1 // SPDX-License-Identifier: GPL-2.0-only
3 * INET An implementation of the TCP/IP protocol suite for the LINUX
4 * operating system. INET is implemented using the BSD Socket
5 * interface as the means of communication with the user level.
7 * Implementation of the Transmission Control Protocol(TCP).
10 * Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
11 * Mark Evans, <evansmp@uhura.aston.ac.uk>
12 * Corey Minyard <wf-rch!minyard@relay.EU.net>
13 * Florian La Roche, <flla@stud.uni-sb.de>
14 * Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
15 * Linus Torvalds, <torvalds@cs.helsinki.fi>
16 * Alan Cox, <gw4pts@gw4pts.ampr.org>
17 * Matthew Dillon, <dillon@apollo.west.oic.com>
18 * Arnt Gulbrandsen, <agulbra@nvg.unit.no>
19 * Jorge Cwik, <jorge@laser.satlink.net>
23 #include <linux/module.h>
24 #include <linux/slab.h>
25 #include <linux/sysctl.h>
26 #include <linux/workqueue.h>
27 #include <linux/static_key.h>
29 #include <net/inet_common.h>
31 #include <net/busy_poll.h>
33 static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
37 if (after(end_seq, s_win) && before(seq, e_win))
39 return seq == e_win && seq == end_seq;
42 static enum tcp_tw_status
43 tcp_timewait_check_oow_rate_limit(struct inet_timewait_sock *tw,
44 const struct sk_buff *skb, int mib_idx)
46 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
48 if (!tcp_oow_rate_limited(twsk_net(tw), skb, mib_idx,
49 &tcptw->tw_last_oow_ack_time)) {
50 /* Send ACK. Note, we do not put the bucket,
51 * it will be released by caller.
56 /* We are rate-limiting, so just release the tw sock and drop skb. */
58 return TCP_TW_SUCCESS;
62 * * Main purpose of TIME-WAIT state is to close connection gracefully,
63 * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
64 * (and, probably, tail of data) and one or more our ACKs are lost.
65 * * What is TIME-WAIT timeout? It is associated with maximal packet
66 * lifetime in the internet, which results in wrong conclusion, that
67 * it is set to catch "old duplicate segments" wandering out of their path.
68 * It is not quite correct. This timeout is calculated so that it exceeds
69 * maximal retransmission timeout enough to allow to lose one (or more)
70 * segments sent by peer and our ACKs. This time may be calculated from RTO.
71 * * When TIME-WAIT socket receives RST, it means that another end
72 * finally closed and we are allowed to kill TIME-WAIT too.
73 * * Second purpose of TIME-WAIT is catching old duplicate segments.
74 * Well, certainly it is pure paranoia, but if we load TIME-WAIT
75 * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
76 * * If we invented some more clever way to catch duplicates
77 * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
79 * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
80 * When you compare it to RFCs, please, read section SEGMENT ARRIVES
81 * from the very beginning.
83 * NOTE. With recycling (and later with fin-wait-2) TW bucket
84 * is _not_ stateless. It means, that strictly speaking we must
85 * spinlock it. I do not want! Well, probability of misbehaviour
86 * is ridiculously low and, seems, we could use some mb() tricks
87 * to avoid misread sequence numbers, states etc. --ANK
89 * We don't need to initialize tmp_out.sack_ok as we don't use the results
92 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
93 const struct tcphdr *th)
95 struct tcp_options_received tmp_opt;
96 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
97 bool paws_reject = false;
99 tmp_opt.saw_tstamp = 0;
100 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
101 tcp_parse_options(twsk_net(tw), skb, &tmp_opt, 0, NULL);
103 if (tmp_opt.saw_tstamp) {
104 if (tmp_opt.rcv_tsecr)
105 tmp_opt.rcv_tsecr -= tcptw->tw_ts_offset;
106 tmp_opt.ts_recent = tcptw->tw_ts_recent;
107 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
108 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
112 if (tw->tw_substate == TCP_FIN_WAIT2) {
113 /* Just repeat all the checks of tcp_rcv_state_process() */
115 /* Out of window, send ACK */
117 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
119 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
120 return tcp_timewait_check_oow_rate_limit(
121 tw, skb, LINUX_MIB_TCPACKSKIPPEDFINWAIT2);
126 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
131 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
132 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
134 return TCP_TW_SUCCESS;
137 /* New data or FIN. If new data arrive after half-duplex close,
141 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1)
144 /* FIN arrived, enter true time-wait state. */
145 tw->tw_substate = TCP_TIME_WAIT;
146 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
147 if (tmp_opt.saw_tstamp) {
148 tcptw->tw_ts_recent_stamp = ktime_get_seconds();
149 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
152 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
157 * Now real TIME-WAIT state.
