xref: /freebsd/sys/netinet/tcp_syncache.c (revision f0cfa1b168014f56c02b83e5f28412cc5f78d117)
1 /*-
2  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
3  *
4  * Copyright (c) 2001 McAfee, Inc.
5  * Copyright (c) 2006,2013 Andre Oppermann, Internet Business Solutions AG
6  * All rights reserved.
7  *
8  * This software was developed for the FreeBSD Project by Jonathan Lemon
9  * and McAfee Research, the Security Research Division of McAfee, Inc. under
10  * DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
11  * DARPA CHATS research program. [2001 McAfee, Inc.]
12  *
13  * Redistribution and use in source and binary forms, with or without
14  * modification, are permitted provided that the following conditions
15  * are met:
16  * 1. Redistributions of source code must retain the above copyright
17  *    notice, this list of conditions and the following disclaimer.
18  * 2. Redistributions in binary form must reproduce the above copyright
19  *    notice, this list of conditions and the following disclaimer in the
20  *    documentation and/or other materials provided with the distribution.
21  *
22  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
23  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
24  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
25  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
26  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
27  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
28  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
29  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
30  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
31  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
32  * SUCH DAMAGE.
33  */
34 
35 #include <sys/cdefs.h>
36 __FBSDID("$FreeBSD$");
37 
38 #include "opt_inet.h"
39 #include "opt_inet6.h"
40 #include "opt_ipsec.h"
41 #include "opt_pcbgroup.h"
42 
43 #include <sys/param.h>
44 #include <sys/systm.h>
45 #include <sys/hash.h>
46 #include <sys/refcount.h>
47 #include <sys/kernel.h>
48 #include <sys/sysctl.h>
49 #include <sys/limits.h>
50 #include <sys/lock.h>
51 #include <sys/mutex.h>
52 #include <sys/malloc.h>
53 #include <sys/mbuf.h>
54 #include <sys/proc.h>		/* for proc0 declaration */
55 #include <sys/random.h>
56 #include <sys/socket.h>
57 #include <sys/socketvar.h>
58 #include <sys/syslog.h>
59 #include <sys/ucred.h>
60 
61 #include <sys/md5.h>
62 #include <crypto/siphash/siphash.h>
63 
64 #include <vm/uma.h>
65 
66 #include <net/if.h>
67 #include <net/if_var.h>
68 #include <net/route.h>
69 #include <net/vnet.h>
70 
71 #include <netinet/in.h>
72 #include <netinet/in_systm.h>
73 #include <netinet/ip.h>
74 #include <netinet/in_var.h>
75 #include <netinet/in_pcb.h>
76 #include <netinet/ip_var.h>
77 #include <netinet/ip_options.h>
78 #ifdef INET6
79 #include <netinet/ip6.h>
80 #include <netinet/icmp6.h>
81 #include <netinet6/nd6.h>
82 #include <netinet6/ip6_var.h>
83 #include <netinet6/in6_pcb.h>
84 #endif
85 #include <netinet/tcp.h>
86 #ifdef TCP_RFC7413
87 #include <netinet/tcp_fastopen.h>
88 #endif
89 #include <netinet/tcp_fsm.h>
90 #include <netinet/tcp_seq.h>
91 #include <netinet/tcp_timer.h>
92 #include <netinet/tcp_var.h>
93 #include <netinet/tcp_syncache.h>
94 #ifdef INET6
95 #include <netinet6/tcp6_var.h>
96 #endif
97 #ifdef TCP_OFFLOAD
98 #include <netinet/toecore.h>
99 #endif
100 
101 #include <netipsec/ipsec_support.h>
102 
103 #include <machine/in_cksum.h>
104 
105 #include <security/mac/mac_framework.h>
106 
107 static VNET_DEFINE(int, tcp_syncookies) = 1;
108 #define	V_tcp_syncookies		VNET(tcp_syncookies)
109 SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies, CTLFLAG_VNET | CTLFLAG_RW,
110     &VNET_NAME(tcp_syncookies), 0,
111     "Use TCP SYN cookies if the syncache overflows");
112 
113 static VNET_DEFINE(int, tcp_syncookiesonly) = 0;
114 #define	V_tcp_syncookiesonly		VNET(tcp_syncookiesonly)
115 SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies_only, CTLFLAG_VNET | CTLFLAG_RW,
116     &VNET_NAME(tcp_syncookiesonly), 0,
117     "Use only TCP SYN cookies");
118 
119 static VNET_DEFINE(int, functions_inherit_listen_socket_stack) = 1;
120 #define V_functions_inherit_listen_socket_stack \
121     VNET(functions_inherit_listen_socket_stack)
122 SYSCTL_INT(_net_inet_tcp, OID_AUTO, functions_inherit_listen_socket_stack,
123     CTLFLAG_VNET | CTLFLAG_RW,
124     &VNET_NAME(functions_inherit_listen_socket_stack), 0,
125     "Inherit listen socket's stack");
126 
127 #ifdef TCP_OFFLOAD
128 #define ADDED_BY_TOE(sc) ((sc)->sc_tod != NULL)
129 #endif
130 
131 static void	 syncache_drop(struct syncache *, struct syncache_head *);
132 static void	 syncache_free(struct syncache *);
133 static void	 syncache_insert(struct syncache *, struct syncache_head *);
134 static int	 syncache_respond(struct syncache *, struct syncache_head *, int,
135 		    const struct mbuf *);
136 static struct	 socket *syncache_socket(struct syncache *, struct socket *,
137 		    struct mbuf *m);
138 static void	 syncache_timeout(struct syncache *sc, struct syncache_head *sch,
139 		    int docallout);
140 static void	 syncache_timer(void *);
141 
142 static uint32_t	 syncookie_mac(struct in_conninfo *, tcp_seq, uint8_t,
143 		    uint8_t *, uintptr_t);
144 static tcp_seq	 syncookie_generate(struct syncache_head *, struct syncache *);
145 static struct syncache
146 		*syncookie_lookup(struct in_conninfo *, struct syncache_head *,
147 		    struct syncache *, struct tcphdr *, struct tcpopt *,
148 		    struct socket *);
149 static void	 syncookie_reseed(void *);
150 #ifdef INVARIANTS
151 static int	 syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
152 		    struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
153 		    struct socket *lso);
154 #endif
155 
156 /*
157  * Transmit the SYN,ACK fewer times than TCP_MAXRXTSHIFT specifies.
158  * 3 retransmits corresponds to a timeout of 3 * (1 + 2 + 4 + 8) == 45 seconds,
159  * the odds are that the user has given up attempting to connect by then.
160  */
161 #define SYNCACHE_MAXREXMTS		3
162 
163 /* Arbitrary values */
164 #define TCP_SYNCACHE_HASHSIZE		512
165 #define TCP_SYNCACHE_BUCKETLIMIT	30
166 
167 static VNET_DEFINE(struct tcp_syncache, tcp_syncache);
168 #define	V_tcp_syncache			VNET(tcp_syncache)
169 
170 static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, syncache, CTLFLAG_RW, 0,
171     "TCP SYN cache");
172 
173 SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, bucketlimit, CTLFLAG_VNET | CTLFLAG_RDTUN,
174     &VNET_NAME(tcp_syncache.bucket_limit), 0,
175     "Per-bucket hash limit for syncache");
176 
177 SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, cachelimit, CTLFLAG_VNET | CTLFLAG_RDTUN,
178     &VNET_NAME(tcp_syncache.cache_limit), 0,
179     "Overall entry limit for syncache");
180 
181 SYSCTL_UMA_CUR(_net_inet_tcp_syncache, OID_AUTO, count, CTLFLAG_VNET,
182     &VNET_NAME(tcp_syncache.zone), "Current number of entries in syncache");
183 
184 SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, hashsize, CTLFLAG_VNET | CTLFLAG_RDTUN,
185     &VNET_NAME(tcp_syncache.hashsize), 0,
186     "Size of TCP syncache hashtable");
187 
188 SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, rexmtlimit, CTLFLAG_VNET | CTLFLAG_RW,
189     &VNET_NAME(tcp_syncache.rexmt_limit), 0,
190     "Limit on SYN/ACK retransmissions");
191 
192 VNET_DEFINE(int, tcp_sc_rst_sock_fail) = 1;
193 SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, rst_on_sock_fail,
194     CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(tcp_sc_rst_sock_fail), 0,
195     "Send reset on socket allocation failure");
196 
197 static MALLOC_DEFINE(M_SYNCACHE, "syncache", "TCP syncache");
198 
199 #define	SCH_LOCK(sch)		mtx_lock(&(sch)->sch_mtx)
200 #define	SCH_UNLOCK(sch)		mtx_unlock(&(sch)->sch_mtx)
201 #define	SCH_LOCK_ASSERT(sch)	mtx_assert(&(sch)->sch_mtx, MA_OWNED)
202 
203 /*
204  * Requires the syncache entry to be already removed from the bucket list.
205  */
206 static void
207 syncache_free(struct syncache *sc)
208 {
209 
210 	if (sc->sc_ipopts)
211 		(void) m_free(sc->sc_ipopts);
212 	if (sc->sc_cred)
213 		crfree(sc->sc_cred);
214 #ifdef MAC
215 	mac_syncache_destroy(&sc->sc_label);
216 #endif
217 
218 	uma_zfree(V_tcp_syncache.zone, sc);
219 }
220 
221 void
222 syncache_init(void)
223 {
224 	int i;
225 
226 	V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
227 	V_tcp_syncache.bucket_limit = TCP_SYNCACHE_BUCKETLIMIT;
228 	V_tcp_syncache.rexmt_limit = SYNCACHE_MAXREXMTS;
229 	V_tcp_syncache.hash_secret = arc4random();
230 
231 	TUNABLE_INT_FETCH("net.inet.tcp.syncache.hashsize",
232 	    &V_tcp_syncache.hashsize);
233 	TUNABLE_INT_FETCH("net.inet.tcp.syncache.bucketlimit",
234 	    &V_tcp_syncache.bucket_limit);
235 	if (!powerof2(V_tcp_syncache.hashsize) ||
236 	    V_tcp_syncache.hashsize == 0) {
237 		printf("WARNING: syncache hash size is not a power of 2.\n");
238 		V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
239 	}
240 	V_tcp_syncache.hashmask = V_tcp_syncache.hashsize - 1;
241 
242 	/* Set limits. */
243 	V_tcp_syncache.cache_limit =
244 	    V_tcp_syncache.hashsize * V_tcp_syncache.bucket_limit;
245 	TUNABLE_INT_FETCH("net.inet.tcp.syncache.cachelimit",
246 	    &V_tcp_syncache.cache_limit);
247 
248 	/* Allocate the hash table. */
249 	V_tcp_syncache.hashbase = malloc(V_tcp_syncache.hashsize *
250 	    sizeof(struct syncache_head), M_SYNCACHE, M_WAITOK | M_ZERO);
251 
252 #ifdef VIMAGE
253 	V_tcp_syncache.vnet = curvnet;
254 #endif
255 
256 	/* Initialize the hash buckets. */
257 	for (i = 0; i < V_tcp_syncache.hashsize; i++) {
258 		TAILQ_INIT(&V_tcp_syncache.hashbase[i].sch_bucket);
259 		mtx_init(&V_tcp_syncache.hashbase[i].sch_mtx, "tcp_sc_head",
260 			 NULL, MTX_DEF);
261 		callout_init_mtx(&V_tcp_syncache.hashbase[i].sch_timer,
262 			 &V_tcp_syncache.hashbase[i].sch_mtx, 0);
263 		V_tcp_syncache.hashbase[i].sch_length = 0;
264 		V_tcp_syncache.hashbase[i].sch_sc = &V_tcp_syncache;
265 		V_tcp_syncache.hashbase[i].sch_last_overflow =
266 		    -(SYNCOOKIE_LIFETIME + 1);
267 	}
268 
269 	/* Create the syncache entry zone. */
270 	V_tcp_syncache.zone = uma_zcreate("syncache", sizeof(struct syncache),
271 	    NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
272 	V_tcp_syncache.cache_limit = uma_zone_set_max(V_tcp_syncache.zone,
273 	    V_tcp_syncache.cache_limit);
274 
275 	/* Start the SYN cookie reseeder callout. */
276 	callout_init(&V_tcp_syncache.secret.reseed, 1);
277 	arc4rand(V_tcp_syncache.secret.key[0], SYNCOOKIE_SECRET_SIZE, 0);
278 	arc4rand(V_tcp_syncache.secret.key[1], SYNCOOKIE_SECRET_SIZE, 0);
279 	callout_reset(&V_tcp_syncache.secret.reseed, SYNCOOKIE_LIFETIME * hz,
280 	    syncookie_reseed, &V_tcp_syncache);
281 }
282 
283 #ifdef VIMAGE
284 void
285 syncache_destroy(void)
286 {
287 	struct syncache_head *sch;
288 	struct syncache *sc, *nsc;
289 	int i;
290 
291 	/*
292 	 * Stop the re-seed timer before freeing resources.  No need to
293 	 * possibly schedule it another time.
