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>
27 #include <net/inet_common.h>
30 int sysctl_tcp_abort_on_overflow __read_mostly;
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;
97 struct inet_timewait_death_row *tcp_death_row = &sock_net((struct sock*)tw)->ipv4.tcp_death_row;
99 tmp_opt.saw_tstamp = 0;
100 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
101 tcp_parse_options(skb, &tmp_opt, 0, NULL);
103 if (tmp_opt.saw_tstamp) {
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 if (tcp_death_row->sysctl_tw_recycle &&
152 tcptw->tw_ts_recent_stamp &&
153 tcp_tw_remember_stamp(tw))
154 inet_twsk_reschedule(tw, tw->tw_timeout);
156 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
161 * Now real TIME-WAIT state.
164 * "When a connection is [...] on TIME-WAIT state [...]
165 * [a TCP] MAY accept a new SYN from the remote TCP to
166 * reopen the connection directly, if it:
168 * (1) assigns its initial sequence number for the new
169 * connection to be larger than the largest sequence
170 * number it used on the previous connection incarnation,
173 * (2) returns to TIME-WAIT state if the SYN turns out
174 * to be an old duplicate".
178 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
179 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
180 /* In window segment, it may be only reset or bare ack. */
183 /* This is TIME_WAIT assassination, in two flavors.
184 * Oh well... nobody has a sufficient solution to this
187 if (sysctl_tcp_rfc1337 == 0) {
189 inet_twsk_deschedule_put(tw);
190 return TCP_TW_SUCCESS;
193 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
195 if (tmp_opt.saw_tstamp) {
196 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
197 tcptw->tw_ts_recent_stamp = get_seconds();
201 return TCP_TW_SUCCESS;
204 /* Out of window segment.
206 All the segments are ACKed immediately.
208 The only exception is new SYN. We accept it, if it is
209 not old duplicate and we are not in danger to be killed
210 by delayed old duplicates. RFC check is that it has
211 newer sequence number works at rates <40Mbit/sec.
212 However, if paws works, it is reliable AND even more,
213 we even may relax silly seq space cutoff.
215 RED-PEN: we violate main RFC requirement, if this SYN will appear
216 old duplicate (i.e. we receive RST in reply to SYN-ACK),
217 we must return socket to time-wait state. It is not good,
221 if (th->syn && !th->rst && !th->ack && !paws_reject &&
222 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
223 (tmp_opt.saw_tstamp &&
224 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
225 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
228 TCP_SKB_CB(skb)->tcp_tw_isn = isn;
233 __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
236 /* In this case we must reset the TIMEWAIT timer.
238 * If it is ACKless SYN it may be both old duplicate
239 * and new good SYN with random sequence number <rcv_nxt.
240 * Do not reschedule in the last case.
242 if (paws_reject || th->ack)
243 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
245 return tcp_timewait_check_oow_rate_limit(
246 tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT);
249 return TCP_TW_SUCCESS;
251 EXPORT_SYMBOL(tcp_timewait_state_process);
254 * Move a socket to time-wait or dead fin-wait-2 state.
256 void tcp_time_wait(struct sock *sk, int state, int timeo)
258 const struct inet_connection_sock *icsk = inet_csk(sk);
259 const struct tcp_sock *tp = tcp_sk(sk);
260 struct inet_timewait_sock *tw;
261 bool recycle_ok = false;
262 struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row;
264 if (tcp_death_row->sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp)
265 recycle_ok = tcp_remember_stamp(sk);
267 tw = inet_twsk_alloc(sk, tcp_death_row, state);
270 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
271 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
272 struct inet_sock *inet = inet_sk(sk);
274 tw->tw_transparent = inet->transparent;
275 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale;
276 tcptw->tw_rcv_nxt = tp->rcv_nxt;
277 tcptw->tw_snd_nxt = tp->snd_nxt;
278 tcptw->tw_rcv_wnd = tcp_receive_window(tp);
279 tcptw->tw_ts_recent = tp->rx_opt.ts_recent;
280 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
281 tcptw->tw_ts_offset = tp->tsoffset;
282 tcptw->tw_last_oow_ack_time = 0;
284 #if IS_ENABLED(CONFIG_IPV6)
285 if (tw->tw_family == PF_INET6) {
286 struct ipv6_pinfo *np = inet6_sk(sk);
288 tw->tw_v6_daddr = sk->sk_v6_daddr;
289 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
290 tw->tw_tclass = np->tclass;
291 tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK);
292 tw->tw_ipv6only = sk->sk_ipv6only;
296 #ifdef CONFIG_TCP_MD5SIG
298 * The timewait bucket does not have the key DB from the
299 * sock structure. We just make a quick copy of the
300 * md5 key being used (if indeed we are using one)
301 * so the timewait ack generating code has the key.
