xref: /freebsd/sys/netinet/tcp_syncache.c (revision 273c26a3c3bea87a241d6879abd4f991db180bf0)
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("$FreeBSD$");
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/hash.h>
44 #include <sys/refcount.h>
45 #include <sys/kernel.h>
46 #include <sys/sysctl.h>
47 #include <sys/limits.h>
48 #include <sys/lock.h>
49 #include <sys/mutex.h>
50 #include <sys/malloc.h>
51 #include <sys/mbuf.h>
52 #include <sys/proc.h>		/* for proc0 declaration */
53 #include <sys/random.h>
54 #include <sys/socket.h>
55 #include <sys/socketvar.h>
56 #include <sys/syslog.h>
57 #include <sys/ucred.h>
58 
59 #include <sys/md5.h>
60 #include <crypto/siphash/siphash.h>
61 
62 #include <vm/uma.h>
63 
64 #include <net/if.h>
65 #include <net/if_var.h>
66 #include <net/route.h>
67 #include <net/vnet.h>
68 
69 #include <netinet/in.h>
70 #include <netinet/in_systm.h>
71 #include <netinet/ip.h>
72 #include <netinet/in_var.h>
73 #include <netinet/in_pcb.h>
74 #include <netinet/ip_var.h>
75 #include <netinet/ip_options.h>
76 #ifdef INET6
77 #include <netinet/ip6.h>
78 #include <netinet/icmp6.h>
79 #include <netinet6/nd6.h>
80 #include <netinet6/ip6_var.h>
81 #include <netinet6/in6_pcb.h>
82 #endif
83 #include <netinet/tcp.h>
84 #ifdef TCP_RFC7413
85 #include <netinet/tcp_fastopen.h>
86 #endif
87 #include <netinet/tcp_fsm.h>
88 #include <netinet/tcp_seq.h>
89 #include <netinet/tcp_timer.h>
90 #include <netinet/tcp_var.h>
91 #include <netinet/tcp_syncache.h>
92 #ifdef INET6
93 #include <netinet6/tcp6_var.h>
94 #endif
95 #ifdef TCP_OFFLOAD
96 #include <netinet/toecore.h>
97 #endif
98 
99 #ifdef IPSEC
100 #include <netipsec/ipsec.h>
101 #ifdef INET6
102 #include <netipsec/ipsec6.h>
103 #endif
104 #include <netipsec/key.h>
105 #endif /*IPSEC*/
106 
107 #include <machine/in_cksum.h>
108 
109 #include <security/mac/mac_framework.h>
110 
111 static VNET_DEFINE(int, tcp_syncookies) = 1;
112 #define	V_tcp_syncookies		VNET(tcp_syncookies)
113 SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies, CTLFLAG_VNET | CTLFLAG_RW,
114     &VNET_NAME(tcp_syncookies), 0,
115     "Use TCP SYN cookies if the syncache overflows");
116 
117 static VNET_DEFINE(int, tcp_syncookiesonly) = 0;
118 #define	V_tcp_syncookiesonly		VNET(tcp_syncookiesonly)
119 SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies_only, CTLFLAG_VNET | CTLFLAG_RW,
120     &VNET_NAME(tcp_syncookiesonly), 0,
121     "Use only TCP SYN cookies");
122 
123 #ifdef TCP_OFFLOAD
124 #define ADDED_BY_TOE(sc) ((sc)->sc_tod != NULL)
125 #endif
126 
127 static void	 syncache_drop(struct syncache *, struct syncache_head *);
128 static void	 syncache_free(struct syncache *);
129 static void	 syncache_insert(struct syncache *, struct syncache_head *);
130 static int	 syncache_respond(struct syncache *, struct syncache_head *, int,
131 		    const struct mbuf *);
132 static struct	 socket *syncache_socket(struct syncache *, struct socket *,
133 		    struct mbuf *m);
134 static void	 syncache_timeout(struct syncache *sc, struct syncache_head *sch,
135 		    int docallout);
136 static void	 syncache_timer(void *);
137 
138 static uint32_t	 syncookie_mac(struct in_conninfo *, tcp_seq, uint8_t,
139 		    uint8_t *, uintptr_t);
140 static tcp_seq	 syncookie_generate(struct syncache_head *, struct syncache *);
141 static struct syncache
142 		*syncookie_lookup(struct in_conninfo *, struct syncache_head *,
143 		    struct syncache *, struct tcphdr *, struct tcpopt *,
144 		    struct socket *);
145 static void	 syncookie_reseed(void *);
146 #ifdef INVARIANTS
147 static int	 syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
148 		    struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
149 		    struct socket *lso);
150 #endif
151 
152 /*
153  * Transmit the SYN,ACK fewer times than TCP_MAXRXTSHIFT specifies.
154  * 3 retransmits corresponds to a timeout of 3 * (1 + 2 + 4 + 8) == 45 seconds,
155  * the odds are that the user has given up attempting to connect by then.
156  */
157 #define SYNCACHE_MAXREXMTS		3
158 
159 /* Arbitrary values */
160 #define TCP_SYNCACHE_HASHSIZE		512
161 #define TCP_SYNCACHE_BUCKETLIMIT	30
162 
163 static VNET_DEFINE(struct tcp_syncache, tcp_syncache);
164 #define	V_tcp_syncache			VNET(tcp_syncache)
165 
166 static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, syncache, CTLFLAG_RW, 0,
167     "TCP SYN cache");
168 
169 SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, bucketlimit, CTLFLAG_VNET | CTLFLAG_RDTUN,
170     &VNET_NAME(tcp_syncache.bucket_limit), 0,
171     "Per-bucket hash limit for syncache");
172 
173 SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, cachelimit, CTLFLAG_VNET | CTLFLAG_RDTUN,
174     &VNET_NAME(tcp_syncache.cache_limit), 0,
175     "Overall entry limit for syncache");
176 
177 SYSCTL_UMA_CUR(_net_inet_tcp_syncache, OID_AUTO, count, CTLFLAG_VNET,
178     &VNET_NAME(tcp_syncache.zone), "Current number of entries in syncache");
179 
180 SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, hashsize, CTLFLAG_VNET | CTLFLAG_RDTUN,
181     &VNET_NAME(tcp_syncache.hashsize), 0,
182     "Size of TCP syncache hashtable");
183 
184 SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, rexmtlimit, CTLFLAG_VNET | CTLFLAG_RW,
185     &VNET_NAME(tcp_syncache.rexmt_limit), 0,
186     "Limit on SYN/ACK retransmissions");
187 
188 VNET_DEFINE(int, tcp_sc_rst_sock_fail) = 1;
189 SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, rst_on_sock_fail,
190     CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(tcp_sc_rst_sock_fail), 0,
191     "Send reset on socket allocation failure");
192 
193 static MALLOC_DEFINE(M_SYNCACHE, "syncache", "TCP syncache");
194 
195 #define	SCH_LOCK(sch)		mtx_lock(&(sch)->sch_mtx)
196 #define	SCH_UNLOCK(sch)		mtx_unlock(&(sch)->sch_mtx)
197 #define	SCH_LOCK_ASSERT(sch)	mtx_assert(&(sch)->sch_mtx, MA_OWNED)
198 
199 /*
200  * Requires the syncache entry to be already removed from the bucket list.
201  */
202 static void
203 syncache_free(struct syncache *sc)
204 {
205 
206 	if (sc->sc_ipopts)
207 		(void) m_free(sc->sc_ipopts);
208 	if (sc->sc_cred)
209 		crfree(sc->sc_cred);
210 #ifdef MAC
211 	mac_syncache_destroy(&sc->sc_label);
212 #endif
213 
214 	uma_zfree(V_tcp_syncache.zone, sc);
215 }
216 
217 void
218 syncache_init(void)
219 {
220 	int i;
221 
222 	V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
223 	V_tcp_syncache.bucket_limit = TCP_SYNCACHE_BUCKETLIMIT;
224 	V_tcp_syncache.rexmt_limit = SYNCACHE_MAXREXMTS;
225 	V_tcp_syncache.hash_secret = arc4random();
226 
227 	TUNABLE_INT_FETCH("net.inet.tcp.syncache.hashsize",
228 	    &V_tcp_syncache.hashsize);
229 	TUNABLE_INT_FETCH("net.inet.tcp.syncache.bucketlimit",
230 	    &V_tcp_syncache.bucket_limit);
231 	if (!powerof2(V_tcp_syncache.hashsize) ||
232 	    V_tcp_syncache.hashsize == 0) {
233 		printf("WARNING: syncache hash size is not a power of 2.\n");
234 		V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
235 	}
236 	V_tcp_syncache.hashmask = V_tcp_syncache.hashsize - 1;
237 
238 	/* Set limits. */
239 	V_tcp_syncache.cache_limit =
240 	    V_tcp_syncache.hashsize * V_tcp_syncache.bucket_limit;
241 	TUNABLE_INT_FETCH("net.inet.tcp.syncache.cachelimit",
242 	    &V_tcp_syncache.cache_limit);
243 
244 	/* Allocate the hash table. */
245 	V_tcp_syncache.hashbase = malloc(V_tcp_syncache.hashsize *
246 	    sizeof(struct syncache_head), M_SYNCACHE, M_WAITOK | M_ZERO);
247 
248 #ifdef VIMAGE
249 	V_tcp_syncache.vnet = curvnet;
250 #endif
251 
252 	/* Initialize the hash buckets. */
253 	for (i = 0; i < V_tcp_syncache.hashsize; i++) {
254 		TAILQ_INIT(&V_tcp_syncache.hashbase[i].sch_bucket);
255 		mtx_init(&V_tcp_syncache.hashbase[i].sch_mtx, "tcp_sc_head",
256 			 NULL, MTX_DEF);
257 		callout_init_mtx(&V_tcp_syncache.hashbase[i].sch_timer,
258 			 &V_tcp_syncache.hashbase[i].sch_mtx, 0);
259 		V_tcp_syncache.hashbase[i].sch_length = 0;
260 		V_tcp_syncache.hashbase[i].sch_sc = &V_tcp_syncache;
261 	}
262 
263 	/* Create the syncache entry zone. */
264 	V_tcp_syncache.zone = uma_zcreate("syncache", sizeof(struct syncache),
265 	    NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
266 	V_tcp_syncache.cache_limit = uma_zone_set_max(V_tcp_syncache.zone,
267 	    V_tcp_syncache.cache_limit);
268 
269 	/* Start the SYN cookie reseeder callout. */
270 	callout_init(&V_tcp_syncache.secret.reseed, 1);
271 	arc4rand(V_tcp_syncache.secret.key[0], SYNCOOKIE_SECRET_SIZE, 0);
272 	arc4rand(V_tcp_syncache.secret.key[1], SYNCOOKIE_SECRET_SIZE, 0);
273 	callout_reset(&V_tcp_syncache.secret.reseed, SYNCOOKIE_LIFETIME * hz,
274 	    syncookie_reseed, &V_tcp_syncache);
275 }
276 
277 #ifdef VIMAGE
278 void
279 syncache_destroy(void)
280 {
281 	struct syncache_head *sch;
282 	struct syncache *sc, *nsc;
283 	int i;
284 
285 	/*
286 	 * Stop the re-seed timer before freeing resources.  No need to
287 	 * possibly schedule it another time.
