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>
26 #include <linux/static_key.h>
28 #include <net/inet_common.h>
30 #include <net/busy_poll.h>
32 static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
36 if (after(end_seq, s_win) && before(seq, e_win))
38 return seq == e_win && seq == end_seq;
41 static enum tcp_tw_status
42 tcp_timewait_check_oow_rate_limit(struct inet_timewait_sock *tw,
43 const struct sk_buff *skb, int mib_idx)
45 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
47 if (!tcp_oow_rate_limited(twsk_net(tw), skb, mib_idx,
48 &tcptw->tw_last_oow_ack_time)) {
49 /* Send ACK. Note, we do not put the bucket,
50 * it will be released by caller.
55 /* We are rate-limiting, so just release the tw sock and drop skb. */
57 return TCP_TW_SUCCESS;
61 * * Main purpose of TIME-WAIT state is to close connection gracefully,
62 * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
63 * (and, probably, tail of data) and one or more our ACKs are lost.
64 * * What is TIME-WAIT timeout? It is associated with maximal packet
65 * lifetime in the internet, which results in wrong conclusion, that
66 * it is set to catch "old duplicate segments" wandering out of their path.
67 * It is not quite correct. This timeout is calculated so that it exceeds
68 * maximal retransmission timeout enough to allow to lose one (or more)
69 * segments sent by peer and our ACKs. This time may be calculated from RTO.
70 * * When TIME-WAIT socket receives RST, it means that another end
71 * finally closed and we are allowed to kill TIME-WAIT too.
72 * * Second purpose of TIME-WAIT is catching old duplicate segments.
73 * Well, certainly it is pure paranoia, but if we load TIME-WAIT
74 * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
75 * * If we invented some more clever way to catch duplicates
76 * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
78 * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
79 * When you compare it to RFCs, please, read section SEGMENT ARRIVES
80 * from the very beginning.
82 * NOTE. With recycling (and later with fin-wait-2) TW bucket
83 * is _not_ stateless. It means, that strictly speaking we must
84 * spinlock it. I do not want! Well, probability of misbehaviour
85 * is ridiculously low and, seems, we could use some mb() tricks
86 * to avoid misread sequence numbers, states etc. --ANK
88 * We don't need to initialize tmp_out.sack_ok as we don't use the results
91 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
92 const struct tcphdr *th)
94 struct tcp_options_received tmp_opt;
95 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
96 bool paws_reject = false;
98 tmp_opt.saw_tstamp = 0;
99 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
100 tcp_parse_options(twsk_net(tw), skb, &tmp_opt, 0, NULL);
102 if (tmp_opt.saw_tstamp) {
103 if (tmp_opt.rcv_tsecr)
104 tmp_opt.rcv_tsecr -= tcptw->tw_ts_offset;
105 tmp_opt.ts_recent = tcptw->tw_ts_recent;
106 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
107 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
111 if (tw->tw_substate == TCP_FIN_WAIT2) {
112 /* Just repeat all the checks of tcp_rcv_state_process() */
114 /* Out of window, send ACK */
116 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
118 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
119 return tcp_timewait_check_oow_rate_limit(
120 tw, skb, LINUX_MIB_TCPACKSKIPPEDFINWAIT2);
125 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
130 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
131 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
133 return TCP_TW_SUCCESS;
136 /* New data or FIN. If new data arrive after half-duplex close,
140 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1)
143 /* FIN arrived, enter true time-wait state. */
144 tw->tw_substate = TCP_TIME_WAIT;
145 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
146 if (tmp_opt.saw_tstamp) {
147 tcptw->tw_ts_recent_stamp = get_seconds();
148 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
151 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
156 * Now real TIME-WAIT state.
159 * "When a connection is [...] on TIME-WAIT state [...]
160 * [a TCP] MAY accept a new SYN from the remote TCP to
161 * reopen the connection directly, if it:
163 * (1) assigns its initial sequence number for the new
164 * connection to be larger than the largest sequence
165 * number it used on the previous connection incarnation,
168 * (2) returns to TIME-WAIT state if the SYN turns out
169 * to be an old duplicate".