304 struct tcp_md5sig_key *key;
305 tcptw->tw_md5_key = NULL;
306 key = tp->af_specific->md5_lookup(sk, sk);
308 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
309 if (tcptw->tw_md5_key && !tcp_alloc_md5sig_pool())
315 /* Get the TIME_WAIT timeout firing. */
320 tw->tw_timeout = rto;
322 tw->tw_timeout = TCP_TIMEWAIT_LEN;
323 if (state == TCP_TIME_WAIT)
324 timeo = TCP_TIMEWAIT_LEN;
327 inet_twsk_schedule(tw, timeo);
328 /* Linkage updates. */
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);
343 void tcp_twsk_destructor(struct sock *sk)
345 #ifdef CONFIG_TCP_MD5SIG
346 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
348 if (twsk->tw_md5_key)
349 kfree_rcu(twsk->tw_md5_key, rcu);
352 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
354 /* Warning : This function is called without sk_listener being locked.
355 * Be sure to read socket fields once, as their value could change under us.
357 void tcp_openreq_init_rwin(struct request_sock *req,
358 const struct sock *sk_listener,
359 const struct dst_entry *dst)
361 struct inet_request_sock *ireq = inet_rsk(req);
362 const struct tcp_sock *tp = tcp_sk(sk_listener);
363 u16 user_mss = READ_ONCE(tp->rx_opt.user_mss);
364 int full_space = tcp_full_space(sk_listener);
365 int mss = dst_metric_advmss(dst);
369 if (user_mss && user_mss < mss)
372 window_clamp = READ_ONCE(tp->window_clamp);
373 /* Set this up on the first call only */
374 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);
376 /* limit the window selection if the user enforce a smaller rx buffer */
377 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK &&
378 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
379 req->rsk_window_clamp = full_space;
381 /* tcp_full_space because it is guaranteed to be the first packet */
382 tcp_select_initial_window(full_space,
383 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
385 &req->rsk_window_clamp,
388 dst_metric(dst, RTAX_INITRWND));
389 ireq->rcv_wscale = rcv_wscale;
391 EXPORT_SYMBOL(tcp_openreq_init_rwin);
393 static void tcp_ecn_openreq_child(struct tcp_sock *tp,
394 const struct request_sock *req)
396 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
399 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
401 struct inet_connection_sock *icsk = inet_csk(sk);
402 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
403 bool ca_got_dst = false;
405 if (ca_key != TCP_CA_UNSPEC) {
406 const struct tcp_congestion_ops *ca;
409 ca = tcp_ca_find_key(ca_key);
410 if (likely(ca && try_module_get(ca->owner))) {
411 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
412 icsk->icsk_ca_ops = ca;
418 /* If no valid choice made yet, assign current system default ca. */
420 (!icsk->icsk_ca_setsockopt ||
421 !try_module_get(icsk->icsk_ca_ops->owner)))
422 tcp_assign_congestion_control(sk);
424 tcp_set_ca_state(sk, TCP_CA_Open);
426 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child);
428 /* This is not only more efficient than what we used to do, it eliminates
429 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
431 * Actually, we could lots of memory writes here. tp of listening
432 * socket contains all necessary default parameters.