288 	 */
289 	callout_drain(&V_tcp_syncache.secret.reseed);
290 
291 	/* Cleanup hash buckets: stop timers, free entries, destroy locks. */
292 	for (i = 0; i < V_tcp_syncache.hashsize; i++) {
293 
294 		sch = &V_tcp_syncache.hashbase[i];
295 		callout_drain(&sch->sch_timer);
296 
297 		SCH_LOCK(sch);
298 		TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc)
299 			syncache_drop(sc, sch);
300 		SCH_UNLOCK(sch);
301 		KASSERT(TAILQ_EMPTY(&sch->sch_bucket),
302 		    ("%s: sch->sch_bucket not empty", __func__));
303 		KASSERT(sch->sch_length == 0, ("%s: sch->sch_length %d not 0",
304 		    __func__, sch->sch_length));
305 		mtx_destroy(&sch->sch_mtx);
306 	}
307 
308 	KASSERT(uma_zone_get_cur(V_tcp_syncache.zone) == 0,
309 	    ("%s: cache_count not 0", __func__));
310 
311 	/* Free the allocated global resources. */
312 	uma_zdestroy(V_tcp_syncache.zone);
313 	free(V_tcp_syncache.hashbase, M_SYNCACHE);
314 }
315 #endif
316 
317 /*
318  * Inserts a syncache entry into the specified bucket row.
319  * Locks and unlocks the syncache_head autonomously.
320  */
321 static void
322 syncache_insert(struct syncache *sc, struct syncache_head *sch)
323 {
324 	struct syncache *sc2;
325 
326 	SCH_LOCK(sch);
327 
328 	/*
329 	 * Make sure that we don't overflow the per-bucket limit.
330 	 * If the bucket is full, toss the oldest element.
331 	 */
332 	if (sch->sch_length >= V_tcp_syncache.bucket_limit) {
333 		KASSERT(!TAILQ_EMPTY(&sch->sch_bucket),
334 			("sch->sch_length incorrect"));
335 		sc2 = TAILQ_LAST(&sch->sch_bucket, sch_head);
336 		syncache_drop(sc2, sch);
337 		TCPSTAT_INC(tcps_sc_bucketoverflow);
338 	}
339 
340 	/* Put it into the bucket. */
341 	TAILQ_INSERT_HEAD(&sch->sch_bucket, sc, sc_hash);
342 	sch->sch_length++;
343 
344 #ifdef TCP_OFFLOAD
345 	if (ADDED_BY_TOE(sc)) {
346 		struct toedev *tod = sc->sc_tod;
347 
348 		tod->tod_syncache_added(tod, sc->sc_todctx);
349 	}
350 #endif
351 
352 	/* Reinitialize the bucket row's timer. */
353 	if (sch->sch_length == 1)
354 		sch->sch_nextc = ticks + INT_MAX;
355 	syncache_timeout(sc, sch, 1);
356 
357 	SCH_UNLOCK(sch);
358 
359 	TCPSTATES_INC(TCPS_SYN_RECEIVED);
360 	TCPSTAT_INC(tcps_sc_added);
361 }
362 
363 /*
364  * Remove and free entry from syncache bucket row.
365  * Expects locked syncache head.
366  */
367 static void
368 syncache_drop(struct syncache *sc, struct syncache_head *sch)
369 {
370 
371 	SCH_LOCK_ASSERT(sch);
372 
373 	TCPSTATES_DEC(TCPS_SYN_RECEIVED);
374 	TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
375 	sch->sch_length--;
376 
377 #ifdef TCP_OFFLOAD
378 	if (ADDED_BY_TOE(sc)) {
379 		struct toedev *tod = sc->sc_tod;
380 
381 		tod->tod_syncache_removed(tod, sc->sc_todctx);
382 	}
383 #endif
384 
385 	syncache_free(sc);
386 }
387 
388 /*
389  * Engage/reengage time on bucket row.
390  */
391 static void
392 syncache_timeout(struct syncache *sc, struct syncache_head *sch, int docallout)
393 {
394 	sc->sc_rxttime = ticks +
395 		TCPTV_RTOBASE * (tcp_syn_backoff[sc->sc_rxmits]);
396 	sc->sc_rxmits++;
397 	if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc)) {
398 		sch->sch_nextc = sc->sc_rxttime;
399 		if (docallout)
400 			callout_reset(&sch->sch_timer, sch->sch_nextc - ticks,
401 			    syncache_timer, (void *)sch);
402 	}
403 }
404 
405 /*
406  * Walk the timer queues, looking for SYN,ACKs that need to be retransmitted.
407  * If we have retransmitted an entry the maximum number of times, expire it.
408  * One separate timer for each bucket row.
409  */
410 static void
411 syncache_timer(void *xsch)
412 {
413 	struct syncache_head *sch = (struct syncache_head *)xsch;
414 	struct syncache *sc, *nsc;
415 	int tick = ticks;
416 	char *s;
417 
418 	CURVNET_SET(sch->sch_sc->vnet);
419 
420 	/* NB: syncache_head has already been locked by the callout. */
421 	SCH_LOCK_ASSERT(sch);
422 
423 	/*
424 	 * In the following cycle we may remove some entries and/or
425 	 * advance some timeouts, so re-initialize the bucket timer.
426 	 */
427 	sch->sch_nextc = tick + INT_MAX;
428 
429 	TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc) {
430 		/*
431 		 * We do not check if the listen socket still exists
432 		 * and accept the case where the listen socket may be
433 		 * gone by the time we resend the SYN/ACK.  We do
434 		 * not expect this to happens often. If it does,
435 		 * then the RST will be sent by the time the remote
436 		 * host does the SYN/ACK->ACK.
437 		 */
438 		if (TSTMP_GT(sc->sc_rxttime, tick)) {
439 			if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc))
440 				sch->sch_nextc = sc->sc_rxttime;
441 			continue;
442 		}
443 		if (sc->sc_rxmits > V_tcp_syncache.rexmt_limit) {
444 			if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
445 				log(LOG_DEBUG, "%s; %s: Retransmits exhausted, "
446 				    "giving up and removing syncache entry\n",
447 				    s, __func__);
448 				free(s, M_TCPLOG);
449 			}
450 			syncache_drop(sc, sch);
451 			TCPSTAT_INC(tcps_sc_stale);
452 			continue;
453 		}
454 		if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
455 			log(LOG_DEBUG, "%s; %s: Response timeout, "
456 			    "retransmitting (%u) SYN|ACK\n",
457 			    s, __func__, sc->sc_rxmits);
458 			free(s, M_TCPLOG);
459 		}
460 
461 		syncache_respond(sc, sch, 1, NULL);
462 		TCPSTAT_INC(tcps_sc_retransmitted);
463 		syncache_timeout(sc, sch, 0);
464 	}
465 	if (!TAILQ_EMPTY(&(sch)->sch_bucket))
466 		callout_reset(&(sch)->sch_timer, (sch)->sch_nextc - tick,
467 			syncache_timer, (void *)(sch));
468 	CURVNET_RESTORE();
469 }
470 
471 /*
472  * Find an entry in the syncache.
473  * Returns always with locked syncache_head plus a matching entry or NULL.
474  */
475 static struct syncache *
476 syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp)
477 {
478 	struct syncache *sc;
479 	struct syncache_head *sch;
480 	uint32_t hash;
481 
482 	/*
483 	 * The hash is built on foreign port + local port + foreign address.
484 	 * We rely on the fact that struct in_conninfo starts with 16 bits
485 	 * of foreign port, then 16 bits of local port then followed by 128
486 	 * bits of foreign address.  In case of IPv4 address, the first 3
487 	 * 32-bit words of the address always are zeroes.
488 	 */
489 	hash = jenkins_hash32((uint32_t *)&inc->inc_ie, 5,
490 	    V_tcp_syncache.hash_secret) & V_tcp_syncache.hashmask;
491 
492 	sch = &V_tcp_syncache.hashbase[hash];
493 	*schp = sch;
494 	SCH_LOCK(sch);
495 
496 	/* Circle through bucket row to find matching entry. */
497 	TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash)
498 		if (bcmp(&inc->inc_ie, &sc->sc_inc.inc_ie,
499 		    sizeof(struct in_endpoints)) == 0)
500 			break;
501 
502 	return (sc);	/* Always returns with locked sch. */
503 }
504 
505 /*
506  * This function is called when we get a RST for a
507  * non-existent connection, so that we can see if the
508  * connection is in the syn cache.  If it is, zap it.
509  */
510 void
511 syncache_chkrst(struct in_conninfo *inc, struct tcphdr *th)
512 {
513 	struct syncache *sc;
514 	struct syncache_head *sch;
515 	char *s = NULL;
516 
517 	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
518 	SCH_LOCK_ASSERT(sch);
519 
520 	/*
521 	 * Any RST to our SYN|ACK must not carry ACK, SYN or FIN flags.
522 	 * See RFC 793 page 65, section SEGMENT ARRIVES.
523 	 */
524 	if (th->th_flags & (TH_ACK|TH_SYN|TH_FIN)) {
525 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
526 			log(LOG_DEBUG, "%s; %s: Spurious RST with ACK, SYN or "
527 			    "FIN flag set, segment ignored\n", s, __func__);
528 		TCPSTAT_INC(tcps_badrst);
529 		goto done;
530 	}
531 
532 	/*
533 	 * No corresponding connection was found in syncache.
534 	 * If syncookies are enabled and possibly exclusively
535 	 * used, or we are under memory pressure, a valid RST
536 	 * may not find a syncache entry.  In that case we're
537 	 * done and no SYN|ACK retransmissions will happen.
538 	 * Otherwise the RST was misdirected or spoofed.
539 	 */
540 	if (sc == NULL) {
541 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
542 			log(LOG_DEBUG, "%s; %s: Spurious RST without matching "
543 			    "syncache entry (possibly syncookie only), "
544 			    "segment ignored\n", s, __func__);
545 		TCPSTAT_INC(tcps_badrst);
546 		goto done;
547 	}
548 
549 	/*
550 	 * If the RST bit is set, check the sequence number to see
551 	 * if this is a valid reset segment.
552 	 * RFC 793 page 37:
553 	 *   In all states except SYN-SENT, all reset (RST) segments
554 	 *   are validated by checking their SEQ-fields.  A reset is
555 	 *   valid if its sequence number is in the window.
556 	 *
557 	 *   The sequence number in the reset segment is normally an
558 	 *   echo of our outgoing acknowlegement numbers, but some hosts
559 	 *   send a reset with the sequence number at the rightmost edge
560 	 *   of our receive window, and we have to handle this case.
561 	 */
562 	if (SEQ_GEQ(th->th_seq, sc->sc_irs) &&
563 	    SEQ_LEQ(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
564 		syncache_drop(sc, sch);
565 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
566 			log(LOG_DEBUG, "%s; %s: Our SYN|ACK was rejected, "
567 			    "connection attempt aborted by remote endpoint\n",
568 			    s, __func__);
569 		TCPSTAT_INC(tcps_sc_reset);
570 	} else {
571 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
572 			log(LOG_DEBUG, "%s; %s: RST with invalid SEQ %u != "
573 			    "IRS %u (+WND %u), segment ignored\n",
574 			    s, __func__, th->th_seq, sc->sc_irs, sc->sc_wnd);
575 		TCPSTAT_INC(tcps_badrst);
576 	}
577 
578 done:
579 	if (s != NULL)
580 		free(s, M_TCPLOG);
581 	SCH_UNLOCK(sch);
582 }
583 
584 void
585 syncache_badack(struct in_conninfo *inc)
586 {
587 	struct syncache *sc;
588 	struct syncache_head *sch;
589 
590 	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
591 	SCH_LOCK_ASSERT(sch);
592 	if (sc != NULL) {
593 		syncache_drop(sc, sch);
594 		TCPSTAT_INC(tcps_sc_badack);
595 	}
596 	SCH_UNLOCK(sch);
597 }
598 
599 void
600 syncache_unreach(struct in_conninfo *inc, struct tcphdr *th)
601 {
602 	struct syncache *sc;
603 	struct syncache_head *sch;
604 
605 	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
606 	SCH_LOCK_ASSERT(sch);
607 	if (sc == NULL)
608 		goto done;
609 
610 	/* If the sequence number != sc_iss, then it's a bogus ICMP msg */
611 	if (ntohl(th->th_seq) != sc->sc_iss)
612 		goto done;
613 
614 	/*
615 	 * If we've rertransmitted 3 times and this is our second error,
616 	 * we remove the entry.  Otherwise, we allow it to continue on.