173 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
174 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
175 /* In window segment, it may be only reset or bare ack. */
178 /* This is TIME_WAIT assassination, in two flavors.
179 * Oh well... nobody has a sufficient solution to this
182 if (twsk_net(tw)->ipv4.sysctl_tcp_rfc1337 == 0) {
184 inet_twsk_deschedule_put(tw);
185 return TCP_TW_SUCCESS;
188 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
190 if (tmp_opt.saw_tstamp) {
191 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
192 tcptw->tw_ts_recent_stamp = get_seconds();
196 return TCP_TW_SUCCESS;
199 /* Out of window segment.
201 All the segments are ACKed immediately.
203 The only exception is new SYN. We accept it, if it is
204 not old duplicate and we are not in danger to be killed
205 by delayed old duplicates. RFC check is that it has
206 newer sequence number works at rates <40Mbit/sec.
207 However, if paws works, it is reliable AND even more,
208 we even may relax silly seq space cutoff.
210 RED-PEN: we violate main RFC requirement, if this SYN will appear
211 old duplicate (i.e. we receive RST in reply to SYN-ACK),
212 we must return socket to time-wait state. It is not good,
216 if (th->syn && !th->rst && !th->ack && !paws_reject &&
217 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
218 (tmp_opt.saw_tstamp &&
219 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
220 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
223 TCP_SKB_CB(skb)->tcp_tw_isn = isn;
228 __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
231 /* In this case we must reset the TIMEWAIT timer.
233 * If it is ACKless SYN it may be both old duplicate
234 * and new good SYN with random sequence number <rcv_nxt.
235 * Do not reschedule in the last case.
237 if (paws_reject || th->ack)
238 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
240 return tcp_timewait_check_oow_rate_limit(
241 tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT);
244 return TCP_TW_SUCCESS;
246 EXPORT_SYMBOL(tcp_timewait_state_process);
249 * Move a socket to time-wait or dead fin-wait-2 state.
251 void tcp_time_wait(struct sock *sk, int state, int timeo)
253 const struct inet_connection_sock *icsk = inet_csk(sk);
254 const struct tcp_sock *tp = tcp_sk(sk);
255 struct inet_timewait_sock *tw;
256 struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row;
258 tw = inet_twsk_alloc(sk, tcp_death_row, state);
261 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
262 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
263 struct inet_sock *inet = inet_sk(sk);
265 tw->tw_transparent = inet->transparent;
266 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale;
267 tcptw->tw_rcv_nxt = tp->rcv_nxt;
268 tcptw->tw_snd_nxt = tp->snd_nxt;
269 tcptw->tw_rcv_wnd = tcp_receive_window(tp);
270 tcptw->tw_ts_recent = tp->rx_opt.ts_recent;
271 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
272 tcptw->tw_ts_offset = tp->tsoffset;
273 tcptw->tw_last_oow_ack_time = 0;
275 #if IS_ENABLED(CONFIG_IPV6)
276 if (tw->tw_family == PF_INET6) {
277 struct ipv6_pinfo *np = inet6_sk(sk);
279 tw->tw_v6_daddr = sk->sk_v6_daddr;
280 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
281 tw->tw_tclass = np->tclass;
282 tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK);
283 tw->tw_ipv6only = sk->sk_ipv6only;
287 #ifdef CONFIG_TCP_MD5SIG
289 * The timewait bucket does not have the key DB from the
290 * sock structure. We just make a quick copy of the
291 * md5 key being used (if indeed we are using one)
292 * so the timewait ack generating code has the key.
