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 (twsk_net(tw)->ipv4.sysctl_tcp_rfc1337 == 0) {
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 && try_module_get(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 !try_module_get(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;
469 newicsk = inet_csk(newsk);
470 newtp = tcp_sk(newsk);
473 smc_check_reset_syn_req(oldtp, req, newtp);
475 /* Now setup tcp_sock */
476 newtp->pred_flags = 0;
478 newtp->rcv_wup = newtp->copied_seq =
479 newtp->rcv_nxt = treq->rcv_isn + 1;
482 newtp->snd_sml = newtp->snd_una =
483 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;
485 INIT_LIST_HEAD(&newtp->tsq_node);
486 INIT_LIST_HEAD(&newtp->tsorted_sent_queue);
488 tcp_init_wl(newtp, treq->rcv_isn);
490 minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U);
491 newicsk->icsk_ack.lrcvtime = tcp_jiffies32;
493 newtp->lsndtime = tcp_jiffies32;
494 newsk->sk_txhash = treq->txhash;
495 newtp->total_retrans = req->num_retrans;
497 tcp_init_xmit_timers(newsk);
498 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
500 if (sock_flag(newsk, SOCK_KEEPOPEN))
501 inet_csk_reset_keepalive_timer(newsk,
502 keepalive_time_when(newtp));
504 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
505 newtp->rx_opt.sack_ok = ireq->sack_ok;
506 newtp->window_clamp = req->rsk_window_clamp;
507 newtp->rcv_ssthresh = req->rsk_rcv_wnd;
508 newtp->rcv_wnd = req->rsk_rcv_wnd;
509 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
510 if (newtp->rx_opt.wscale_ok) {
511 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
512 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
514 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
515 newtp->window_clamp = min(newtp->window_clamp, 65535U);
517 newtp->snd_wnd = ntohs(tcp_hdr(skb)->window) << newtp->rx_opt.snd_wscale;
518 newtp->max_window = newtp->snd_wnd;
520 if (newtp->rx_opt.tstamp_ok) {
521 newtp->rx_opt.ts_recent = req->ts_recent;
522 newtp->rx_opt.ts_recent_stamp = ktime_get_seconds();
523 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
525 newtp->rx_opt.ts_recent_stamp = 0;
526 newtp->tcp_header_len = sizeof(struct tcphdr);
528 if (req->num_timeout) {
529 newtp->undo_marker = treq->snt_isn;
530 newtp->retrans_stamp = div_u64(treq->snt_synack,
531 USEC_PER_SEC / TCP_TS_HZ);
533 newtp->tsoffset = treq->ts_off;
534 #ifdef CONFIG_TCP_MD5SIG
535 newtp->md5sig_info = NULL; /*XXX*/
536 if (newtp->af_specific->md5_lookup(sk, newsk))
537 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
539 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
540 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
541 newtp->rx_opt.mss_clamp = req->mss;
542 tcp_ecn_openreq_child(newtp, req);
543 newtp->fastopen_req = NULL;
544 newtp->fastopen_rsk = NULL;
546 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS);
550 EXPORT_SYMBOL(tcp_create_openreq_child);
553 * Process an incoming packet for SYN_RECV sockets represented as a
554 * request_sock. Normally sk is the listener socket but for TFO it
555 * points to the child socket.
557 * XXX (TFO) - The current impl contains a special check for ack
558 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
560 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
563 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
564 struct request_sock *req,
565 bool fastopen, bool *req_stolen)
567 struct tcp_options_received tmp_opt;
569 const struct tcphdr *th = tcp_hdr(skb);
570 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
571 bool paws_reject = false;
574 tmp_opt.saw_tstamp = 0;
575 if (th->doff > (sizeof(struct tcphdr)>>2)) {
576 tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL);
578 if (tmp_opt.saw_tstamp) {
579 tmp_opt.ts_recent = req->ts_recent;
580 if (tmp_opt.rcv_tsecr)
581 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off;
582 /* We do not store true stamp, but it is not required,
583 * it can be estimated (approximately)
586 tmp_opt.ts_recent_stamp = ktime_get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
587 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
591 /* Check for pure retransmitted SYN. */
592 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
593 flg == TCP_FLAG_SYN &&
596 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
597 * this case on figure 6 and figure 8, but formal
598 * protocol description says NOTHING.
599 * To be more exact, it says that we should send ACK,
600 * because this segment (at least, if it has no data)
603 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
604 * describe SYN-RECV state. All the description
605 * is wrong, we cannot believe to it and should
606 * rely only on common sense and implementation
609 * Enforce "SYN-ACK" according to figure 8, figure 6
610 * of RFC793, fixed by RFC1122.
612 * Note that even if there is new data in the SYN packet
613 * they will be thrown away too.
615 * Reset timer after retransmitting SYNACK, similar to
616 * the idea of fast retransmit in recovery.
618 if (!tcp_oow_rate_limited(sock_net(sk), skb,
619 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
620 &tcp_rsk(req)->last_oow_ack_time) &&
622 !inet_rtx_syn_ack(sk, req)) {
623 unsigned long expires = jiffies;
625 expires += min(TCP_TIMEOUT_INIT << req->num_timeout,
628 mod_timer_pending(&req->rsk_timer, expires);
630 req->rsk_timer.expires = expires;
635 /* Further reproduces section "SEGMENT ARRIVES"
636 for state SYN-RECEIVED of RFC793.
637 It is broken, however, it does not work only
638 when SYNs are crossed.