617 	 * This prevents us from incorrectly nuking an entry during a
618 	 * spurious network outage.
619 	 *
620 	 * See tcp_notify().
621 	 */
622 	if ((sc->sc_flags & SCF_UNREACH) == 0 || sc->sc_rxmits < 3 + 1) {
623 		sc->sc_flags |= SCF_UNREACH;
624 		goto done;
625 	}
626 	syncache_drop(sc, sch);
627 	TCPSTAT_INC(tcps_sc_unreach);
628 done:
629 	SCH_UNLOCK(sch);
630 }
631 
632 /*
633  * Build a new TCP socket structure from a syncache entry.
634  *
635  * On success return the newly created socket with its underlying inp locked.
636  */
637 static struct socket *
638 syncache_socket(struct syncache *sc, struct socket *lso, struct mbuf *m)
639 {
640 	struct tcp_function_block *blk;
641 	struct inpcb *inp = NULL;
642 	struct socket *so;
643 	struct tcpcb *tp;
644 	int error;
645 	char *s;
646 
647 	INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
648 
649 	/*
650 	 * Ok, create the full blown connection, and set things up
651 	 * as they would have been set up if we had created the
652 	 * connection when the SYN arrived.  If we can't create
653 	 * the connection, abort it.
654 	 */
655 	so = sonewconn(lso, 0);
656 	if (so == NULL) {
657 		/*
658 		 * Drop the connection; we will either send a RST or
659 		 * have the peer retransmit its SYN again after its
660 		 * RTO and try again.
661 		 */
662 		TCPSTAT_INC(tcps_listendrop);
663 		if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
664 			log(LOG_DEBUG, "%s; %s: Socket create failed "
665 			    "due to limits or memory shortage\n",
666 			    s, __func__);
667 			free(s, M_TCPLOG);
668 		}
669 		goto abort2;
670 	}
671 #ifdef MAC
672 	mac_socketpeer_set_from_mbuf(m, so);
673 #endif
674 
675 	inp = sotoinpcb(so);
676 	inp->inp_inc.inc_fibnum = so->so_fibnum;
677 	INP_WLOCK(inp);
678 	/*
679 	 * Exclusive pcbinfo lock is not required in syncache socket case even
680 	 * if two inpcb locks can be acquired simultaneously:
681 	 *  - the inpcb in LISTEN state,
682 	 *  - the newly created inp.
683 	 *
684 	 * In this case, an inp cannot be at same time in LISTEN state and
685 	 * just created by an accept() call.
686 	 */
687 	INP_HASH_WLOCK(&V_tcbinfo);
688 
689 	/* Insert new socket into PCB hash list. */
690 	inp->inp_inc.inc_flags = sc->sc_inc.inc_flags;
691 #ifdef INET6
692 	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
693 		inp->in6p_laddr = sc->sc_inc.inc6_laddr;
694 	} else {
695 		inp->inp_vflag &= ~INP_IPV6;
696 		inp->inp_vflag |= INP_IPV4;
697 #endif
698 		inp->inp_laddr = sc->sc_inc.inc_laddr;
699 #ifdef INET6
700 	}
701 #endif
702 
703 	/*
704 	 * If there's an mbuf and it has a flowid, then let's initialise the
705 	 * inp with that particular flowid.
706 	 */
707 	if (m != NULL && M_HASHTYPE_GET(m) != M_HASHTYPE_NONE) {
708 		inp->inp_flowid = m->m_pkthdr.flowid;
709 		inp->inp_flowtype = M_HASHTYPE_GET(m);
710 	}
711 
712 	/*
713 	 * Install in the reservation hash table for now, but don't yet
714 	 * install a connection group since the full 4-tuple isn't yet
715 	 * configured.
716 	 */
717 	inp->inp_lport = sc->sc_inc.inc_lport;
718 	if ((error = in_pcbinshash_nopcbgroup(inp)) != 0) {
719 		/*
720 		 * Undo the assignments above if we failed to
721 		 * put the PCB on the hash lists.
722 		 */
723 #ifdef INET6
724 		if (sc->sc_inc.inc_flags & INC_ISIPV6)
725 			inp->in6p_laddr = in6addr_any;
726 		else
727 #endif
728 			inp->inp_laddr.s_addr = INADDR_ANY;
729 		inp->inp_lport = 0;
730 		if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
731 			log(LOG_DEBUG, "%s; %s: in_pcbinshash failed "
732 			    "with error %i\n",
733 			    s, __func__, error);
734 			free(s, M_TCPLOG);
735 		}
736 		INP_HASH_WUNLOCK(&V_tcbinfo);
737 		goto abort;
738 	}
739 #ifdef IPSEC
740 	/* Copy old policy into new socket's. */
741 	if (ipsec_copy_policy(sotoinpcb(lso)->inp_sp, inp->inp_sp))
742 		printf("syncache_socket: could not copy policy\n");
743 #endif
744 #ifdef INET6
745 	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
746 		struct inpcb *oinp = sotoinpcb(lso);
747 		struct in6_addr laddr6;
748 		struct sockaddr_in6 sin6;
749 		/*
750 		 * Inherit socket options from the listening socket.
751 		 * Note that in6p_inputopts are not (and should not be)
752 		 * copied, since it stores previously received options and is
753 		 * used to detect if each new option is different than the
754 		 * previous one and hence should be passed to a user.
755 		 * If we copied in6p_inputopts, a user would not be able to
756 		 * receive options just after calling the accept system call.
757 		 */
758 		inp->inp_flags |= oinp->inp_flags & INP_CONTROLOPTS;
759 		if (oinp->in6p_outputopts)
760 			inp->in6p_outputopts =
761 			    ip6_copypktopts(oinp->in6p_outputopts, M_NOWAIT);
762 
763 		sin6.sin6_family = AF_INET6;
764 		sin6.sin6_len = sizeof(sin6);
765 		sin6.sin6_addr = sc->sc_inc.inc6_faddr;
766 		sin6.sin6_port = sc->sc_inc.inc_fport;
767 		sin6.sin6_flowinfo = sin6.sin6_scope_id = 0;
768 		laddr6 = inp->in6p_laddr;
769 		if (IN6_IS_ADDR_UNSPECIFIED(&inp->in6p_laddr))
770 			inp->in6p_laddr = sc->sc_inc.inc6_laddr;
771 		if ((error = in6_pcbconnect_mbuf(inp, (struct sockaddr *)&sin6,
772 		    thread0.td_ucred, m)) != 0) {
773 			inp->in6p_laddr = laddr6;
774 			if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
775 				log(LOG_DEBUG, "%s; %s: in6_pcbconnect failed "
776 				    "with error %i\n",
777 				    s, __func__, error);
778 				free(s, M_TCPLOG);
779 			}
780 			INP_HASH_WUNLOCK(&V_tcbinfo);
781 			goto abort;
782 		}
783 		/* Override flowlabel from in6_pcbconnect. */
784 		inp->inp_flow &= ~IPV6_FLOWLABEL_MASK;
785 		inp->inp_flow |= sc->sc_flowlabel;
786 	}
787 #endif /* INET6 */
788 #if defined(INET) && defined(INET6)
789 	else
790 #endif
791 #ifdef INET
792 	{
793 		struct in_addr laddr;
794 		struct sockaddr_in sin;
795 
796 		inp->inp_options = (m) ? ip_srcroute(m) : NULL;
797 
798 		if (inp->inp_options == NULL) {
799 			inp->inp_options = sc->sc_ipopts;
800 			sc->sc_ipopts = NULL;
801 		}
802 
803 		sin.sin_family = AF_INET;
804 		sin.sin_len = sizeof(sin);
805 		sin.sin_addr = sc->sc_inc.inc_faddr;
806 		sin.sin_port = sc->sc_inc.inc_fport;
807 		bzero((caddr_t)sin.sin_zero, sizeof(sin.sin_zero));
808 		laddr = inp->inp_laddr;
809 		if (inp->inp_laddr.s_addr == INADDR_ANY)
810 			inp->inp_laddr = sc->sc_inc.inc_laddr;
811 		if ((error = in_pcbconnect_mbuf(inp, (struct sockaddr *)&sin,
812 		    thread0.td_ucred, m)) != 0) {
813 			inp->inp_laddr = laddr;
814 			if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
815 				log(LOG_DEBUG, "%s; %s: in_pcbconnect failed "
816 				    "with error %i\n",
817 				    s, __func__, error);
818 				free(s, M_TCPLOG);
819 			}
820 			INP_HASH_WUNLOCK(&V_tcbinfo);
821 			goto abort;
822 		}
823 	}
824 #endif /* INET */
825 	INP_HASH_WUNLOCK(&V_tcbinfo);
826 	tp = intotcpcb(inp);
827 	tcp_state_change(tp, TCPS_SYN_RECEIVED);
828 	tp->iss = sc->sc_iss;
829 	tp->irs = sc->sc_irs;
830 	tcp_rcvseqinit(tp);
831 	tcp_sendseqinit(tp);
832 	blk = sototcpcb(lso)->t_fb;
833 	if (blk != tp->t_fb) {
834 		/*
835 		 * Our parents t_fb was not the default,
836 		 * we need to release our ref on tp->t_fb and
837 		 * pickup one on the new entry.
838 		 */
839 		struct tcp_function_block *rblk;
840 
841 		rblk = find_and_ref_tcp_fb(blk);
842 		KASSERT(rblk != NULL,
843 		    ("cannot find blk %p out of syncache?", blk));
844 		if (tp->t_fb->tfb_tcp_fb_fini)
845 			(*tp->t_fb->tfb_tcp_fb_fini)(tp, 0);
846 		refcount_release(&tp->t_fb->tfb_refcnt);
847 		tp->t_fb = rblk;
848 		if (tp->t_fb->tfb_tcp_fb_init) {
849 			(*tp->t_fb->tfb_tcp_fb_init)(tp);
850 		}
851 	}
852 	tp->snd_wl1 = sc->sc_irs;
853 	tp->snd_max = tp->iss + 1;
854 	tp->snd_nxt = tp->iss + 1;
855 	tp->rcv_up = sc->sc_irs + 1;
856 	tp->rcv_wnd = sc->sc_wnd;
857 	tp->rcv_adv += tp->rcv_wnd;
858 	tp->last_ack_sent = tp->rcv_nxt;
859 
860 	tp->t_flags = sototcpcb(lso)->t_flags & (TF_NOPUSH|TF_NODELAY);
861 	if (sc->sc_flags & SCF_NOOPT)
862 		tp->t_flags |= TF_NOOPT;
863 	else {
864 		if (sc->sc_flags & SCF_WINSCALE) {
865 			tp->t_flags |= TF_REQ_SCALE|TF_RCVD_SCALE;
866 			tp->snd_scale = sc->sc_requested_s_scale;
867 			tp->request_r_scale = sc->sc_requested_r_scale;
868 		}
869 		if (sc->sc_flags & SCF_TIMESTAMP) {
870 			tp->t_flags |= TF_REQ_TSTMP|TF_RCVD_TSTMP;
871 			tp->ts_recent = sc->sc_tsreflect;
872 			tp->ts_recent_age = tcp_ts_getticks();
873 			tp->ts_offset = sc->sc_tsoff;
874 		}
875 #ifdef TCP_SIGNATURE
876 		if (sc->sc_flags & SCF_SIGNATURE)
877 			tp->t_flags |= TF_SIGNATURE;
878 #endif
879 		if (sc->sc_flags & SCF_SACK)
880 			tp->t_flags |= TF_SACK_PERMIT;
881 	}
882 
883 	if (sc->sc_flags & SCF_ECN)
884 		tp->t_flags |= TF_ECN_PERMIT;
885 
886 	/*
887 	 * Set up MSS and get cached values from tcp_hostcache.