294 	 */
295 	callout_drain(&V_tcp_syncache.secret.reseed);
296 
297 	/* Cleanup hash buckets: stop timers, free entries, destroy locks. */
298 	for (i = 0; i < V_tcp_syncache.hashsize; i++) {
299 
300 		sch = &V_tcp_syncache.hashbase[i];
301 		callout_drain(&sch->sch_timer);
302 
303 		SCH_LOCK(sch);
304 		TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc)
305 			syncache_drop(sc, sch);
306 		SCH_UNLOCK(sch);
307 		KASSERT(TAILQ_EMPTY(&sch->sch_bucket),
308 		    ("%s: sch->sch_bucket not empty", __func__));
309 		KASSERT(sch->sch_length == 0, ("%s: sch->sch_length %d not 0",
310 		    __func__, sch->sch_length));
311 		mtx_destroy(&sch->sch_mtx);
312 	}
313 
314 	KASSERT(uma_zone_get_cur(V_tcp_syncache.zone) == 0,
315 	    ("%s: cache_count not 0", __func__));
316 
317 	/* Free the allocated global resources. */
318 	uma_zdestroy(V_tcp_syncache.zone);
319 	free(V_tcp_syncache.hashbase, M_SYNCACHE);
320 }
321 #endif
322 
323 /*
324  * Inserts a syncache entry into the specified bucket row.
325  * Locks and unlocks the syncache_head autonomously.
326  */
327 static void
328 syncache_insert(struct syncache *sc, struct syncache_head *sch)
329 {
330 	struct syncache *sc2;
331 
332 	SCH_LOCK(sch);
333 
334 	/*
335 	 * Make sure that we don't overflow the per-bucket limit.
336 	 * If the bucket is full, toss the oldest element.
337 	 */
338 	if (sch->sch_length >= V_tcp_syncache.bucket_limit) {
339 		KASSERT(!TAILQ_EMPTY(&sch->sch_bucket),
340 			("sch->sch_length incorrect"));
341 		sc2 = TAILQ_LAST(&sch->sch_bucket, sch_head);
342 		sch->sch_last_overflow = time_uptime;
343 		syncache_drop(sc2, sch);
344 		TCPSTAT_INC(tcps_sc_bucketoverflow);
345 	}
346 
347 	/* Put it into the bucket. */
348 	TAILQ_INSERT_HEAD(&sch->sch_bucket, sc, sc_hash);
349 	sch->sch_length++;
350 
351 #ifdef TCP_OFFLOAD
352 	if (ADDED_BY_TOE(sc)) {
353 		struct toedev *tod = sc->sc_tod;
354 
355 		tod->tod_syncache_added(tod, sc->sc_todctx);
356 	}
357 #endif
358 
359 	/* Reinitialize the bucket row's timer. */
360 	if (sch->sch_length == 1)
361 		sch->sch_nextc = ticks + INT_MAX;
362 	syncache_timeout(sc, sch, 1);
363 
364 	SCH_UNLOCK(sch);
365 
366 	TCPSTATES_INC(TCPS_SYN_RECEIVED);
367 	TCPSTAT_INC(tcps_sc_added);
368 }
369 
370 /*
371  * Remove and free entry from syncache bucket row.
372  * Expects locked syncache head.
373  */
374 static void
375 syncache_drop(struct syncache *sc, struct syncache_head *sch)
376 {
377 
378 	SCH_LOCK_ASSERT(sch);
379 
380 	TCPSTATES_DEC(TCPS_SYN_RECEIVED);
381 	TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
382 	sch->sch_length--;
383 
384 #ifdef TCP_OFFLOAD
385 	if (ADDED_BY_TOE(sc)) {
386 		struct toedev *tod = sc->sc_tod;
387 
388 		tod->tod_syncache_removed(tod, sc->sc_todctx);
389 	}
390 #endif
391 
392 	syncache_free(sc);
393 }
394 
395 /*
396  * Engage/reengage time on bucket row.
397  */
398 static void
399 syncache_timeout(struct syncache *sc, struct syncache_head *sch, int docallout)
400 {
401 	sc->sc_rxttime = ticks +
402 		TCPTV_RTOBASE * (tcp_syn_backoff[sc->sc_rxmits]);
403 	sc->sc_rxmits++;
404 	if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc)) {
405 		sch->sch_nextc = sc->sc_rxttime;
406 		if (docallout)
407 			callout_reset(&sch->sch_timer, sch->sch_nextc - ticks,
408 			    syncache_timer, (void *)sch);
409 	}
410 }
411 
412 /*
413  * Walk the timer queues, looking for SYN,ACKs that need to be retransmitted.
414  * If we have retransmitted an entry the maximum number of times, expire it.
415  * One separate timer for each bucket row.
416  */
417 static void
418 syncache_timer(void *xsch)
419 {
420 	struct syncache_head *sch = (struct syncache_head *)xsch;
421 	struct syncache *sc, *nsc;
422 	int tick = ticks;
423 	char *s;
424 
425 	CURVNET_SET(sch->sch_sc->vnet);
426 
427 	/* NB: syncache_head has already been locked by the callout. */
428 	SCH_LOCK_ASSERT(sch);
429 
430 	/*
431 	 * In the following cycle we may remove some entries and/or
432 	 * advance some timeouts, so re-initialize the bucket timer.
433 	 */
434 	sch->sch_nextc = tick + INT_MAX;
435 
436 	TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc) {
437 		/*
438 		 * We do not check if the listen socket still exists
439 		 * and accept the case where the listen socket may be
440 		 * gone by the time we resend the SYN/ACK.  We do
441 		 * not expect this to happens often. If it does,
442 		 * then the RST will be sent by the time the remote
443 		 * host does the SYN/ACK->ACK.
444 		 */
445 		if (TSTMP_GT(sc->sc_rxttime, tick)) {
446 			if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc))
447 				sch->sch_nextc = sc->sc_rxttime;
448 			continue;
449 		}
450 		if (sc->sc_rxmits > V_tcp_syncache.rexmt_limit) {
451 			if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
452 				log(LOG_DEBUG, "%s; %s: Retransmits exhausted, "
453 				    "giving up and removing syncache entry\n",
454 				    s, __func__);
455 				free(s, M_TCPLOG);
456 			}
457 			syncache_drop(sc, sch);
458 			TCPSTAT_INC(tcps_sc_stale);
459 			continue;
460 		}
461 		if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
462 			log(LOG_DEBUG, "%s; %s: Response timeout, "
463 			    "retransmitting (%u) SYN|ACK\n",
464 			    s, __func__, sc->sc_rxmits);
465 			free(s, M_TCPLOG);
466 		}
467 
468 		syncache_respond(sc, sch, 1, NULL);
469 		TCPSTAT_INC(tcps_sc_retransmitted);
470 		syncache_timeout(sc, sch, 0);
471 	}
472 	if (!TAILQ_EMPTY(&(sch)->sch_bucket))
473 		callout_reset(&(sch)->sch_timer, (sch)->sch_nextc - tick,
474 			syncache_timer, (void *)(sch));
475 	CURVNET_RESTORE();
476 }
477 
478 /*
479  * Find an entry in the syncache.
480  * Returns always with locked syncache_head plus a matching entry or NULL.
481  */
482 static struct syncache *
483 syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp)
484 {
485 	struct syncache *sc;
486 	struct syncache_head *sch;
487 	uint32_t hash;
488 
489 	/*
490 	 * The hash is built on foreign port + local port + foreign address.
491 	 * We rely on the fact that struct in_conninfo starts with 16 bits
492 	 * of foreign port, then 16 bits of local port then followed by 128
493 	 * bits of foreign address.  In case of IPv4 address, the first 3
494 	 * 32-bit words of the address always are zeroes.
495 	 */
496 	hash = jenkins_hash32((uint32_t *)&inc->inc_ie, 5,
497 	    V_tcp_syncache.hash_secret) & V_tcp_syncache.hashmask;
498 
499 	sch = &V_tcp_syncache.hashbase[hash];
500 	*schp = sch;
501 	SCH_LOCK(sch);
502 
503 	/* Circle through bucket row to find matching entry. */
504 	TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash)
505 		if (bcmp(&inc->inc_ie, &sc->sc_inc.inc_ie,
506 		    sizeof(struct in_endpoints)) == 0)
507 			break;
508 
509 	return (sc);	/* Always returns with locked sch. */
510 }
511 
512 /*
513  * This function is called when we get a RST for a
514  * non-existent connection, so that we can see if the
515  * connection is in the syn cache.  If it is, zap it.
516  */
517 void
518 syncache_chkrst(struct in_conninfo *inc, struct tcphdr *th)
519 {
520 	struct syncache *sc;
521 	struct syncache_head *sch;
522 	char *s = NULL;
523 
524 	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
525 	SCH_LOCK_ASSERT(sch);
526 
527 	/*
528 	 * Any RST to our SYN|ACK must not carry ACK, SYN or FIN flags.
529 	 * See RFC 793 page 65, section SEGMENT ARRIVES.
530 	 */
531 	if (th->th_flags & (TH_ACK|TH_SYN|TH_FIN)) {
532 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
533 			log(LOG_DEBUG, "%s; %s: Spurious RST with ACK, SYN or "
534 			    "FIN flag set, segment ignored\n", s, __func__);
535 		TCPSTAT_INC(tcps_badrst);
536 		goto done;
537 	}
538 
539 	/*
540 	 * No corresponding connection was found in syncache.
541 	 * If syncookies are enabled and possibly exclusively
542 	 * used, or we are under memory pressure, a valid RST
543 	 * may not find a syncache entry.  In that case we're
544 	 * done and no SYN|ACK retransmissions will happen.
545 	 * Otherwise the RST was misdirected or spoofed.
546 	 */
547 	if (sc == NULL) {
548 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
549 			log(LOG_DEBUG, "%s; %s: Spurious RST without matching "
550 			    "syncache entry (possibly syncookie only), "
551 			    "segment ignored\n", s, __func__);
552 		TCPSTAT_INC(tcps_badrst);
553 		goto done;
554 	}
555 
556 	/*
557 	 * If the RST bit is set, check the sequence number to see
558 	 * if this is a valid reset segment.
559 	 * RFC 793 page 37:
560 	 *   In all states except SYN-SENT, all reset (RST) segments
561 	 *   are validated by checking their SEQ-fields.  A reset is
562 	 *   valid if its sequence number is in the window.
563 	 *
564 	 *   The sequence number in the reset segment is normally an
565 	 *   echo of our outgoing acknowlegement numbers, but some hosts
566 	 *   send a reset with the sequence number at the rightmost edge
567 	 *   of our receive window, and we have to handle this case.
