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 tcp_prequeue_init(newtp);
449 INIT_LIST_HEAD(&newtp->tsq_node);
451 tcp_init_wl(newtp, treq->rcv_isn);
454 newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT);
455 minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U);
456 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
457 newicsk->icsk_ack.lrcvtime = tcp_jiffies32;
459 newtp->packets_out = 0;
460 newtp->retrans_out = 0;
461 newtp->sacked_out = 0;
462 newtp->fackets_out = 0;
463 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
464 newtp->tlp_high_seq = 0;
465 newtp->lsndtime = tcp_jiffies32;
466 newsk->sk_txhash = treq->txhash;
467 newtp->last_oow_ack_time = 0;
468 newtp->total_retrans = req->num_retrans;
470 /* So many TCP implementations out there (incorrectly) count the
471 * initial SYN frame in their delayed-ACK and congestion control
472 * algorithms that we must have the following bandaid to talk
473 * efficiently to them. -DaveM
475 newtp->snd_cwnd = TCP_INIT_CWND;
476 newtp->snd_cwnd_cnt = 0;
478 /* There's a bubble in the pipe until at least the first ACK. */
479 newtp->app_limited = ~0U;
481 tcp_init_xmit_timers(newsk);
482 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
484 newtp->rx_opt.saw_tstamp = 0;
486 newtp->rx_opt.dsack = 0;
487 newtp->rx_opt.num_sacks = 0;
491 if (sock_flag(newsk, SOCK_KEEPOPEN))
492 inet_csk_reset_keepalive_timer(newsk,
493 keepalive_time_when(newtp));
495 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
496 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
498 tcp_enable_fack(newtp);
500 newtp->window_clamp = req->rsk_window_clamp;
501 newtp->rcv_ssthresh = req->rsk_rcv_wnd;
502 newtp->rcv_wnd = req->rsk_rcv_wnd;
503 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
504 if (newtp->rx_opt.wscale_ok) {
505 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
506 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
508 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
509 newtp->window_clamp = min(newtp->window_clamp, 65535U);
511 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
512 newtp->rx_opt.snd_wscale);
513 newtp->max_window = newtp->snd_wnd;
515 if (newtp->rx_opt.tstamp_ok) {
516 newtp->rx_opt.ts_recent = req->ts_recent;
517 newtp->rx_opt.ts_recent_stamp = get_seconds();
518 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
520 newtp->rx_opt.ts_recent_stamp = 0;
521 newtp->tcp_header_len = sizeof(struct tcphdr);
523 newtp->tsoffset = treq->ts_off;
524 #ifdef CONFIG_TCP_MD5SIG
525 newtp->md5sig_info = NULL; /*XXX*/
526 if (newtp->af_specific->md5_lookup(sk, newsk))
527 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
529 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
530 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
531 newtp->rx_opt.mss_clamp = req->mss;
532 tcp_ecn_openreq_child(newtp, req);
533 newtp->fastopen_req = NULL;
534 newtp->fastopen_rsk = NULL;
535 newtp->syn_data_acked = 0;
536 newtp->rack.mstamp = 0;
537 newtp->rack.advanced = 0;
539 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS);
543 EXPORT_SYMBOL(tcp_create_openreq_child);
546 * Process an incoming packet for SYN_RECV sockets represented as a
547 * request_sock. Normally sk is the listener socket but for TFO it
548 * points to the child socket.
550 * XXX (TFO) - The current impl contains a special check for ack
551 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
553 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
556 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
557 struct request_sock *req,
560 struct tcp_options_received tmp_opt;
562 const struct tcphdr *th = tcp_hdr(skb);
563 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
564 bool paws_reject = false;
567 tmp_opt.saw_tstamp = 0;
568 if (th->doff > (sizeof(struct tcphdr)>>2)) {
569 tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL);
571 if (tmp_opt.saw_tstamp) {
572 tmp_opt.ts_recent = req->ts_recent;
573 if (tmp_opt.rcv_tsecr)
574 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off;
575 /* We do not store true stamp, but it is not required,
576 * it can be estimated (approximately)
579 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
580 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
584 /* Check for pure retransmitted SYN. */
585 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
586 flg == TCP_FLAG_SYN &&
589 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
590 * this case on figure 6 and figure 8, but formal
591 * protocol description says NOTHING.
