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>
24 #include <net/busy_poll.h>
26 static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
30 if (after(end_seq, s_win) && before(seq, e_win))
32 return seq == e_win && seq == end_seq;
35 static enum tcp_tw_status
36 tcp_timewait_check_oow_rate_limit(struct inet_timewait_sock *tw,
37 const struct sk_buff *skb, int mib_idx)
39 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
41 if (!tcp_oow_rate_limited(twsk_net(tw), skb, mib_idx,
42 &tcptw->tw_last_oow_ack_time)) {
43 /* Send ACK. Note, we do not put the bucket,
44 * it will be released by caller.
49 /* We are rate-limiting, so just release the tw sock and drop skb. */
51 return TCP_TW_SUCCESS;
55 * * Main purpose of TIME-WAIT state is to close connection gracefully,
56 * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
57 * (and, probably, tail of data) and one or more our ACKs are lost.
58 * * What is TIME-WAIT timeout? It is associated with maximal packet
59 * lifetime in the internet, which results in wrong conclusion, that
60 * it is set to catch "old duplicate segments" wandering out of their path.
61 * It is not quite correct. This timeout is calculated so that it exceeds
62 * maximal retransmission timeout enough to allow to lose one (or more)
63 * segments sent by peer and our ACKs. This time may be calculated from RTO.
64 * * When TIME-WAIT socket receives RST, it means that another end
65 * finally closed and we are allowed to kill TIME-WAIT too.
66 * * Second purpose of TIME-WAIT is catching old duplicate segments.
67 * Well, certainly it is pure paranoia, but if we load TIME-WAIT
68 * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
69 * * If we invented some more clever way to catch duplicates
70 * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
72 * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
73 * When you compare it to RFCs, please, read section SEGMENT ARRIVES
74 * from the very beginning.
76 * NOTE. With recycling (and later with fin-wait-2) TW bucket
77 * is _not_ stateless. It means, that strictly speaking we must
78 * spinlock it. I do not want! Well, probability of misbehaviour
79 * is ridiculously low and, seems, we could use some mb() tricks
80 * to avoid misread sequence numbers, states etc. --ANK
82 * We don't need to initialize tmp_out.sack_ok as we don't use the results
85 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
86 const struct tcphdr *th)
88 struct tcp_options_received tmp_opt;
89 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
90 bool paws_reject = false;
92 tmp_opt.saw_tstamp = 0;
93 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
94 tcp_parse_options(twsk_net(tw), skb, &tmp_opt, 0, NULL);
96 if (tmp_opt.saw_tstamp) {
97 if (tmp_opt.rcv_tsecr)
98 tmp_opt.rcv_tsecr -= tcptw->tw_ts_offset;
99 tmp_opt.ts_recent = tcptw->tw_ts_recent;
100 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
101 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
105 if (tw->tw_substate == TCP_FIN_WAIT2) {
106 /* Just repeat all the checks of tcp_rcv_state_process() */
108 /* Out of window, send ACK */
110 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
112 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
113 return tcp_timewait_check_oow_rate_limit(
114 tw, skb, LINUX_MIB_TCPACKSKIPPEDFINWAIT2);
119 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
124 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
125 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
127 return TCP_TW_SUCCESS;
130 /* New data or FIN. If new data arrive after half-duplex close,
134 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1)
137 /* FIN arrived, enter true time-wait state. */
138 tw->tw_substate = TCP_TIME_WAIT;
139 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
140 if (tmp_opt.saw_tstamp) {
141 tcptw->tw_ts_recent_stamp = ktime_get_seconds();
142 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
145 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
150 * Now real TIME-WAIT state.
153 * "When a connection is [...] on TIME-WAIT state [...]
154 * [a TCP] MAY accept a new SYN from the remote TCP to
155 * reopen the connection directly, if it:
157 * (1) assigns its initial sequence number for the new
158 * connection to be larger than the largest sequence
159 * number it used on the previous connection incarnation,
162 * (2) returns to TIME-WAIT state if the SYN turns out
163 * to be an old duplicate".
