2 * INET An implementation of the TCP/IP protocol suite for the LINUX
3 * operating system. INET is implemented using the BSD Socket
4 * interface as the means of communication with the user level.
6 * Implementation of the Transmission Control Protocol(TCP).
9 * Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
10 * Mark Evans, <evansmp@uhura.aston.ac.uk>
11 * Corey Minyard <wf-rch!minyard@relay.EU.net>
12 * Florian La Roche, <flla@stud.uni-sb.de>
13 * Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
14 * Linus Torvalds, <torvalds@cs.helsinki.fi>
15 * Alan Cox, <gw4pts@gw4pts.ampr.org>
16 * Matthew Dillon, <dillon@apollo.west.oic.com>
17 * Arnt Gulbrandsen, <agulbra@nvg.unit.no>
18 * Jorge Cwik, <jorge@laser.satlink.net>
22 #include <linux/module.h>
23 #include <linux/slab.h>
24 #include <linux/sysctl.h>
25 #include <linux/workqueue.h>
27 #include <net/inet_common.h>
30 int sysctl_tcp_syncookies __read_mostly = 1;
31 EXPORT_SYMBOL(sysctl_tcp_syncookies);
33 int sysctl_tcp_abort_on_overflow __read_mostly;
35 struct inet_timewait_death_row tcp_death_row = {
36 .sysctl_max_tw_buckets = NR_FILE * 2,
37 .period = TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS,
38 .death_lock = __SPIN_LOCK_UNLOCKED(tcp_death_row.death_lock),
39 .hashinfo = &tcp_hashinfo,
40 .tw_timer = TIMER_INITIALIZER(inet_twdr_hangman, 0,
41 (unsigned long)&tcp_death_row),
42 .twkill_work = __WORK_INITIALIZER(tcp_death_row.twkill_work,
43 inet_twdr_twkill_work),
44 /* Short-time timewait calendar */
47 .twcal_timer = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0,
48 (unsigned long)&tcp_death_row),
50 EXPORT_SYMBOL_GPL(tcp_death_row);
52 static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
56 if (after(end_seq, s_win) && before(seq, e_win))
58 return seq == e_win && seq == end_seq;
61 static enum tcp_tw_status
62 tcp_timewait_check_oow_rate_limit(struct inet_timewait_sock *tw,
63 const struct sk_buff *skb, int mib_idx)
65 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
67 if (!tcp_oow_rate_limited(twsk_net(tw), skb, mib_idx,
68 &tcptw->tw_last_oow_ack_time)) {
69 /* Send ACK. Note, we do not put the bucket,
70 * it will be released by caller.
75 /* We are rate-limiting, so just release the tw sock and drop skb. */
77 return TCP_TW_SUCCESS;
81 * * Main purpose of TIME-WAIT state is to close connection gracefully,
82 * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
83 * (and, probably, tail of data) and one or more our ACKs are lost.
84 * * What is TIME-WAIT timeout? It is associated with maximal packet
85 * lifetime in the internet, which results in wrong conclusion, that
86 * it is set to catch "old duplicate segments" wandering out of their path.
87 * It is not quite correct. This timeout is calculated so that it exceeds
88 * maximal retransmission timeout enough to allow to lose one (or more)
89 * segments sent by peer and our ACKs. This time may be calculated from RTO.
90 * * When TIME-WAIT socket receives RST, it means that another end
91 * finally closed and we are allowed to kill TIME-WAIT too.
92 * * Second purpose of TIME-WAIT is catching old duplicate segments.
93 * Well, certainly it is pure paranoia, but if we load TIME-WAIT
94 * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
95 * * If we invented some more clever way to catch duplicates
96 * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
98 * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
99 * When you compare it to RFCs, please, read section SEGMENT ARRIVES
100 * from the very beginning.
