ack - 1;
 1975         seq = th->th_seq - 1;
 1976 
 1977         /*
 1978          * Unpack the flags containing enough information to restore the
 1979          * connection.
 1980          */
 1981         cookie.cookie = (ack & 0xff) ^ (ack >> 24);
 1982 
 1983         /* Which of the two secrets to use. */
 1984         secbits = sch->sch_sc->secret.key[cookie.flags.odd_even];
 1985 
 1986         hash = syncookie_mac(inc, seq, cookie.cookie, secbits, (uintptr_t)sch);
 1987 
 1988         /* The recomputed hash matches the ACK if this was a genuine cookie. */
 1989         if ((ack & ~0xff) != (hash & ~0xff))
 1990                 return (NULL);
 1991 
 1992         /* Fill in the syncache values. */
 1993         sc->sc_flags = 0;
 1994         bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
 1995         sc->sc_ipopts = NULL;
 1996         
 1997         sc->sc_irs = seq;
 1998         sc->sc_iss = ack;
 1999 
 2000         switch (inc->inc_flags & INC_ISIPV6) {
 2001 #ifdef INET
 2002         case 0:
 2003                 sc->sc_ip_ttl = sotoinpcb(lso)->inp_ip_ttl;
 2004                 sc->sc_ip_tos = sotoinpcb(lso)->inp_ip_tos;
 2005                 break;
 2006 #endif
 2007 #ifdef INET6
 2008         case INC_ISIPV6:
 2009                 if (sotoinpcb(lso)->inp_flags & IN6P_AUTOFLOWLABEL)
 2010                         sc->sc_flowlabel = sc->sc_iss & IPV6_FLOWLABEL_MASK;
 2011                 break;
 2012 #endif
 2013         }
 2014 
 2015         sc->sc_peer_mss = tcp_sc_msstab[cookie.flags.mss_idx];
 2016 
 2017         /* We can simply recompute receive window scale we sent earlier. */
 2018         while (wscale < TCP_MAX_WINSHIFT && (TCP_MAXWIN << wscale) < sb_max)
 2019                 wscale++;
 2020 
 2021         /* Only use wscale if it was enabled in the orignal SYN. */
 2022         if (cookie.flags.wscale_idx > 0) {
 2023                 sc->sc_requested_r_scale = wscale;
 2024                 sc->sc_requested_s_scale = tcp_sc_wstab[cookie.flags.wscale_idx];
 2025                 sc->sc_flags |= SCF_WINSCALE;
 2026         }
 2027 
 2028         wnd = sbspace(&lso->so_rcv);
 2029         wnd = imax(wnd, 0);
 2030         wnd = imin(wnd, TCP_MAXWIN);
 2031         sc->sc_wnd = wnd;
 2032 
 2033         if (cookie.flags.sack_ok)
 2034                 sc->sc_flags |= SCF_SACK;
 2035 
 2036         if (to->to_flags & TOF_TS) {
 2037                 sc->sc_flags |= SCF_TIMESTAMP;
 2038                 sc->sc_tsreflect = to->to_tsval;
 2039                 sc->sc_ts = to->to_tsecr;
 2040                 sc->sc_tsoff = to->to_tsecr - tcp_ts_getticks();
 2041         }
 2042 
 2043         if (to->to_flags & TOF_SIGNATURE)
 2044                 sc->sc_flags |= SCF_SIGNATURE;
 2045 
 2046         sc->sc_rxmits = 0;
 2047 
 2048         TCPSTAT_INC(tcps_sc_recvcookie);
 2049         return (sc);
 2050 }
 2051 
 2052 #ifdef INVARIANTS
 2053 static int
 2054 syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
 2055     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
 2056     struct socket *lso)
 2057 {
 2058         struct syncache scs, *scx;
 2059         char *s;
 2060 
 2061         bzero(&scs, sizeof(scs));
 2062         scx = syncookie_lookup(inc, sch, &scs, th, to, lso);
 2063 
 2064         if ((s = tcp_log_addrs(inc, th, NULL, NULL)) == NULL)
 2065                 return (0);
 2066 
 2067         if (scx != NULL) {
 2068                 if (sc->sc_peer_mss != scx->sc_peer_mss)
 2069                         log(LOG_DEBUG, "%s; %s: mss different %i vs %i\n",
 2070                             s, __func__, sc->sc_peer_mss, scx->sc_peer_mss);
 2071 
 2072                 if (sc->sc_requested_r_scale != scx->sc_requested_r_scale)
 2073                         log(LOG_DEBUG, "%s; %s: rwscale different %i vs %i\n",
 2074                             s, __func__, sc->sc_requested_r_scale,
 2075                             scx->sc_requested_r_scale);
 2076 
 2077                 if (sc->sc_requested_s_scale != scx->sc_requested_s_scale)
 2078                         log(LOG_DEBUG, "%s; %s: swscale different %i vs %i\n",
 2079                             s, __func__, sc->sc_requested_s_scale,
 2080                             scx->sc_requested_s_scale);
 2081 
 2082                 if ((sc->sc_flags & SCF_SACK) != (scx->sc_flags & SCF_SACK))
 2083                         log(LOG_DEBUG, "%s; %s: SACK different\n", s, __func__);
 2084         }
 2085 
 2086         if (s != NULL)
 2087                 free(s, M_TCPLOG);
 2088         return (0);
 2089 }
 2090 #endif /* INVARIANTS */
 2091 
 2092 static void
 2093 syncookie_reseed(void *arg)
 2094 {
 2095         struct tcp_syncache *sc = arg;
 2096         uint8_t *secbits;
 2097         int secbit;
 2098 
 2099         /*
 2100          * Reseeding the secret doesn't have to be protected by a lock.