167 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
168 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
169 /* In window segment, it may be only reset or bare ack. */
172 /* This is TIME_WAIT assassination, in two flavors.
173 * Oh well... nobody has a sufficient solution to this
176 if (!READ_ONCE(twsk_net(tw)->ipv4.sysctl_tcp_rfc1337)) {
178 inet_twsk_deschedule_put(tw);
179 return TCP_TW_SUCCESS;
182 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
185 if (tmp_opt.saw_tstamp) {
186 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
187 tcptw->tw_ts_recent_stamp = ktime_get_seconds();
191 return TCP_TW_SUCCESS;
194 /* Out of window segment.
196 All the segments are ACKed immediately.
198 The only exception is new SYN. We accept it, if it is
199 not old duplicate and we are not in danger to be killed
200 by delayed old duplicates. RFC check is that it has
201 newer sequence number works at rates <40Mbit/sec.
202 However, if paws works, it is reliable AND even more,
203 we even may relax silly seq space cutoff.
205 RED-PEN: we violate main RFC requirement, if this SYN will appear
206 old duplicate (i.e. we receive RST in reply to SYN-ACK),
207 we must return socket to time-wait state. It is not good,
211 if (th->syn && !th->rst && !th->ack && !paws_reject &&
212 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
213 (tmp_opt.saw_tstamp &&
214 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
215 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
218 TCP_SKB_CB(skb)->tcp_tw_isn = isn;
223 __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
226 /* In this case we must reset the TIMEWAIT timer.
228 * If it is ACKless SYN it may be both old duplicate
229 * and new good SYN with random sequence number <rcv_nxt.
230 * Do not reschedule in the last case.
232 if (paws_reject || th->ack)
233 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
235 return tcp_timewait_check_oow_rate_limit(
236 tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT);
239 return TCP_TW_SUCCESS;
241 EXPORT_SYMBOL(tcp_timewait_state_process);
244 * Move a socket to time-wait or dead fin-wait-2 state.
246 void tcp_time_wait(struct sock *sk, int state, int timeo)
248 const struct inet_connection_sock *icsk = inet_csk(sk);
249 const struct tcp_sock *tp = tcp_sk(sk);
250 struct net *net = sock_net(sk);
251 struct inet_timewait_sock *tw;
253 tw = inet_twsk_alloc(sk, &net->ipv4.tcp_death_row, state);
256 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
257 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
258 struct inet_sock *inet = inet_sk(sk);
260 tw->tw_transparent = inet->transparent;
261 tw->tw_mark = sk->sk_mark;
262 tw->tw_priority = sk->sk_priority;
263 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale;
264 tcptw->tw_rcv_nxt = tp->rcv_nxt;
265 tcptw->tw_snd_nxt = tp->snd_nxt;
266 tcptw->tw_rcv_wnd = tcp_receive_window(tp);
267 tcptw->tw_ts_recent = tp->rx_opt.ts_recent;
268 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
269 tcptw->tw_ts_offset = tp->tsoffset;
270 tcptw->tw_last_oow_ack_time = 0;
271 tcptw->tw_tx_delay = tp->tcp_tx_delay;
272 #if IS_ENABLED(CONFIG_IPV6)
273 if (tw->tw_family == PF_INET6) {
274 struct ipv6_pinfo *np = inet6_sk(sk);
276 tw->tw_v6_daddr = sk->sk_v6_daddr;
277 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
278 tw->tw_tclass = np->tclass;
279 tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK);
280 tw->tw_txhash = sk->sk_txhash;
281 tw->tw_ipv6only = sk->sk_ipv6only;
285 #ifdef CONFIG_TCP_MD5SIG
287 * The timewait bucket does not have the key DB from the
288 * sock structure. We just make a quick copy of the
289 * md5 key being used (if indeed we are using one)
290 * so the timewait ack generating code has the key.