102 * NOTE. With recycling (and later with fin-wait-2) TW bucket
103 * is _not_ stateless. It means, that strictly speaking we must
104 * spinlock it. I do not want! Well, probability of misbehaviour
105 * is ridiculously low and, seems, we could use some mb() tricks
106 * to avoid misread sequence numbers, states etc. --ANK
108 * We don't need to initialize tmp_out.sack_ok as we don't use the results
111 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
112 const struct tcphdr *th)
114 struct tcp_options_received tmp_opt;
115 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
116 bool paws_reject = false;
118 tmp_opt.saw_tstamp = 0;
119 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
120 tcp_parse_options(skb, &tmp_opt, 0, NULL);
122 if (tmp_opt.saw_tstamp) {
123 tmp_opt.rcv_tsecr -= tcptw->tw_ts_offset;
124 tmp_opt.ts_recent = tcptw->tw_ts_recent;
125 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
126 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
130 if (tw->tw_substate == TCP_FIN_WAIT2) {
131 /* Just repeat all the checks of tcp_rcv_state_process() */
133 /* Out of window, send ACK */
135 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
137 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
138 return tcp_timewait_check_oow_rate_limit(
139 tw, skb, LINUX_MIB_TCPACKSKIPPEDFINWAIT2);
144 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
149 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
150 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
152 return TCP_TW_SUCCESS;
155 /* New data or FIN. If new data arrive after half-duplex close,
159 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) {
161 inet_twsk_deschedule(tw, &tcp_death_row);
166 /* FIN arrived, enter true time-wait state. */
167 tw->tw_substate = TCP_TIME_WAIT;
168 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
169 if (tmp_opt.saw_tstamp) {
170 tcptw->tw_ts_recent_stamp = get_seconds();
171 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
174 if (tcp_death_row.sysctl_tw_recycle &&
175 tcptw->tw_ts_recent_stamp &&
176 tcp_tw_remember_stamp(tw))
177 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout,
180 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
186 * Now real TIME-WAIT state.
189 * "When a connection is [...] on TIME-WAIT state [...]
190 * [a TCP] MAY accept a new SYN from the remote TCP to
191 * reopen the connection directly, if it:
193 * (1) assigns its initial sequence number for the new
194 * connection to be larger than the largest sequence
195 * number it used on the previous connection incarnation,
198 * (2) returns to TIME-WAIT state if the SYN turns out
199 * to be an old duplicate".
203 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
204 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
205 /* In window segment, it may be only reset or bare ack. */
208 /* This is TIME_WAIT assassination, in two flavors.
209 * Oh well... nobody has a sufficient solution to this
212 if (sysctl_tcp_rfc1337 == 0) {
214 inet_twsk_deschedule(tw, &tcp_death_row);
216 return TCP_TW_SUCCESS;
219 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
222 if (tmp_opt.saw_tstamp) {
223 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
224 tcptw->tw_ts_recent_stamp = get_seconds();
228 return TCP_TW_SUCCESS;
231 /* Out of window segment.
233 All the segments are ACKed immediately.
235 The only exception is new SYN. We accept it, if it is
236 not old duplicate and we are not in danger to be killed
237 by delayed old duplicates. RFC check is that it has
238 newer sequence number works at rates <40Mbit/sec.
239 However, if paws works, it is reliable AND even more,
240 we even may relax silly seq space cutoff.
242 RED-PEN: we violate main RFC requirement, if this SYN will appear
243 old duplicate (i.e. we receive RST in reply to SYN-ACK),
244 we must return socket to time-wait state. It is not good,
248 if (th->syn && !th->rst && !th->ack && !paws_reject &&
249 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
250 (tmp_opt.saw_tstamp &&
251 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
252 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
255 TCP_SKB_CB(skb)->tcp_tw_isn = isn;
260 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
263 /* In this case we must reset the TIMEWAIT timer.
265 * If it is ACKless SYN it may be both old duplicate
266 * and new good SYN with random sequence number <rcv_nxt.
267 * Do not reschedule in the last case.
269 if (paws_reject || th->ack)
270 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
273 return tcp_timewait_check_oow_rate_limit(
274 tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT);
277 return TCP_TW_SUCCESS;
279 EXPORT_SYMBOL(tcp_timewait_state_process);
282 * Move a socket to time-wait or dead fin-wait-2 state.