568 	 */
569 	if (SEQ_GEQ(th->th_seq, sc->sc_irs) &&
570 	    SEQ_LEQ(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
571 		syncache_drop(sc, sch);
572 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
573 			log(LOG_DEBUG, "%s; %s: Our SYN|ACK was rejected, "
574 			    "connection attempt aborted by remote endpoint\n",
575 			    s, __func__);
576 		TCPSTAT_INC(tcps_sc_reset);
577 	} else {
578 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
579 			log(LOG_DEBUG, "%s; %s: RST with invalid SEQ %u != "
580 			    "IRS %u (+WND %u), segment ignored\n",
581 			    s, __func__, th->th_seq, sc->sc_irs, sc->sc_wnd);
582 		TCPSTAT_INC(tcps_badrst);
583 	}
584 
585 done:
586 	if (s != NULL)
587 		free(s, M_TCPLOG);
588 	SCH_UNLOCK(sch);
589 }
590 
591 void
592 syncache_badack(struct in_conninfo *inc)
593 {
594 	struct syncache *sc;
595 	struct syncache_head *sch;
596 
597 	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
598 	SCH_LOCK_ASSERT(sch);
599 	if (sc != NULL) {
600 		syncache_drop(sc, sch);
601 		TCPSTAT_INC(tcps_sc_badack);
602 	}
603 	SCH_UNLOCK(sch);
604 }
605 
606 void
607 syncache_unreach(struct in_conninfo *inc, tcp_seq th_seq)
608 {
609 	struct syncache *sc;
610 	struct syncache_head *sch;
611 
612 	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
613 	SCH_LOCK_ASSERT(sch);
614 	if (sc == NULL)
615 		goto done;
616 
617 	/* If the sequence number != sc_iss, then it's a bogus ICMP msg */
618 	if (ntohl(th_seq) != sc->sc_iss)
619 		goto done;
620 
621 	/*
622 	 * If we've rertransmitted 3 times and this is our second error,
623 	 * we remove the entry.  Otherwise, we allow it to continue on.
624 	 * This prevents us from incorrectly nuking an entry during a
625 	 * spurious network outage.
626 	 *
627 	 * See tcp_notify().
628 	 */
629 	if ((sc->sc_flags & SCF_UNREACH) == 0 || sc->sc_rxmits < 3 + 1) {
630 		sc->sc_flags |= SCF_UNREACH;
631 		goto done;
632 	}
633 	syncache_drop(sc, sch);
634 	TCPSTAT_INC(tcps_sc_unreach);
635 done:
636 	SCH_UNLOCK(sch);
637 }
638 
639 /*
640  * Build a new TCP socket structure from a syncache entry.
641  *
642  * On success return the newly created socket with its underlying inp locked.
643  */
644 static struct socket *
645 syncache_socket(struct syncache *sc, struct socket *lso, struct mbuf *m)
646 {
647 	struct tcp_function_block *blk;
648 	struct inpcb *inp = NULL;
649 	struct socket *so;
650 	struct tcpcb *tp;
651 	int error;
652 	char *s;
653 
654 	INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
655 
656 	/*
657 	 * Ok, create the full blown connection, and set things up
658 	 * as they would have been set up if we had created the
659 	 * connection when the SYN arrived.  If we can't create
660 	 * the connection, abort it.
661 	 */
662 	so = sonewconn(lso, 0);
663 	if (so == NULL) {
664 		/*
665 		 * Drop the connection; we will either send a RST or
666 		 * have the peer retransmit its SYN again after its
667 		 * RTO and try again.
668 		 */
669 		TCPSTAT_INC(tcps_listendrop);
670 		if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
671 			log(LOG_DEBUG, "%s; %s: Socket create failed "
672 			    "due to limits or memory shortage\n",
673 			    s, __func__);
674 			free(s, M_TCPLOG);
675 		}
676 		goto abort2;
677 	}
678 #ifdef MAC
679 	mac_socketpeer_set_from_mbuf(m, so);
680 #endif
681 
682 	inp = sotoinpcb(so);
683 	inp->inp_inc.inc_fibnum = so->so_fibnum;
684 	INP_WLOCK(inp);
685 	/*
686 	 * Exclusive pcbinfo lock is not required in syncache socket case even
687 	 * if two inpcb locks can be acquired simultaneously:
688 	 *  - the inpcb in LISTEN state,
689 	 *  - the newly created inp.
690 	 *
691 	 * In this case, an inp cannot be at same time in LISTEN state and
692 	 * just created by an accept() call.
693 	 */
694 	INP_HASH_WLOCK(&V_tcbinfo);
695 
696 	/* Insert new socket into PCB hash list. */
697 	inp->inp_inc.inc_flags = sc->sc_inc.inc_flags;
698 #ifdef INET6
699 	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
700 		inp->in6p_laddr = sc->sc_inc.inc6_laddr;
701 	} else {
702 		inp->inp_vflag &= ~INP_IPV6;
703 		inp->inp_vflag |= INP_IPV4;
704 #endif
705 		inp->inp_laddr = sc->sc_inc.inc_laddr;
706 #ifdef INET6
707 	}
708 #endif
709 
710 	/*
711 	 * If there's an mbuf and it has a flowid, then let's initialise the
712 	 * inp with that particular flowid.
713 	 */
714 	if (m != NULL && M_HASHTYPE_GET(m) != M_HASHTYPE_NONE) {
715 		inp->inp_flowid = m->m_pkthdr.flowid;
716 		inp->inp_flowtype = M_HASHTYPE_GET(m);
717 	}
718 
719 	/*
720 	 * Install in the reservation hash table for now, but don't yet
721 	 * install a connection group since the full 4-tuple isn't yet
722 	 * configured.
723 	 */
724 	inp->inp_lport = sc->sc_inc.inc_lport;
725 	if ((error = in_pcbinshash_nopcbgroup(inp)) != 0) {
726 		/*
727 		 * Undo the assignments above if we failed to
728 		 * put the PCB on the hash lists.
729 		 */
730 #ifdef INET6
731 		if (sc->sc_inc.inc_flags & INC_ISIPV6)
732 			inp->in6p_laddr = in6addr_any;
733 		else
734 #endif
735 			inp->inp_laddr.s_addr = INADDR_ANY;
736 		inp->inp_lport = 0;
737 		if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
738 			log(LOG_DEBUG, "%s; %s: in_pcbinshash failed "
739 			    "with error %i\n",
740 			    s, __func__, error);
741 			free(s, M_TCPLOG);
742 		}
743 		INP_HASH_WUNLOCK(&V_tcbinfo);
744 		goto abort;
745 	}
746 #ifdef INET6
747 	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
748 		struct inpcb *oinp = sotoinpcb(lso);
749 		struct in6_addr laddr6;
750 		struct sockaddr_in6 sin6;
751 		/*
752 		 * Inherit socket options from the listening socket.
753 		 * Note that in6p_inputopts are not (and should not be)
754 		 * copied, since it stores previously received options and is
755 		 * used to detect if each new option is different than the
756 		 * previous one and hence should be passed to a user.
757 		 * If we copied in6p_inputopts, a user would not be able to
758 		 * receive options just after calling the accept system call.
759 		 */
760 		inp->inp_flags |= oinp->inp_flags & INP_CONTROLOPTS;
761 		if (oinp->in6p_outputopts)
762 			inp->in6p_outputopts =
763 			    ip6_copypktopts(oinp->in6p_outputopts, M_NOWAIT);
764 
765 		sin6.sin6_family = AF_INET6;
766 		sin6.sin6_len = sizeof(sin6);
767 		sin6.sin6_addr = sc->sc_inc.inc6_faddr;
768 		sin6.sin6_port = sc->sc_inc.inc_fport;
769 		sin6.sin6_flowinfo = sin6.sin6_scope_id = 0;
770 		laddr6 = inp->in6p_laddr;
771 		if (IN6_IS_ADDR_UNSPECIFIED(&inp->in6p_laddr))
772 			inp->in6p_laddr = sc->sc_inc.inc6_laddr;
773 		if ((error = in6_pcbconnect_mbuf(inp, (struct sockaddr *)&sin6,
774 		    thread0.td_ucred, m)) != 0) {
775 			inp->in6p_laddr = laddr6;
776 			if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
777 				log(LOG_DEBUG, "%s; %s: in6_pcbconnect failed "
778 				    "with error %i\n",
779 				    s, __func__, error);
780 				free(s, M_TCPLOG);
781 			}
782 			INP_HASH_WUNLOCK(&V_tcbinfo);
783 			goto abort;
784 		}
785 		/* Override flowlabel from in6_pcbconnect. */
786 		inp->inp_flow &= ~IPV6_FLOWLABEL_MASK;
787 		inp->inp_flow |= sc->sc_flowlabel;
788 	}
789 #endif /* INET6 */
790 #if defined(INET) && defined(INET6)
791 	else
792 #endif
793 #ifdef INET
794 	{
795 		struct in_addr laddr;
796 		struct sockaddr_in sin;
797 
798 		inp->inp_options = (m) ? ip_srcroute(m) : NULL;
799 
800 		if (inp->inp_options == NULL) {
801 			inp->inp_options = sc->sc_ipopts;
802 			sc->sc_ipopts = NULL;
803 		}
804 
805 		sin.sin_family = AF_INET;
806 		sin.sin_len = sizeof(sin);
807 		sin.sin_addr = sc->sc_inc.inc_faddr;
808 		sin.sin_port = sc->sc_inc.inc_fport;
809 		bzero((caddr_t)sin.sin_zero, sizeof(sin.sin_zero));
810 		laddr = inp->inp_laddr;
811 		if (inp->inp_laddr.s_addr == INADDR_ANY)
812 			inp->inp_laddr = sc->sc_inc.inc_laddr;
813 		if ((error = in_pcbconnect_mbuf(inp, (struct sockaddr *)&sin,
814 		    thread0.td_ucred, m)) != 0) {
815 			inp->inp_laddr = laddr;
816 			if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
817 				log(LOG_DEBUG, "%s; %s: in_pcbconnect failed "
818 				    "with error %i\n",
819 				    s, __func__, error);
820 				free(s, M_TCPLOG);
821 			}
822 			INP_HASH_WUNLOCK(&V_tcbinfo);
823 			goto abort;
824 		}
825 	}
826 #endif /* INET */
827 #if defined(IPSEC) || defined(IPSEC_SUPPORT)
828 	/* Copy old policy into new socket's. */
829 	if (ipsec_copy_pcbpolicy(sotoinpcb(lso), inp) != 0)
830 		printf("syncache_socket: could not copy policy\n");
831 #endif
832 	INP_HASH_WUNLOCK(&V_tcbinfo);
833 	tp = intotcpcb(inp);
834 	tcp_state_change(tp, TCPS_SYN_RECEIVED);
835 	tp->iss = sc->sc_iss;
836 	tp->irs = sc->sc_irs;
837 	tcp_rcvseqinit(tp);
838 	tcp_sendseqinit(tp);
839 	blk = sototcpcb(lso)->t_fb;
840 	if (V_functions_inherit_listen_socket_stack && blk != tp->t_fb) {
841 		/*
842 		 * Our parents t_fb was not the default,
843 		 * we need to release our ref on tp->t_fb and
844 		 * pickup one on the new entry.
845 		 */
846 		struct tcp_function_block *rblk;
847 
848 		rblk = find_and_ref_tcp_fb(blk);
849 		KASSERT(rblk != NULL,
850 		    ("cannot find blk %p out of syncache?", blk));
851 		if (tp->t_fb->tfb_tcp_fb_fini)
852 			(*tp->t_fb->tfb_tcp_fb_fini)(tp, 0);
853 		refcount_release(&tp->t_fb->tfb_refcnt);
854 		tp->t_fb = rblk;
855 		if (tp->t_fb->tfb_tcp_fb_init) {
856 			(*tp->t_fb->tfb_tcp_fb_init)(tp);
857 		}
858 	}
859 	tp->snd_wl1 = sc->sc_irs;
860 	tp->snd_max = tp->iss + 1;
861 	tp->snd_nxt = tp->iss + 1;
862 	tp->rcv_up = sc->sc_irs + 1;
863 	tp->rcv_wnd = sc->sc_wnd;
864 	tp->rcv_adv += tp->rcv_wnd;
865 	tp->last_ack_sent = tp->rcv_nxt;
866 
867 	tp->t_flags = sototcpcb(lso)->t_flags & (TF_NOPUSH|TF_NODELAY);
868 	if (sc->sc_flags & SCF_NOOPT)
869 		tp->t_flags |= TF_NOOPT;
870 	else {
871 		if (sc->sc_flags & SCF_WINSCALE) {
872 			tp->t_flags |= TF_REQ_SCALE|TF_RCVD_SCALE;
873 			tp->snd_scale = sc->sc_requested_s_scale;
874 			tp->request_r_scale = sc->sc_requested_r_scale;
875 		}
876 		if (sc->sc_flags & SCF_TIMESTAMP) {
877 			tp->t_flags |= TF_REQ_TSTMP|TF_RCVD_TSTMP;
878 			tp->ts_recent = sc->sc_tsreflect;
879 			tp->ts_recent_age = tcp_ts_getticks();
880 			tp->ts_offset = sc->sc_tsoff;
881 		}
882 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
883 		if (sc->sc_flags & SCF_SIGNATURE)
884 			tp->t_flags |= TF_SIGNATURE;
885 #endif
886 		if (sc->sc_flags & SCF_SACK)
887 			tp->t_flags |= TF_SACK_PERMIT;
888 	}
889 
890 	if (sc->sc_flags & SCF_ECN)
891 		tp->t_flags |= TF_ECN_PERMIT;
892 
893 	/*
894 	 * Set up MSS and get cached values from tcp_hostcache.