293 tcptw->tw_md5_key = NULL;
294 if (static_branch_unlikely(&tcp_md5_needed)) {
295 struct tcp_md5sig_key *key;
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());
306 /* Get the TIME_WAIT timeout firing. */
310 if (state == TCP_TIME_WAIT)
311 timeo = TCP_TIMEWAIT_LEN;
313 /* tw_timer is pinned, so we need to make sure BH are disabled
314 * in following section, otherwise timer handler could run before
315 * we complete the initialization.
318 inet_twsk_schedule(tw, timeo);
320 * Note that access to tw after this point is illegal.
322 inet_twsk_hashdance(tw, sk, net->ipv4.tcp_death_row.hashinfo);
325 /* Sorry, if we're out of memory, just CLOSE this
326 * socket up. We've got bigger problems than
327 * non-graceful socket closings.
329 NET_INC_STATS(net, LINUX_MIB_TCPTIMEWAITOVERFLOW);
332 tcp_update_metrics(sk);
335 EXPORT_SYMBOL(tcp_time_wait);
337 void tcp_twsk_destructor(struct sock *sk)
339 #ifdef CONFIG_TCP_MD5SIG
340 if (static_branch_unlikely(&tcp_md5_needed)) {
341 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
343 if (twsk->tw_md5_key)
344 kfree_rcu(twsk->tw_md5_key, rcu);
348 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
350 void tcp_twsk_purge(struct list_head *net_exit_list, int family)
352 bool purged_once = false;
355 list_for_each_entry(net, net_exit_list, exit_list) {
356 if (net->ipv4.tcp_death_row.hashinfo->pernet) {
357 /* Even if tw_refcount == 1, we must clean up kernel reqsk */
358 inet_twsk_purge(net->ipv4.tcp_death_row.hashinfo, family);
359 } else if (!purged_once) {
360 inet_twsk_purge(&tcp_hashinfo, family);
365 EXPORT_SYMBOL_GPL(tcp_twsk_purge);
367 /* Warning : This function is called without sk_listener being locked.
368 * Be sure to read socket fields once, as their value could change under us.
370 void tcp_openreq_init_rwin(struct request_sock *req,
371 const struct sock *sk_listener,
372 const struct dst_entry *dst)
374 struct inet_request_sock *ireq = inet_rsk(req);
375 const struct tcp_sock *tp = tcp_sk(sk_listener);
376 int full_space = tcp_full_space(sk_listener);
382 mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
383 window_clamp = READ_ONCE(tp->window_clamp);
384 /* Set this up on the first call only */
385 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);
387 /* limit the window selection if the user enforce a smaller rx buffer */
388 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK &&
389 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
390 req->rsk_window_clamp = full_space;
392 rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req);
394 rcv_wnd = dst_metric(dst, RTAX_INITRWND);
395 else if (full_space < rcv_wnd * mss)
396 full_space = rcv_wnd * mss;
398 /* tcp_full_space because it is guaranteed to be the first packet */
399 tcp_select_initial_window(sk_listener, full_space,
400 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
402 &req->rsk_window_clamp,
406 ireq->rcv_wscale = rcv_wscale;
408 EXPORT_SYMBOL(tcp_openreq_init_rwin);
410 static void tcp_ecn_openreq_child(struct tcp_sock *tp,
411 const struct request_sock *req)
413 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
416 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
418 struct inet_connection_sock *icsk = inet_csk(sk);
419 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
420 bool ca_got_dst = false;
422 if (ca_key != TCP_CA_UNSPEC) {
423 const struct tcp_congestion_ops *ca;
426 ca = tcp_ca_find_key(ca_key);
427 if (likely(ca && bpf_try_module_get(ca, ca->owner))) {
428 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
429 icsk->icsk_ca_ops = ca;
435 /* If no valid choice made yet, assign current system default ca. */
437 (!icsk->icsk_ca_setsockopt ||
438 !bpf_try_module_get(icsk->icsk_ca_ops, icsk->icsk_ca_ops->owner)))
439 tcp_assign_congestion_control(sk);
441 tcp_set_ca_state(sk, TCP_CA_Open);
443 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child);
445 static void smc_check_reset_syn_req(struct tcp_sock *oldtp,
446 struct request_sock *req,
447 struct tcp_sock *newtp)
449 #if IS_ENABLED(CONFIG_SMC)
450 struct inet_request_sock *ireq;
452 if (static_branch_unlikely(&tcp_have_smc)) {
453 ireq = inet_rsk(req);
454 if (oldtp->syn_smc && !ireq->smc_ok)
460 /* This is not only more efficient than what we used to do, it eliminates
461 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
463 * Actually, we could lots of memory writes here. tp of listening
464 * socket contains all necessary default parameters.