284 void tcp_time_wait(struct sock *sk, int state, int timeo)
286 struct inet_timewait_sock *tw = NULL;
287 const struct inet_connection_sock *icsk = inet_csk(sk);
288 const struct tcp_sock *tp = tcp_sk(sk);
289 bool recycle_ok = false;
291 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp)
292 recycle_ok = tcp_remember_stamp(sk);
294 if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets)
295 tw = inet_twsk_alloc(sk, state);
298 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
299 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
300 struct inet_sock *inet = inet_sk(sk);
302 tw->tw_transparent = inet->transparent;
303 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale;
304 tcptw->tw_rcv_nxt = tp->rcv_nxt;
305 tcptw->tw_snd_nxt = tp->snd_nxt;
306 tcptw->tw_rcv_wnd = tcp_receive_window(tp);
307 tcptw->tw_ts_recent = tp->rx_opt.ts_recent;
308 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
309 tcptw->tw_ts_offset = tp->tsoffset;
310 tcptw->tw_last_oow_ack_time = 0;
312 #if IS_ENABLED(CONFIG_IPV6)
313 if (tw->tw_family == PF_INET6) {
314 struct ipv6_pinfo *np = inet6_sk(sk);
316 tw->tw_v6_daddr = sk->sk_v6_daddr;
317 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
318 tw->tw_tclass = np->tclass;
319 tw->tw_flowlabel = np->flow_label >> 12;
320 tw->tw_ipv6only = sk->sk_ipv6only;
324 #ifdef CONFIG_TCP_MD5SIG
326 * The timewait bucket does not have the key DB from the
327 * sock structure. We just make a quick copy of the
328 * md5 key being used (if indeed we are using one)
329 * so the timewait ack generating code has the key.
332 struct tcp_md5sig_key *key;
333 tcptw->tw_md5_key = NULL;
334 key = tp->af_specific->md5_lookup(sk, sk);
336 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
337 if (tcptw->tw_md5_key && !tcp_alloc_md5sig_pool())
343 /* Linkage updates. */
344 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
346 /* Get the TIME_WAIT timeout firing. */
351 tw->tw_timeout = rto;
353 tw->tw_timeout = TCP_TIMEWAIT_LEN;
354 if (state == TCP_TIME_WAIT)
355 timeo = TCP_TIMEWAIT_LEN;
358 inet_twsk_schedule(tw, &tcp_death_row, timeo,
362 /* Sorry, if we're out of memory, just CLOSE this
363 * socket up. We've got bigger problems than
364 * non-graceful socket closings.
366 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW);
369 tcp_update_metrics(sk);
373 void tcp_twsk_destructor(struct sock *sk)
375 #ifdef CONFIG_TCP_MD5SIG
376 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
378 if (twsk->tw_md5_key)
379 kfree_rcu(twsk->tw_md5_key, rcu);
382 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
384 void tcp_openreq_init_rwin(struct request_sock *req,
385 struct sock *sk, struct dst_entry *dst)
387 struct inet_request_sock *ireq = inet_rsk(req);
388 struct tcp_sock *tp = tcp_sk(sk);
390 int mss = dst_metric_advmss(dst);
392 if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss)
393 mss = tp->rx_opt.user_mss;
395 /* Set this up on the first call only */
396 req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW);
398 /* limit the window selection if the user enforce a smaller rx buffer */
399 if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
400 (req->window_clamp > tcp_full_space(sk) || req->window_clamp == 0))
401 req->window_clamp = tcp_full_space(sk);
403 /* tcp_full_space because it is guaranteed to be the first packet */
404 tcp_select_initial_window(tcp_full_space(sk),
405 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
410 dst_metric(dst, RTAX_INITRWND));
411 ireq->rcv_wscale = rcv_wscale;
413 EXPORT_SYMBOL(tcp_openreq_init_rwin);
415 static void tcp_ecn_openreq_child(struct tcp_sock *tp,
416 const struct request_sock *req)
418 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
421 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
423 struct inet_connection_sock *icsk = inet_csk(sk);
424 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
425 bool ca_got_dst = false;
427 if (ca_key != TCP_CA_UNSPEC) {
428 const struct tcp_congestion_ops *ca;
431 ca = tcp_ca_find_key(ca_key);
432 if (likely(ca && try_module_get(ca->owner))) {
433 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
434 icsk->icsk_ca_ops = ca;
440 if (!ca_got_dst && !try_module_get(icsk->icsk_ca_ops->owner))
441 tcp_assign_congestion_control(sk);
443 tcp_set_ca_state(sk, TCP_CA_Open);
445 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child);
447 /* This is not only more efficient than what we used to do, it eliminates
448 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
450 * Actually, we could lots of memory writes here. tp of listening
451 * socket contains all necessary default parameters.