895 	 * This might overwrite some of the defaults we just set.
896 	 */
897 	tcp_mss(tp, sc->sc_peer_mss);
898 
899 	/*
900 	 * If the SYN,ACK was retransmitted, indicate that CWND to be
901 	 * limited to one segment in cc_conn_init().
902 	 * NB: sc_rxmits counts all SYN,ACK transmits, not just retransmits.
903 	 */
904 	if (sc->sc_rxmits > 1)
905 		tp->snd_cwnd = 1;
906 
907 #ifdef TCP_OFFLOAD
908 	/*
909 	 * Allow a TOE driver to install its hooks.  Note that we hold the
910 	 * pcbinfo lock too and that prevents tcp_usr_accept from accepting a
911 	 * new connection before the TOE driver has done its thing.
912 	 */
913 	if (ADDED_BY_TOE(sc)) {
914 		struct toedev *tod = sc->sc_tod;
915 
916 		tod->tod_offload_socket(tod, sc->sc_todctx, so);
917 	}
918 #endif
919 	/*
920 	 * Copy and activate timers.
921 	 */
922 	tp->t_keepinit = sototcpcb(lso)->t_keepinit;
923 	tp->t_keepidle = sototcpcb(lso)->t_keepidle;
924 	tp->t_keepintvl = sototcpcb(lso)->t_keepintvl;
925 	tp->t_keepcnt = sototcpcb(lso)->t_keepcnt;
926 	tcp_timer_activate(tp, TT_KEEP, TP_KEEPINIT(tp));
927 
928 	TCPSTAT_INC(tcps_accepts);
929 	return (so);
930 
931 abort:
932 	INP_WUNLOCK(inp);
933 abort2:
934 	if (so != NULL)
935 		soabort(so);
936 	return (NULL);
937 }
938 
939 /*
940  * This function gets called when we receive an ACK for a
941  * socket in the LISTEN state.  We look up the connection
942  * in the syncache, and if its there, we pull it out of
943  * the cache and turn it into a full-blown connection in
944  * the SYN-RECEIVED state.
945  *
946  * On syncache_socket() success the newly created socket
947  * has its underlying inp locked.
948  */
949 int
950 syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
951     struct socket **lsop, struct mbuf *m)
952 {
953 	struct syncache *sc;
954 	struct syncache_head *sch;
955 	struct syncache scs;
956 	char *s;
957 
958 	/*
959 	 * Global TCP locks are held because we manipulate the PCB lists
960 	 * and create a new socket.
961 	 */
962 	INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
963 	KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK,
964 	    ("%s: can handle only ACK", __func__));
965 
966 	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
967 	SCH_LOCK_ASSERT(sch);
968 
969 #ifdef INVARIANTS
970 	/*
971 	 * Test code for syncookies comparing the syncache stored
972 	 * values with the reconstructed values from the cookie.
973 	 */
974 	if (sc != NULL)
975 		syncookie_cmp(inc, sch, sc, th, to, *lsop);
976 #endif
977 
978 	if (sc == NULL) {
979 		/*
980 		 * There is no syncache entry, so see if this ACK is
981 		 * a returning syncookie.  To do this, first:
982 		 *  A. Check if syncookies are used in case of syncache
983 		 *     overflows
984 		 *  B. See if this socket has had a syncache entry dropped in
985 		 *     the recent past. We don't want to accept a bogus
986 		 *     syncookie if we've never received a SYN or accept it
987 		 *     twice.
988 		 *  C. check that the syncookie is valid.  If it is, then
989 		 *     cobble up a fake syncache entry, and return.
990 		 */
991 		if (!V_tcp_syncookies) {
992 			SCH_UNLOCK(sch);
993 			if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
994 				log(LOG_DEBUG, "%s; %s: Spurious ACK, "
995 				    "segment rejected (syncookies disabled)\n",
996 				    s, __func__);
997 			goto failed;
998 		}
999 		if (!V_tcp_syncookiesonly &&
1000 		    sch->sch_last_overflow < time_uptime - SYNCOOKIE_LIFETIME) {
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 (no syncache entry)\n",
1005 				    s, __func__);
1006 			goto failed;
1007 		}
1008 		bzero(&scs, sizeof(scs));
1009 		sc = syncookie_lookup(inc, sch, &scs, th, to, *lsop);
1010 		SCH_UNLOCK(sch);
1011 		if (sc == NULL) {
1012 			if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
1013 				log(LOG_DEBUG, "%s; %s: Segment failed "
1014 				    "SYNCOOKIE authentication, segment rejected "
1015 				    "(probably spoofed)\n", s, __func__);
1016 			goto failed;
1017 		}
1018 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
1019 		/* If received ACK has MD5 signature, check it. */
1020 		if ((to->to_flags & TOF_SIGNATURE) != 0 &&
1021 		    (!TCPMD5_ENABLED() ||
1022 		    TCPMD5_INPUT(m, th, to->to_signature) != 0)) {
1023 			/* Drop the ACK. */
1024 			if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
1025 				log(LOG_DEBUG, "%s; %s: Segment rejected, "
1026 				    "MD5 signature doesn't match.\n",
1027 				    s, __func__);
1028 				free(s, M_TCPLOG);
1029 			}
1030 			TCPSTAT_INC(tcps_sig_err_sigopt);
1031 			return (-1); /* Do not send RST */
1032 		}
1033 #endif /* TCP_SIGNATURE */
1034 	} else {
1035 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
1036 		/*
1037 		 * If listening socket requested TCP digests, check that
1038 		 * received ACK has signature and it is correct.
1039 		 * If not, drop the ACK and leave sc entry in th cache,
1040 		 * because SYN was received with correct signature.
1041 		 */
1042 		if (sc->sc_flags & SCF_SIGNATURE) {
1043 			if ((to->to_flags & TOF_SIGNATURE) == 0) {
1044 				/* No signature */
1045 				TCPSTAT_INC(tcps_sig_err_nosigopt);
1046 				SCH_UNLOCK(sch);
1047 				if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
1048 					log(LOG_DEBUG, "%s; %s: Segment "
1049 					    "rejected, MD5 signature wasn't "
1050 					    "provided.\n", s, __func__);
1051 					free(s, M_TCPLOG);
1052 				}
1053 				return (-1); /* Do not send RST */
1054 			}
1055 			if (!TCPMD5_ENABLED() ||
1056 			    TCPMD5_INPUT(m, th, to->to_signature) != 0) {
1057 				/* Doesn't match or no SA */
1058 				SCH_UNLOCK(sch);
1059 				if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
1060 					log(LOG_DEBUG, "%s; %s: Segment "
1061 					    "rejected, MD5 signature doesn't "
1062 					    "match.\n", s, __func__);
1063 					free(s, M_TCPLOG);
1064 				}
1065 				return (-1); /* Do not send RST */
1066 			}
1067 		}
1068 #endif /* TCP_SIGNATURE */
1069 		/*
1070 		 * Pull out the entry to unlock the bucket row.
1071 		 *
1072 		 * NOTE: We must decrease TCPS_SYN_RECEIVED count here, not
1073 		 * tcp_state_change().  The tcpcb is not existent at this
1074 		 * moment.  A new one will be allocated via syncache_socket->
1075 		 * sonewconn->tcp_usr_attach in TCPS_CLOSED state, then
1076 		 * syncache_socket() will change it to TCPS_SYN_RECEIVED.
1077 		 */
1078 		TCPSTATES_DEC(TCPS_SYN_RECEIVED);
1079 		TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
1080 		sch->sch_length--;
1081 #ifdef TCP_OFFLOAD
1082 		if (ADDED_BY_TOE(sc)) {
1083 			struct toedev *tod = sc->sc_tod;
1084 
1085 			tod->tod_syncache_removed(tod, sc->sc_todctx);
1086 		}
1087 #endif
1088 		SCH_UNLOCK(sch);
1089 	}
1090 
1091 	/*
1092 	 * Segment validation:
1093 	 * ACK must match our initial sequence number + 1 (the SYN|ACK).
1094 	 */
1095 	if (th->th_ack != sc->sc_iss + 1) {
1096 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
1097 			log(LOG_DEBUG, "%s; %s: ACK %u != ISS+1 %u, segment "
1098 			    "rejected\n", s, __func__, th->th_ack, sc->sc_iss);
1099 		goto failed;
1100 	}
1101 
1102 	/*
1103 	 * The SEQ must fall in the window starting at the received
1104 	 * initial receive sequence number + 1 (the SYN).
1105 	 */
1106 	if (SEQ_LEQ(th->th_seq, sc->sc_irs) ||
1107 	    SEQ_GT(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
1108 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
1109 			log(LOG_DEBUG, "%s; %s: SEQ %u != IRS+1 %u, segment "
1110 			    "rejected\n", s, __func__, th->th_seq, sc->sc_irs);
1111 		goto failed;
1112 	}
1113 
1114 	/*
1115 	 * If timestamps were not negotiated during SYN/ACK they
1116 	 * must not appear on any segment during this session.
1117 	 */
1118 	if (!(sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS)) {
1119 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
1120 			log(LOG_DEBUG, "%s; %s: Timestamp not expected, "
1121 			    "segment rejected\n", s, __func__);
1122 		goto failed;
1123 	}
1124 
1125 	/*
1126 	 * If timestamps were negotiated during SYN/ACK they should
1127 	 * appear on every segment during this session.
1128 	 * XXXAO: This is only informal as there have been unverified
1129 	 * reports of non-compliants stacks.
1130 	 */
1131 	if ((sc->sc_flags & SCF_TIMESTAMP) && !(to->to_flags & TOF_TS)) {
1132 		if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
1133 			log(LOG_DEBUG, "%s; %s: Timestamp missing, "
1134 			    "no action\n", s, __func__);
1135 			free(s, M_TCPLOG);
1136 			s = NULL;
1137 		}
1138 	}
1139 
1140 	/*
1141 	 * If timestamps were negotiated, the reflected timestamp
1142 	 * must be equal to what we actually sent in the SYN|ACK
1143 	 * except in the case of 0. Some boxes are known for sending
1144 	 * broken timestamp replies during the 3whs (and potentially
1145 	 * during the connection also).
1146 	 *
1147 	 * Accept the final ACK of 3whs with reflected timestamp of 0
1148 	 * instead of sending a RST and deleting the syncache entry.