466 struct sock *tcp_create_openreq_child(const struct sock *sk,
467 struct request_sock *req,
470 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
471 const struct inet_request_sock *ireq = inet_rsk(req);
472 struct tcp_request_sock *treq = tcp_rsk(req);
473 struct inet_connection_sock *newicsk;
474 struct tcp_sock *oldtp, *newtp;
480 newicsk = inet_csk(newsk);
481 newtp = tcp_sk(newsk);
484 smc_check_reset_syn_req(oldtp, req, newtp);
486 /* Now setup tcp_sock */
487 newtp->pred_flags = 0;
489 seq = treq->rcv_isn + 1;
490 newtp->rcv_wup = seq;
491 WRITE_ONCE(newtp->copied_seq, seq);
492 WRITE_ONCE(newtp->rcv_nxt, seq);
495 seq = treq->snt_isn + 1;
496 newtp->snd_sml = newtp->snd_una = seq;
497 WRITE_ONCE(newtp->snd_nxt, seq);
500 INIT_LIST_HEAD(&newtp->tsq_node);
501 INIT_LIST_HEAD(&newtp->tsorted_sent_queue);
503 tcp_init_wl(newtp, treq->rcv_isn);
505 minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U);
506 newicsk->icsk_ack.lrcvtime = tcp_jiffies32;
508 newtp->lsndtime = tcp_jiffies32;
509 newsk->sk_txhash = READ_ONCE(treq->txhash);
510 newtp->total_retrans = req->num_retrans;
512 tcp_init_xmit_timers(newsk);
513 WRITE_ONCE(newtp->write_seq, newtp->pushed_seq = treq->snt_isn + 1);
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) << 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 = READ_ONCE(req->ts_recent);
537 newtp->rx_opt.ts_recent_stamp = ktime_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);
543 if (req->num_timeout) {
544 newtp->undo_marker = treq->snt_isn;
545 newtp->retrans_stamp = div_u64(treq->snt_synack,
546 USEC_PER_SEC / TCP_TS_HZ);
548 newtp->tsoffset = treq->ts_off;
549 #ifdef CONFIG_TCP_MD5SIG
550 newtp->md5sig_info = NULL; /*XXX*/
551 if (treq->af_specific->req_md5_lookup(sk, req_to_sk(req)))
552 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
554 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
555 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
556 newtp->rx_opt.mss_clamp = req->mss;
557 tcp_ecn_openreq_child(newtp, req);
558 newtp->fastopen_req = NULL;
559 RCU_INIT_POINTER(newtp->fastopen_rsk, NULL);
561 newtp->bpf_chg_cc_inprogress = 0;
562 tcp_bpf_clone(sk, newsk);
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
580 * Note: If @fastopen is true, this can be called from process context.
581 * Otherwise, this is from BH context.
584 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
585 struct request_sock *req,
586 bool fastopen, bool *req_stolen)
588 struct tcp_options_received tmp_opt;
590 const struct tcphdr *th = tcp_hdr(skb);
591 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
592 bool paws_reject = false;
595 tmp_opt.saw_tstamp = 0;
596 if (th->doff > (sizeof(struct tcphdr)>>2)) {
597 tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL);
599 if (tmp_opt.saw_tstamp) {
600 tmp_opt.ts_recent = READ_ONCE(req->ts_recent);
601 if (tmp_opt.rcv_tsecr)
602 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off;
603 /* We do not store true stamp, but it is not required,
604 * it can be estimated (approximately)
607 tmp_opt.ts_recent_stamp = ktime_get_seconds() - reqsk_timeout(req, TCP_RTO_MAX) / HZ;
608 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
612 /* Check for pure retransmitted SYN. */
613 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
614 flg == TCP_FLAG_SYN &&
617 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
618 * this case on figure 6 and figure 8, but formal
619 * protocol description says NOTHING.