160 * "When a connection is [...] on TIME-WAIT state [...]
161 * [a TCP] MAY accept a new SYN from the remote TCP to
162 * reopen the connection directly, if it:
164 * (1) assigns its initial sequence number for the new
165 * connection to be larger than the largest sequence
166 * number it used on the previous connection incarnation,
169 * (2) returns to TIME-WAIT state if the SYN turns out
170 * to be an old duplicate".
174 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
175 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
176 /* In window segment, it may be only reset or bare ack. */
179 /* This is TIME_WAIT assassination, in two flavors.
180 * Oh well... nobody has a sufficient solution to this
183 if (!READ_ONCE(twsk_net(tw)->ipv4.sysctl_tcp_rfc1337)) {
185 inet_twsk_deschedule_put(tw);
186 return TCP_TW_SUCCESS;
189 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
192 if (tmp_opt.saw_tstamp) {
193 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
194 tcptw->tw_ts_recent_stamp = ktime_get_seconds();
198 return TCP_TW_SUCCESS;
201 /* Out of window segment.
203 All the segments are ACKed immediately.
205 The only exception is new SYN. We accept it, if it is
206 not old duplicate and we are not in danger to be killed
207 by delayed old duplicates. RFC check is that it has
208 newer sequence number works at rates <40Mbit/sec.
209 However, if paws works, it is reliable AND even more,
210 we even may relax silly seq space cutoff.
212 RED-PEN: we violate main RFC requirement, if this SYN will appear
213 old duplicate (i.e. we receive RST in reply to SYN-ACK),
214 we must return socket to time-wait state. It is not good,
218 if (th->syn && !th->rst && !th->ack && !paws_reject &&
219 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
220 (tmp_opt.saw_tstamp &&
221 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
222 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
225 TCP_SKB_CB(skb)->tcp_tw_isn = isn;
230 __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
233 /* In this case we must reset the TIMEWAIT timer.
235 * If it is ACKless SYN it may be both old duplicate
236 * and new good SYN with random sequence number <rcv_nxt.
237 * Do not reschedule in the last case.
239 if (paws_reject || th->ack)
240 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
242 return tcp_timewait_check_oow_rate_limit(
243 tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT);
246 return TCP_TW_SUCCESS;
248 EXPORT_SYMBOL(tcp_timewait_state_process);
251 * Move a socket to time-wait or dead fin-wait-2 state.
253 void tcp_time_wait(struct sock *sk, int state, int timeo)
255 const struct inet_connection_sock *icsk = inet_csk(sk);
256 const struct tcp_sock *tp = tcp_sk(sk);
257 struct inet_timewait_sock *tw;
258 struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row;
260 tw = inet_twsk_alloc(sk, tcp_death_row, state);
263 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
264 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
265 struct inet_sock *inet = inet_sk(sk);
267 tw->tw_transparent = inet->transparent;
268 tw->tw_mark = sk->sk_mark;
269 tw->tw_priority = sk->sk_priority;
270 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale;
271 tcptw->tw_rcv_nxt = tp->rcv_nxt;
272 tcptw->tw_snd_nxt = tp->snd_nxt;
273 tcptw->tw_rcv_wnd = tcp_receive_window(tp);
274 tcptw->tw_ts_recent = tp->rx_opt.ts_recent;
275 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
276 tcptw->tw_ts_offset = tp->tsoffset;
277 tcptw->tw_last_oow_ack_time = 0;
278 tcptw->tw_tx_delay = tp->tcp_tx_delay;
279 #if IS_ENABLED(CONFIG_IPV6)
280 if (tw->tw_family == PF_INET6) {
281 struct ipv6_pinfo *np = inet6_sk(sk);
283 tw->tw_v6_daddr = sk->sk_v6_daddr;
284 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
285 tw->tw_tclass = np->tclass;
286 tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK);
287 tw->tw_txhash = sk->sk_txhash;
288 tw->tw_ipv6only = sk->sk_ipv6only;
292 #ifdef CONFIG_TCP_MD5SIG
294 * The timewait bucket does not have the key DB from the
295 * sock structure. We just make a quick copy of the
296 * md5 key being used (if indeed we are using one)
297 * so the timewait ack generating code has the key.