453 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
455 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
458 const struct inet_request_sock *ireq = inet_rsk(req);
459 struct tcp_request_sock *treq = tcp_rsk(req);
460 struct inet_connection_sock *newicsk = inet_csk(newsk);
461 struct tcp_sock *newtp = tcp_sk(newsk);
463 /* Now setup tcp_sock */
464 newtp->pred_flags = 0;
466 newtp->rcv_wup = newtp->copied_seq =
467 newtp->rcv_nxt = treq->rcv_isn + 1;
469 newtp->snd_sml = newtp->snd_una =
470 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;
472 tcp_prequeue_init(newtp);
473 INIT_LIST_HEAD(&newtp->tsq_node);
475 tcp_init_wl(newtp, treq->rcv_isn);
478 newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT);
479 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
481 newtp->packets_out = 0;
482 newtp->retrans_out = 0;
483 newtp->sacked_out = 0;
484 newtp->fackets_out = 0;
485 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
486 tcp_enable_early_retrans(newtp);
487 newtp->tlp_high_seq = 0;
488 newtp->lsndtime = treq->snt_synack;
489 newtp->last_oow_ack_time = 0;
490 newtp->total_retrans = req->num_retrans;
492 /* So many TCP implementations out there (incorrectly) count the
493 * initial SYN frame in their delayed-ACK and congestion control
494 * algorithms that we must have the following bandaid to talk
495 * efficiently to them. -DaveM
497 newtp->snd_cwnd = TCP_INIT_CWND;
498 newtp->snd_cwnd_cnt = 0;
500 tcp_init_xmit_timers(newsk);
501 __skb_queue_head_init(&newtp->out_of_order_queue);
502 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
504 newtp->rx_opt.saw_tstamp = 0;
506 newtp->rx_opt.dsack = 0;
507 newtp->rx_opt.num_sacks = 0;
511 if (sock_flag(newsk, SOCK_KEEPOPEN))
512 inet_csk_reset_keepalive_timer(newsk,
513 keepalive_time_when(newtp));
515 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
516 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
518 tcp_enable_fack(newtp);
520 newtp->window_clamp = req->window_clamp;
521 newtp->rcv_ssthresh = req->rcv_wnd;
522 newtp->rcv_wnd = req->rcv_wnd;
523 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
524 if (newtp->rx_opt.wscale_ok) {
525 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
526 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
528 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
529 newtp->window_clamp = min(newtp->window_clamp, 65535U);
531 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
532 newtp->rx_opt.snd_wscale);
533 newtp->max_window = newtp->snd_wnd;
535 if (newtp->rx_opt.tstamp_ok) {
536 newtp->rx_opt.ts_recent = req->ts_recent;
537 newtp->rx_opt.ts_recent_stamp = get_seconds();
538 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
540 newtp->rx_opt.ts_recent_stamp = 0;
541 newtp->tcp_header_len = sizeof(struct tcphdr);
544 #ifdef CONFIG_TCP_MD5SIG
545 newtp->md5sig_info = NULL; /*XXX*/
546 if (newtp->af_specific->md5_lookup(sk, newsk))
547 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
549 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
550 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
551 newtp->rx_opt.mss_clamp = req->mss;
552 tcp_ecn_openreq_child(newtp, req);
553 newtp->fastopen_rsk = NULL;
554 newtp->syn_data_acked = 0;
556 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS);
560 EXPORT_SYMBOL(tcp_create_openreq_child);
563 * Process an incoming packet for SYN_RECV sockets represented as a
564 * request_sock. Normally sk is the listener socket but for TFO it
565 * points to the child socket.