620 * To be more exact, it says that we should send ACK,
621 * because this segment (at least, if it has no data)
624 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
625 * describe SYN-RECV state. All the description
626 * is wrong, we cannot believe to it and should
627 * rely only on common sense and implementation
630 * Enforce "SYN-ACK" according to figure 8, figure 6
631 * of RFC793, fixed by RFC1122.
633 * Note that even if there is new data in the SYN packet
634 * they will be thrown away too.
636 * Reset timer after retransmitting SYNACK, similar to
637 * the idea of fast retransmit in recovery.
639 if (!tcp_oow_rate_limited(sock_net(sk), skb,
640 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
641 &tcp_rsk(req)->last_oow_ack_time) &&
643 !inet_rtx_syn_ack(sk, req)) {
644 unsigned long expires = jiffies;
646 expires += reqsk_timeout(req, TCP_RTO_MAX);
648 mod_timer_pending(&req->rsk_timer, expires);
650 req->rsk_timer.expires = expires;
655 /* Further reproduces section "SEGMENT ARRIVES"
656 for state SYN-RECEIVED of RFC793.
657 It is broken, however, it does not work only
658 when SYNs are crossed.
660 You would think that SYN crossing is impossible here, since
661 we should have a SYN_SENT socket (from connect()) on our end,
662 but this is not true if the crossed SYNs were sent to both
663 ends by a malicious third party. We must defend against this,
664 and to do that we first verify the ACK (as per RFC793, page
665 36) and reset if it is invalid. Is this a true full defense?
666 To convince ourselves, let us consider a way in which the ACK
667 test can still pass in this 'malicious crossed SYNs' case.
668 Malicious sender sends identical SYNs (and thus identical sequence
669 numbers) to both A and B:
674 By our good fortune, both A and B select the same initial
675 send sequence number of seven :-)
677 A: sends SYN|ACK, seq=7, ack_seq=8
678 B: sends SYN|ACK, seq=7, ack_seq=8
680 So we are now A eating this SYN|ACK, ACK test passes. So
681 does sequence test, SYN is truncated, and thus we consider
684 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
685 bare ACK. Otherwise, we create an established connection. Both
686 ends (listening sockets) accept the new incoming connection and try
687 to talk to each other. 8-)
689 Note: This case is both harmless, and rare. Possibility is about the
690 same as us discovering intelligent life on another plant tomorrow.
692 But generally, we should (RFC lies!) to accept ACK
693 from SYNACK both here and in tcp_rcv_state_process().
694 tcp_rcv_state_process() does not, hence, we do not too.
696 Note that the case is absolutely generic:
697 we cannot optimize anything here without
698 violating protocol. All the checks must be made
699 before attempt to create socket.
702 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
703 * and the incoming segment acknowledges something not yet
704 * sent (the segment carries an unacceptable ACK) ...
707 * Invalid ACK: reset will be sent by listening socket.
708 * Note that the ACK validity check for a Fast Open socket is done
709 * elsewhere and is checked directly against the child socket rather
710 * than req because user data may have been sent out.
712 if ((flg & TCP_FLAG_ACK) && !fastopen &&
713 (TCP_SKB_CB(skb)->ack_seq !=
714 tcp_rsk(req)->snt_isn + 1))
717 /* Also, it would be not so bad idea to check rcv_tsecr, which
718 * is essentially ACK extension and too early or too late values
719 * should cause reset in unsynchronized states.
