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_mark = sk->sk_mark;
267 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale;
268 tcptw->tw_rcv_nxt = tp->rcv_nxt;
269 tcptw->tw_snd_nxt = tp->snd_nxt;
270 tcptw->tw_rcv_wnd = tcp_receive_window(tp);
271 tcptw->tw_ts_recent = tp->rx_opt.ts_recent;
272 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
273 tcptw->tw_ts_offset = tp->tsoffset;
274 tcptw->tw_last_oow_ack_time = 0;
276 #if IS_ENABLED(CONFIG_IPV6)
277 if (tw->tw_family == PF_INET6) {
278 struct ipv6_pinfo *np = inet6_sk(sk);
280 tw->tw_v6_daddr = sk->sk_v6_daddr;
281 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
282 tw->tw_tclass = np->tclass;
283 tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK);
284 tw->tw_ipv6only = sk->sk_ipv6only;
288 #ifdef CONFIG_TCP_MD5SIG
290 * The timewait bucket does not have the key DB from the
291 * sock structure. We just make a quick copy of the
292 * md5 key being used (if indeed we are using one)
293 * so the timewait ack generating code has the key.
296 struct tcp_md5sig_key *key;
297 tcptw->tw_md5_key = NULL;
298 key = tp->af_specific->md5_lookup(sk, sk);
300 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
301 BUG_ON(tcptw->tw_md5_key && !tcp_alloc_md5sig_pool());
306 /* Get the TIME_WAIT timeout firing. */
310 tw->tw_timeout = TCP_TIMEWAIT_LEN;
311 if (state == TCP_TIME_WAIT)
312 timeo = TCP_TIMEWAIT_LEN;
314 /* tw_timer is pinned, so we need to make sure BH are disabled
315 * in following section, otherwise timer handler could run before
316 * we complete the initialization.
319 inet_twsk_schedule(tw, timeo);
321 * Note that access to tw after this point is illegal.
323 inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
326 /* Sorry, if we're out of memory, just CLOSE this
327 * socket up. We've got bigger problems than
328 * non-graceful socket closings.
330 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW);
333 tcp_update_metrics(sk);
336 EXPORT_SYMBOL(tcp_time_wait);
338 void tcp_twsk_destructor(struct sock *sk)
340 #ifdef CONFIG_TCP_MD5SIG
341 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
343 if (twsk->tw_md5_key)
344 kfree_rcu(twsk->tw_md5_key, rcu);
347 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
349 /* Warning : This function is called without sk_listener being locked.
350 * Be sure to read socket fields once, as their value could change under us.
352 void tcp_openreq_init_rwin(struct request_sock *req,
353 const struct sock *sk_listener,
354 const struct dst_entry *dst)
356 struct inet_request_sock *ireq = inet_rsk(req);
357 const struct tcp_sock *tp = tcp_sk(sk_listener);
358 int full_space = tcp_full_space(sk_listener);
364 mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
365 window_clamp = READ_ONCE(tp->window_clamp);
366 /* Set this up on the first call only */
367 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);
369 /* limit the window selection if the user enforce a smaller rx buffer */
370 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK &&
371 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
372 req->rsk_window_clamp = full_space;
374 rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req);
376 rcv_wnd = dst_metric(dst, RTAX_INITRWND);
377 else if (full_space < rcv_wnd * mss)
378 full_space = rcv_wnd * mss;
380 /* tcp_full_space because it is guaranteed to be the first packet */
381 tcp_select_initial_window(sk_listener, full_space,
382 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
384 &req->rsk_window_clamp,
388 ireq->rcv_wscale = rcv_wscale;
390 EXPORT_SYMBOL(tcp_openreq_init_rwin);
392 static void tcp_ecn_openreq_child(struct tcp_sock *tp,
393 const struct request_sock *req)
395 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
398 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
400 struct inet_connection_sock *icsk = inet_csk(sk);
401 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
402 bool ca_got_dst = false;
404 if (ca_key != TCP_CA_UNSPEC) {
405 const struct tcp_congestion_ops *ca;
408 ca = tcp_ca_find_key(ca_key);
409 if (likely(ca && try_module_get(ca->owner))) {
410 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
411 icsk->icsk_ca_ops = ca;
417 /* If no valid choice made yet, assign current system default ca. */
419 (!icsk->icsk_ca_setsockopt ||
420 !try_module_get(icsk->icsk_ca_ops->owner)))
421 tcp_assign_congestion_control(sk);
423 tcp_set_ca_state(sk, TCP_CA_Open);
425 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child);
427 static void smc_check_reset_syn_req(struct tcp_sock *oldtp,
428 struct request_sock *req,
429 struct tcp_sock *newtp)
431 #if IS_ENABLED(CONFIG_SMC)
432 struct inet_request_sock *ireq;
434 if (static_branch_unlikely(&tcp_have_smc)) {
435 ireq = inet_rsk(req);
436 if (oldtp->syn_smc && !ireq->smc_ok)
442 /* This is not only more efficient than what we used to do, it eliminates
443 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
445 * Actually, we could lots of memory writes here. tp of listening
446 * socket contains all necessary default parameters.