888 	 * This might overwrite some of the defaults we just set.
889 	 */
890 	tcp_mss(tp, sc->sc_peer_mss);
891 
892 	/*
893 	 * If the SYN,ACK was retransmitted, indicate that CWND to be
894 	 * limited to one segment in cc_conn_init().
895 	 * NB: sc_rxmits counts all SYN,ACK transmits, not just retransmits.
896 	 */
897 	if (sc->sc_rxmits > 1)
898 		tp->snd_cwnd = 1;
899 
900 #ifdef TCP_OFFLOAD
901 	/*
902 	 * Allow a TOE driver to install its hooks.  Note that we hold the
903 	 * pcbinfo lock too and that prevents tcp_usr_accept from accepting a
904 	 * new connection before the TOE driver has done its thing.
905 	 */
906 	if (ADDED_BY_TOE(sc)) {
907 		struct toedev *tod = sc->sc_tod;
908 
909 		tod->tod_offload_socket(tod, sc->sc_todctx, so);
910 	}
911 #endif
912 	/*
913 	 * Copy and activate timers.
914 	 */
915 	tp->t_keepinit = sototcpcb(lso)->t_keepinit;
916 	tp->t_keepidle = sototcpcb(lso)->t_keepidle;
917 	tp->t_keepintvl = sototcpcb(lso)->t_keepintvl;
918 	tp->t_keepcnt = sototcpcb(lso)->t_keepcnt;
919 	tcp_timer_activate(tp, TT_KEEP, TP_KEEPINIT(tp));
920 
921 	if ((so->so_options & SO_ACCEPTFILTER) == 0) {
922 		soisconnected(so);
923 	}
924 
925 	TCPSTAT_INC(tcps_accepts);
926 	return (so);
927 
928 abort:
929 	INP_WUNLOCK(inp);
930 abort2:
931 	if (so != NULL)
932 		soabort(so);
933 	return (NULL);
934 }
935 
936 /*
937  * This function gets called when we receive an ACK for a
938  * socket in the LISTEN state.  We look up the connection
939  * in the syncache, and if its there, we pull it out of
940  * the cache and turn it into a full-blown connection in
941  * the SYN-RECEIVED state.
942  *
943  * On syncache_socket() success the newly created socket
944  * has its underlying inp locked.
945  */
946 int
947 syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
948     struct socket **lsop, struct mbuf *m)
949 {
950 	struct syncache *sc;
951 	struct syncache_head *sch;
952 	struct syncache scs;
953 	char *s;
954 
955 	/*
956 	 * Global TCP locks are held because we manipulate the PCB lists
957 	 * and create a new socket.
958 	 */
959 	INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
960 	KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK,
961 	    ("%s: can handle only ACK", __func__));
962 
963 	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
964 	SCH_LOCK_ASSERT(sch);
965 
966 #ifdef INVARIANTS
967 	/*
968 	 * Test code for syncookies comparing the syncache stored
969 	 * values with the reconstructed values from the cookie.
970 	 */
971 	if (sc != NULL)
972 		syncookie_cmp(inc, sch, sc, th, to, *lsop);
973 #endif
974 
975 	if (sc == NULL) {
976 		/*
977 		 * There is no syncache entry, so see if this ACK is
978 		 * a returning syncookie.  To do this, first:
979 		 *  A. See if this socket has had a syncache entry dropped in
980 		 *     the past.  We don't want to accept a bogus syncookie
981 		 *     if we've never received a SYN.
982 		 *  B. check that the syncookie is valid.  If it is, then
983 		 *     cobble up a fake syncache entry, and return.
984 		 */
985 		if (!V_tcp_syncookies) {
986 			SCH_UNLOCK(sch);
987 			if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
988 				log(LOG_DEBUG, "%s; %s: Spurious ACK, "
989 				    "segment rejected (syncookies disabled)\n",
990 				    s, __func__);
991 			goto failed;
992 		}
993 		bzero(&scs, sizeof(scs));
994 		sc = syncookie_lookup(inc, sch, &scs, th, to, *lsop);
995 		SCH_UNLOCK(sch);
996 		if (sc == NULL) {
997 			if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
998 				log(LOG_DEBUG, "%s; %s: Segment failed "
999 				    "SYNCOOKIE authentication, segment rejected "
1000 				    "(probably spoofed)\n", s, __func__);
1001 			goto failed;
1002 		}
1003 	} else {
1004 		/*
1005 		 * Pull out the entry to unlock the bucket row.
1006 		 *
1007 		 * NOTE: We must decrease TCPS_SYN_RECEIVED count here, not
1008 		 * tcp_state_change().  The tcpcb is not existent at this
1009 		 * moment.  A new one will be allocated via syncache_socket->
1010 		 * sonewconn->tcp_usr_attach in TCPS_CLOSED state, then
1011 		 * syncache_socket() will change it to TCPS_SYN_RECEIVED.
1012 		 */
1013 		TCPSTATES_DEC(TCPS_SYN_RECEIVED);
1014 		TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
1015 		sch->sch_length--;
1016 #ifdef TCP_OFFLOAD
1017 		if (ADDED_BY_TOE(sc)) {
1018 			struct toedev *tod = sc->sc_tod;
1019 
1020 			tod->tod_syncache_removed(tod, sc->sc_todctx);
1021 		}
1022 #endif
1023 		SCH_UNLOCK(sch);
1024 	}
1025 
1026 	/*
1027 	 * Segment validation:
1028 	 * ACK must match our initial sequence number + 1 (the SYN|ACK).
1029 	 */
1030 	if (th->th_ack != sc->sc_iss + 1) {
1031 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
1032 			log(LOG_DEBUG, "%s; %s: ACK %u != ISS+1 %u, segment "
1033 			    "rejected\n", s, __func__, th->th_ack, sc->sc_iss);
1034 		goto failed;
1035 	}
1036 
1037 	/*
1038 	 * The SEQ must fall in the window starting at the received
1039 	 * initial receive sequence number + 1 (the SYN).
1040 	 */
1041 	if (SEQ_LEQ(th->th_seq, sc->sc_irs) ||
1042 	    SEQ_GT(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
1043 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
1044 			log(LOG_DEBUG, "%s; %s: SEQ %u != IRS+1 %u, segment "
1045 			    "rejected\n", s, __func__, th->th_seq, sc->sc_irs);
1046 		goto failed;
1047 	}
1048 
1049 	/*
1050 	 * If timestamps were not negotiated during SYN/ACK they
1051 	 * must not appear on any segment during this session.
1052 	 */
1053 	if (!(sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS)) {
1054 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
1055 			log(LOG_DEBUG, "%s; %s: Timestamp not expected, "
1056 			    "segment rejected\n", s, __func__);
1057 		goto failed;
1058 	}
1059 
1060 	/*
1061 	 * If timestamps were negotiated during SYN/ACK they should
1062 	 * appear on every segment during this session.
1063 	 * XXXAO: This is only informal as there have been unverified
1064 	 * reports of non-compliants stacks.
1065 	 */
1066 	if ((sc->sc_flags & SCF_TIMESTAMP) && !(to->to_flags & TOF_TS)) {
1067 		if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
1068 			log(LOG_DEBUG, "%s; %s: Timestamp missing, "
1069 			    "no action\n", s, __func__);
1070 			free(s, M_TCPLOG);
1071 			s = NULL;
1072 		}
1073 	}
1074 
1075 	/*
1076 	 * If timestamps were negotiated the reflected timestamp
1077 	 * must be equal to what we actually sent in the SYN|ACK.
1078 	 */
1079 	if ((to->to_flags & TOF_TS) && to->to_tsecr != sc->sc_ts) {
1080 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
1081 			log(LOG_DEBUG, "%s; %s: TSECR %u != TS %u, "
1082 			    "segment rejected\n",
1083 			    s, __func__, to->to_tsecr, sc->sc_ts);
1084 		goto failed;
1085 	}
1086 
1087 	*lsop = syncache_socket(sc, *lsop, m);
1088 
1089 	if (*lsop == NULL)
1090 		TCPSTAT_INC(tcps_sc_aborted);
1091 	else
1092 		TCPSTAT_INC(tcps_sc_completed);
1093 
1094 /* how do we find the inp for the new socket? */
1095 	if (sc != &scs)
1096 		syncache_free(sc);
1097 	return (1);
1098 failed:
1099 	if (sc != NULL && sc != &scs)
1100 		syncache_free(sc);
1101 	if (s != NULL)
1102 		free(s, M_TCPLOG);
1103 	*lsop = NULL;
1104 	return (0);
1105 }
1106 
1107 #ifdef TCP_RFC7413
1108 static void
1109 syncache_tfo_expand(struct syncache *sc, struct socket **lsop, struct mbuf *m,
1110     uint64_t response_cookie)
1111 {
1112 	struct inpcb *inp;
1113 	struct tcpcb *tp;
1114 	unsigned int *pending_counter;
1115 
1116 	/*
1117 	 * Global TCP locks are held because we manipulate the PCB lists
1118 	 * and create a new socket.
1119 	 */
1120 	INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
1121 
1122 	pending_counter = intotcpcb(sotoinpcb(*lsop))->t_tfo_pending;
1123 	*lsop = syncache_socket(sc, *lsop, m);
1124 	if (*lsop == NULL) {
1125 		TCPSTAT_INC(tcps_sc_aborted);
1126 		atomic_subtract_int(pending_counter, 1);
1127 	} else {
1128 		inp = sotoinpcb(*lsop);
1129 		tp = intotcpcb(inp);
1130 		tp->t_flags |= TF_FASTOPEN;
1131 		tp->t_tfo_cookie = response_cookie;
1132 		tp->snd_max = tp->iss;
1133 		tp->snd_nxt = tp->iss;
1134 		tp->t_tfo_pending = pending_counter;
1135 		TCPSTAT_INC(tcps_sc_completed);
1136 	}
1137 }
1138 #endif /* TCP_RFC7413 */
1139 
1140 /*
1141  * Given a LISTEN socket and an inbound SYN request, add
1142  * this to the syn cache, and send back a segment:
1143  *	<SEQ=ISS><ACK=RCV_NXT><CTL=SYN,ACK>
1144  * to the source.