1149 	 */
1150 	if ((to->to_flags & TOF_TS) && to->to_tsecr &&
1151 	    to->to_tsecr != sc->sc_ts) {
1152 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
1153 			log(LOG_DEBUG, "%s; %s: TSECR %u != TS %u, "
1154 			    "segment rejected\n",
1155 			    s, __func__, to->to_tsecr, sc->sc_ts);
1156 		goto failed;
1157 	}
1158 
1159 	*lsop = syncache_socket(sc, *lsop, m);
1160 
1161 	if (*lsop == NULL)
1162 		TCPSTAT_INC(tcps_sc_aborted);
1163 	else
1164 		TCPSTAT_INC(tcps_sc_completed);
1165 
1166 /* how do we find the inp for the new socket? */
1167 	if (sc != &scs)
1168 		syncache_free(sc);
1169 	return (1);
1170 failed:
1171 	if (sc != NULL && sc != &scs)
1172 		syncache_free(sc);
1173 	if (s != NULL)
1174 		free(s, M_TCPLOG);
1175 	*lsop = NULL;
1176 	return (0);
1177 }
1178 
1179 #ifdef TCP_RFC7413
1180 static void
1181 syncache_tfo_expand(struct syncache *sc, struct socket **lsop, struct mbuf *m,
1182     uint64_t response_cookie)
1183 {
1184 	struct inpcb *inp;
1185 	struct tcpcb *tp;
1186 	unsigned int *pending_counter;
1187 
1188 	/*
1189 	 * Global TCP locks are held because we manipulate the PCB lists
1190 	 * and create a new socket.
1191 	 */
1192 	INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
1193 
1194 	pending_counter = intotcpcb(sotoinpcb(*lsop))->t_tfo_pending;
1195 	*lsop = syncache_socket(sc, *lsop, m);
1196 	if (*lsop == NULL) {
1197 		TCPSTAT_INC(tcps_sc_aborted);
1198 		atomic_subtract_int(pending_counter, 1);
1199 	} else {
1200 		soisconnected(*lsop);
1201 		inp = sotoinpcb(*lsop);
1202 		tp = intotcpcb(inp);
1203 		tp->t_flags |= TF_FASTOPEN;
1204 		tp->t_tfo_cookie = response_cookie;
1205 		tp->snd_max = tp->iss;
1206 		tp->snd_nxt = tp->iss;
1207 		tp->t_tfo_pending = pending_counter;
1208 		TCPSTAT_INC(tcps_sc_completed);
1209 	}
1210 }
1211 #endif /* TCP_RFC7413 */
1212 
1213 /*
1214  * Given a LISTEN socket and an inbound SYN request, add
1215  * this to the syn cache, and send back a segment:
1216  *	<SEQ=ISS><ACK=RCV_NXT><CTL=SYN,ACK>
1217  * to the source.
1218  *
1219  * IMPORTANT NOTE: We do _NOT_ ACK data that might accompany the SYN.
1220  * Doing so would require that we hold onto the data and deliver it
1221  * to the application.  However, if we are the target of a SYN-flood
1222  * DoS attack, an attacker could send data which would eventually
1223  * consume all available buffer space if it were ACKed.  By not ACKing
1224  * the data, we avoid this DoS scenario.
1225  *
1226  * The exception to the above is when a SYN with a valid TCP Fast Open (TFO)
1227  * cookie is processed and a new socket is created.  In this case, any data
1228  * accompanying the SYN will be queued to the socket by tcp_input() and will
1229  * be ACKed either when the application sends response data or the delayed
1230  * ACK timer expires, whichever comes first.
1231  */
1232 int
1233 syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
1234     struct inpcb *inp, struct socket **lsop, struct mbuf *m, void *tod,
1235     void *todctx)
1236 {
1237 	struct tcpcb *tp;
1238 	struct socket *so;
1239 	struct syncache *sc = NULL;
1240 	struct syncache_head *sch;
1241 	struct mbuf *ipopts = NULL;
1242 	u_int ltflags;
1243 	int win, ip_ttl, ip_tos;
1244 	char *s;
1245 	int rv = 0;
1246 #ifdef INET6
1247 	int autoflowlabel = 0;
1248 #endif
1249 #ifdef MAC
1250 	struct label *maclabel;
1251 #endif
1252 	struct syncache scs;
1253 	struct ucred *cred;
1254 #ifdef TCP_RFC7413
1255 	uint64_t tfo_response_cookie;
1256 	unsigned int *tfo_pending = NULL;
1257 	int tfo_cookie_valid = 0;
1258 	int tfo_response_cookie_valid = 0;
1259 #endif
1260 
1261 	INP_WLOCK_ASSERT(inp);			/* listen socket */
1262 	KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_SYN,
1263 	    ("%s: unexpected tcp flags", __func__));
1264 
1265 	/*
1266 	 * Combine all so/tp operations very early to drop the INP lock as
1267 	 * soon as possible.
1268 	 */
1269 	so = *lsop;
1270 	KASSERT(SOLISTENING(so), ("%s: %p not listening", __func__, so));
1271 	tp = sototcpcb(so);
1272 	cred = crhold(so->so_cred);
1273 
1274 #ifdef INET6
1275 	if ((inc->inc_flags & INC_ISIPV6) &&
1276 	    (inp->inp_flags & IN6P_AUTOFLOWLABEL))
1277 		autoflowlabel = 1;
1278 #endif
1279 	ip_ttl = inp->inp_ip_ttl;
1280 	ip_tos = inp->inp_ip_tos;
1281 	win = so->sol_sbrcv_hiwat;
1282 	ltflags = (tp->t_flags & (TF_NOOPT | TF_SIGNATURE));
1283 
1284 #ifdef TCP_RFC7413
1285 	if (V_tcp_fastopen_enabled && IS_FASTOPEN(tp->t_flags) &&
1286 	    (tp->t_tfo_pending != NULL) && (to->to_flags & TOF_FASTOPEN)) {
1287 		/*
1288 		 * Limit the number of pending TFO connections to
1289 		 * approximately half of the queue limit.  This prevents TFO
1290 		 * SYN floods from starving the service by filling the
1291 		 * listen queue with bogus TFO connections.
1292 		 */
1293 		if (atomic_fetchadd_int(tp->t_tfo_pending, 1) <=
1294 		    (so->sol_qlimit / 2)) {
1295 			int result;
1296 
1297 			result = tcp_fastopen_check_cookie(inc,
1298 			    to->to_tfo_cookie, to->to_tfo_len,
1299 			    &tfo_response_cookie);
1300 			tfo_cookie_valid = (result > 0);
1301 			tfo_response_cookie_valid = (result >= 0);
1302 		}
1303 
1304 		/*
1305 		 * Remember the TFO pending counter as it will have to be
1306 		 * decremented below if we don't make it to syncache_tfo_expand().
1307 		 */
1308 		tfo_pending = tp->t_tfo_pending;
1309 	}
1310 #endif
1311 
1312 	/* By the time we drop the lock these should no longer be used. */
1313 	so = NULL;
1314 	tp = NULL;
1315 
1316 #ifdef MAC
1317 	if (mac_syncache_init(&maclabel) != 0) {
1318 		INP_WUNLOCK(inp);
1319 		goto done;
1320 	} else
1321 		mac_syncache_create(maclabel, inp);
1322 #endif
1323 #ifdef TCP_RFC7413
1324 	if (!tfo_cookie_valid)
1325 #endif
1326 		INP_WUNLOCK(inp);
1327 
1328 	/*
1329 	 * Remember the IP options, if any.
1330 	 */
1331 #ifdef INET6
1332 	if (!(inc->inc_flags & INC_ISIPV6))
1333 #endif
1334 #ifdef INET
1335 		ipopts = (m) ? ip_srcroute(m) : NULL;
1336 #else
1337 		ipopts = NULL;
1338 #endif
1339 
1340 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
1341 	/*
1342 	 * If listening socket requested TCP digests, check that received
1343 	 * SYN has signature and it is correct. If signature doesn't match
1344 	 * or TCP_SIGNATURE support isn't enabled, drop the packet.
1345 	 */
1346 	if (ltflags & TF_SIGNATURE) {
1347 		if ((to->to_flags & TOF_SIGNATURE) == 0) {
1348 			TCPSTAT_INC(tcps_sig_err_nosigopt);
1349 			goto done;
1350 		}
1351 		if (!TCPMD5_ENABLED() ||
1352 		    TCPMD5_INPUT(m, th, to->to_signature) != 0)
1353 			goto done;
1354 	}
1355 #endif	/* TCP_SIGNATURE */
1356 	/*
1357 	 * See if we already have an entry for this connection.
1358 	 * If we do, resend the SYN,ACK, and reset the retransmit timer.
1359 	 *
1360 	 * XXX: should the syncache be re-initialized with the contents
1361 	 * of the new SYN here (which may have different options?)
1362 	 *
1363 	 * XXX: We do not check the sequence number to see if this is a
1364 	 * real retransmit or a new connection attempt.  The question is
1365 	 * how to handle such a case; either ignore it as spoofed, or
1366 	 * drop the current entry and create a new one?
1367 	 */
1368 	sc = syncache_lookup(inc, &sch);	/* returns locked entry */
1369 	SCH_LOCK_ASSERT(sch);
1370 	if (sc != NULL) {
1371 #ifdef TCP_RFC7413
1372 		if (tfo_cookie_valid)
1373 			INP_WUNLOCK(inp);
1374 #endif
1375 		TCPSTAT_INC(tcps_sc_dupsyn);
1376 		if (ipopts) {
1377 			/*
1378 			 * If we were remembering a previous source route,
1379 			 * forget it and use the new one we've been given.
1380 			 */
1381 			if (sc->sc_ipopts)
1382 				(void) m_free(sc->sc_ipopts);
1383 			sc->sc_ipopts = ipopts;
1384 		}
1385 		/*
1386 		 * Update timestamp if present.
1387 		 */
1388 		if ((sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS))
1389 			sc->sc_tsreflect = to->to_tsval;
1390 		else
1391 			sc->sc_flags &= ~SCF_TIMESTAMP;
1392 #ifdef MAC
1393 		/*
1394 		 * Since we have already unconditionally allocated label
1395 		 * storage, free it up.  The syncache entry will already
1396 		 * have an initialized label we can use.
1397 		 */
1398 		mac_syncache_destroy(&maclabel);
1399 #endif
1400 		/* Retransmit SYN|ACK and reset retransmit count. */
1401 		if ((s = tcp_log_addrs(&sc->sc_inc, th, NULL, NULL))) {
1402 			log(LOG_DEBUG, "%s; %s: Received duplicate SYN, "
1403 			    "resetting timer and retransmitting SYN|ACK\n",
1404 			    s, __func__);
1405 			free(s, M_TCPLOG);
1406 		}
1407 		if (syncache_respond(sc, sch, 1, m) == 0) {
1408 			sc->sc_rxmits = 0;
1409 			syncache_timeout(sc, sch, 1);
1410 			TCPSTAT_INC(tcps_sndacks);
1411 			TCPSTAT_INC(tcps_sndtotal);
1412 		}
1413 		SCH_UNLOCK(sch);
1414 		goto done;
1415 	}
1416 
1417 #ifdef TCP_RFC7413
1418 	if (tfo_cookie_valid) {
1419 		bzero(&scs, sizeof(scs));
1420 		sc = &scs;
1421 		goto skip_alloc;
1422 	}
1423 #endif
1424 
1425 	sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
1426 	if (sc == NULL) {
1427 		/*
1428 		 * The zone allocator couldn't provide more entries.
1429 		 * Treat this as if the cache was full; drop the oldest
1430 		 * entry and insert the new one.
1431 		 */
1432 		TCPSTAT_INC(tcps_sc_zonefail);
1433 		if ((sc = TAILQ_LAST(&sch->sch_bucket, sch_head)) != NULL) {
1434 			sch->sch_last_overflow = time_uptime;
1435 			syncache_drop(sc, sch);
1436 		}
1437 		sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
1438 		if (sc == NULL) {
1439 			if (V_tcp_syncookies) {
1440 				bzero(&scs, sizeof(scs));
1441 				sc = &scs;
1442 			} else {
1443 				SCH_UNLOCK(sch);
1444 				if (ipopts)
1445 					(void) m_free(ipopts);
1446 				goto done;
1447 			}
1448 		}
1449 	}
1450 
1451 #ifdef TCP_RFC7413
1452 skip_alloc:
1453 	if (!tfo_cookie_valid && tfo_response_cookie_valid)
1454 		sc->sc_tfo_cookie = &tfo_response_cookie;
1455 #endif
1456 
1457 	/*
1458 	 * Fill in the syncache values.