 2101          * It only must be ensured that the new random values are visible
 2102          * to all CPUs in a SMP environment.  The atomic with release
 2103          * semantics ensures that.
 2104          */
 2105         secbit = (sc->secret.oddeven & 0x1) ? 0 : 1;
 2106         secbits = sc->secret.key[secbit];
 2107         arc4rand(secbits, SYNCOOKIE_SECRET_SIZE, 0);
 2108         atomic_add_rel_int(&sc->secret.oddeven, 1);
 2109 
 2110         /* Reschedule ourself. */
 2111         callout_schedule(&sc->secret.reseed, SYNCOOKIE_LIFETIME * hz);
 2112 }
 2113 
 2114 /*
 2115  * Returns the current number of syncache entries.  This number
 2116  * will probably change before you get around to calling 
 2117  * syncache_pcblist.
 2118  */
 2119 int
 2120 syncache_pcbcount(void)
 2121 {
 2122         struct syncache_head *sch;
 2123         int count, i;
 2124 
 2125         for (count = 0, i = 0; i < V_tcp_syncache.hashsize; i++) {
 2126                 /* No need to lock for a read. */
 2127                 sch = &V_tcp_syncache.hashbase[i];
 2128                 count += sch->sch_length;
 2129         }
 2130         return count;
 2131 }
 2132 
 2133 /*
 2134  * Exports the syncache entries to userland so that netstat can display
 2135  * them alongside the other sockets.  This function is intended to be
 2136  * called only from tcp_pcblist.
 2137  *
 2138  * Due to concurrency on an active system, the number of pcbs exported
 2139  * may have no relation to max_pcbs.  max_pcbs merely indicates the
 2140  * amount of space the caller allocated for this function to use.
 2141  */
 2142 int
 2143 syncache_pcblist(struct sysctl_req *req, int max_pcbs, int *pcbs_exported)
 2144 {
 2145         struct xtcpcb xt;
 2146         struct syncache *sc;
 2147         struct syncache_head *sch;
 2148         int count, error, i;
 2149 
 2150         for (count = 0, error = 0, i = 0; i < V_tcp_syncache.hashsize; i++) {
 2151                 sch = &V_tcp_syncache.hashbase[i];
 2152                 SCH_LOCK(sch);
 2153                 TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
 2154                         if (count >= max_pcbs) {
 2155                                 SCH_UNLOCK(sch);
 2156                                 goto exit;
 2157                         }
 2158                         if (cr_cansee(req->td->td_ucred, sc->sc_cred) != 0)
 2159                                 continue;
 2160                         bzero(&xt, sizeof(xt));
 2161                         xt.xt_len = sizeof(xt);
 2162                         if (sc->sc_inc.inc_flags & INC_ISIPV6)
 2163                                 xt.xt_inp.inp_vflag = INP_IPV6;
 2164                         else
 2165                                 xt.xt_inp.inp_vflag = INP_IPV4;
 2166                         bcopy(&sc->sc_inc, &xt.xt_inp.inp_inc, sizeof (struct in_conninfo));
 2167                         xt.xt_tp.t_inpcb = &xt.xt_inp;
 2168                         xt.xt_tp.t_state = TCPS_SYN_RECEIVED;
 2169                         xt.xt_socket.xso_protocol = IPPROTO_TCP;
 2170                         xt.xt_socket.xso_len = sizeof (struct xsocket);
 2171                         xt.xt_socket.so_type = SOCK_STREAM;
 2172                         xt.xt_socket.so_state = SS_ISCONNECTING;
 2173                         error = SYSCTL_OUT(req, &xt, sizeof xt);
 2174                         if (error) {
 2175                                 SCH_UNLOCK(sch);
 2176                                 goto exit;
 2177                         }
 2178                         count++;
 2179                 }
 2180                 SCH_UNLOCK(sch);
 2181         }
 2182 exit:
 2183         *pcbs_exported = count;
 2184         return error;
 2185 }

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