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>
29 #include <net/busy_poll.h>
31 int sysctl_tcp_abort_on_overflow __read_mostly;
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 = 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 (sysctl_tcp_rfc1337 == 0) {
185 inet_twsk_deschedule_put(tw);
186 return TCP_TW_SUCCESS;
189 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
191 if (tmp_opt.saw_tstamp) {
192 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
193 tcptw->tw_ts_recent_stamp = get_seconds();
197 return TCP_TW_SUCCESS;
200 /* Out of window segment.
202 All the segments are ACKed immediately.
204 The only exception is new SYN. We accept it, if it is
205 not old duplicate and we are not in danger to be killed
206 by delayed old duplicates. RFC check is that it has
207 newer sequence number works at rates <40Mbit/sec.
208 However, if paws works, it is reliable AND even more,
209 we even may relax silly seq space cutoff.
211 RED-PEN: we violate main RFC requirement, if this SYN will appear
212 old duplicate (i.e. we receive RST in reply to SYN-ACK),
213 we must return socket to time-wait state. It is not good,
217 if (th->syn && !th->rst && !th->ack && !paws_reject &&
218 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
219 (tmp_opt.saw_tstamp &&
220 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
221 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
224 TCP_SKB_CB(skb)->tcp_tw_isn = isn;
229 __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
232 /* In this case we must reset the TIMEWAIT timer.
234 * If it is ACKless SYN it may be both old duplicate
235 * and new good SYN with random sequence number <rcv_nxt.
236 * Do not reschedule in the last case.
238 if (paws_reject || th->ack)
239 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
241 return tcp_timewait_check_oow_rate_limit(
242 tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT);
245 return TCP_TW_SUCCESS;
247 EXPORT_SYMBOL(tcp_timewait_state_process);
250 * Move a socket to time-wait or dead fin-wait-2 state.
252 void tcp_time_wait(struct sock *sk, int state, int timeo)
254 const struct inet_connection_sock *icsk = inet_csk(sk);
255 const struct tcp_sock *tp = tcp_sk(sk);
256 struct inet_timewait_sock *tw;
257 struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row;
259 tw = inet_twsk_alloc(sk, tcp_death_row, state);
262 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
263 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
264 struct inet_sock *inet = inet_sk(sk);
266 tw->tw_transparent = inet->transparent;
267 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale;
268 tcptw->tw_rcv_nxt = tp->rcv_nxt;
269 tcptw->tw_snd_nxt = tp->snd_nxt;
270 tcptw->tw_rcv_wnd = tcp_receive_window(tp);
271 tcptw->tw_ts_recent = tp->rx_opt.ts_recent;
272 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
273 tcptw->tw_ts_offset = tp->tsoffset;
274 tcptw->tw_last_oow_ack_time = 0;
276 #if IS_ENABLED(CONFIG_IPV6)
277 if (tw->tw_family == PF_INET6) {
278 struct ipv6_pinfo *np = inet6_sk(sk);
280 tw->tw_v6_daddr = sk->sk_v6_daddr;
281 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
282 tw->tw_tclass = np->tclass;
283 tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK);
284 tw->tw_ipv6only = sk->sk_ipv6only;
288 #ifdef CONFIG_TCP_MD5SIG
290 * The timewait bucket does not have the key DB from the
291 * sock structure. We just make a quick copy of the
292 * md5 key being used (if indeed we are using one)
293 * so the timewait ack generating code has the key.
296 struct tcp_md5sig_key *key;
297 tcptw->tw_md5_key = NULL;
298 key = tp->af_specific->md5_lookup(sk, sk);
300 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
301 if (tcptw->tw_md5_key && !tcp_alloc_md5sig_pool())
307 /* Get the TIME_WAIT timeout firing. */
311 tw->tw_timeout = TCP_TIMEWAIT_LEN;
312 if (state == TCP_TIME_WAIT)
313 timeo = TCP_TIMEWAIT_LEN;
315 inet_twsk_schedule(tw, timeo);
316 /* Linkage updates. */
317 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
320 /* Sorry, if we're out of memory, just CLOSE this
321 * socket up. We've got bigger problems than
322 * non-graceful socket closings.
324 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW);
327 tcp_update_metrics(sk);
331 void tcp_twsk_destructor(struct sock *sk)
333 #ifdef CONFIG_TCP_MD5SIG
334 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
336 if (twsk->tw_md5_key)
337 kfree_rcu(twsk->tw_md5_key, rcu);
340 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
342 /* Warning : This function is called without sk_listener being locked.
343 * Be sure to read socket fields once, as their value could change under us.