295 struct tcp_md5sig_key *key;
296 tcptw->tw_md5_key = NULL;
297 key = tp->af_specific->md5_lookup(sk, sk);
299 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
300 BUG_ON(tcptw->tw_md5_key && !tcp_alloc_md5sig_pool());
305 /* Get the TIME_WAIT timeout firing. */
309 tw->tw_timeout = TCP_TIMEWAIT_LEN;
310 if (state == TCP_TIME_WAIT)
311 timeo = TCP_TIMEWAIT_LEN;
313 inet_twsk_schedule(tw, timeo);
314 /* Linkage updates. */
315 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
318 /* Sorry, if we're out of memory, just CLOSE this
319 * socket up. We've got bigger problems than
320 * non-graceful socket closings.
322 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW);
325 tcp_update_metrics(sk);
329 void tcp_twsk_destructor(struct sock *sk)
331 #ifdef CONFIG_TCP_MD5SIG
332 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
334 if (twsk->tw_md5_key)
335 kfree_rcu(twsk->tw_md5_key, rcu);
338 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
340 /* Warning : This function is called without sk_listener being locked.
341 * Be sure to read socket fields once, as their value could change under us.
343 void tcp_openreq_init_rwin(struct request_sock *req,
344 const struct sock *sk_listener,
345 const struct dst_entry *dst)
347 struct inet_request_sock *ireq = inet_rsk(req);
348 const struct tcp_sock *tp = tcp_sk(sk_listener);
349 int full_space = tcp_full_space(sk_listener);
355 mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
356 window_clamp = READ_ONCE(tp->window_clamp);
357 /* Set this up on the first call only */
358 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);
360 /* limit the window selection if the user enforce a smaller rx buffer */
361 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK &&
362 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
363 req->rsk_window_clamp = full_space;
365 rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req);
367 rcv_wnd = dst_metric(dst, RTAX_INITRWND);
368 else if (full_space < rcv_wnd * mss)
369 full_space = rcv_wnd * mss;
371 /* tcp_full_space because it is guaranteed to be the first packet */
372 tcp_select_initial_window(sk_listener, full_space,
373 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
375 &req->rsk_window_clamp,
379 ireq->rcv_wscale = rcv_wscale;
381 EXPORT_SYMBOL(tcp_openreq_init_rwin);
383 static void tcp_ecn_openreq_child(struct tcp_sock *tp,
384 const struct request_sock *req)
386 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
389 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
391 struct inet_connection_sock *icsk = inet_csk(sk);
392 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
393 bool ca_got_dst = false;
395 if (ca_key != TCP_CA_UNSPEC) {
396 const struct tcp_congestion_ops *ca;
399 ca = tcp_ca_find_key(ca_key);
400 if (likely(ca && try_module_get(ca->owner))) {
401 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
402 icsk->icsk_ca_ops = ca;
408 /* If no valid choice made yet, assign current system default ca. */
410 (!icsk->icsk_ca_setsockopt ||
411 !try_module_get(icsk->icsk_ca_ops->owner)))
412 tcp_assign_congestion_control(sk);
414 tcp_set_ca_state(sk, TCP_CA_Open);
416 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child);
418 static void smc_check_reset_syn_req(struct tcp_sock *oldtp,
419 struct request_sock *req,
420 struct tcp_sock *newtp)
422 #if IS_ENABLED(CONFIG_SMC)
423 struct inet_request_sock *ireq;
425 if (static_branch_unlikely(&tcp_have_smc)) {
426 ireq = inet_rsk(req);
427 if (oldtp->syn_smc && !ireq->smc_ok)
433 /* This is not only more efficient than what we used to do, it eliminates
434 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
436 * Actually, we could lots of memory writes here. tp of listening
437 * socket contains all necessary default parameters.