434 struct sock *tcp_create_openreq_child(const struct sock *sk,
435 struct request_sock *req,
438 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
441 const struct inet_request_sock *ireq = inet_rsk(req);
442 struct tcp_request_sock *treq = tcp_rsk(req);
443 struct inet_connection_sock *newicsk = inet_csk(newsk);
444 struct tcp_sock *newtp = tcp_sk(newsk);
446 /* Now setup tcp_sock */
447 newtp->pred_flags = 0;
449 newtp->rcv_wup = newtp->copied_seq =
450 newtp->rcv_nxt = treq->rcv_isn + 1;
453 newtp->snd_sml = newtp->snd_una =
454 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;
456 tcp_prequeue_init(newtp);
457 INIT_LIST_HEAD(&newtp->tsq_node);
459 tcp_init_wl(newtp, treq->rcv_isn);
462 newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT);
463 minmax_reset(&newtp->rtt_min, tcp_time_stamp, ~0U);
464 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
466 newtp->packets_out = 0;
467 newtp->retrans_out = 0;
468 newtp->sacked_out = 0;
469 newtp->fackets_out = 0;
470 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
471 newtp->tlp_high_seq = 0;
472 newtp->lsndtime = treq->snt_synack.stamp_jiffies;
473 newsk->sk_txhash = treq->txhash;
474 newtp->last_oow_ack_time = 0;
475 newtp->total_retrans = req->num_retrans;
477 /* So many TCP implementations out there (incorrectly) count the
478 * initial SYN frame in their delayed-ACK and congestion control
479 * algorithms that we must have the following bandaid to talk
480 * efficiently to them. -DaveM
482 newtp->snd_cwnd = TCP_INIT_CWND;
483 newtp->snd_cwnd_cnt = 0;
485 /* There's a bubble in the pipe until at least the first ACK. */
486 newtp->app_limited = ~0U;
488 tcp_init_xmit_timers(newsk);
489 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
491 newtp->rx_opt.saw_tstamp = 0;
493 newtp->rx_opt.dsack = 0;
494 newtp->rx_opt.num_sacks = 0;
498 if (sock_flag(newsk, SOCK_KEEPOPEN))
499 inet_csk_reset_keepalive_timer(newsk,
500 keepalive_time_when(newtp));
502 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
503 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
505 tcp_enable_fack(newtp);
507 newtp->window_clamp = req->rsk_window_clamp;
508 newtp->rcv_ssthresh = req->rsk_rcv_wnd;
509 newtp->rcv_wnd = req->rsk_rcv_wnd;
510 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
511 if (newtp->rx_opt.wscale_ok) {
512 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
513 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
515 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
516 newtp->window_clamp = min(newtp->window_clamp, 65535U);
518 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
519 newtp->rx_opt.snd_wscale);
520 newtp->max_window = newtp->snd_wnd;
522 if (newtp->rx_opt.tstamp_ok) {
523 newtp->rx_opt.ts_recent = req->ts_recent;
524 newtp->rx_opt.ts_recent_stamp = get_seconds();
525 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
527 newtp->rx_opt.ts_recent_stamp = 0;
528 newtp->tcp_header_len = sizeof(struct tcphdr);
530 newtp->tsoffset = treq->ts_off;
531 #ifdef CONFIG_TCP_MD5SIG
532 newtp->md5sig_info = NULL; /*XXX*/
533 if (newtp->af_specific->md5_lookup(sk, newsk))
534 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
536 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
537 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
538 newtp->rx_opt.mss_clamp = req->mss;
539 tcp_ecn_openreq_child(newtp, req);
540 newtp->fastopen_rsk = NULL;
541 newtp->syn_data_acked = 0;
542 newtp->rack.mstamp.v64 = 0;
543 newtp->rack.advanced = 0;
545 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS);
549 EXPORT_SYMBOL(tcp_create_openreq_child);
552 * Process an incoming packet for SYN_RECV sockets represented as a
553 * request_sock. Normally sk is the listener socket but for TFO it
554 * points to the child socket.