345 void tcp_openreq_init_rwin(struct request_sock *req,
346 const struct sock *sk_listener,
347 const struct dst_entry *dst)
349 struct inet_request_sock *ireq = inet_rsk(req);
350 const struct tcp_sock *tp = tcp_sk(sk_listener);
351 int full_space = tcp_full_space(sk_listener);
357 mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
358 window_clamp = READ_ONCE(tp->window_clamp);
359 /* Set this up on the first call only */
360 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);
362 /* limit the window selection if the user enforce a smaller rx buffer */
363 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK &&
364 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
365 req->rsk_window_clamp = full_space;
367 rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req);
369 rcv_wnd = dst_metric(dst, RTAX_INITRWND);
370 else if (full_space < rcv_wnd * mss)
371 full_space = rcv_wnd * mss;
373 /* tcp_full_space because it is guaranteed to be the first packet */
374 tcp_select_initial_window(full_space,
375 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
377 &req->rsk_window_clamp,
381 ireq->rcv_wscale = rcv_wscale;
383 EXPORT_SYMBOL(tcp_openreq_init_rwin);
385 static void tcp_ecn_openreq_child(struct tcp_sock *tp,
386 const struct request_sock *req)
388 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
391 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
393 struct inet_connection_sock *icsk = inet_csk(sk);
394 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
395 bool ca_got_dst = false;
397 if (ca_key != TCP_CA_UNSPEC) {
398 const struct tcp_congestion_ops *ca;
401 ca = tcp_ca_find_key(ca_key);
402 if (likely(ca && try_module_get(ca->owner))) {
403 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
404 icsk->icsk_ca_ops = ca;
410 /* If no valid choice made yet, assign current system default ca. */
412 (!icsk->icsk_ca_setsockopt ||
413 !try_module_get(icsk->icsk_ca_ops->owner)))
414 tcp_assign_congestion_control(sk);
416 tcp_set_ca_state(sk, TCP_CA_Open);
418 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child);
420 /* This is not only more efficient than what we used to do, it eliminates
421 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
423 * Actually, we could lots of memory writes here. tp of listening
424 * socket contains all necessary default parameters.
426 struct sock *tcp_create_openreq_child(const struct sock *sk,
427 struct request_sock *req,
430 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
433 const struct inet_request_sock *ireq = inet_rsk(req);
434 struct tcp_request_sock *treq = tcp_rsk(req);
435 struct inet_connection_sock *newicsk = inet_csk(newsk);
436 struct tcp_sock *newtp = tcp_sk(newsk);
438 /* Now setup tcp_sock */
439 newtp->pred_flags = 0;
441 newtp->rcv_wup = newtp->copied_seq =
442 newtp->rcv_nxt = treq->rcv_isn + 1;
445 newtp->snd_sml = newtp->snd_una =
446 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;
448 INIT_LIST_HEAD(&newtp->tsq_node);
450 tcp_init_wl(newtp, treq->rcv_isn);
453 newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT);
454 minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U);
455 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
456 newicsk->icsk_ack.lrcvtime = tcp_jiffies32;
458 newtp->packets_out = 0;
459 newtp->retrans_out = 0;
460 newtp->sacked_out = 0;
461 newtp->fackets_out = 0;
462 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
463 newtp->tlp_high_seq = 0;
464 newtp->lsndtime = tcp_jiffies32;
465 newsk->sk_txhash = treq->txhash;
466 newtp->last_oow_ack_time = 0;
467 newtp->total_retrans = req->num_retrans;
469 /* So many TCP implementations out there (incorrectly) count the
470 * initial SYN frame in their delayed-ACK and congestion control
471 * algorithms that we must have the following bandaid to talk
472 * efficiently to them. -DaveM
474 newtp->snd_cwnd = TCP_INIT_CWND;
475 newtp->snd_cwnd_cnt = 0;
477 /* There's a bubble in the pipe until at least the first ACK. */
478 newtp->app_limited = ~0U;
480 tcp_init_xmit_timers(newsk);
481 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
483 newtp->rx_opt.saw_tstamp = 0;
485 newtp->rx_opt.dsack = 0;
486 newtp->rx_opt.num_sacks = 0;
490 if (sock_flag(newsk, SOCK_KEEPOPEN))
491 inet_csk_reset_keepalive_timer(newsk,
492 keepalive_time_when(newtp));
494 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
495 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
497 tcp_enable_fack(newtp);
499 newtp->window_clamp = req->rsk_window_clamp;
500 newtp->rcv_ssthresh = req->rsk_rcv_wnd;
501 newtp->rcv_wnd = req->rsk_rcv_wnd;
502 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
503 if (newtp->rx_opt.wscale_ok) {
504 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
505 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
507 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
508 newtp->window_clamp = min(newtp->window_clamp, 65535U);