567 * XXX (TFO) - The current impl contains a special check for ack
568 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
570 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
573 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
574 struct request_sock *req,
577 struct tcp_options_received tmp_opt;
579 const struct tcphdr *th = tcp_hdr(skb);
580 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
581 bool paws_reject = false;
583 BUG_ON(fastopen == (sk->sk_state == TCP_LISTEN));
585 tmp_opt.saw_tstamp = 0;
586 if (th->doff > (sizeof(struct tcphdr)>>2)) {
587 tcp_parse_options(skb, &tmp_opt, 0, NULL);
589 if (tmp_opt.saw_tstamp) {
590 tmp_opt.ts_recent = req->ts_recent;
591 /* We do not store true stamp, but it is not required,
592 * it can be estimated (approximately)
595 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
596 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
600 /* Check for pure retransmitted SYN. */
601 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
602 flg == TCP_FLAG_SYN &&
605 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
606 * this case on figure 6 and figure 8, but formal
607 * protocol description says NOTHING.
608 * To be more exact, it says that we should send ACK,
609 * because this segment (at least, if it has no data)
612 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
613 * describe SYN-RECV state. All the description
614 * is wrong, we cannot believe to it and should
615 * rely only on common sense and implementation
618 * Enforce "SYN-ACK" according to figure 8, figure 6
619 * of RFC793, fixed by RFC1122.
621 * Note that even if there is new data in the SYN packet
622 * they will be thrown away too.
624 * Reset timer after retransmitting SYNACK, similar to
625 * the idea of fast retransmit in recovery.
627 if (!tcp_oow_rate_limited(sock_net(sk), skb,
628 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
629 &tcp_rsk(req)->last_oow_ack_time) &&
631 !inet_rtx_syn_ack(sk, req)) {
632 unsigned long expires = jiffies;
634 expires += min(TCP_TIMEOUT_INIT << req->num_timeout,
637 mod_timer_pending(&req->rsk_timer, expires);
639 req->rsk_timer.expires = expires;
644 /* Further reproduces section "SEGMENT ARRIVES"
645 for state SYN-RECEIVED of RFC793.
646 It is broken, however, it does not work only
647 when SYNs are crossed.
649 You would think that SYN crossing is impossible here, since
650 we should have a SYN_SENT socket (from connect()) on our end,
651 but this is not true if the crossed SYNs were sent to both
652 ends by a malicious third party. We must defend against this,
653 and to do that we first verify the ACK (as per RFC793, page
654 36) and reset if it is invalid. Is this a true full defense?
655 To convince ourselves, let us consider a way in which the ACK
656 test can still pass in this 'malicious crossed SYNs' case.
657 Malicious sender sends identical SYNs (and thus identical sequence
658 numbers) to both A and B:
663 By our good fortune, both A and B select the same initial
664 send sequence number of seven :-)
666 A: sends SYN|ACK, seq=7, ack_seq=8
667 B: sends SYN|ACK, seq=7, ack_seq=8
669 So we are now A eating this SYN|ACK, ACK test passes. So
670 does sequence test, SYN is truncated, and thus we consider
673 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
674 bare ACK. Otherwise, we create an established connection. Both
675 ends (listening sockets) accept the new incoming connection and try
676 to talk to each other. 8-)
678 Note: This case is both harmless, and rare. Possibility is about the
679 same as us discovering intelligent life on another plant tomorrow.
681 But generally, we should (RFC lies!) to accept ACK
682 from SYNACK both here and in tcp_rcv_state_process().
683 tcp_rcv_state_process() does not, hence, we do not too.