439 struct sock *tcp_create_openreq_child(const struct sock *sk,
440 struct request_sock *req,
443 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
446 const struct inet_request_sock *ireq = inet_rsk(req);
447 struct tcp_request_sock *treq = tcp_rsk(req);
448 struct inet_connection_sock *newicsk = inet_csk(newsk);
449 struct tcp_sock *newtp = tcp_sk(newsk);
450 struct tcp_sock *oldtp = tcp_sk(sk);
452 smc_check_reset_syn_req(oldtp, req, newtp);
454 /* Now setup tcp_sock */
455 newtp->pred_flags = 0;
457 newtp->rcv_wup = newtp->copied_seq =
458 newtp->rcv_nxt = treq->rcv_isn + 1;
461 newtp->snd_sml = newtp->snd_una =
462 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;
464 INIT_LIST_HEAD(&newtp->tsq_node);
465 INIT_LIST_HEAD(&newtp->tsorted_sent_queue);
467 tcp_init_wl(newtp, treq->rcv_isn);
470 newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT);
471 minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U);
472 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
473 newicsk->icsk_ack.lrcvtime = tcp_jiffies32;
475 newtp->packets_out = 0;
476 newtp->retrans_out = 0;
477 newtp->sacked_out = 0;
478 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
479 newtp->tlp_high_seq = 0;
480 newtp->lsndtime = tcp_jiffies32;
481 newsk->sk_txhash = treq->txhash;
482 newtp->last_oow_ack_time = 0;
483 newtp->total_retrans = req->num_retrans;
485 /* So many TCP implementations out there (incorrectly) count the
486 * initial SYN frame in their delayed-ACK and congestion control
487 * algorithms that we must have the following bandaid to talk
488 * efficiently to them. -DaveM
490 newtp->snd_cwnd = TCP_INIT_CWND;
491 newtp->snd_cwnd_cnt = 0;
493 /* There's a bubble in the pipe until at least the first ACK. */
494 newtp->app_limited = ~0U;
496 tcp_init_xmit_timers(newsk);
497 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
499 newtp->rx_opt.saw_tstamp = 0;
501 newtp->rx_opt.dsack = 0;
502 newtp->rx_opt.num_sacks = 0;
506 if (sock_flag(newsk, SOCK_KEEPOPEN))
507 inet_csk_reset_keepalive_timer(newsk,
508 keepalive_time_when(newtp));
510 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
511 newtp->rx_opt.sack_ok = ireq->sack_ok;
512 newtp->window_clamp = req->rsk_window_clamp;
513 newtp->rcv_ssthresh = req->rsk_rcv_wnd;
514 newtp->rcv_wnd = req->rsk_rcv_wnd;
515 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
516 if (newtp->rx_opt.wscale_ok) {
517 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
518 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
520 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
521 newtp->window_clamp = min(newtp->window_clamp, 65535U);
523 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
524 newtp->rx_opt.snd_wscale);
525 newtp->max_window = newtp->snd_wnd;
527 if (newtp->rx_opt.tstamp_ok) {
528 newtp->rx_opt.ts_recent = req->ts_recent;
529 newtp->rx_opt.ts_recent_stamp = get_seconds();
530 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
532 newtp->rx_opt.ts_recent_stamp = 0;
533 newtp->tcp_header_len = sizeof(struct tcphdr);
535 newtp->tsoffset = treq->ts_off;
536 #ifdef CONFIG_TCP_MD5SIG
537 newtp->md5sig_info = NULL; /*XXX*/
538 if (newtp->af_specific->md5_lookup(sk, newsk))
539 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
541 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
542 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
543 newtp->rx_opt.mss_clamp = req->mss;
544 tcp_ecn_openreq_child(newtp, req);
545 newtp->fastopen_req = NULL;
546 newtp->fastopen_rsk = NULL;
547 newtp->syn_data_acked = 0;
548 newtp->rack.mstamp = 0;
549 newtp->rack.advanced = 0;
550 newtp->rack.reo_wnd_steps = 1;
551 newtp->rack.last_delivered = 0;
552 newtp->rack.reo_wnd_persist = 0;
553 newtp->rack.dsack_seen = 0;
555 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS);
559 EXPORT_SYMBOL(tcp_create_openreq_child);
562 * Process an incoming packet for SYN_RECV sockets represented as a
563 * request_sock. Normally sk is the listener socket but for TFO it
564 * points to the child socket.