1145  *
1146  * IMPORTANT NOTE: We do _NOT_ ACK data that might accompany the SYN.
1147  * Doing so would require that we hold onto the data and deliver it
1148  * to the application.  However, if we are the target of a SYN-flood
1149  * DoS attack, an attacker could send data which would eventually
1150  * consume all available buffer space if it were ACKed.  By not ACKing
1151  * the data, we avoid this DoS scenario.
1152  *
1153  * The exception to the above is when a SYN with a valid TCP Fast Open (TFO)
1154  * cookie is processed, V_tcp_fastopen_enabled set to true, and the
1155  * TCP_FASTOPEN socket option is set.  In this case, a new socket is created
1156  * and returned via lsop, the mbuf is not freed so that tcp_input() can
1157  * queue its data to the socket, and 1 is returned to indicate the
1158  * TFO-socket-creation path was taken.
1159  */
1160 int
1161 syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
1162     struct inpcb *inp, struct socket **lsop, struct mbuf *m, void *tod,
1163     void *todctx)
1164 {
1165 	struct tcpcb *tp;
1166 	struct socket *so;
1167 	struct syncache *sc = NULL;
1168 	struct syncache_head *sch;
1169 	struct mbuf *ipopts = NULL;
1170 	u_int ltflags;
1171 	int win, sb_hiwat, ip_ttl, ip_tos;
1172 	char *s;
1173 	int rv = 0;
1174 #ifdef INET6
1175 	int autoflowlabel = 0;
1176 #endif
1177 #ifdef MAC
1178 	struct label *maclabel;
1179 #endif
1180 	struct syncache scs;
1181 	struct ucred *cred;
1182 #ifdef TCP_RFC7413
1183 	uint64_t tfo_response_cookie;
1184 	int tfo_cookie_valid = 0;
1185 	int tfo_response_cookie_valid = 0;
1186 #endif
1187 
1188 	INP_WLOCK_ASSERT(inp);			/* listen socket */
1189 	KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_SYN,
1190 	    ("%s: unexpected tcp flags", __func__));
1191 
1192 	/*
1193 	 * Combine all so/tp operations very early to drop the INP lock as
1194 	 * soon as possible.
1195 	 */
1196 	so = *lsop;
1197 	tp = sototcpcb(so);
1198 	cred = crhold(so->so_cred);
1199 
1200 #ifdef INET6
1201 	if ((inc->inc_flags & INC_ISIPV6) &&
1202 	    (inp->inp_flags & IN6P_AUTOFLOWLABEL))
1203 		autoflowlabel = 1;
1204 #endif
1205 	ip_ttl = inp->inp_ip_ttl;
1206 	ip_tos = inp->inp_ip_tos;
1207 	win = sbspace(&so->so_rcv);
1208 	sb_hiwat = so->so_rcv.sb_hiwat;
1209 	ltflags = (tp->t_flags & (TF_NOOPT | TF_SIGNATURE));
1210 
1211 #ifdef TCP_RFC7413
1212 	if (V_tcp_fastopen_enabled && (tp->t_flags & TF_FASTOPEN) &&
1213 	    (tp->t_tfo_pending != NULL) && (to->to_flags & TOF_FASTOPEN)) {
1214 		/*
1215 		 * Limit the number of pending TFO connections to
1216 		 * approximately half of the queue limit.  This prevents TFO
1217 		 * SYN floods from starving the service by filling the
1218 		 * listen queue with bogus TFO connections.
1219 		 */
1220 		if (atomic_fetchadd_int(tp->t_tfo_pending, 1) <=
1221 		    (so->so_qlimit / 2)) {
1222 			int result;
1223 
1224 			result = tcp_fastopen_check_cookie(inc,
1225 			    to->to_tfo_cookie, to->to_tfo_len,
1226 			    &tfo_response_cookie);
1227 			tfo_cookie_valid = (result > 0);
1228 			tfo_response_cookie_valid = (result >= 0);
1229 		} else
1230 			atomic_subtract_int(tp->t_tfo_pending, 1);
1231 	}
1232 #endif
1233 
1234 	/* By the time we drop the lock these should no longer be used. */
1235 	so = NULL;
1236 	tp = NULL;
1237 
1238 #ifdef MAC
1239 	if (mac_syncache_init(&maclabel) != 0) {
1240 		INP_WUNLOCK(inp);
1241 		goto done;
1242 	} else
1243 		mac_syncache_create(maclabel, inp);
1244 #endif
1245 #ifdef TCP_RFC7413
1246 	if (!tfo_cookie_valid)
1247 #endif
1248 		INP_WUNLOCK(inp);
1249 
1250 	/*
1251 	 * Remember the IP options, if any.
1252 	 */
1253 #ifdef INET6
1254 	if (!(inc->inc_flags & INC_ISIPV6))
1255 #endif
1256 #ifdef INET
1257 		ipopts = (m) ? ip_srcroute(m) : NULL;
1258 #else
1259 		ipopts = NULL;
1260 #endif
1261 
1262 	/*
1263 	 * See if we already have an entry for this connection.
1264 	 * If we do, resend the SYN,ACK, and reset the retransmit timer.
1265 	 *
1266 	 * XXX: should the syncache be re-initialized with the contents
1267 	 * of the new SYN here (which may have different options?)
1268 	 *
1269 	 * XXX: We do not check the sequence number to see if this is a
1270 	 * real retransmit or a new connection attempt.  The question is
1271 	 * how to handle such a case; either ignore it as spoofed, or
1272 	 * drop the current entry and create a new one?
1273 	 */
1274 	sc = syncache_lookup(inc, &sch);	/* returns locked entry */
1275 	SCH_LOCK_ASSERT(sch);
1276 	if (sc != NULL) {
1277 #ifdef TCP_RFC7413
1278 		if (tfo_cookie_valid)
1279 			INP_WUNLOCK(inp);
1280 #endif
1281 		TCPSTAT_INC(tcps_sc_dupsyn);
1282 		if (ipopts) {
1283 			/*
1284 			 * If we were remembering a previous source route,
1285 			 * forget it and use the new one we've been given.
1286 			 */
1287 			if (sc->sc_ipopts)
1288 				(void) m_free(sc->sc_ipopts);
1289 			sc->sc_ipopts = ipopts;
1290 		}
1291 		/*
1292 		 * Update timestamp if present.
1293 		 */
1294 		if ((sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS))
1295 			sc->sc_tsreflect = to->to_tsval;
1296 		else
1297 			sc->sc_flags &= ~SCF_TIMESTAMP;
1298 #ifdef MAC
1299 		/*
1300 		 * Since we have already unconditionally allocated label
1301 		 * storage, free it up.  The syncache entry will already
1302 		 * have an initialized label we can use.
1303 		 */
1304 		mac_syncache_destroy(&maclabel);
1305 #endif
1306 		/* Retransmit SYN|ACK and reset retransmit count. */
1307 		if ((s = tcp_log_addrs(&sc->sc_inc, th, NULL, NULL))) {
1308 			log(LOG_DEBUG, "%s; %s: Received duplicate SYN, "
1309 			    "resetting timer and retransmitting SYN|ACK\n",
1310 			    s, __func__);
1311 			free(s, M_TCPLOG);
1312 		}
1313 		if (syncache_respond(sc, sch, 1, m) == 0) {
1314 			sc->sc_rxmits = 0;
1315 			syncache_timeout(sc, sch, 1);
1316 			TCPSTAT_INC(tcps_sndacks);
1317 			TCPSTAT_INC(tcps_sndtotal);
1318 		}
1319 		SCH_UNLOCK(sch);
1320 		goto done;
1321 	}
1322 
1323 #ifdef TCP_RFC7413
1324 	if (tfo_cookie_valid) {
1325 		bzero(&scs, sizeof(scs));
1326 		sc = &scs;
1327 		goto skip_alloc;
1328 	}
1329 #endif
1330 
1331 	sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
1332 	if (sc == NULL) {
1333 		/*
1334 		 * The zone allocator couldn't provide more entries.
1335 		 * Treat this as if the cache was full; drop the oldest
1336 		 * entry and insert the new one.
1337 		 */
1338 		TCPSTAT_INC(tcps_sc_zonefail);
1339 		if ((sc = TAILQ_LAST(&sch->sch_bucket, sch_head)) != NULL)
1340 			syncache_drop(sc, sch);
1341 		sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
1342 		if (sc == NULL) {
1343 			if (V_tcp_syncookies) {
1344 				bzero(&scs, sizeof(scs));
1345 				sc = &scs;
1346 			} else {
1347 				SCH_UNLOCK(sch);
1348 				if (ipopts)
1349 					(void) m_free(ipopts);
1350 				goto done;
1351 			}
1352 		}
1353 	}
1354 
1355 #ifdef TCP_RFC7413
1356 skip_alloc:
1357 	if (!tfo_cookie_valid && tfo_response_cookie_valid)
1358 		sc->sc_tfo_cookie = &tfo_response_cookie;
1359 #endif
1360 
1361 	/*
1362 	 * Fill in the syncache values.
1363 	 */
1364 #ifdef MAC
1365 	sc->sc_label = maclabel;
1366 #endif
1367 	sc->sc_cred = cred;
1368 	cred = NULL;
1369 	sc->sc_ipopts = ipopts;
1370 	bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
1371 #ifdef INET6
1372 	if (!(inc->inc_flags & INC_ISIPV6))
1373 #endif
1374 	{
1375 		sc->sc_ip_tos = ip_tos;
1376 		sc->sc_ip_ttl = ip_ttl;
1377 	}
1378 #ifdef TCP_OFFLOAD
1379 	sc->sc_tod = tod;
1380 	sc->sc_todctx = todctx;
1381 #endif
1382 	sc->sc_irs = th->th_seq;
1383 	sc->sc_iss = arc4random();
1384 	sc->sc_flags = 0;
1385 	sc->sc_flowlabel = 0;
1386 
1387 	/*
1388 	 * Initial receive window: clip sbspace to [0 .. TCP_MAXWIN].
1389 	 * win was derived from socket earlier in the function.
1390 	 */
1391 	win = imax(win, 0);
1392 	win = imin(win, TCP_MAXWIN);
1393 	sc->sc_wnd = win;
1394 
1395 	if (V_tcp_do_rfc1323) {
1396 		/*
1397 		 * A timestamp received in a SYN makes
1398 		 * it ok to send timestamp requests and replies.
1399 		 */
1400 		if (to->to_flags & TOF_TS) {
1401 			sc->sc_tsreflect = to->to_tsval;
1402 			sc->sc_ts = tcp_ts_getticks();
1403 			sc->sc_flags |= SCF_TIMESTAMP;
1404 		}
1405 		if (to->to_flags & TOF_SCALE) {
1406 			int wscale = 0;
1407 
1408 			/*
1409 			 * Pick the smallest possible scaling factor that
1410 			 * will still allow us to scale up to sb_max, aka
1411 			 * kern.ipc.maxsockbuf.
1412 			 *
1413 			 * We do this because there are broken firewalls that
1414 			 * will corrupt the window scale option, leading to
1415 			 * the other endpoint believing that our advertised
1416 			 * window is unscaled.  At scale factors larger than
1417 			 * 5 the unscaled window will drop below 1500 bytes,
1418 			 * leading to serious problems when traversing these
1419 			 * broken firewalls.
1420 			 *
1421 			 * With the default maxsockbuf of 256K, a scale factor
1422 			 * of 3 will be chosen by this algorithm.  Those who
1423 			 * choose a larger maxsockbuf should watch out
1424 			 * for the compatibility problems mentioned above.