448 struct sock *tcp_create_openreq_child(const struct sock *sk,
449 struct request_sock *req,
452 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
455 const struct inet_request_sock *ireq = inet_rsk(req);
456 struct tcp_request_sock *treq = tcp_rsk(req);
457 struct inet_connection_sock *newicsk = inet_csk(newsk);
458 struct tcp_sock *newtp = tcp_sk(newsk);
459 struct tcp_sock *oldtp = tcp_sk(sk);
461 smc_check_reset_syn_req(oldtp, req, newtp);
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;
470 newtp->snd_sml = newtp->snd_una =
471 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;
473 INIT_LIST_HEAD(&newtp->tsq_node);
474 INIT_LIST_HEAD(&newtp->tsorted_sent_queue);
476 tcp_init_wl(newtp, treq->rcv_isn);
479 newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT);
480 minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U);
481 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
482 newicsk->icsk_ack.lrcvtime = tcp_jiffies32;
484 newtp->packets_out = 0;
485 newtp->retrans_out = 0;
486 newtp->sacked_out = 0;
487 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
488 newtp->tlp_high_seq = 0;
489 newtp->lsndtime = tcp_jiffies32;
490 newsk->sk_txhash = treq->txhash;
491 newtp->last_oow_ack_time = 0;
492 newtp->total_retrans = req->num_retrans;
494 /* So many TCP implementations out there (incorrectly) count the
495 * initial SYN frame in their delayed-ACK and congestion control
496 * algorithms that we must have the following bandaid to talk
497 * efficiently to them. -DaveM
499 newtp->snd_cwnd = TCP_INIT_CWND;
500 newtp->snd_cwnd_cnt = 0;
502 /* There's a bubble in the pipe until at least the first ACK. */
503 newtp->app_limited = ~0U;
505 tcp_init_xmit_timers(newsk);
506 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
508 newtp->rx_opt.saw_tstamp = 0;
510 newtp->rx_opt.dsack = 0;
511 newtp->rx_opt.num_sacks = 0;
515 if (sock_flag(newsk, SOCK_KEEPOPEN))
516 inet_csk_reset_keepalive_timer(newsk,
517 keepalive_time_when(newtp));
519 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
520 newtp->rx_opt.sack_ok = ireq->sack_ok;
521 newtp->window_clamp = req->rsk_window_clamp;
522 newtp->rcv_ssthresh = req->rsk_rcv_wnd;
523 newtp->rcv_wnd = req->rsk_rcv_wnd;
524 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
525 if (newtp->rx_opt.wscale_ok) {
526 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
527 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
529 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
530 newtp->window_clamp = min(newtp->window_clamp, 65535U);
532 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
533 newtp->rx_opt.snd_wscale);
534 newtp->max_window = newtp->snd_wnd;
536 if (newtp->rx_opt.tstamp_ok) {
537 newtp->rx_opt.ts_recent = req->ts_recent;
538 newtp->rx_opt.ts_recent_stamp = get_seconds();
539 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
541 newtp->rx_opt.ts_recent_stamp = 0;
542 newtp->tcp_header_len = sizeof(struct tcphdr);
544 newtp->tsoffset = treq->ts_off;
545 #ifdef CONFIG_TCP_MD5SIG
546 newtp->md5sig_info = NULL; /*XXX*/
547 if (newtp->af_specific->md5_lookup(sk, newsk))
548 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
550 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
551 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
552 newtp->rx_opt.mss_clamp = req->mss;
553 tcp_ecn_openreq_child(newtp, req);
554 newtp->fastopen_req = NULL;
555 newtp->fastopen_rsk = NULL;
556 newtp->syn_data_acked = 0;
557 newtp->rack.mstamp = 0;
558 newtp->rack.advanced = 0;
559 newtp->rack.reo_wnd_steps = 1;
560 newtp->rack.last_delivered = 0;
561 newtp->rack.reo_wnd_persist = 0;
562 newtp->rack.dsack_seen = 0;
564 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS);
568 EXPORT_SYMBOL(tcp_create_openreq_child);
571 * Process an incoming packet for SYN_RECV sockets represented as a
572 * request_sock. Normally sk is the listener socket but for TFO it
573 * points to the child socket.