300 tcptw->tw_md5_key = NULL;
301 if (static_branch_unlikely(&tcp_md5_needed)) {
302 struct tcp_md5sig_key *key;
304 key = tp->af_specific->md5_lookup(sk, sk);
306 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
307 BUG_ON(tcptw->tw_md5_key && !tcp_alloc_md5sig_pool());
313 /* Get the TIME_WAIT timeout firing. */
317 if (state == TCP_TIME_WAIT)
318 timeo = TCP_TIMEWAIT_LEN;
320 /* tw_timer is pinned, so we need to make sure BH are disabled
321 * in following section, otherwise timer handler could run before
322 * we complete the initialization.
325 inet_twsk_schedule(tw, timeo);
327 * Note that access to tw after this point is illegal.
329 inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
332 /* Sorry, if we're out of memory, just CLOSE this
333 * socket up. We've got bigger problems than
334 * non-graceful socket closings.
336 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW);
339 tcp_update_metrics(sk);
342 EXPORT_SYMBOL(tcp_time_wait);
344 void tcp_twsk_destructor(struct sock *sk)
346 #ifdef CONFIG_TCP_MD5SIG
347 if (static_branch_unlikely(&tcp_md5_needed)) {
348 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
350 if (twsk->tw_md5_key)
351 kfree_rcu(twsk->tw_md5_key, rcu);
355 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
357 /* Warning : This function is called without sk_listener being locked.
358 * Be sure to read socket fields once, as their value could change under us.
360 void tcp_openreq_init_rwin(struct request_sock *req,
361 const struct sock *sk_listener,
362 const struct dst_entry *dst)
364 struct inet_request_sock *ireq = inet_rsk(req);
365 const struct tcp_sock *tp = tcp_sk(sk_listener);
366 int full_space = tcp_full_space(sk_listener);
372 mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
373 window_clamp = READ_ONCE(tp->window_clamp);
374 /* Set this up on the first call only */
375 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);
377 /* limit the window selection if the user enforce a smaller rx buffer */
378 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK &&
379 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
380 req->rsk_window_clamp = full_space;
382 rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req);
384 rcv_wnd = dst_metric(dst, RTAX_INITRWND);
385 else if (full_space < rcv_wnd * mss)
386 full_space = rcv_wnd * mss;
388 /* tcp_full_space because it is guaranteed to be the first packet */
389 tcp_select_initial_window(sk_listener, full_space,
390 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
392 &req->rsk_window_clamp,
396 ireq->rcv_wscale = rcv_wscale;
398 EXPORT_SYMBOL(tcp_openreq_init_rwin);
400 static void tcp_ecn_openreq_child(struct tcp_sock *tp,
401 const struct request_sock *req)
403 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
406 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
408 struct inet_connection_sock *icsk = inet_csk(sk);
409 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
410 bool ca_got_dst = false;
412 if (ca_key != TCP_CA_UNSPEC) {
413 const struct tcp_congestion_ops *ca;
416 ca = tcp_ca_find_key(ca_key);
417 if (likely(ca && bpf_try_module_get(ca, ca->owner))) {
418 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
419 icsk->icsk_ca_ops = ca;
425 /* If no valid choice made yet, assign current system default ca. */
427 (!icsk->icsk_ca_setsockopt ||
428 !bpf_try_module_get(icsk->icsk_ca_ops, icsk->icsk_ca_ops->owner)))
429 tcp_assign_congestion_control(sk);
431 tcp_set_ca_state(sk, TCP_CA_Open);
433 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child);
435 static void smc_check_reset_syn_req(struct tcp_sock *oldtp,
436 struct request_sock *req,
437 struct tcp_sock *newtp)
439 #if IS_ENABLED(CONFIG_SMC)
440 struct inet_request_sock *ireq;
442 if (static_branch_unlikely(&tcp_have_smc)) {
443 ireq = inet_rsk(req);
444 if (oldtp->syn_smc && !ireq->smc_ok)
450 /* This is not only more efficient than what we used to do, it eliminates
451 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
453 * Actually, we could lots of memory writes here. tp of listening
454 * socket contains all necessary default parameters.