1459 	 */
1460 #ifdef MAC
1461 	sc->sc_label = maclabel;
1462 #endif
1463 	sc->sc_cred = cred;
1464 	cred = NULL;
1465 	sc->sc_ipopts = ipopts;
1466 	bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
1467 #ifdef INET6
1468 	if (!(inc->inc_flags & INC_ISIPV6))
1469 #endif
1470 	{
1471 		sc->sc_ip_tos = ip_tos;
1472 		sc->sc_ip_ttl = ip_ttl;
1473 	}
1474 #ifdef TCP_OFFLOAD
1475 	sc->sc_tod = tod;
1476 	sc->sc_todctx = todctx;
1477 #endif
1478 	sc->sc_irs = th->th_seq;
1479 	sc->sc_iss = arc4random();
1480 	sc->sc_flags = 0;
1481 	sc->sc_flowlabel = 0;
1482 
1483 	/*
1484 	 * Initial receive window: clip sbspace to [0 .. TCP_MAXWIN].
1485 	 * win was derived from socket earlier in the function.
1486 	 */
1487 	win = imax(win, 0);
1488 	win = imin(win, TCP_MAXWIN);
1489 	sc->sc_wnd = win;
1490 
1491 	if (V_tcp_do_rfc1323) {
1492 		/*
1493 		 * A timestamp received in a SYN makes
1494 		 * it ok to send timestamp requests and replies.
1495 		 */
1496 		if (to->to_flags & TOF_TS) {
1497 			sc->sc_tsreflect = to->to_tsval;
1498 			sc->sc_ts = tcp_ts_getticks();
1499 			sc->sc_flags |= SCF_TIMESTAMP;
1500 		}
1501 		if (to->to_flags & TOF_SCALE) {
1502 			int wscale = 0;
1503 
1504 			/*
1505 			 * Pick the smallest possible scaling factor that
1506 			 * will still allow us to scale up to sb_max, aka
1507 			 * kern.ipc.maxsockbuf.
1508 			 *
1509 			 * We do this because there are broken firewalls that
1510 			 * will corrupt the window scale option, leading to
1511 			 * the other endpoint believing that our advertised
1512 			 * window is unscaled.  At scale factors larger than
1513 			 * 5 the unscaled window will drop below 1500 bytes,
1514 			 * leading to serious problems when traversing these
1515 			 * broken firewalls.
1516 			 *
1517 			 * With the default maxsockbuf of 256K, a scale factor
1518 			 * of 3 will be chosen by this algorithm.  Those who
1519 			 * choose a larger maxsockbuf should watch out
1520 			 * for the compatibility problems mentioned above.
1521 			 *
1522 			 * RFC1323: The Window field in a SYN (i.e., a <SYN>
1523 			 * or <SYN,ACK>) segment itself is never scaled.
1524 			 */
1525 			while (wscale < TCP_MAX_WINSHIFT &&
1526 			    (TCP_MAXWIN << wscale) < sb_max)
1527 				wscale++;
1528 			sc->sc_requested_r_scale = wscale;
1529 			sc->sc_requested_s_scale = to->to_wscale;
1530 			sc->sc_flags |= SCF_WINSCALE;
1531 		}
1532 	}
1533 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
1534 	/*
1535 	 * If listening socket requested TCP digests, flag this in the
1536 	 * syncache so that syncache_respond() will do the right thing
1537 	 * with the SYN+ACK.
1538 	 */
1539 	if (ltflags & TF_SIGNATURE)
1540 		sc->sc_flags |= SCF_SIGNATURE;
1541 #endif	/* TCP_SIGNATURE */
1542 	if (to->to_flags & TOF_SACKPERM)
1543 		sc->sc_flags |= SCF_SACK;
1544 	if (to->to_flags & TOF_MSS)
1545 		sc->sc_peer_mss = to->to_mss;	/* peer mss may be zero */
1546 	if (ltflags & TF_NOOPT)
1547 		sc->sc_flags |= SCF_NOOPT;
1548 	if ((th->th_flags & (TH_ECE|TH_CWR)) && V_tcp_do_ecn)
1549 		sc->sc_flags |= SCF_ECN;
1550 
1551 	if (V_tcp_syncookies)
1552 		sc->sc_iss = syncookie_generate(sch, sc);
1553 #ifdef INET6
1554 	if (autoflowlabel) {
1555 		if (V_tcp_syncookies)
1556 			sc->sc_flowlabel = sc->sc_iss;
1557 		else
1558 			sc->sc_flowlabel = ip6_randomflowlabel();
1559 		sc->sc_flowlabel = htonl(sc->sc_flowlabel) & IPV6_FLOWLABEL_MASK;
1560 	}
1561 #endif
1562 	SCH_UNLOCK(sch);
1563 
1564 #ifdef TCP_RFC7413
1565 	if (tfo_cookie_valid) {
1566 		syncache_tfo_expand(sc, lsop, m, tfo_response_cookie);
1567 		/* INP_WUNLOCK(inp) will be performed by the caller */
1568 		rv = 1;
1569 		goto tfo_expanded;
1570 	}
1571 #endif
1572 
1573 	/*
1574 	 * Do a standard 3-way handshake.
1575 	 */
1576 	if (syncache_respond(sc, sch, 0, m) == 0) {
1577 		if (V_tcp_syncookies && V_tcp_syncookiesonly && sc != &scs)
1578 			syncache_free(sc);
1579 		else if (sc != &scs)
1580 			syncache_insert(sc, sch);   /* locks and unlocks sch */
1581 		TCPSTAT_INC(tcps_sndacks);
1582 		TCPSTAT_INC(tcps_sndtotal);
1583 	} else {
1584 		if (sc != &scs)
1585 			syncache_free(sc);
1586 		TCPSTAT_INC(tcps_sc_dropped);
1587 	}
1588 
1589 done:
1590 	if (m) {
1591 		*lsop = NULL;
1592 		m_freem(m);
1593 	}
1594 #ifdef TCP_RFC7413
1595 	/*
1596 	 * If tfo_pending is not NULL here, then a TFO SYN that did not
1597 	 * result in a new socket was processed and the associated pending
1598 	 * counter has not yet been decremented.  All such TFO processing paths
1599 	 * transit this point.
1600 	 */
1601 	if (tfo_pending != NULL)
1602 		tcp_fastopen_decrement_counter(tfo_pending);
1603 
1604 tfo_expanded:
1605 #endif
1606 	if (cred != NULL)
1607 		crfree(cred);
1608 #ifdef MAC
1609 	if (sc == &scs)
1610 		mac_syncache_destroy(&maclabel);
1611 #endif
1612 	return (rv);
1613 }
1614 
1615 /*
1616  * Send SYN|ACK to the peer.  Either in response to the peer's SYN,
1617  * i.e. m0 != NULL, or upon 3WHS ACK timeout, i.e. m0 == NULL.
1618  */
1619 static int
1620 syncache_respond(struct syncache *sc, struct syncache_head *sch, int locked,
1621     const struct mbuf *m0)
1622 {
1623 	struct ip *ip = NULL;
1624 	struct mbuf *m;
1625 	struct tcphdr *th = NULL;
1626 	int optlen, error = 0;	/* Make compiler happy */
1627 	u_int16_t hlen, tlen, mssopt;
1628 	struct tcpopt to;
1629 #ifdef INET6
1630 	struct ip6_hdr *ip6 = NULL;
1631 #endif
1632 	hlen =
1633 #ifdef INET6
1634 	       (sc->sc_inc.inc_flags & INC_ISIPV6) ? sizeof(struct ip6_hdr) :
1635 #endif
1636 		sizeof(struct ip);
1637 	tlen = hlen + sizeof(struct tcphdr);
1638 
1639 	/* Determine MSS we advertize to other end of connection. */
1640 	mssopt = max(tcp_mssopt(&sc->sc_inc), V_tcp_minmss);
1641 
1642 	/* XXX: Assume that the entire packet will fit in a header mbuf. */
1643 	KASSERT(max_linkhdr + tlen + TCP_MAXOLEN <= MHLEN,
1644 	    ("syncache: mbuf too small"));
1645 
1646 	/* Create the IP+TCP header from scratch. */
1647 	m = m_gethdr(M_NOWAIT, MT_DATA);
1648 	if (m == NULL)
1649 		return (ENOBUFS);
1650 #ifdef MAC
1651 	mac_syncache_create_mbuf(sc->sc_label, m);
1652 #endif
1653 	m->m_data += max_linkhdr;
1654 	m->m_len = tlen;
1655 	m->m_pkthdr.len = tlen;
1656 	m->m_pkthdr.rcvif = NULL;
1657 
1658 #ifdef INET6
1659 	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
1660 		ip6 = mtod(m, struct ip6_hdr *);
1661 		ip6->ip6_vfc = IPV6_VERSION;
1662 		ip6->ip6_nxt = IPPROTO_TCP;
1663 		ip6->ip6_src = sc->sc_inc.inc6_laddr;
1664 		ip6->ip6_dst = sc->sc_inc.inc6_faddr;
1665 		ip6->ip6_plen = htons(tlen - hlen);
1666 		/* ip6_hlim is set after checksum */
1667 		ip6->ip6_flow &= ~IPV6_FLOWLABEL_MASK;
1668 		ip6->ip6_flow |= sc->sc_flowlabel;
1669 
1670 		th = (struct tcphdr *)(ip6 + 1);
1671 	}
1672 #endif
1673 #if defined(INET6) && defined(INET)
1674 	else
1675 #endif
1676 #ifdef INET
1677 	{
1678 		ip = mtod(m, struct ip *);
1679 		ip->ip_v = IPVERSION;
1680 		ip->ip_hl = sizeof(struct ip) >> 2;
1681 		ip->ip_len = htons(tlen);
1682 		ip->ip_id = 0;
1683 		ip->ip_off = 0;
1684 		ip->ip_sum = 0;
1685 		ip->ip_p = IPPROTO_TCP;
1686 		ip->ip_src = sc->sc_inc.inc_laddr;
1687 		ip->ip_dst = sc->sc_inc.inc_faddr;
1688 		ip->ip_ttl = sc->sc_ip_ttl;
1689 		ip->ip_tos = sc->sc_ip_tos;
1690 
1691 		/*
1692 		 * See if we should do MTU discovery.  Route lookups are
1693 		 * expensive, so we will only unset the DF bit if:
1694 		 *
1695 		 *	1) path_mtu_discovery is disabled
1696 		 *	2) the SCF_UNREACH flag has been set
1697 		 */
1698 		if (V_path_mtu_discovery && ((sc->sc_flags & SCF_UNREACH) == 0))
1699 		       ip->ip_off |= htons(IP_DF);
1700 
1701 		th = (struct tcphdr *)(ip + 1);
1702 	}
1703 #endif /* INET */
1704 	th->th_sport = sc->sc_inc.inc_lport;
1705 	th->th_dport = sc->sc_inc.inc_fport;
1706 
1707 	th->th_seq = htonl(sc->sc_iss);
1708 	th->th_ack = htonl(sc->sc_irs + 1);
1709 	th->th_off = sizeof(struct tcphdr) >> 2;
1710 	th->th_x2 = 0;
1711 	th->th_flags = TH_SYN|TH_ACK;
1712 	th->th_win = htons(sc->sc_wnd);
1713 	th->th_urp = 0;
1714 
1715 	if (sc->sc_flags & SCF_ECN) {
1716 		th->th_flags |= TH_ECE;
1717 		TCPSTAT_INC(tcps_ecn_shs);
1718 	}
1719 
1720 	/* Tack on the TCP options. */
1721 	if ((sc->sc_flags & SCF_NOOPT) == 0) {
1722 		to.to_flags = 0;
1723 
1724 		to.to_mss = mssopt;
1725 		to.to_flags = TOF_MSS;
1726 		if (sc->sc_flags & SCF_WINSCALE) {
1727 			to.to_wscale = sc->sc_requested_r_scale;
1728 			to.to_flags |= TOF_SCALE;
1729 		}
1730 		if (sc->sc_flags & SCF_TIMESTAMP) {
1731 			/* Virgin timestamp or TCP cookie enhanced one. */
1732 			to.to_tsval = sc->sc_ts;
1733 			to.to_tsecr = sc->sc_tsreflect;
1734 			to.to_flags |= TOF_TS;
1735 		}
1736 		if (sc->sc_flags & SCF_SACK)
1737 			to.to_flags |= TOF_SACKPERM;
1738 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
1739 		if (sc->sc_flags & SCF_SIGNATURE)
1740 			to.to_flags |= TOF_SIGNATURE;
1741 #endif
1742 #ifdef TCP_RFC7413
1743 		if (sc->sc_tfo_cookie) {
1744 			to.to_flags |= TOF_FASTOPEN;
1745 			to.to_tfo_len = TCP_FASTOPEN_COOKIE_LEN;
1746 			to.to_tfo_cookie = sc->sc_tfo_cookie;
1747 			/* don't send cookie again when retransmitting response */
1748 			sc->sc_tfo_cookie = NULL;
1749 		}
1750 #endif
1751 		optlen = tcp_addoptions(&to, (u_char *)(th + 1));
1752 
1753 		/* Adjust headers by option size. */
1754 		th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
1755 		m->m_len += optlen;
1756 		m->m_pkthdr.len += optlen;
1757 #ifdef INET6
1758 		if (sc->sc_inc.inc_flags & INC_ISIPV6)
1759 			ip6->ip6_plen = htons(ntohs(ip6->ip6_plen) + optlen);
1760 		else
1761 #endif
1762 			ip->ip_len = htons(ntohs(ip->ip_len) + optlen);
1763 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
1764 		if (sc->sc_flags & SCF_SIGNATURE) {
1765 			KASSERT(to.to_flags & TOF_SIGNATURE,
1766 			    ("tcp_addoptions() didn't set tcp_signature"));
1767 
1768 			/* NOTE: to.to_signature is inside of mbuf */
1769 			if (!TCPMD5_ENABLED() ||
1770 			    TCPMD5_OUTPUT(m, th, to.to_signature) != 0) {
1771 				m_freem(m);
1772 				return (EACCES);
1773 			}
1774 		}
1775 #endif
1776 	} else
1777 		optlen = 0;
1778 
1779 	M_SETFIB(m, sc->sc_inc.inc_fibnum);
1780 	m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
1781 	/*
1782 	 * If we have peer's SYN and it has a flowid, then let's assign it to
1783 	 * our SYN|ACK.  ip6_output() and ip_output() will not assign flowid
1784 	 * to SYN|ACK due to lack of inp here.