556 * XXX (TFO) - The current impl contains a special check for ack
557 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
559 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
562 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
563 struct request_sock *req,
566 struct tcp_options_received tmp_opt;
568 const struct tcphdr *th = tcp_hdr(skb);
569 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
570 bool paws_reject = false;
573 tmp_opt.saw_tstamp = 0;
574 if (th->doff > (sizeof(struct tcphdr)>>2)) {
575 tcp_parse_options(skb, &tmp_opt, 0, NULL);
577 if (tmp_opt.saw_tstamp) {
578 tmp_opt.ts_recent = req->ts_recent;
579 if (tmp_opt.rcv_tsecr)
580 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off;
581 /* We do not store true stamp, but it is not required,
582 * it can be estimated (approximately)
585 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
586 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
590 /* Check for pure retransmitted SYN. */
591 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
592 flg == TCP_FLAG_SYN &&
595 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
596 * this case on figure 6 and figure 8, but formal
597 * protocol description says NOTHING.
598 * To be more exact, it says that we should send ACK,
599 * because this segment (at least, if it has no data)
602 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
603 * describe SYN-RECV state. All the description
604 * is wrong, we cannot believe to it and should
605 * rely only on common sense and implementation
608 * Enforce "SYN-ACK" according to figure 8, figure 6
609 * of RFC793, fixed by RFC1122.
611 * Note that even if there is new data in the SYN packet
612 * they will be thrown away too.
614 * Reset timer after retransmitting SYNACK, similar to
615 * the idea of fast retransmit in recovery.
617 if (!tcp_oow_rate_limited(sock_net(sk), skb,
618 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
619 &tcp_rsk(req)->last_oow_ack_time) &&
621 !inet_rtx_syn_ack(sk, req)) {
622 unsigned long expires = jiffies;
624 expires += min(TCP_TIMEOUT_INIT << req->num_timeout,
627 mod_timer_pending(&req->rsk_timer, expires);
629 req->rsk_timer.expires = expires;
634 /* Further reproduces section "SEGMENT ARRIVES"
635 for state SYN-RECEIVED of RFC793.
636 It is broken, however, it does not work only
637 when SYNs are crossed.
639 You would think that SYN crossing is impossible here, since
640 we should have a SYN_SENT socket (from connect()) on our end,
641 but this is not true if the crossed SYNs were sent to both
642 ends by a malicious third party. We must defend against this,
643 and to do that we first verify the ACK (as per RFC793, page
644 36) and reset if it is invalid. Is this a true full defense?
645 To convince ourselves, let us consider a way in which the ACK
646 test can still pass in this 'malicious crossed SYNs' case.
647 Malicious sender sends identical SYNs (and thus identical sequence
648 numbers) to both A and B:
653 By our good fortune, both A and B select the same initial
654 send sequence number of seven :-)
656 A: sends SYN|ACK, seq=7, ack_seq=8
657 B: sends SYN|ACK, seq=7, ack_seq=8
659 So we are now A eating this SYN|ACK, ACK test passes. So
660 does sequence test, SYN is truncated, and thus we consider
663 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
664 bare ACK. Otherwise, we create an established connection. Both
665 ends (listening sockets) accept the new incoming connection and try
666 to talk to each other. 8-)
668 Note: This case is both harmless, and rare. Possibility is about the
669 same as us discovering intelligent life on another plant tomorrow.
671 But generally, we should (RFC lies!) to accept ACK
672 from SYNACK both here and in tcp_rcv_state_process().
673 tcp_rcv_state_process() does not, hence, we do not too.
675 Note that the case is absolutely generic:
676 we cannot optimize anything here without
677 violating protocol. All the checks must be made
678 before attempt to create socket.
681 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
682 * and the incoming segment acknowledges something not yet
683 * sent (the segment carries an unacceptable ACK) ...