510 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
511 newtp->rx_opt.snd_wscale);
512 newtp->max_window = newtp->snd_wnd;
514 if (newtp->rx_opt.tstamp_ok) {
515 newtp->rx_opt.ts_recent = req->ts_recent;
516 newtp->rx_opt.ts_recent_stamp = get_seconds();
517 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
519 newtp->rx_opt.ts_recent_stamp = 0;
520 newtp->tcp_header_len = sizeof(struct tcphdr);
522 newtp->tsoffset = treq->ts_off;
523 #ifdef CONFIG_TCP_MD5SIG
524 newtp->md5sig_info = NULL; /*XXX*/
525 if (newtp->af_specific->md5_lookup(sk, newsk))
526 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
528 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
529 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
530 newtp->rx_opt.mss_clamp = req->mss;
531 tcp_ecn_openreq_child(newtp, req);
532 newtp->fastopen_req = NULL;
533 newtp->fastopen_rsk = NULL;
534 newtp->syn_data_acked = 0;
535 newtp->rack.mstamp = 0;
536 newtp->rack.advanced = 0;
538 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS);
542 EXPORT_SYMBOL(tcp_create_openreq_child);
545 * Process an incoming packet for SYN_RECV sockets represented as a
546 * request_sock. Normally sk is the listener socket but for TFO it
547 * points to the child socket.
549 * XXX (TFO) - The current impl contains a special check for ack
550 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
552 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
555 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
556 struct request_sock *req,
559 struct tcp_options_received tmp_opt;
561 const struct tcphdr *th = tcp_hdr(skb);
562 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
563 bool paws_reject = false;
566 tmp_opt.saw_tstamp = 0;
567 if (th->doff > (sizeof(struct tcphdr)>>2)) {
568 tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL);
570 if (tmp_opt.saw_tstamp) {
571 tmp_opt.ts_recent = req->ts_recent;
572 if (tmp_opt.rcv_tsecr)
573 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off;
574 /* We do not store true stamp, but it is not required,
575 * it can be estimated (approximately)
578 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
579 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
583 /* Check for pure retransmitted SYN. */
584 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
585 flg == TCP_FLAG_SYN &&
588 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
589 * this case on figure 6 and figure 8, but formal
590 * protocol description says NOTHING.
591 * To be more exact, it says that we should send ACK,
592 * because this segment (at least, if it has no data)
595 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
596 * describe SYN-RECV state. All the description
597 * is wrong, we cannot believe to it and should
598 * rely only on common sense and implementation
601 * Enforce "SYN-ACK" according to figure 8, figure 6
602 * of RFC793, fixed by RFC1122.
604 * Note that even if there is new data in the SYN packet
605 * they will be thrown away too.
607 * Reset timer after retransmitting SYNACK, similar to
608 * the idea of fast retransmit in recovery.
610 if (!tcp_oow_rate_limited(sock_net(sk), skb,
611 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
612 &tcp_rsk(req)->last_oow_ack_time) &&
614 !inet_rtx_syn_ack(sk, req)) {
615 unsigned long expires = jiffies;
617 expires += min(TCP_TIMEOUT_INIT << req->num_timeout,
620 mod_timer_pending(&req->rsk_timer, expires);
622 req->rsk_timer.expires = expires;
627 /* Further reproduces section "SEGMENT ARRIVES"
628 for state SYN-RECEIVED of RFC793.
629 It is broken, however, it does not work only
630 when SYNs are crossed.
632 You would think that SYN crossing is impossible here, since
633 we should have a SYN_SENT socket (from connect()) on our end,
634 but this is not true if the crossed SYNs were sent to both
635 ends by a malicious third party. We must defend against this,
636 and to do that we first verify the ACK (as per RFC793, page
637 36) and reset if it is invalid. Is this a true full defense?
638 To convince ourselves, let us consider a way in which the ACK
639 test can still pass in this 'malicious crossed SYNs' case.
640 Malicious sender sends identical SYNs (and thus identical sequence
641 numbers) to both A and B:
646 By our good fortune, both A and B select the same initial
647 send sequence number of seven :-)
649 A: sends SYN|ACK, seq=7, ack_seq=8
650 B: sends SYN|ACK, seq=7, ack_seq=8
652 So we are now A eating this SYN|ACK, ACK test passes. So
653 does sequence test, SYN is truncated, and thus we consider
656 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
657 bare ACK. Otherwise, we create an established connection. Both
658 ends (listening sockets) accept the new incoming connection and try
659 to talk to each other. 8-)
661 Note: This case is both harmless, and rare. Possibility is about the
662 same as us discovering intelligent life on another plant tomorrow.