456 struct sock *tcp_create_openreq_child(const struct sock *sk,
457 struct request_sock *req,
460 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
461 const struct inet_request_sock *ireq = inet_rsk(req);
462 struct tcp_request_sock *treq = tcp_rsk(req);
463 struct inet_connection_sock *newicsk;
464 struct tcp_sock *oldtp, *newtp;
470 newicsk = inet_csk(newsk);
471 newtp = tcp_sk(newsk);
474 smc_check_reset_syn_req(oldtp, req, newtp);
476 /* Now setup tcp_sock */
477 newtp->pred_flags = 0;
479 seq = treq->rcv_isn + 1;
480 newtp->rcv_wup = seq;
481 WRITE_ONCE(newtp->copied_seq, seq);
482 WRITE_ONCE(newtp->rcv_nxt, seq);
485 seq = treq->snt_isn + 1;
486 newtp->snd_sml = newtp->snd_una = seq;
487 WRITE_ONCE(newtp->snd_nxt, seq);
490 INIT_LIST_HEAD(&newtp->tsq_node);
491 INIT_LIST_HEAD(&newtp->tsorted_sent_queue);
493 tcp_init_wl(newtp, treq->rcv_isn);
495 minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U);
496 newicsk->icsk_ack.lrcvtime = tcp_jiffies32;
498 newtp->lsndtime = tcp_jiffies32;
499 newsk->sk_txhash = treq->txhash;
500 newtp->total_retrans = req->num_retrans;
502 tcp_init_xmit_timers(newsk);
503 WRITE_ONCE(newtp->write_seq, newtp->pushed_seq = treq->snt_isn + 1);
505 if (sock_flag(newsk, SOCK_KEEPOPEN))
506 inet_csk_reset_keepalive_timer(newsk,
507 keepalive_time_when(newtp));
509 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
510 newtp->rx_opt.sack_ok = ireq->sack_ok;
511 newtp->window_clamp = req->rsk_window_clamp;
512 newtp->rcv_ssthresh = req->rsk_rcv_wnd;
513 newtp->rcv_wnd = req->rsk_rcv_wnd;
514 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
515 if (newtp->rx_opt.wscale_ok) {
516 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
517 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
519 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
520 newtp->window_clamp = min(newtp->window_clamp, 65535U);
522 newtp->snd_wnd = ntohs(tcp_hdr(skb)->window) << newtp->rx_opt.snd_wscale;
523 newtp->max_window = newtp->snd_wnd;
525 if (newtp->rx_opt.tstamp_ok) {
526 newtp->rx_opt.ts_recent = READ_ONCE(req->ts_recent);
527 newtp->rx_opt.ts_recent_stamp = ktime_get_seconds();
528 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
530 newtp->rx_opt.ts_recent_stamp = 0;
531 newtp->tcp_header_len = sizeof(struct tcphdr);
533 if (req->num_timeout) {
534 newtp->undo_marker = treq->snt_isn;
535 newtp->retrans_stamp = div_u64(treq->snt_synack,
536 USEC_PER_SEC / TCP_TS_HZ);
538 newtp->tsoffset = treq->ts_off;
539 #ifdef CONFIG_TCP_MD5SIG
540 newtp->md5sig_info = NULL; /*XXX*/
541 if (treq->af_specific->req_md5_lookup(sk, req_to_sk(req)))
542 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
544 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
545 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
546 newtp->rx_opt.mss_clamp = req->mss;
547 tcp_ecn_openreq_child(newtp, req);
548 newtp->fastopen_req = NULL;
549 RCU_INIT_POINTER(newtp->fastopen_rsk, NULL);
551 tcp_bpf_clone(sk, newsk);
553 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS);
557 EXPORT_SYMBOL(tcp_create_openreq_child);
560 * Process an incoming packet for SYN_RECV sockets represented as a
561 * request_sock. Normally sk is the listener socket but for TFO it
562 * points to the child socket.