686 * Invalid ACK: reset will be sent by listening socket.
687 * Note that the ACK validity check for a Fast Open socket is done
688 * elsewhere and is checked directly against the child socket rather
689 * than req because user data may have been sent out.
691 if ((flg & TCP_FLAG_ACK) && !fastopen &&
692 (TCP_SKB_CB(skb)->ack_seq !=
693 tcp_rsk(req)->snt_isn + 1))
696 /* Also, it would be not so bad idea to check rcv_tsecr, which
697 * is essentially ACK extension and too early or too late values
698 * should cause reset in unsynchronized states.
701 /* RFC793: "first check sequence number". */
703 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
704 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) {
705 /* Out of window: send ACK and drop. */
706 if (!(flg & TCP_FLAG_RST) &&
707 !tcp_oow_rate_limited(sock_net(sk), skb,
708 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
709 &tcp_rsk(req)->last_oow_ack_time))
710 req->rsk_ops->send_ack(sk, skb, req);
712 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
716 /* In sequence, PAWS is OK. */
718 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
719 req->ts_recent = tmp_opt.rcv_tsval;
721 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
722 /* Truncate SYN, it is out of window starting
723 at tcp_rsk(req)->rcv_isn + 1. */
724 flg &= ~TCP_FLAG_SYN;
727 /* RFC793: "second check the RST bit" and
728 * "fourth, check the SYN bit"
730 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
731 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
732 goto embryonic_reset;
735 /* ACK sequence verified above, just make sure ACK is
736 * set. If ACK not set, just silently drop the packet.
738 * XXX (TFO) - if we ever allow "data after SYN", the
739 * following check needs to be removed.
741 if (!(flg & TCP_FLAG_ACK))
744 /* For Fast Open no more processing is needed (sk is the
750 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
751 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
752 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
753 inet_rsk(req)->acked = 1;
754 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
758 /* OK, ACK is valid, create big socket and
759 * feed this segment to it. It will repeat all
760 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
761 * ESTABLISHED STATE. If it will be dropped after
762 * socket is created, wait for troubles.
764 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
767 goto listen_overflow;
769 sock_rps_save_rxhash(child, skb);
770 tcp_synack_rtt_meas(child, req);
771 return inet_csk_complete_hashdance(sk, child, req, own_req);
774 if (!sysctl_tcp_abort_on_overflow) {
775 inet_rsk(req)->acked = 1;
780 if (!(flg & TCP_FLAG_RST)) {
781 /* Received a bad SYN pkt - for TFO We try not to reset
782 * the local connection unless it's really necessary to
783 * avoid becoming vulnerable to outside attack aiming at
784 * resetting legit local connections.
786 req->rsk_ops->send_reset(sk, skb);
787 } else if (fastopen) { /* received a valid RST pkt */
788 reqsk_fastopen_remove(sk, req, true);
792 inet_csk_reqsk_queue_drop(sk, req);
793 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
797 EXPORT_SYMBOL(tcp_check_req);
800 * Queue segment on the new socket if the new socket is active,
801 * otherwise we just shortcircuit this and continue with
804 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
805 * when entering. But other states are possible due to a race condition
806 * where after __inet_lookup_established() fails but before the listener
807 * locked is obtained, other packets cause the same connection to
811 int tcp_child_process(struct sock *parent, struct sock *child,
815 int state = child->sk_state;
817 tcp_segs_in(tcp_sk(child), skb);
818 if (!sock_owned_by_user(child)) {
819 ret = tcp_rcv_state_process(child, skb);
820 /* Wakeup parent, send SIGIO */
821 if (state == TCP_SYN_RECV && child->sk_state != state)
822 parent->sk_data_ready(parent);
824 /* Alas, it is possible again, because we do lookup
825 * in main socket hash table and lock on listening
826 * socket does not protect us more.
828 __sk_add_backlog(child, skb);
831 bh_unlock_sock(child);
835 EXPORT_SYMBOL(tcp_child_process);