1785 	 */
1786 	if (m0 != NULL && M_HASHTYPE_GET(m0) != M_HASHTYPE_NONE) {
1787 		m->m_pkthdr.flowid = m0->m_pkthdr.flowid;
1788 		M_HASHTYPE_SET(m, M_HASHTYPE_GET(m0));
1789 	}
1790 #ifdef INET6
1791 	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
1792 		m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
1793 		th->th_sum = in6_cksum_pseudo(ip6, tlen + optlen - hlen,
1794 		    IPPROTO_TCP, 0);
1795 		ip6->ip6_hlim = in6_selecthlim(NULL, NULL);
1796 #ifdef TCP_OFFLOAD
1797 		if (ADDED_BY_TOE(sc)) {
1798 			struct toedev *tod = sc->sc_tod;
1799 
1800 			error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
1801 
1802 			return (error);
1803 		}
1804 #endif
1805 		error = ip6_output(m, NULL, NULL, 0, NULL, NULL, NULL);
1806 	}
1807 #endif
1808 #if defined(INET6) && defined(INET)
1809 	else
1810 #endif
1811 #ifdef INET
1812 	{
1813 		m->m_pkthdr.csum_flags = CSUM_TCP;
1814 		th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
1815 		    htons(tlen + optlen - hlen + IPPROTO_TCP));
1816 #ifdef TCP_OFFLOAD
1817 		if (ADDED_BY_TOE(sc)) {
1818 			struct toedev *tod = sc->sc_tod;
1819 
1820 			error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
1821 
1822 			return (error);
1823 		}
1824 #endif
1825 		error = ip_output(m, sc->sc_ipopts, NULL, 0, NULL, NULL);
1826 	}
1827 #endif
1828 	return (error);
1829 }
1830 
1831 /*
1832  * The purpose of syncookies is to handle spoofed SYN flooding DoS attacks
1833  * that exceed the capacity of the syncache by avoiding the storage of any
1834  * of the SYNs we receive.  Syncookies defend against blind SYN flooding
1835  * attacks where the attacker does not have access to our responses.
1836  *
1837  * Syncookies encode and include all necessary information about the
1838  * connection setup within the SYN|ACK that we send back.  That way we
1839  * can avoid keeping any local state until the ACK to our SYN|ACK returns
1840  * (if ever).  Normally the syncache and syncookies are running in parallel
1841  * with the latter taking over when the former is exhausted.  When matching
1842  * syncache entry is found the syncookie is ignored.
1843  *
1844  * The only reliable information persisting the 3WHS is our initial sequence
1845  * number ISS of 32 bits.  Syncookies embed a cryptographically sufficient
1846  * strong hash (MAC) value and a few bits of TCP SYN options in the ISS
1847  * of our SYN|ACK.  The MAC can be recomputed when the ACK to our SYN|ACK
1848  * returns and signifies a legitimate connection if it matches the ACK.
1849  *
1850  * The available space of 32 bits to store the hash and to encode the SYN
1851  * option information is very tight and we should have at least 24 bits for
1852  * the MAC to keep the number of guesses by blind spoofing reasonably high.
1853  *
1854  * SYN option information we have to encode to fully restore a connection:
1855  * MSS: is imporant to chose an optimal segment size to avoid IP level
1856  *   fragmentation along the path.  The common MSS values can be encoded
1857  *   in a 3-bit table.  Uncommon values are captured by the next lower value
1858  *   in the table leading to a slight increase in packetization overhead.
1859  * WSCALE: is necessary to allow large windows to be used for high delay-
1860  *   bandwidth product links.  Not scaling the window when it was initially
1861  *   negotiated is bad for performance as lack of scaling further decreases
1862  *   the apparent available send window.  We only need to encode the WSCALE
1863  *   we received from the remote end.  Our end can be recalculated at any
1864  *   time.  The common WSCALE values can be encoded in a 3-bit table.
1865  *   Uncommon values are captured by the next lower value in the table
1866  *   making us under-estimate the available window size halving our
1867  *   theoretically possible maximum throughput for that connection.
1868  * SACK: Greatly assists in packet loss recovery and requires 1 bit.
1869  * TIMESTAMP and SIGNATURE is not encoded because they are permanent options
1870  *   that are included in all segments on a connection.  We enable them when
1871  *   the ACK has them.
1872  *
1873  * Security of syncookies and attack vectors:
1874  *
1875  * The MAC is computed over (faddr||laddr||fport||lport||irs||flags||secmod)
1876  * together with the gloabl secret to make it unique per connection attempt.
1877  * Thus any change of any of those parameters results in a different MAC output
1878  * in an unpredictable way unless a collision is encountered.  24 bits of the
1879  * MAC are embedded into the ISS.
1880  *
1881  * To prevent replay attacks two rotating global secrets are updated with a
1882  * new random value every 15 seconds.  The life-time of a syncookie is thus
1883  * 15-30 seconds.
1884  *
1885  * Vector 1: Attacking the secret.  This requires finding a weakness in the
1886  * MAC itself or the way it is used here.  The attacker can do a chosen plain
1887  * text attack by varying and testing the all parameters under his control.
1888  * The strength depends on the size and randomness of the secret, and the
1889  * cryptographic security of the MAC function.  Due to the constant updating
1890  * of the secret the attacker has at most 29.999 seconds to find the secret
1891  * and launch spoofed connections.  After that he has to start all over again.
1892  *
1893  * Vector 2: Collision attack on the MAC of a single ACK.  With a 24 bit MAC
1894  * size an average of 4,823 attempts are required for a 50% chance of success
1895  * to spoof a single syncookie (birthday collision paradox).  However the
1896  * attacker is blind and doesn't know if one of his attempts succeeded unless
1897  * he has a side channel to interfere success from.  A single connection setup
1898  * success average of 90% requires 8,790 packets, 99.99% requires 17,578 packets.
1899  * This many attempts are required for each one blind spoofed connection.  For
1900  * every additional spoofed connection he has to launch another N attempts.
1901  * Thus for a sustained rate 100 spoofed connections per second approximately
1902  * 1,800,000 packets per second would have to be sent.
1903  *
1904  * NB: The MAC function should be fast so that it doesn't become a CPU
1905  * exhaustion attack vector itself.
1906  *
1907  * References:
1908  *  RFC4987 TCP SYN Flooding Attacks and Common Mitigations
1909  *  SYN cookies were first proposed by cryptographer Dan J. Bernstein in 1996
1910  *   http://cr.yp.to/syncookies.html    (overview)
1911  *   http://cr.yp.to/syncookies/archive (details)
1912  *
1913  *
1914  * Schematic construction of a syncookie enabled Initial Sequence Number:
1915  *  0        1         2         3
1916  *  12345678901234567890123456789012
1917  * |xxxxxxxxxxxxxxxxxxxxxxxxWWWMMMSP|
1918  *
1919  *  x 24 MAC (truncated)
1920  *  W  3 Send Window Scale index
1921  *  M  3 MSS index
1922  *  S  1 SACK permitted
1923  *  P  1 Odd/even secret
1924  */
1925 
1926 /*
1927  * Distribution and probability of certain MSS values.  Those in between are
1928  * rounded down to the next lower one.
1929  * [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011]
1930  *                            .2%  .3%   5%    7%    7%    20%   15%   45%
1931  */
1932 static int tcp_sc_msstab[] = { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };
1933 
1934 /*
1935  * Distribution and probability of certain WSCALE values.  We have to map the
1936  * (send) window scale (shift) option with a range of 0-14 from 4 bits into 3
1937  * bits based on prevalence of certain values.  Where we don't have an exact
1938  * match for are rounded down to the next lower one letting us under-estimate
1939  * the true available window.  At the moment this would happen only for the
1940  * very uncommon values 3, 5 and those above 8 (more than 16MB socket buffer
1941  * and window size).  The absence of the WSCALE option (no scaling in either
1942  * direction) is encoded with index zero.
1943  * [WSCALE values histograms, Allman, 2012]
1944  *                            X 10 10 35  5  6 14 10%   by host
1945  *                            X 11  4  5  5 18 49  3%   by connections
1946  */
1947 static int tcp_sc_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };
1948 
1949 /*
1950  * Compute the MAC for the SYN cookie.  SIPHASH-2-4 is chosen for its speed
1951  * and good cryptographic properties.
1952  */
1953 static uint32_t
1954 syncookie_mac(struct in_conninfo *inc, tcp_seq irs, uint8_t flags,
1955     uint8_t *secbits, uintptr_t secmod)
1956 {
1957 	SIPHASH_CTX ctx;
1958 	uint32_t siphash[2];
1959 
1960 	SipHash24_Init(&ctx);
1961 	SipHash_SetKey(&ctx, secbits);
1962 	switch (inc->inc_flags & INC_ISIPV6) {
1963 #ifdef INET
1964 	case 0:
1965 		SipHash_Update(&ctx, &inc->inc_faddr, sizeof(inc->inc_faddr));
1966 		SipHash_Update(&ctx, &inc->inc_laddr, sizeof(inc->inc_laddr));
1967 		break;
1968 #endif
1969 #ifdef INET6
1970 	case INC_ISIPV6:
1971 		SipHash_Update(&ctx, &inc->inc6_faddr, sizeof(inc->inc6_faddr));
1972 		SipHash_Update(&ctx, &inc->inc6_laddr, sizeof(inc->inc6_laddr));
1973 		break;
1974 #endif
1975 	}
1976 	SipHash_Update(&ctx, &inc->inc_fport, sizeof(inc->inc_fport));
1977 	SipHash_Update(&ctx, &inc->inc_lport, sizeof(inc->inc_lport));
1978 	SipHash_Update(&ctx, &irs, sizeof(irs));
1979 	SipHash_Update(&ctx, &flags, sizeof(flags));
1980 	SipHash_Update(&ctx, &secmod, sizeof(secmod));
1981 	SipHash_Final((u_int8_t *)&siphash, &ctx);
1982 
1983 	return (siphash[0] ^ siphash[1]);
1984 }
1985 
1986 static tcp_seq
1987 syncookie_generate(struct syncache_head *sch, struct syncache *sc)
1988 {
1989 	u_int i, secbit, wscale;
1990 	uint32_t iss, hash;
1991 	uint8_t *secbits;
1992 	union syncookie cookie;
1993 
1994 	SCH_LOCK_ASSERT(sch);
1995 
1996 	cookie.cookie = 0;
1997 
1998 	/* Map our computed MSS into the 3-bit index. */
1999 	for (i = nitems(tcp_sc_msstab) - 1;
2000 	     tcp_sc_msstab[i] > sc->sc_peer_mss && i > 0;
2001 	     i--)
2002 		;
2003 	cookie.flags.mss_idx = i;
2004 
2005 	/*
2006 	 * Map the send window scale into the 3-bit index but only if
2007 	 * the wscale option was received.