564 * XXX (TFO) - The current impl contains a special check for ack
565 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
567 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
569 * Note: If @fastopen is true, this can be called from process context.
570 * Otherwise, this is from BH context.
573 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
574 struct request_sock *req,
575 bool fastopen, bool *req_stolen)
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;
584 tmp_opt.saw_tstamp = 0;
585 if (th->doff > (sizeof(struct tcphdr)>>2)) {
586 tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL);
588 if (tmp_opt.saw_tstamp) {
589 tmp_opt.ts_recent = READ_ONCE(req->ts_recent);
590 if (tmp_opt.rcv_tsecr)
591 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off;
592 /* We do not store true stamp, but it is not required,
593 * it can be estimated (approximately)
596 tmp_opt.ts_recent_stamp = ktime_get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
597 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
601 /* Check for pure retransmitted SYN. */
602 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
603 flg == TCP_FLAG_SYN &&
606 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
607 * this case on figure 6 and figure 8, but formal
608 * protocol description says NOTHING.
609 * To be more exact, it says that we should send ACK,
610 * because this segment (at least, if it has no data)
613 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
614 * describe SYN-RECV state. All the description
615 * is wrong, we cannot believe to it and should
616 * rely only on common sense and implementation
619 * Enforce "SYN-ACK" according to figure 8, figure 6
620 * of RFC793, fixed by RFC1122.
622 * Note that even if there is new data in the SYN packet
623 * they will be thrown away too.
625 * Reset timer after retransmitting SYNACK, similar to
626 * the idea of fast retransmit in recovery.
628 if (!tcp_oow_rate_limited(sock_net(sk), skb,
629 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
630 &tcp_rsk(req)->last_oow_ack_time) &&
632 !inet_rtx_syn_ack(sk, req)) {
633 unsigned long expires = jiffies;
635 expires += min(TCP_TIMEOUT_INIT << req->num_timeout,
638 mod_timer_pending(&req->rsk_timer, expires);
640 req->rsk_timer.expires = expires;
645 /* Further reproduces section "SEGMENT ARRIVES"
646 for state SYN-RECEIVED of RFC793.
647 It is broken, however, it does not work only
648 when SYNs are crossed.
650 You would think that SYN crossing is impossible here, since
651 we should have a SYN_SENT socket (from connect()) on our end,
652 but this is not true if the crossed SYNs were sent to both
653 ends by a malicious third party. We must defend against this,
654 and to do that we first verify the ACK (as per RFC793, page
655 36) and reset if it is invalid. Is this a true full defense?
656 To convince ourselves, let us consider a way in which the ACK
657 test can still pass in this 'malicious crossed SYNs' case.
658 Malicious sender sends identical SYNs (and thus identical sequence
659 numbers) to both A and B:
664 By our good fortune, both A and B select the same initial
665 send sequence number of seven :-)
667 A: sends SYN|ACK, seq=7, ack_seq=8
668 B: sends SYN|ACK, seq=7, ack_seq=8
670 So we are now A eating this SYN|ACK, ACK test passes. So
671 does sequence test, SYN is truncated, and thus we consider
674 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
675 bare ACK. Otherwise, we create an established connection. Both
676 ends (listening sockets) accept the new incoming connection and try
677 to talk to each other. 8-)
679 Note: This case is both harmless, and rare. Possibility is about the
680 same as us discovering intelligent life on another plant tomorrow.
682 But generally, we should (RFC lies!) to accept ACK
683 from SYNACK both here and in tcp_rcv_state_process().
684 tcp_rcv_state_process() does not, hence, we do not too.
686 Note that the case is absolutely generic:
687 we cannot optimize anything here without
688 violating protocol. All the checks must be made
689 before attempt to create socket.
692 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
693 * and the incoming segment acknowledges something not yet
694 * sent (the segment carries an unacceptable ACK) ...
697 * Invalid ACK: reset will be sent by listening socket.
698 * Note that the ACK validity check for a Fast Open socket is done
699 * elsewhere and is checked directly against the child socket rather
700 * than req because user data may have been sent out.