575 * XXX (TFO) - The current impl contains a special check for ack
576 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
578 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
581 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
582 struct request_sock *req,
583 bool fastopen, bool *req_stolen)
585 struct tcp_options_received tmp_opt;
587 const struct tcphdr *th = tcp_hdr(skb);
588 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
589 bool paws_reject = false;
592 tmp_opt.saw_tstamp = 0;
593 if (th->doff > (sizeof(struct tcphdr)>>2)) {
594 tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL);
596 if (tmp_opt.saw_tstamp) {
597 tmp_opt.ts_recent = req->ts_recent;
598 if (tmp_opt.rcv_tsecr)
599 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off;
600 /* We do not store true stamp, but it is not required,
601 * it can be estimated (approximately)
604 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
605 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
609 /* Check for pure retransmitted SYN. */
610 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
611 flg == TCP_FLAG_SYN &&
614 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
615 * this case on figure 6 and figure 8, but formal
616 * protocol description says NOTHING.
617 * To be more exact, it says that we should send ACK,
618 * because this segment (at least, if it has no data)
621 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
622 * describe SYN-RECV state. All the description
623 * is wrong, we cannot believe to it and should
624 * rely only on common sense and implementation
627 * Enforce "SYN-ACK" according to figure 8, figure 6
628 * of RFC793, fixed by RFC1122.
630 * Note that even if there is new data in the SYN packet
631 * they will be thrown away too.
633 * Reset timer after retransmitting SYNACK, similar to
634 * the idea of fast retransmit in recovery.
636 if (!tcp_oow_rate_limited(sock_net(sk), skb,
637 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
638 &tcp_rsk(req)->last_oow_ack_time) &&
640 !inet_rtx_syn_ack(sk, req)) {
641 unsigned long expires = jiffies;
643 expires += min(TCP_TIMEOUT_INIT << req->num_timeout,
646 mod_timer_pending(&req->rsk_timer, expires);
648 req->rsk_timer.expires = expires;
653 /* Further reproduces section "SEGMENT ARRIVES"
654 for state SYN-RECEIVED of RFC793.
655 It is broken, however, it does not work only
656 when SYNs are crossed.
658 You would think that SYN crossing is impossible here, since
659 we should have a SYN_SENT socket (from connect()) on our end,
660 but this is not true if the crossed SYNs were sent to both
661 ends by a malicious third party. We must defend against this,
662 and to do that we first verify the ACK (as per RFC793, page
663 36) and reset if it is invalid. Is this a true full defense?
664 To convince ourselves, let us consider a way in which the ACK
665 test can still pass in this 'malicious crossed SYNs' case.
666 Malicious sender sends identical SYNs (and thus identical sequence
667 numbers) to both A and B:
672 By our good fortune, both A and B select the same initial
673 send sequence number of seven :-)
675 A: sends SYN|ACK, seq=7, ack_seq=8
676 B: sends SYN|ACK, seq=7, ack_seq=8
678 So we are now A eating this SYN|ACK, ACK test passes. So
679 does sequence test, SYN is truncated, and thus we consider
682 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
683 bare ACK. Otherwise, we create an established connection. Both
684 ends (listening sockets) accept the new incoming connection and try
685 to talk to each other. 8-)
687 Note: This case is both harmless, and rare. Possibility is about the
688 same as us discovering intelligent life on another plant tomorrow.
690 But generally, we should (RFC lies!) to accept ACK
691 from SYNACK both here and in tcp_rcv_state_process().
692 tcp_rcv_state_process() does not, hence, we do not too.