2008 	 */
2009 	if (sc->sc_flags & SCF_WINSCALE) {
2010 		wscale = sc->sc_requested_s_scale;
2011 		for (i = nitems(tcp_sc_wstab) - 1;
2012 		    tcp_sc_wstab[i] > wscale && i > 0;
2013 		     i--)
2014 			;
2015 		cookie.flags.wscale_idx = i;
2016 	}
2017 
2018 	/* Can we do SACK? */
2019 	if (sc->sc_flags & SCF_SACK)
2020 		cookie.flags.sack_ok = 1;
2021 
2022 	/* Which of the two secrets to use. */
2023 	secbit = sch->sch_sc->secret.oddeven & 0x1;
2024 	cookie.flags.odd_even = secbit;
2025 
2026 	secbits = sch->sch_sc->secret.key[secbit];
2027 	hash = syncookie_mac(&sc->sc_inc, sc->sc_irs, cookie.cookie, secbits,
2028 	    (uintptr_t)sch);
2029 
2030 	/*
2031 	 * Put the flags into the hash and XOR them to get better ISS number
2032 	 * variance.  This doesn't enhance the cryptographic strength and is
2033 	 * done to prevent the 8 cookie bits from showing up directly on the
2034 	 * wire.
2035 	 */
2036 	iss = hash & ~0xff;
2037 	iss |= cookie.cookie ^ (hash >> 24);
2038 
2039 	/* Randomize the timestamp. */
2040 	if (sc->sc_flags & SCF_TIMESTAMP) {
2041 		sc->sc_ts = arc4random();
2042 		sc->sc_tsoff = sc->sc_ts - tcp_ts_getticks();
2043 	}
2044 
2045 	TCPSTAT_INC(tcps_sc_sendcookie);
2046 	return (iss);
2047 }
2048 
2049 static struct syncache *
2050 syncookie_lookup(struct in_conninfo *inc, struct syncache_head *sch,
2051     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
2052     struct socket *lso)
2053 {
2054 	uint32_t hash;
2055 	uint8_t *secbits;
2056 	tcp_seq ack, seq;
2057 	int wnd, wscale = 0;
2058 	union syncookie cookie;
2059 
2060 	SCH_LOCK_ASSERT(sch);
2061 
2062 	/*
2063 	 * Pull information out of SYN-ACK/ACK and revert sequence number
2064 	 * advances.
2065 	 */
2066 	ack = th->th_ack - 1;
2067 	seq = th->th_seq - 1;
2068 
2069 	/*
2070 	 * Unpack the flags containing enough information to restore the
2071 	 * connection.
2072 	 */
2073 	cookie.cookie = (ack & 0xff) ^ (ack >> 24);
2074 
2075 	/* Which of the two secrets to use. */
2076 	secbits = sch->sch_sc->secret.key[cookie.flags.odd_even];
2077 
2078 	hash = syncookie_mac(inc, seq, cookie.cookie, secbits, (uintptr_t)sch);
2079 
2080 	/* The recomputed hash matches the ACK if this was a genuine cookie. */
2081 	if ((ack & ~0xff) != (hash & ~0xff))
2082 		return (NULL);
2083 
2084 	/* Fill in the syncache values. */
2085 	sc->sc_flags = 0;
2086 	bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
2087 	sc->sc_ipopts = NULL;
2088 
2089 	sc->sc_irs = seq;
2090 	sc->sc_iss = ack;
2091 
2092 	switch (inc->inc_flags & INC_ISIPV6) {
2093 #ifdef INET
2094 	case 0:
2095 		sc->sc_ip_ttl = sotoinpcb(lso)->inp_ip_ttl;
2096 		sc->sc_ip_tos = sotoinpcb(lso)->inp_ip_tos;
2097 		break;
2098 #endif
2099 #ifdef INET6
2100 	case INC_ISIPV6:
2101 		if (sotoinpcb(lso)->inp_flags & IN6P_AUTOFLOWLABEL)
2102 			sc->sc_flowlabel = sc->sc_iss & IPV6_FLOWLABEL_MASK;
2103 		break;
2104 #endif
2105 	}
2106 
2107 	sc->sc_peer_mss = tcp_sc_msstab[cookie.flags.mss_idx];
2108 
2109 	/* We can simply recompute receive window scale we sent earlier. */
2110 	while (wscale < TCP_MAX_WINSHIFT && (TCP_MAXWIN << wscale) < sb_max)
2111 		wscale++;
2112 
2113 	/* Only use wscale if it was enabled in the orignal SYN. */
2114 	if (cookie.flags.wscale_idx > 0) {
2115 		sc->sc_requested_r_scale = wscale;
2116 		sc->sc_requested_s_scale = tcp_sc_wstab[cookie.flags.wscale_idx];
2117 		sc->sc_flags |= SCF_WINSCALE;
2118 	}
2119 
2120 	wnd = lso->sol_sbrcv_hiwat;
2121 	wnd = imax(wnd, 0);
2122 	wnd = imin(wnd, TCP_MAXWIN);
2123 	sc->sc_wnd = wnd;
2124 
2125 	if (cookie.flags.sack_ok)
2126 		sc->sc_flags |= SCF_SACK;
2127 
2128 	if (to->to_flags & TOF_TS) {
2129 		sc->sc_flags |= SCF_TIMESTAMP;
2130 		sc->sc_tsreflect = to->to_tsval;
2131 		sc->sc_ts = to->to_tsecr;
2132 		sc->sc_tsoff = to->to_tsecr - tcp_ts_getticks();
2133 	}
2134 
2135 	if (to->to_flags & TOF_SIGNATURE)
2136 		sc->sc_flags |= SCF_SIGNATURE;
2137 
2138 	sc->sc_rxmits = 0;
2139 
2140 	TCPSTAT_INC(tcps_sc_recvcookie);
2141 	return (sc);
2142 }
2143 
2144 #ifdef INVARIANTS
2145 static int
2146 syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
2147     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
2148     struct socket *lso)
2149 {
2150 	struct syncache scs, *scx;
2151 	char *s;
2152 
2153 	bzero(&scs, sizeof(scs));
2154 	scx = syncookie_lookup(inc, sch, &scs, th, to, lso);
2155 
2156 	if ((s = tcp_log_addrs(inc, th, NULL, NULL)) == NULL)
2157 		return (0);
2158 
2159 	if (scx != NULL) {
2160 		if (sc->sc_peer_mss != scx->sc_peer_mss)
2161 			log(LOG_DEBUG, "%s; %s: mss different %i vs %i\n",
2162 			    s, __func__, sc->sc_peer_mss, scx->sc_peer_mss);
2163 
2164 		if (sc->sc_requested_r_scale != scx->sc_requested_r_scale)
2165 			log(LOG_DEBUG, "%s; %s: rwscale different %i vs %i\n",
2166 			    s, __func__, sc->sc_requested_r_scale,
2167 			    scx->sc_requested_r_scale);
2168 
2169 		if (sc->sc_requested_s_scale != scx->sc_requested_s_scale)
2170 			log(LOG_DEBUG, "%s; %s: swscale different %i vs %i\n",
2171 			    s, __func__, sc->sc_requested_s_scale,
2172 			    scx->sc_requested_s_scale);
2173 
2174 		if ((sc->sc_flags & SCF_SACK) != (scx->sc_flags & SCF_SACK))
2175 			log(LOG_DEBUG, "%s; %s: SACK different\n", s, __func__);
2176 	}
2177 
2178 	if (s != NULL)
2179 		free(s, M_TCPLOG);
2180 	return (0);
2181 }
2182 #endif /* INVARIANTS */
2183 
2184 static void
2185 syncookie_reseed(void *arg)
2186 {
2187 	struct tcp_syncache *sc = arg;
2188 	uint8_t *secbits;
2189 	int secbit;
2190 
2191 	/*
2192 	 * Reseeding the secret doesn't have to be protected by a lock.
2193 	 * It only must be ensured that the new random values are visible
2194 	 * to all CPUs in a SMP environment.  The atomic with release
2195 	 * semantics ensures that.
2196 	 */
2197 	secbit = (sc->secret.oddeven & 0x1) ? 0 : 1;
2198 	secbits = sc->secret.key[secbit];
2199 	arc4rand(secbits, SYNCOOKIE_SECRET_SIZE, 0);
2200 	atomic_add_rel_int(&sc->secret.oddeven, 1);
2201 
2202 	/* Reschedule ourself. */
2203 	callout_schedule(&sc->secret.reseed, SYNCOOKIE_LIFETIME * hz);
2204 }
2205 
2206 /*
2207  * Exports the syncache entries to userland so that netstat can display
2208  * them alongside the other sockets.  This function is intended to be
2209  * called only from tcp_pcblist.
2210  *
2211  * Due to concurrency on an active system, the number of pcbs exported
2212  * may have no relation to max_pcbs.  max_pcbs merely indicates the
2213  * amount of space the caller allocated for this function to use.
2214  */
2215 int
2216 syncache_pcblist(struct sysctl_req *req, int max_pcbs, int *pcbs_exported)
2217 {
2218 	struct xtcpcb xt;
2219 	struct syncache *sc;
2220 	struct syncache_head *sch;
2221 	int count, error, i;
2222 
2223 	for (count = 0, error = 0, i = 0; i < V_tcp_syncache.hashsize; i++) {
2224 		sch = &V_tcp_syncache.hashbase[i];
2225 		SCH_LOCK(sch);
2226 		TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
2227 			if (count >= max_pcbs) {
2228 				SCH_UNLOCK(sch);
2229 				goto exit;
2230 			}
2231 			if (cr_cansee(req->td->td_ucred, sc->sc_cred) != 0)
2232 				continue;
2233 			bzero(&xt, sizeof(xt));
2234 			xt.xt_len = sizeof(xt);
2235 			if (sc->sc_inc.inc_flags & INC_ISIPV6)
2236 				xt.xt_inp.inp_vflag = INP_IPV6;
2237 			else
2238 				xt.xt_inp.inp_vflag = INP_IPV4;
2239 			bcopy(&sc->sc_inc, &xt.xt_inp.inp_inc,
2240 			    sizeof (struct in_conninfo));
2241 			xt.t_state = TCPS_SYN_RECEIVED;
2242 			xt.xt_inp.xi_socket.xso_protocol = IPPROTO_TCP;
2243 			xt.xt_inp.xi_socket.xso_len = sizeof (struct xsocket);
2244 			xt.xt_inp.xi_socket.so_type = SOCK_STREAM;
2245 			xt.xt_inp.xi_socket.so_state = SS_ISCONNECTING;
2246 			error = SYSCTL_OUT(req, &xt, sizeof xt);
2247 			if (error) {
2248 				SCH_UNLOCK(sch);
2249 				goto exit;
2250 			}
2251 			count++;
2252 		}
2253 		SCH_UNLOCK(sch);
2254 	}
2255 exit:
2256 	*pcbs_exported = count;
2257 	return error;
2258 }
2259