1425 			 *
1426 			 * RFC1323: The Window field in a SYN (i.e., a <SYN>
1427 			 * or <SYN,ACK>) segment itself is never scaled.
1428 			 */
1429 			while (wscale < TCP_MAX_WINSHIFT &&
1430 			    (TCP_MAXWIN << wscale) < sb_max)
1431 				wscale++;
1432 			sc->sc_requested_r_scale = wscale;
1433 			sc->sc_requested_s_scale = to->to_wscale;
1434 			sc->sc_flags |= SCF_WINSCALE;
1435 		}
1436 	}
1437 #ifdef TCP_SIGNATURE
1438 	/*
1439 	 * If listening socket requested TCP digests, OR received SYN
1440 	 * contains the option, flag this in the syncache so that
1441 	 * syncache_respond() will do the right thing with the SYN+ACK.
1442 	 */
1443 	if (to->to_flags & TOF_SIGNATURE || ltflags & TF_SIGNATURE)
1444 		sc->sc_flags |= SCF_SIGNATURE;
1445 #endif
1446 	if (to->to_flags & TOF_SACKPERM)
1447 		sc->sc_flags |= SCF_SACK;
1448 	if (to->to_flags & TOF_MSS)
1449 		sc->sc_peer_mss = to->to_mss;	/* peer mss may be zero */
1450 	if (ltflags & TF_NOOPT)
1451 		sc->sc_flags |= SCF_NOOPT;
1452 	if ((th->th_flags & (TH_ECE|TH_CWR)) && V_tcp_do_ecn)
1453 		sc->sc_flags |= SCF_ECN;
1454 
1455 	if (V_tcp_syncookies)
1456 		sc->sc_iss = syncookie_generate(sch, sc);
1457 #ifdef INET6
1458 	if (autoflowlabel) {
1459 		if (V_tcp_syncookies)
1460 			sc->sc_flowlabel = sc->sc_iss;
1461 		else
1462 			sc->sc_flowlabel = ip6_randomflowlabel();
1463 		sc->sc_flowlabel = htonl(sc->sc_flowlabel) & IPV6_FLOWLABEL_MASK;
1464 	}
1465 #endif
1466 	SCH_UNLOCK(sch);
1467 
1468 #ifdef TCP_RFC7413
1469 	if (tfo_cookie_valid) {
1470 		syncache_tfo_expand(sc, lsop, m, tfo_response_cookie);
1471 		/* INP_WUNLOCK(inp) will be performed by the called */
1472 		rv = 1;
1473 		goto tfo_done;
1474 	}
1475 #endif
1476 
1477 	/*
1478 	 * Do a standard 3-way handshake.
1479 	 */
1480 	if (syncache_respond(sc, sch, 0, m) == 0) {
1481 		if (V_tcp_syncookies && V_tcp_syncookiesonly && sc != &scs)
1482 			syncache_free(sc);
1483 		else if (sc != &scs)
1484 			syncache_insert(sc, sch);   /* locks and unlocks sch */
1485 		TCPSTAT_INC(tcps_sndacks);
1486 		TCPSTAT_INC(tcps_sndtotal);
1487 	} else {
1488 		if (sc != &scs)
1489 			syncache_free(sc);
1490 		TCPSTAT_INC(tcps_sc_dropped);
1491 	}
1492 
1493 done:
1494 	if (m) {
1495 		*lsop = NULL;
1496 		m_freem(m);
1497 	}
1498 #ifdef TCP_RFC7413
1499 tfo_done:
1500 #endif
1501 	if (cred != NULL)
1502 		crfree(cred);
1503 #ifdef MAC
1504 	if (sc == &scs)
1505 		mac_syncache_destroy(&maclabel);
1506 #endif
1507 	return (rv);
1508 }
1509 
1510 /*
1511  * Send SYN|ACK to the peer.  Either in response to the peer's SYN,
1512  * i.e. m0 != NULL, or upon 3WHS ACK timeout, i.e. m0 == NULL.
1513  */
1514 static int
1515 syncache_respond(struct syncache *sc, struct syncache_head *sch, int locked,
1516     const struct mbuf *m0)
1517 {
1518 	struct ip *ip = NULL;
1519 	struct mbuf *m;
1520 	struct tcphdr *th = NULL;
1521 	int optlen, error = 0;	/* Make compiler happy */
1522 	u_int16_t hlen, tlen, mssopt;
1523 	struct tcpopt to;
1524 #ifdef INET6
1525 	struct ip6_hdr *ip6 = NULL;
1526 #endif
1527 #ifdef TCP_SIGNATURE
1528 	struct secasvar *sav;
1529 #endif
1530 
1531 	hlen =
1532 #ifdef INET6
1533 	       (sc->sc_inc.inc_flags & INC_ISIPV6) ? sizeof(struct ip6_hdr) :
1534 #endif
1535 		sizeof(struct ip);
1536 	tlen = hlen + sizeof(struct tcphdr);
1537 
1538 	/* Determine MSS we advertize to other end of connection. */
1539 	mssopt = tcp_mssopt(&sc->sc_inc);
1540 	if (sc->sc_peer_mss)
1541 		mssopt = max( min(sc->sc_peer_mss, mssopt), V_tcp_minmss);
1542 
1543 	/* XXX: Assume that the entire packet will fit in a header mbuf. */
1544 	KASSERT(max_linkhdr + tlen + TCP_MAXOLEN <= MHLEN,
1545 	    ("syncache: mbuf too small"));
1546 
1547 	/* Create the IP+TCP header from scratch. */
1548 	m = m_gethdr(M_NOWAIT, MT_DATA);
1549 	if (m == NULL)
1550 		return (ENOBUFS);
1551 #ifdef MAC
1552 	mac_syncache_create_mbuf(sc->sc_label, m);
1553 #endif
1554 	m->m_data += max_linkhdr;
1555 	m->m_len = tlen;
1556 	m->m_pkthdr.len = tlen;
1557 	m->m_pkthdr.rcvif = NULL;
1558 
1559 #ifdef INET6
1560 	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
1561 		ip6 = mtod(m, struct ip6_hdr *);
1562 		ip6->ip6_vfc = IPV6_VERSION;
1563 		ip6->ip6_nxt = IPPROTO_TCP;
1564 		ip6->ip6_src = sc->sc_inc.inc6_laddr;
1565 		ip6->ip6_dst = sc->sc_inc.inc6_faddr;
1566 		ip6->ip6_plen = htons(tlen - hlen);
1567 		/* ip6_hlim is set after checksum */
1568 		ip6->ip6_flow &= ~IPV6_FLOWLABEL_MASK;
1569 		ip6->ip6_flow |= sc->sc_flowlabel;
1570 
1571 		th = (struct tcphdr *)(ip6 + 1);
1572 	}
1573 #endif
1574 #if defined(INET6) && defined(INET)
1575 	else
1576 #endif
1577 #ifdef INET
1578 	{
1579 		ip = mtod(m, struct ip *);
1580 		ip->ip_v = IPVERSION;
1581 		ip->ip_hl = sizeof(struct ip) >> 2;
1582 		ip->ip_len = htons(tlen);
1583 		ip->ip_id = 0;
1584 		ip->ip_off = 0;
1585 		ip->ip_sum = 0;
1586 		ip->ip_p = IPPROTO_TCP;
1587 		ip->ip_src = sc->sc_inc.inc_laddr;
1588 		ip->ip_dst = sc->sc_inc.inc_faddr;
1589 		ip->ip_ttl = sc->sc_ip_ttl;
1590 		ip->ip_tos = sc->sc_ip_tos;
1591 
1592 		/*
1593 		 * See if we should do MTU discovery.  Route lookups are
1594 		 * expensive, so we will only unset the DF bit if:
1595 		 *
1596 		 *	1) path_mtu_discovery is disabled
1597 		 *	2) the SCF_UNREACH flag has been set
1598 		 */
1599 		if (V_path_mtu_discovery && ((sc->sc_flags & SCF_UNREACH) == 0))
1600 		       ip->ip_off |= htons(IP_DF);
1601 
1602 		th = (struct tcphdr *)(ip + 1);
1603 	}
1604 #endif /* INET */
1605 	th->th_sport = sc->sc_inc.inc_lport;
1606 	th->th_dport = sc->sc_inc.inc_fport;
1607 
1608 	th->th_seq = htonl(sc->sc_iss);
1609 	th->th_ack = htonl(sc->sc_irs + 1);
1610 	th->th_off = sizeof(struct tcphdr) >> 2;
1611 	th->th_x2 = 0;
1612 	th->th_flags = TH_SYN|TH_ACK;
1613 	th->th_win = htons(sc->sc_wnd);
1614 	th->th_urp = 0;
1615 
1616 	if (sc->sc_flags & SCF_ECN) {
1617 		th->th_flags |= TH_ECE;
1618 		TCPSTAT_INC(tcps_ecn_shs);
1619 	}
1620 
1621 	/* Tack on the TCP options. */
1622 	if ((sc->sc_flags & SCF_NOOPT) == 0) {
1623 		to.to_flags = 0;
1624 
1625 		to.to_mss = mssopt;
1626 		to.to_flags = TOF_MSS;
1627 		if (sc->sc_flags & SCF_WINSCALE) {
1628 			to.to_wscale = sc->sc_requested_r_scale;
1629 			to.to_flags |= TOF_SCALE;
1630 		}
1631 		if (sc->sc_flags & SCF_TIMESTAMP) {
1632 			/* Virgin timestamp or TCP cookie enhanced one. */
1633 			to.to_tsval = sc->sc_ts;
1634 			to.to_tsecr = sc->sc_tsreflect;
1635 			to.to_flags |= TOF_TS;
1636 		}
1637 		if (sc->sc_flags & SCF_SACK)
1638 			to.to_flags |= TOF_SACKPERM;
1639 #ifdef TCP_SIGNATURE
1640 		sav = NULL;
1641 		if (sc->sc_flags & SCF_SIGNATURE) {
1642 			sav = tcp_get_sav(m, IPSEC_DIR_OUTBOUND);
1643 			if (sav != NULL)
1644 				to.to_flags |= TOF_SIGNATURE;
1645 			else {
1646 
1647 				/*
1648 				 * We've got SCF_SIGNATURE flag
1649 				 * inherited from listening socket,
1650 				 * but no SADB key for given source
1651 				 * address. Assume signature is not
1652 				 * required and remove signature flag
1653 				 * instead of silently dropping
1654 				 * connection.