685 Note that the case is absolutely generic:
686 we cannot optimize anything here without
687 violating protocol. All the checks must be made
688 before attempt to create socket.
691 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
692 * and the incoming segment acknowledges something not yet
693 * sent (the segment carries an unacceptable ACK) ...
696 * Invalid ACK: reset will be sent by listening socket.
697 * Note that the ACK validity check for a Fast Open socket is done
698 * elsewhere and is checked directly against the child socket rather
699 * than req because user data may have been sent out.
701 if ((flg & TCP_FLAG_ACK) && !fastopen &&
702 (TCP_SKB_CB(skb)->ack_seq !=
703 tcp_rsk(req)->snt_isn + 1))
706 /* Also, it would be not so bad idea to check rcv_tsecr, which
707 * is essentially ACK extension and too early or too late values
708 * should cause reset in unsynchronized states.
711 /* RFC793: "first check sequence number". */
713 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
714 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rcv_wnd)) {
715 /* Out of window: send ACK and drop. */
716 if (!(flg & TCP_FLAG_RST))
717 req->rsk_ops->send_ack(sk, skb, req);
719 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
723 /* In sequence, PAWS is OK. */
725 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
726 req->ts_recent = tmp_opt.rcv_tsval;
728 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
729 /* Truncate SYN, it is out of window starting
730 at tcp_rsk(req)->rcv_isn + 1. */
731 flg &= ~TCP_FLAG_SYN;
734 /* RFC793: "second check the RST bit" and
735 * "fourth, check the SYN bit"
737 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
738 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
739 goto embryonic_reset;
742 /* ACK sequence verified above, just make sure ACK is
743 * set. If ACK not set, just silently drop the packet.
745 * XXX (TFO) - if we ever allow "data after SYN", the
746 * following check needs to be removed.
748 if (!(flg & TCP_FLAG_ACK))
751 /* For Fast Open no more processing is needed (sk is the
757 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
758 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
759 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
760 inet_rsk(req)->acked = 1;
761 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
765 /* OK, ACK is valid, create big socket and
766 * feed this segment to it. It will repeat all
767 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
768 * ESTABLISHED STATE. If it will be dropped after
769 * socket is created, wait for troubles.
771 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL);
773 goto listen_overflow;
775 inet_csk_reqsk_queue_unlink(sk, req);
776 inet_csk_reqsk_queue_removed(sk, req);
778 inet_csk_reqsk_queue_add(sk, req, child);
782 if (!sysctl_tcp_abort_on_overflow) {
783 inet_rsk(req)->acked = 1;
788 if (!(flg & TCP_FLAG_RST)) {
789 /* Received a bad SYN pkt - for TFO We try not to reset
790 * the local connection unless it's really necessary to
791 * avoid becoming vulnerable to outside attack aiming at
792 * resetting legit local connections.
794 req->rsk_ops->send_reset(sk, skb);
795 } else if (fastopen) { /* received a valid RST pkt */
796 reqsk_fastopen_remove(sk, req, true);
800 inet_csk_reqsk_queue_drop(sk, req);
801 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
805 EXPORT_SYMBOL(tcp_check_req);
808 * Queue segment on the new socket if the new socket is active,
809 * otherwise we just shortcircuit this and continue with
812 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
813 * when entering. But other states are possible due to a race condition
814 * where after __inet_lookup_established() fails but before the listener
815 * locked is obtained, other packets cause the same connection to
819 int tcp_child_process(struct sock *parent, struct sock *child,
823 int state = child->sk_state;
825 if (!sock_owned_by_user(child)) {
826 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
828 /* Wakeup parent, send SIGIO */
829 if (state == TCP_SYN_RECV && child->sk_state != state)
830 parent->sk_data_ready(parent);
832 /* Alas, it is possible again, because we do lookup
833 * in main socket hash table and lock on listening
834 * socket does not protect us more.
836 __sk_add_backlog(child, skb);
839 bh_unlock_sock(child);
843 EXPORT_SYMBOL(tcp_child_process);