566 * XXX (TFO) - The current impl contains a special check for ack
567 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
569 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
572 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
573 struct request_sock *req,
576 struct tcp_options_received tmp_opt;
578 const struct tcphdr *th = tcp_hdr(skb);
579 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
580 bool paws_reject = false;
583 tmp_opt.saw_tstamp = 0;
584 if (th->doff > (sizeof(struct tcphdr)>>2)) {
585 tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL);
587 if (tmp_opt.saw_tstamp) {
588 tmp_opt.ts_recent = req->ts_recent;
589 if (tmp_opt.rcv_tsecr)
590 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off;
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->rsk_rcv_wnd)) {
715 /* Out of window: send ACK and drop. */
716 if (!(flg & TCP_FLAG_RST) &&
717 !tcp_oow_rate_limited(sock_net(sk), skb,
718 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
719 &tcp_rsk(req)->last_oow_ack_time))
720 req->rsk_ops->send_ack(sk, skb, req);
722 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
726 /* In sequence, PAWS is OK. */
728 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
729 req->ts_recent = tmp_opt.rcv_tsval;
731 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
732 /* Truncate SYN, it is out of window starting
733 at tcp_rsk(req)->rcv_isn + 1. */
734 flg &= ~TCP_FLAG_SYN;
737 /* RFC793: "second check the RST bit" and
738 * "fourth, check the SYN bit"
740 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
741 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
742 goto embryonic_reset;
745 /* ACK sequence verified above, just make sure ACK is
746 * set. If ACK not set, just silently drop the packet.
748 * XXX (TFO) - if we ever allow "data after SYN", the
749 * following check needs to be removed.
751 if (!(flg & TCP_FLAG_ACK))
754 /* For Fast Open no more processing is needed (sk is the
760 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
761 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
762 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
763 inet_rsk(req)->acked = 1;
764 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
768 /* OK, ACK is valid, create big socket and
769 * feed this segment to it. It will repeat all
770 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
771 * ESTABLISHED STATE. If it will be dropped after
772 * socket is created, wait for troubles.
774 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
777 goto listen_overflow;
779 sock_rps_save_rxhash(child, skb);
780 tcp_synack_rtt_meas(child, req);
781 return inet_csk_complete_hashdance(sk, child, req, own_req);
784 if (!sock_net(sk)->ipv4.sysctl_tcp_abort_on_overflow) {
785 inet_rsk(req)->acked = 1;
790 if (!(flg & TCP_FLAG_RST)) {
791 /* Received a bad SYN pkt - for TFO We try not to reset
792 * the local connection unless it's really necessary to
793 * avoid becoming vulnerable to outside attack aiming at
794 * resetting legit local connections.
796 req->rsk_ops->send_reset(sk, skb);
797 } else if (fastopen) { /* received a valid RST pkt */
798 reqsk_fastopen_remove(sk, req, true);
802 inet_csk_reqsk_queue_drop(sk, req);
803 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
807 EXPORT_SYMBOL(tcp_check_req);
810 * Queue segment on the new socket if the new socket is active,
811 * otherwise we just shortcircuit this and continue with
814 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
815 * when entering. But other states are possible due to a race condition
816 * where after __inet_lookup_established() fails but before the listener
817 * locked is obtained, other packets cause the same connection to
821 int tcp_child_process(struct sock *parent, struct sock *child,
825 int state = child->sk_state;
827 /* record NAPI ID of child */
828 sk_mark_napi_id(child, skb);
830 tcp_segs_in(tcp_sk(child), skb);
831 if (!sock_owned_by_user(child)) {
832 ret = tcp_rcv_state_process(child, skb);
833 /* Wakeup parent, send SIGIO */
834 if (state == TCP_SYN_RECV && child->sk_state != state)
835 parent->sk_data_ready(parent);
837 /* Alas, it is possible again, because we do lookup
838 * in main socket hash table and lock on listening
839 * socket does not protect us more.
841 __sk_add_backlog(child, skb);
844 bh_unlock_sock(child);
848 EXPORT_SYMBOL(tcp_child_process);