1655 				 */
1656 				if (locked == 0)
1657 					SCH_LOCK(sch);
1658 				sc->sc_flags &= ~SCF_SIGNATURE;
1659 				if (locked == 0)
1660 					SCH_UNLOCK(sch);
1661 			}
1662 		}
1663 #endif
1664 
1665 #ifdef TCP_RFC7413
1666 		if (sc->sc_tfo_cookie) {
1667 			to.to_flags |= TOF_FASTOPEN;
1668 			to.to_tfo_len = TCP_FASTOPEN_COOKIE_LEN;
1669 			to.to_tfo_cookie = sc->sc_tfo_cookie;
1670 			/* don't send cookie again when retransmitting response */
1671 			sc->sc_tfo_cookie = NULL;
1672 		}
1673 #endif
1674 		optlen = tcp_addoptions(&to, (u_char *)(th + 1));
1675 
1676 		/* Adjust headers by option size. */
1677 		th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
1678 		m->m_len += optlen;
1679 		m->m_pkthdr.len += optlen;
1680 
1681 #ifdef TCP_SIGNATURE
1682 		if (sc->sc_flags & SCF_SIGNATURE)
1683 			tcp_signature_do_compute(m, 0, optlen,
1684 			    to.to_signature, sav);
1685 #endif
1686 #ifdef INET6
1687 		if (sc->sc_inc.inc_flags & INC_ISIPV6)
1688 			ip6->ip6_plen = htons(ntohs(ip6->ip6_plen) + optlen);
1689 		else
1690 #endif
1691 			ip->ip_len = htons(ntohs(ip->ip_len) + optlen);
1692 	} else
1693 		optlen = 0;
1694 
1695 	M_SETFIB(m, sc->sc_inc.inc_fibnum);
1696 	m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
1697 	/*
1698 	 * If we have peer's SYN and it has a flowid, then let's assign it to
1699 	 * our SYN|ACK.  ip6_output() and ip_output() will not assign flowid
1700 	 * to SYN|ACK due to lack of inp here.
1701 	 */
1702 	if (m0 != NULL && M_HASHTYPE_GET(m0) != M_HASHTYPE_NONE) {
1703 		m->m_pkthdr.flowid = m0->m_pkthdr.flowid;
1704 		M_HASHTYPE_SET(m, M_HASHTYPE_GET(m0));
1705 	}
1706 #ifdef INET6
1707 	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
1708 		m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
1709 		th->th_sum = in6_cksum_pseudo(ip6, tlen + optlen - hlen,
1710 		    IPPROTO_TCP, 0);
1711 		ip6->ip6_hlim = in6_selecthlim(NULL, NULL);
1712 #ifdef TCP_OFFLOAD
1713 		if (ADDED_BY_TOE(sc)) {
1714 			struct toedev *tod = sc->sc_tod;
1715 
1716 			error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
1717 
1718 			return (error);
1719 		}
1720 #endif
1721 		error = ip6_output(m, NULL, NULL, 0, NULL, NULL, NULL);
1722 	}
1723 #endif
1724 #if defined(INET6) && defined(INET)
1725 	else
1726 #endif
1727 #ifdef INET
1728 	{
1729 		m->m_pkthdr.csum_flags = CSUM_TCP;
1730 		th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
1731 		    htons(tlen + optlen - hlen + IPPROTO_TCP));
1732 #ifdef TCP_OFFLOAD
1733 		if (ADDED_BY_TOE(sc)) {
1734 			struct toedev *tod = sc->sc_tod;
1735 
1736 			error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
1737 
1738 			return (error);
1739 		}
1740 #endif
1741 		error = ip_output(m, sc->sc_ipopts, NULL, 0, NULL, NULL);
1742 	}
1743 #endif
1744 	return (error);
1745 }
1746 
1747 /*
1748  * The purpose of syncookies is to handle spoofed SYN flooding DoS attacks
1749  * that exceed the capacity of the syncache by avoiding the storage of any
1750  * of the SYNs we receive.  Syncookies defend against blind SYN flooding
1751  * attacks where the attacker does not have access to our responses.
1752  *
1753  * Syncookies encode and include all necessary information about the
1754  * connection setup within the SYN|ACK that we send back.  That way we
1755  * can avoid keeping any local state until the ACK to our SYN|ACK returns
1756  * (if ever).  Normally the syncache and syncookies are running in parallel
1757  * with the latter taking over when the former is exhausted.  When matching
1758  * syncache entry is found the syncookie is ignored.
1759  *
1760  * The only reliable information persisting the 3WHS is our initial sequence
1761  * number ISS of 32 bits.  Syncookies embed a cryptographically sufficient
1762  * strong hash (MAC) value and a few bits of TCP SYN options in the ISS
1763  * of our SYN|ACK.  The MAC can be recomputed when the ACK to our SYN|ACK
1764  * returns and signifies a legitimate connection if it matches the ACK.
1765  *
1766  * The available space of 32 bits to store the hash and to encode the SYN
1767  * option information is very tight and we should have at least 24 bits for
1768  * the MAC to keep the number of guesses by blind spoofing reasonably high.
1769  *
1770  * SYN option information we have to encode to fully restore a connection:
1771  * MSS: is imporant to chose an optimal segment size to avoid IP level
1772  *   fragmentation along the path.  The common MSS values can be encoded
1773  *   in a 3-bit table.  Uncommon values are captured by the next lower value
1774  *   in the table leading to a slight increase in packetization overhead.
1775  * WSCALE: is necessary to allow large windows to be used for high delay-
1776  *   bandwidth product links.  Not scaling the window when it was initially
1777  *   negotiated is bad for performance as lack of scaling further decreases
1778  *   the apparent available send window.  We only need to encode the WSCALE
1779  *   we received from the remote end.  Our end can be recalculated at any
1780  *   time.  The common WSCALE values can be encoded in a 3-bit table.
1781  *   Uncommon values are captured by the next lower value in the table
1782  *   making us under-estimate the available window size halving our
1783  *   theoretically possible maximum throughput for that connection.
1784  * SACK: Greatly assists in packet loss recovery and requires 1 bit.
1785  * TIMESTAMP and SIGNATURE is not encoded because they are permanent options
1786  *   that are included in all segments on a connection.  We enable them when
1787  *   the ACK has them.
1788  *
1789  * Security of syncookies and attack vectors:
1790  *
1791  * The MAC is computed over (faddr||laddr||fport||lport||irs||flags||secmod)
1792  * together with the gloabl secret to make it unique per connection attempt.
1793  * Thus any change of any of those parameters results in a different MAC output
1794  * in an unpredictable way unless a collision is encountered.  24 bits of the
1795  * MAC are embedded into the ISS.
1796  *
1797  * To prevent replay attacks two rotating global secrets are updated with a
1798  * new random value every 15 seconds.  The life-time of a syncookie is thus
1799  * 15-30 seconds.
1800  *
1801  * Vector 1: Attacking the secret.  This requires finding a weakness in the
1802  * MAC itself or the way it is used here.  The attacker can do a chosen plain
1803  * text attack by varying and testing the all parameters under his control.
1804  * The strength depends on the size and randomness of the secret, and the
1805  * cryptographic security of the MAC function.  Due to the constant updating
1806  * of the secret the attacker has at most 29.999 seconds to find the secret
1807  * and launch spoofed connections.  After that he has to start all over again.
1808  *
1809  * Vector 2: Collision attack on the MAC of a single ACK.  With a 24 bit MAC
1810  * size an average of 4,823 attempts are required for a 50% chance of success
1811  * to spoof a single syncookie (birthday collision paradox).  However the
1812  * attacker is blind and doesn't know if one of his attempts succeeded unless
1813  * he has a side channel to interfere success from.  A single connection setup
1814  * success average of 90% requires 8,790 packets, 99.99% requires 17,578 packets.
1815  * This many attempts are required for each one blind spoofed connection.  For
1816  * every additional spoofed connection he has to launch another N attempts.
1817  * Thus for a sustained rate 100 spoofed connections per second approximately
1818  * 1,800,000 packets per second would have to be sent.
1819  *
1820  * NB: The MAC function should be fast so that it doesn't become a CPU
1821  * exhaustion attack vector itself.
1822  *
1823  * References:
1824  *  RFC4987 TCP SYN Flooding Attacks and Common Mitigations
1825  *  SYN cookies were first proposed by cryptographer Dan J. Bernstein in 1996
1826  *   http://cr.yp.to/syncookies.html    (overview)
1827  *   http://cr.yp.to/syncookies/archive (details)
1828  *
1829  *
1830  * Schematic construction of a syncookie enabled Initial Sequence Number:
1831  *  0        1         2         3
1832  *  12345678901234567890123456789012
1833  * |xxxxxxxxxxxxxxxxxxxxxxxxWWWMMMSP|
1834  *
1835  *  x 24 MAC (truncated)
1836  *  W  3 Send Window Scale index
1837  *  M  3 MSS index
1838  *  S  1 SACK permitted
1839  *  P  1 Odd/even secret
1840  */
1841 
1842 /*
1843  * Distribution and probability of certain MSS values.  Those in between are
1844  * rounded down to the next lower one.
1845  * [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011]
1846  *                            .2%  .3%   5%    7%    7%    20%   15%   45%
1847  */
1848 static int tcp_sc_msstab[] = { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };
1849 
1850 /*
1851  * Distribution and probability of certain WSCALE values.  We have to map the
1852  * (send) window scale (shift) option with a range of 0-14 from 4 bits into 3
1853  * bits based on prevalence of certain values.  Where we don't have an exact
1854  * match for are rounded down to the next lower one letting us under-estimate
1855  * the true available window.  At the moment this would happen only for the
1856  * very uncommon values 3, 5 and those above 8 (more than 16MB socket buffer
1857  * and window size).  The absence of the WSCALE option (no scaling in either
1858  * direction) is encoded with index zero.
1859  * [WSCALE values histograms, Allman, 2012]
1860  *                            X 10 10 35  5  6 14 10%   by host
1861  *                            X 11  4  5  5 18 49  3%   by connections
1862  */
1863 static int tcp_sc_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };
1864 
1865 /*
1866  * Compute the MAC for the SYN cookie.  SIPHASH-2-4 is chosen for its speed
1867  * and good cryptographic properties.
1868  */
1869 static uint32_t
1870 syncookie_mac(struct in_conninfo *inc, tcp_seq irs, uint8_t flags,
1871     uint8_t *secbits, uintptr_t secmod)
1872 {
1873 	SIPHASH_CTX ctx;
1874 	uint32_t siphash[2];
1875 
1876 	SipHash24_Init(&ctx);
1877 	SipHash_SetKey(&ctx, secbits);
1878 	switch (inc->inc_flags & INC_ISIPV6) {
1879 #ifdef INET
1880 	case 0:
1881 		SipHash_Update(&ctx, &inc->inc_faddr, sizeof(inc->inc_faddr));
1882 		SipHash_Update(&ctx, &inc->inc_laddr, sizeof(inc->inc_laddr));
1883 		break;
1884 #endif
1885 #ifdef INET6
1886 	case INC_ISIPV6:
1887 		SipHash_Update(&ctx, &inc->inc6_faddr, sizeof(inc->inc6_faddr));
1888 		SipHash_Update(&ctx, &inc->inc6_laddr, sizeof(inc->inc6_laddr));
1889 		break;
1890 #endif
1891 	}
1892 	SipHash_Update(&ctx, &inc->inc_fport, sizeof(inc->inc_fport));
1893 	SipHash_Update(&ctx, &inc->inc_lport, sizeof(inc->inc_lport));
1894 	SipHash_Update(&ctx, &irs, sizeof(irs));
1895 	SipHash_Update(&ctx, &flags, sizeof(flags));
1896 	SipHash_Update(&ctx, &secmod, sizeof(secmod));
1897 	SipHash_Final((u_int8_t *)&siphash, &ctx);
1898 
1899 	return (siphash[0] ^ siphash[1]);
1900 }
1901 
1902 static tcp_seq
1903 syncookie_generate(struct syncache_head *sch, struct syncache *sc)
1904 {
1905 	u_int i, mss, secbit, wscale;
1906 	uint32_t iss, hash;
1907 	uint8_t *secbits;
1908 	union syncookie cookie;
1909 
1910 	SCH_LOCK_ASSERT(sch);
1911 
1912 	cookie.cookie = 0;
1913 
1914 	/* Map our computed MSS into the 3-bit index. */
1915 	mss = min(tcp_mssopt(&sc->sc_inc), max(sc->sc_peer_mss, V_tcp_minmss));
1916 	for (i = nitems(tcp_sc_msstab) - 1; tcp_sc_msstab[i] > mss && i > 0;
1917 	     i--)
1918 		;
1919 	cookie.flags.mss_idx = i;
1920 
1921 	/*
1922 	 * Map the send window scale into the 3-bit index but only if
1923 	 * the wscale option was received.