640 You would think that SYN crossing is impossible here, since
641 we should have a SYN_SENT socket (from connect()) on our end,
642 but this is not true if the crossed SYNs were sent to both
643 ends by a malicious third party. We must defend against this,
644 and to do that we first verify the ACK (as per RFC793, page
645 36) and reset if it is invalid. Is this a true full defense?
646 To convince ourselves, let us consider a way in which the ACK
647 test can still pass in this 'malicious crossed SYNs' case.
648 Malicious sender sends identical SYNs (and thus identical sequence
649 numbers) to both A and B:
654 By our good fortune, both A and B select the same initial
655 send sequence number of seven :-)
657 A: sends SYN|ACK, seq=7, ack_seq=8
658 B: sends SYN|ACK, seq=7, ack_seq=8
660 So we are now A eating this SYN|ACK, ACK test passes. So
661 does sequence test, SYN is truncated, and thus we consider
664 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
665 bare ACK. Otherwise, we create an established connection. Both
666 ends (listening sockets) accept the new incoming connection and try
667 to talk to each other. 8-)
669 Note: This case is both harmless, and rare. Possibility is about the
670 same as us discovering intelligent life on another plant tomorrow.
672 But generally, we should (RFC lies!) to accept ACK
673 from SYNACK both here and in tcp_rcv_state_process().
674 tcp_rcv_state_process() does not, hence, we do not too.
676 Note that the case is absolutely generic:
677 we cannot optimize anything here without
678 violating protocol. All the checks must be made
679 before attempt to create socket.
682 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
683 * and the incoming segment acknowledges something not yet
684 * sent (the segment carries an unacceptable ACK) ...
687 * Invalid ACK: reset will be sent by listening socket.
688 * Note that the ACK validity check for a Fast Open socket is done
689 * elsewhere and is checked directly against the child socket rather
690 * than req because user data may have been sent out.
692 if ((flg & TCP_FLAG_ACK) && !fastopen &&
693 (TCP_SKB_CB(skb)->ack_seq !=
694 tcp_rsk(req)->snt_isn + 1))
697 /* Also, it would be not so bad idea to check rcv_tsecr, which
698 * is essentially ACK extension and too early or too late values
699 * should cause reset in unsynchronized states.
702 /* RFC793: "first check sequence number". */
704 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
705 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) {
706 /* Out of window: send ACK and drop. */
707 if (!(flg & TCP_FLAG_RST) &&
708 !tcp_oow_rate_limited(sock_net(sk), skb,
709 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
710 &tcp_rsk(req)->last_oow_ack_time))
711 req->rsk_ops->send_ack(sk, skb, req);
713 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
717 /* In sequence, PAWS is OK. */
719 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
720 req->ts_recent = tmp_opt.rcv_tsval;
722 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
723 /* Truncate SYN, it is out of window starting
724 at tcp_rsk(req)->rcv_isn + 1. */
725 flg &= ~TCP_FLAG_SYN;
728 /* RFC793: "second check the RST bit" and
729 * "fourth, check the SYN bit"
731 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
732 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
733 goto embryonic_reset;
736 /* ACK sequence verified above, just make sure ACK is
737 * set. If ACK not set, just silently drop the packet.
739 * XXX (TFO) - if we ever allow "data after SYN", the
740 * following check needs to be removed.
742 if (!(flg & TCP_FLAG_ACK))
745 /* For Fast Open no more processing is needed (sk is the
751 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
752 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
753 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
754 inet_rsk(req)->acked = 1;
755 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
759 /* OK, ACK is valid, create big socket and
760 * feed this segment to it. It will repeat all
761 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
762 * ESTABLISHED STATE. If it will be dropped after
763 * socket is created, wait for troubles.
765 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
768 goto listen_overflow;
770 sock_rps_save_rxhash(child, skb);
771 tcp_synack_rtt_meas(child, req);
772 *req_stolen = !own_req;
773 return inet_csk_complete_hashdance(sk, child, req, own_req);
776 if (!sock_net(sk)->ipv4.sysctl_tcp_abort_on_overflow) {
777 inet_rsk(req)->acked = 1;
782 if (!(flg & TCP_FLAG_RST)) {
783 /* Received a bad SYN pkt - for TFO We try not to reset
784 * the local connection unless it's really necessary to
785 * avoid becoming vulnerable to outside attack aiming at
786 * resetting legit local connections.
788 req->rsk_ops->send_reset(sk, skb);
789 } else if (fastopen) { /* received a valid RST pkt */
790 reqsk_fastopen_remove(sk, req, true);
794 inet_csk_reqsk_queue_drop(sk, req);
795 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
799 EXPORT_SYMBOL(tcp_check_req);
802 * Queue segment on the new socket if the new socket is active,
803 * otherwise we just shortcircuit this and continue with
806 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
807 * when entering. But other states are possible due to a race condition
808 * where after __inet_lookup_established() fails but before the listener
809 * locked is obtained, other packets cause the same connection to
813 int tcp_child_process(struct sock *parent, struct sock *child,
817 int state = child->sk_state;
819 /* record NAPI ID of child */
820 sk_mark_napi_id(child, skb);
822 tcp_segs_in(tcp_sk(child), skb);
823 if (!sock_owned_by_user(child)) {
824 ret = tcp_rcv_state_process(child, skb);
825 /* Wakeup parent, send SIGIO */
826 if (state == TCP_SYN_RECV && child->sk_state != state)
827 parent->sk_data_ready(parent);
829 /* Alas, it is possible again, because we do lookup
830 * in main socket hash table and lock on listening
831 * socket does not protect us more.
833 __sk_add_backlog(child, skb);
836 bh_unlock_sock(child);
840 EXPORT_SYMBOL(tcp_child_process);