702 if ((flg & TCP_FLAG_ACK) && !fastopen &&
703 (TCP_SKB_CB(skb)->ack_seq !=
704 tcp_rsk(req)->snt_isn + 1))
707 /* Also, it would be not so bad idea to check rcv_tsecr, which
708 * is essentially ACK extension and too early or too late values
709 * should cause reset in unsynchronized states.
712 /* RFC793: "first check sequence number". */
714 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
715 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) {
716 /* Out of window: send ACK and drop. */
717 if (!(flg & TCP_FLAG_RST) &&
718 !tcp_oow_rate_limited(sock_net(sk), skb,
719 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
720 &tcp_rsk(req)->last_oow_ack_time))
721 req->rsk_ops->send_ack(sk, skb, req);
723 NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
727 /* In sequence, PAWS is OK. */
729 /* TODO: We probably should defer ts_recent change once
730 * we take ownership of @req.
732 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
733 WRITE_ONCE(req->ts_recent, tmp_opt.rcv_tsval);
735 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
736 /* Truncate SYN, it is out of window starting
737 at tcp_rsk(req)->rcv_isn + 1. */
738 flg &= ~TCP_FLAG_SYN;
741 /* RFC793: "second check the RST bit" and
742 * "fourth, check the SYN bit"
744 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
745 TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
746 goto embryonic_reset;
749 /* ACK sequence verified above, just make sure ACK is
750 * set. If ACK not set, just silently drop the packet.
752 * XXX (TFO) - if we ever allow "data after SYN", the
753 * following check needs to be removed.
755 if (!(flg & TCP_FLAG_ACK))
758 /* For Fast Open no more processing is needed (sk is the
764 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
765 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
766 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
767 inet_rsk(req)->acked = 1;
768 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
772 /* OK, ACK is valid, create big socket and
773 * feed this segment to it. It will repeat all
774 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
775 * ESTABLISHED STATE. If it will be dropped after
776 * socket is created, wait for troubles.
778 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
781 goto listen_overflow;
783 if (own_req && rsk_drop_req(req)) {
784 reqsk_queue_removed(&inet_csk(req->rsk_listener)->icsk_accept_queue, req);
785 inet_csk_reqsk_queue_drop_and_put(req->rsk_listener, req);
789 sock_rps_save_rxhash(child, skb);
790 tcp_synack_rtt_meas(child, req);
791 *req_stolen = !own_req;
792 return inet_csk_complete_hashdance(sk, child, req, own_req);
795 if (sk != req->rsk_listener)
796 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPMIGRATEREQFAILURE);
798 if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_abort_on_overflow)) {
799 inet_rsk(req)->acked = 1;
804 if (!(flg & TCP_FLAG_RST)) {
805 /* Received a bad SYN pkt - for TFO We try not to reset
806 * the local connection unless it's really necessary to
807 * avoid becoming vulnerable to outside attack aiming at
808 * resetting legit local connections.
810 req->rsk_ops->send_reset(sk, skb);
811 } else if (fastopen) { /* received a valid RST pkt */
812 reqsk_fastopen_remove(sk, req, true);
816 bool unlinked = inet_csk_reqsk_queue_drop(sk, req);
819 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
820 *req_stolen = !unlinked;
824 EXPORT_SYMBOL(tcp_check_req);
827 * Queue segment on the new socket if the new socket is active,
828 * otherwise we just shortcircuit this and continue with
831 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
832 * when entering. But other states are possible due to a race condition
833 * where after __inet_lookup_established() fails but before the listener
834 * locked is obtained, other packets cause the same connection to
838 int tcp_child_process(struct sock *parent, struct sock *child,
840 __releases(&((child)->sk_lock.slock))
843 int state = child->sk_state;
845 /* record NAPI ID of child */
846 sk_mark_napi_id(child, skb);
848 tcp_segs_in(tcp_sk(child), skb);
849 if (!sock_owned_by_user(child)) {
850 ret = tcp_rcv_state_process(child, skb);
851 /* Wakeup parent, send SIGIO */
852 if (state == TCP_SYN_RECV && child->sk_state != state)
853 parent->sk_data_ready(parent);
855 /* Alas, it is possible again, because we do lookup
856 * in main socket hash table and lock on listening
857 * socket does not protect us more.
859 __sk_add_backlog(child, skb);
862 bh_unlock_sock(child);
866 EXPORT_SYMBOL(tcp_child_process);