722 /* RFC793: "first check sequence number". */
724 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
725 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) {
726 /* Out of window: send ACK and drop. */
727 if (!(flg & TCP_FLAG_RST) &&
728 !tcp_oow_rate_limited(sock_net(sk), skb,
729 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
730 &tcp_rsk(req)->last_oow_ack_time))
731 req->rsk_ops->send_ack(sk, skb, req);
733 NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
737 /* In sequence, PAWS is OK. */
739 /* TODO: We probably should defer ts_recent change once
740 * we take ownership of @req.
742 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
743 WRITE_ONCE(req->ts_recent, tmp_opt.rcv_tsval);
745 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
746 /* Truncate SYN, it is out of window starting
747 at tcp_rsk(req)->rcv_isn + 1. */
748 flg &= ~TCP_FLAG_SYN;
751 /* RFC793: "second check the RST bit" and
752 * "fourth, check the SYN bit"
754 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
755 TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
756 goto embryonic_reset;
759 /* ACK sequence verified above, just make sure ACK is
760 * set. If ACK not set, just silently drop the packet.
762 * XXX (TFO) - if we ever allow "data after SYN", the
763 * following check needs to be removed.
765 if (!(flg & TCP_FLAG_ACK))
768 /* For Fast Open no more processing is needed (sk is the
774 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
775 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
776 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
777 inet_rsk(req)->acked = 1;
778 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
782 /* OK, ACK is valid, create big socket and
783 * feed this segment to it. It will repeat all
784 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
785 * ESTABLISHED STATE. If it will be dropped after
786 * socket is created, wait for troubles.
788 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
791 goto listen_overflow;
793 if (own_req && rsk_drop_req(req)) {
794 reqsk_queue_removed(&inet_csk(req->rsk_listener)->icsk_accept_queue, req);
795 inet_csk_reqsk_queue_drop_and_put(req->rsk_listener, req);
799 sock_rps_save_rxhash(child, skb);
800 tcp_synack_rtt_meas(child, req);
801 *req_stolen = !own_req;
802 return inet_csk_complete_hashdance(sk, child, req, own_req);
805 if (sk != req->rsk_listener)
806 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPMIGRATEREQFAILURE);
808 if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_abort_on_overflow)) {
809 inet_rsk(req)->acked = 1;
814 if (!(flg & TCP_FLAG_RST)) {
815 /* Received a bad SYN pkt - for TFO We try not to reset
816 * the local connection unless it's really necessary to
817 * avoid becoming vulnerable to outside attack aiming at
818 * resetting legit local connections.
820 req->rsk_ops->send_reset(sk, skb);
821 } else if (fastopen) { /* received a valid RST pkt */
822 reqsk_fastopen_remove(sk, req, true);
826 bool unlinked = inet_csk_reqsk_queue_drop(sk, req);
829 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
830 *req_stolen = !unlinked;
834 EXPORT_SYMBOL(tcp_check_req);
837 * Queue segment on the new socket if the new socket is active,
838 * otherwise we just shortcircuit this and continue with
841 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
842 * when entering. But other states are possible due to a race condition
843 * where after __inet_lookup_established() fails but before the listener
844 * locked is obtained, other packets cause the same connection to
848 int tcp_child_process(struct sock *parent, struct sock *child,
850 __releases(&((child)->sk_lock.slock))
853 int state = child->sk_state;
855 /* record sk_napi_id and sk_rx_queue_mapping of child. */
856 sk_mark_napi_id_set(child, skb);
858 tcp_segs_in(tcp_sk(child), skb);
859 if (!sock_owned_by_user(child)) {
860 ret = tcp_rcv_state_process(child, skb);
861 /* Wakeup parent, send SIGIO */
862 if (state == TCP_SYN_RECV && child->sk_state != state)
863 parent->sk_data_ready(parent);
865 /* Alas, it is possible again, because we do lookup
866 * in main socket hash table and lock on listening
867 * socket does not protect us more.
869 __sk_add_backlog(child, skb);
872 bh_unlock_sock(child);
876 EXPORT_SYMBOL(tcp_child_process);