694 Note that the case is absolutely generic:
695 we cannot optimize anything here without
696 violating protocol. All the checks must be made
697 before attempt to create socket.
700 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
701 * and the incoming segment acknowledges something not yet
702 * sent (the segment carries an unacceptable ACK) ...
705 * Invalid ACK: reset will be sent by listening socket.
706 * Note that the ACK validity check for a Fast Open socket is done
707 * elsewhere and is checked directly against the child socket rather
708 * than req because user data may have been sent out.
710 if ((flg & TCP_FLAG_ACK) && !fastopen &&
711 (TCP_SKB_CB(skb)->ack_seq !=
712 tcp_rsk(req)->snt_isn + 1))
715 /* Also, it would be not so bad idea to check rcv_tsecr, which
716 * is essentially ACK extension and too early or too late values
717 * should cause reset in unsynchronized states.
720 /* RFC793: "first check sequence number". */
722 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
723 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) {
724 /* Out of window: send ACK and drop. */
725 if (!(flg & TCP_FLAG_RST) &&
726 !tcp_oow_rate_limited(sock_net(sk), skb,
727 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
728 &tcp_rsk(req)->last_oow_ack_time))
729 req->rsk_ops->send_ack(sk, skb, req);
731 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
735 /* In sequence, PAWS is OK. */
737 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
738 req->ts_recent = tmp_opt.rcv_tsval;
740 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
741 /* Truncate SYN, it is out of window starting
742 at tcp_rsk(req)->rcv_isn + 1. */
743 flg &= ~TCP_FLAG_SYN;
746 /* RFC793: "second check the RST bit" and
747 * "fourth, check the SYN bit"
749 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
750 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
751 goto embryonic_reset;
754 /* ACK sequence verified above, just make sure ACK is
755 * set. If ACK not set, just silently drop the packet.
757 * XXX (TFO) - if we ever allow "data after SYN", the
758 * following check needs to be removed.
760 if (!(flg & TCP_FLAG_ACK))
763 /* For Fast Open no more processing is needed (sk is the
769 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
770 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
771 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
772 inet_rsk(req)->acked = 1;
773 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
777 /* OK, ACK is valid, create big socket and
778 * feed this segment to it. It will repeat all
779 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
780 * ESTABLISHED STATE. If it will be dropped after
781 * socket is created, wait for troubles.
783 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
786 goto listen_overflow;
788 sock_rps_save_rxhash(child, skb);
789 tcp_synack_rtt_meas(child, req);
790 *req_stolen = !own_req;
791 return inet_csk_complete_hashdance(sk, child, req, own_req);
794 if (!sock_net(sk)->ipv4.sysctl_tcp_abort_on_overflow) {
795 inet_rsk(req)->acked = 1;
800 if (!(flg & TCP_FLAG_RST)) {
801 /* Received a bad SYN pkt - for TFO We try not to reset
802 * the local connection unless it's really necessary to
803 * avoid becoming vulnerable to outside attack aiming at
804 * resetting legit local connections.
806 req->rsk_ops->send_reset(sk, skb);
807 } else if (fastopen) { /* received a valid RST pkt */
808 reqsk_fastopen_remove(sk, req, true);
812 inet_csk_reqsk_queue_drop(sk, req);
813 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
817 EXPORT_SYMBOL(tcp_check_req);
820 * Queue segment on the new socket if the new socket is active,
821 * otherwise we just shortcircuit this and continue with
824 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
825 * when entering. But other states are possible due to a race condition
826 * where after __inet_lookup_established() fails but before the listener
827 * locked is obtained, other packets cause the same connection to
831 int tcp_child_process(struct sock *parent, struct sock *child,
835 int state = child->sk_state;
837 /* record NAPI ID of child */
838 sk_mark_napi_id(child, skb);
840 tcp_segs_in(tcp_sk(child), skb);
841 if (!sock_owned_by_user(child)) {
842 ret = tcp_rcv_state_process(child, skb);
843 /* Wakeup parent, send SIGIO */
844 if (state == TCP_SYN_RECV && child->sk_state != state)
845 parent->sk_data_ready(parent);
847 /* Alas, it is possible again, because we do lookup
848 * in main socket hash table and lock on listening
849 * socket does not protect us more.
851 __sk_add_backlog(child, skb);
854 bh_unlock_sock(child);
858 EXPORT_SYMBOL(tcp_child_process);