1924 	 */
1925 	if (sc->sc_flags & SCF_WINSCALE) {
1926 		wscale = sc->sc_requested_s_scale;
1927 		for (i = nitems(tcp_sc_wstab) - 1;
1928 		    tcp_sc_wstab[i] > wscale && i > 0;
1929 		     i--)
1930 			;
1931 		cookie.flags.wscale_idx = i;
1932 	}
1933 
1934 	/* Can we do SACK? */
1935 	if (sc->sc_flags & SCF_SACK)
1936 		cookie.flags.sack_ok = 1;
1937 
1938 	/* Which of the two secrets to use. */
1939 	secbit = sch->sch_sc->secret.oddeven & 0x1;
1940 	cookie.flags.odd_even = secbit;
1941 
1942 	secbits = sch->sch_sc->secret.key[secbit];
1943 	hash = syncookie_mac(&sc->sc_inc, sc->sc_irs, cookie.cookie, secbits,
1944 	    (uintptr_t)sch);
1945 
1946 	/*
1947 	 * Put the flags into the hash and XOR them to get better ISS number
1948 	 * variance.  This doesn't enhance the cryptographic strength and is
1949 	 * done to prevent the 8 cookie bits from showing up directly on the
1950 	 * wire.
1951 	 */
1952 	iss = hash & ~0xff;
1953 	iss |= cookie.cookie ^ (hash >> 24);
1954 
1955 	/* Randomize the timestamp. */
1956 	if (sc->sc_flags & SCF_TIMESTAMP) {
1957 		sc->sc_ts = arc4random();
1958 		sc->sc_tsoff = sc->sc_ts - tcp_ts_getticks();
1959 	}
1960 
1961 	TCPSTAT_INC(tcps_sc_sendcookie);
1962 	return (iss);
1963 }
1964 
1965 static struct syncache *
1966 syncookie_lookup(struct in_conninfo *inc, struct syncache_head *sch,
1967     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
1968     struct socket *lso)
1969 {
1970 	uint32_t hash;
1971 	uint8_t *secbits;
1972 	tcp_seq ack, seq;
1973 	int wnd, wscale = 0;
1974 	union syncookie cookie;
1975 
1976 	SCH_LOCK_ASSERT(sch);
1977 
1978 	/*
1979 	 * Pull information out of SYN-ACK/ACK and revert sequence number
1980 	 * advances.
1981 	 */
1982 	ack = th->th_ack - 1;
1983 	seq = th->th_seq - 1;
1984 
1985 	/*
1986 	 * Unpack the flags containing enough information to restore the
1987 	 * connection.
1988 	 */
1989 	cookie.cookie = (ack & 0xff) ^ (ack >> 24);
1990 
1991 	/* Which of the two secrets to use. */
1992 	secbits = sch->sch_sc->secret.key[cookie.flags.odd_even];
1993 
1994 	hash = syncookie_mac(inc, seq, cookie.cookie, secbits, (uintptr_t)sch);
1995 
1996 	/* The recomputed hash matches the ACK if this was a genuine cookie. */
1997 	if ((ack & ~0xff) != (hash & ~0xff))
1998 		return (NULL);
1999 
2000 	/* Fill in the syncache values. */
2001 	sc->sc_flags = 0;
2002 	bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
2003 	sc->sc_ipopts = NULL;
2004 
2005 	sc->sc_irs = seq;
2006 	sc->sc_iss = ack;
2007 
2008 	switch (inc->inc_flags & INC_ISIPV6) {
2009 #ifdef INET
2010 	case 0:
2011 		sc->sc_ip_ttl = sotoinpcb(lso)->inp_ip_ttl;
2012 		sc->sc_ip_tos = sotoinpcb(lso)->inp_ip_tos;
2013 		break;
2014 #endif
2015 #ifdef INET6
2016 	case INC_ISIPV6:
2017 		if (sotoinpcb(lso)->inp_flags & IN6P_AUTOFLOWLABEL)
2018 			sc->sc_flowlabel = sc->sc_iss & IPV6_FLOWLABEL_MASK;
2019 		break;
2020 #endif
2021 	}
2022 
2023 	sc->sc_peer_mss = tcp_sc_msstab[cookie.flags.mss_idx];
2024 
2025 	/* We can simply recompute receive window scale we sent earlier. */
2026 	while (wscale < TCP_MAX_WINSHIFT && (TCP_MAXWIN << wscale) < sb_max)
2027 		wscale++;
2028 
2029 	/* Only use wscale if it was enabled in the orignal SYN. */
2030 	if (cookie.flags.wscale_idx > 0) {
2031 		sc->sc_requested_r_scale = wscale;
2032 		sc->sc_requested_s_scale = tcp_sc_wstab[cookie.flags.wscale_idx];
2033 		sc->sc_flags |= SCF_WINSCALE;
2034 	}
2035 
2036 	wnd = sbspace(&lso->so_rcv);
2037 	wnd = imax(wnd, 0);
2038 	wnd = imin(wnd, TCP_MAXWIN);
2039 	sc->sc_wnd = wnd;
2040 
2041 	if (cookie.flags.sack_ok)
2042 		sc->sc_flags |= SCF_SACK;
2043 
2044 	if (to->to_flags & TOF_TS) {
2045 		sc->sc_flags |= SCF_TIMESTAMP;
2046 		sc->sc_tsreflect = to->to_tsval;
2047 		sc->sc_ts = to->to_tsecr;
2048 		sc->sc_tsoff = to->to_tsecr - tcp_ts_getticks();
2049 	}
2050 
2051 	if (to->to_flags & TOF_SIGNATURE)
2052 		sc->sc_flags |= SCF_SIGNATURE;
2053 
2054 	sc->sc_rxmits = 0;
2055 
2056 	TCPSTAT_INC(tcps_sc_recvcookie);
2057 	return (sc);
2058 }
2059 
2060 #ifdef INVARIANTS
2061 static int
2062 syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
2063     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
2064     struct socket *lso)
2065 {
2066 	struct syncache scs, *scx;
2067 	char *s;
2068 
2069 	bzero(&scs, sizeof(scs));
2070 	scx = syncookie_lookup(inc, sch, &scs, th, to, lso);
2071 
2072 	if ((s = tcp_log_addrs(inc, th, NULL, NULL)) == NULL)
2073 		return (0);
2074 
2075 	if (scx != NULL) {
2076 		if (sc->sc_peer_mss != scx->sc_peer_mss)
2077 			log(LOG_DEBUG, "%s; %s: mss different %i vs %i\n",
2078 			    s, __func__, sc->sc_peer_mss, scx->sc_peer_mss);
2079 
2080 		if (sc->sc_requested_r_scale != scx->sc_requested_r_scale)
2081 			log(LOG_DEBUG, "%s; %s: rwscale different %i vs %i\n",
2082 			    s, __func__, sc->sc_requested_r_scale,
2083 			    scx->sc_requested_r_scale);
2084 
2085 		if (sc->sc_requested_s_scale != scx->sc_requested_s_scale)
2086 			log(LOG_DEBUG, "%s; %s: swscale different %i vs %i\n",
2087 			    s, __func__, sc->sc_requested_s_scale,
2088 			    scx->sc_requested_s_scale);
2089 
2090 		if ((sc->sc_flags & SCF_SACK) != (scx->sc_flags & SCF_SACK))
2091 			log(LOG_DEBUG, "%s; %s: SACK different\n", s, __func__);
2092 	}
2093 
2094 	if (s != NULL)
2095 		free(s, M_TCPLOG);
2096 	return (0);
2097 }
2098 #endif /* INVARIANTS */
2099 
2100 static void
2101 syncookie_reseed(void *arg)
2102 {
2103 	struct tcp_syncache *sc = arg;
2104 	uint8_t *secbits;
2105 	int secbit;
2106 
2107 	/*
2108 	 * Reseeding the secret doesn't have to be protected by a lock.
2109 	 * It only must be ensured that the new random values are visible
2110 	 * to all CPUs in a SMP environment.  The atomic with release
2111 	 * semantics ensures that.
2112 	 */
2113 	secbit = (sc->secret.oddeven & 0x1) ? 0 : 1;
2114 	secbits = sc->secret.key[secbit];
2115 	arc4rand(secbits, SYNCOOKIE_SECRET_SIZE, 0);
2116 	atomic_add_rel_int(&sc->secret.oddeven, 1);
2117 
2118 	/* Reschedule ourself. */
2119 	callout_schedule(&sc->secret.reseed, SYNCOOKIE_LIFETIME * hz);
2120 }
2121 
2122 /*
2123  * Exports the syncache entries to userland so that netstat can display
2124  * them alongside the other sockets.  This function is intended to be
2125  * called only from tcp_pcblist.
2126  *
2127  * Due to concurrency on an active system, the number of pcbs exported
2128  * may have no relation to max_pcbs.  max_pcbs merely indicates the
2129  * amount of space the caller allocated for this function to use.
2130  */
2131 int
2132 syncache_pcblist(struct sysctl_req *req, int max_pcbs, int *pcbs_exported)
2133 {
2134 	struct xtcpcb xt;
2135 	struct syncache *sc;
2136 	struct syncache_head *sch;
2137 	int count, error, i;
2138 
2139 	for (count = 0, error = 0, i = 0; i < V_tcp_syncache.hashsize; i++) {
2140 		sch = &V_tcp_syncache.hashbase[i];
2141 		SCH_LOCK(sch);
2142 		TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
2143 			if (count >= max_pcbs) {
2144 				SCH_UNLOCK(sch);
2145 				goto exit;
2146 			}
2147 			if (cr_cansee(req->td->td_ucred, sc->sc_cred) != 0)
2148 				continue;
2149 			bzero(&xt, sizeof(xt));
2150 			xt.xt_len = sizeof(xt);
2151 			if (sc->sc_inc.inc_flags & INC_ISIPV6)
2152 				xt.xt_inp.inp_vflag = INP_IPV6;
2153 			else
2154 				xt.xt_inp.inp_vflag = INP_IPV4;
2155 			bcopy(&sc->sc_inc, &xt.xt_inp.inp_inc, sizeof (struct in_conninfo));
2156 			xt.xt_tp.t_inpcb = &xt.xt_inp;
2157 			xt.xt_tp.t_state = TCPS_SYN_RECEIVED;
2158 			xt.xt_socket.xso_protocol = IPPROTO_TCP;
2159 			xt.xt_socket.xso_len = sizeof (struct xsocket);
2160 			xt.xt_socket.so_type = SOCK_STREAM;
2161 			xt.xt_socket.so_state = SS_ISCONNECTING;
2162 			error = SYSCTL_OUT(req, &xt, sizeof xt);
2163 			if (error) {
2164 				SCH_UNLOCK(sch);
2165 				goto exit;
2166 			}
2167 			count++;
2168 		}
2169 		SCH_UNLOCK(sch);
2170 	}
2171 exit:
2172 	*pcbs_exported = count;
2173 	return error;
2174 }
2175