664 But generally, we should (RFC lies!) to accept ACK
665 from SYNACK both here and in tcp_rcv_state_process().
666 tcp_rcv_state_process() does not, hence, we do not too.
668 Note that the case is absolutely generic:
669 we cannot optimize anything here without
670 violating protocol. All the checks must be made
671 before attempt to create socket.
674 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
675 * and the incoming segment acknowledges something not yet
676 * sent (the segment carries an unacceptable ACK) ...
679 * Invalid ACK: reset will be sent by listening socket.
680 * Note that the ACK validity check for a Fast Open socket is done
681 * elsewhere and is checked directly against the child socket rather
682 * than req because user data may have been sent out.
684 if ((flg & TCP_FLAG_ACK) && !fastopen &&
685 (TCP_SKB_CB(skb)->ack_seq !=
686 tcp_rsk(req)->snt_isn + 1))
689 /* Also, it would be not so bad idea to check rcv_tsecr, which
690 * is essentially ACK extension and too early or too late values
691 * should cause reset in unsynchronized states.
694 /* RFC793: "first check sequence number". */
696 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
697 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) {
698 /* Out of window: send ACK and drop. */
699 if (!(flg & TCP_FLAG_RST) &&
700 !tcp_oow_rate_limited(sock_net(sk), skb,
701 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
702 &tcp_rsk(req)->last_oow_ack_time))
703 req->rsk_ops->send_ack(sk, skb, req);
705 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
709 /* In sequence, PAWS is OK. */
711 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
712 req->ts_recent = tmp_opt.rcv_tsval;
714 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
715 /* Truncate SYN, it is out of window starting
716 at tcp_rsk(req)->rcv_isn + 1. */
717 flg &= ~TCP_FLAG_SYN;
720 /* RFC793: "second check the RST bit" and
721 * "fourth, check the SYN bit"
723 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
724 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
725 goto embryonic_reset;
728 /* ACK sequence verified above, just make sure ACK is
729 * set. If ACK not set, just silently drop the packet.
731 * XXX (TFO) - if we ever allow "data after SYN", the
732 * following check needs to be removed.
734 if (!(flg & TCP_FLAG_ACK))
737 /* For Fast Open no more processing is needed (sk is the
743 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
744 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
745 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
746 inet_rsk(req)->acked = 1;
747 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
751 /* OK, ACK is valid, create big socket and
752 * feed this segment to it. It will repeat all
753 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
754 * ESTABLISHED STATE. If it will be dropped after
755 * socket is created, wait for troubles.
757 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
760 goto listen_overflow;
762 sock_rps_save_rxhash(child, skb);
763 tcp_synack_rtt_meas(child, req);
764 return inet_csk_complete_hashdance(sk, child, req, own_req);
767 if (!sysctl_tcp_abort_on_overflow) {
768 inet_rsk(req)->acked = 1;
773 if (!(flg & TCP_FLAG_RST)) {
774 /* Received a bad SYN pkt - for TFO We try not to reset
775 * the local connection unless it's really necessary to
776 * avoid becoming vulnerable to outside attack aiming at
777 * resetting legit local connections.
779 req->rsk_ops->send_reset(sk, skb);
780 } else if (fastopen) { /* received a valid RST pkt */
781 reqsk_fastopen_remove(sk, req, true);
785 inet_csk_reqsk_queue_drop(sk, req);
786 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
790 EXPORT_SYMBOL(tcp_check_req);
793 * Queue segment on the new socket if the new socket is active,
794 * otherwise we just shortcircuit this and continue with
797 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
798 * when entering. But other states are possible due to a race condition
799 * where after __inet_lookup_established() fails but before the listener
800 * locked is obtained, other packets cause the same connection to
804 int tcp_child_process(struct sock *parent, struct sock *child,
808 int state = child->sk_state;
810 /* record NAPI ID of child */
811 sk_mark_napi_id(child, skb);
813 tcp_segs_in(tcp_sk(child), skb);
814 if (!sock_owned_by_user(child)) {
815 ret = tcp_rcv_state_process(child, skb);
816 /* Wakeup parent, send SIGIO */
817 if (state == TCP_SYN_RECV && child->sk_state != state)
818 parent->sk_data_ready(parent);
820 /* Alas, it is possible again, because we do lookup
821 * in main socket hash table and lock on listening
822 * socket does not protect us more.
824 __sk_add_backlog(child, skb);
827 bh_unlock_sock(child);
831 EXPORT_SYMBOL(tcp_child_process);