592 * To be more exact, it says that we should send ACK,
593 * because this segment (at least, if it has no data)
596 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
597 * describe SYN-RECV state. All the description
598 * is wrong, we cannot believe to it and should
599 * rely only on common sense and implementation
602 * Enforce "SYN-ACK" according to figure 8, figure 6
603 * of RFC793, fixed by RFC1122.
605 * Note that even if there is new data in the SYN packet
606 * they will be thrown away too.
608 * Reset timer after retransmitting SYNACK, similar to
609 * the idea of fast retransmit in recovery.
611 if (!tcp_oow_rate_limited(sock_net(sk), skb,
612 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
613 &tcp_rsk(req)->last_oow_ack_time) &&
615 !inet_rtx_syn_ack(sk, req)) {
616 unsigned long expires = jiffies;
618 expires += min(TCP_TIMEOUT_INIT << req->num_timeout,
621 mod_timer_pending(&req->rsk_timer, expires);
623 req->rsk_timer.expires = expires;
628 /* Further reproduces section "SEGMENT ARRIVES"
629 for state SYN-RECEIVED of RFC793.
630 It is broken, however, it does not work only
631 when SYNs are crossed.
633 You would think that SYN crossing is impossible here, since
634 we should have a SYN_SENT socket (from connect()) on our end,
635 but this is not true if the crossed SYNs were sent to both
636 ends by a malicious third party. We must defend against this,
637 and to do that we first verify the ACK (as per RFC793, page
638 36) and reset if it is invalid. Is this a true full defense?
639 To convince ourselves, let us consider a way in which the ACK
640 test can still pass in this 'malicious crossed SYNs' case.
641 Malicious sender sends identical SYNs (and thus identical sequence
642 numbers) to both A and B:
647 By our good fortune, both A and B select the same initial
648 send sequence number of seven :-)
650 A: sends SYN|ACK, seq=7, ack_seq=8
651 B: sends SYN|ACK, seq=7, ack_seq=8
653 So we are now A eating this SYN|ACK, ACK test passes. So
654 does sequence test, SYN is truncated, and thus we consider
657 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
658 bare ACK. Otherwise, we create an established connection. Both
659 ends (listening sockets) accept the new incoming connection and try
660 to talk to each other. 8-)
662 Note: This case is both harmless, and rare. Possibility is about the
663 same as us discovering intelligent life on another plant tomorrow.
665 But generally, we should (RFC lies!) to accept ACK
666 from SYNACK both here and in tcp_rcv_state_process().
667 tcp_rcv_state_process() does not, hence, we do not too.
669 Note that the case is absolutely generic:
670 we cannot optimize anything here without
671 violating protocol. All the checks must be made
672 before attempt to create socket.
675 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
676 * and the incoming segment acknowledges something not yet
677 * sent (the segment carries an unacceptable ACK) ...
680 * Invalid ACK: reset will be sent by listening socket.
681 * Note that the ACK validity check for a Fast Open socket is done
682 * elsewhere and is checked directly against the child socket rather
683 * than req because user data may have been sent out.
685 if ((flg & TCP_FLAG_ACK) && !fastopen &&
686 (TCP_SKB_CB(skb)->ack_seq !=
687 tcp_rsk(req)->snt_isn + 1))
690 /* Also, it would be not so bad idea to check rcv_tsecr, which
691 * is essentially ACK extension and too early or too late values
692 * should cause reset in unsynchronized states.
695 /* RFC793: "first check sequence number". */
697 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
698 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) {
699 /* Out of window: send ACK and drop. */
700 if (!(flg & TCP_FLAG_RST) &&
701 !tcp_oow_rate_limited(sock_net(sk), skb,
702 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
703 &tcp_rsk(req)->last_oow_ack_time))
704 req->rsk_ops->send_ack(sk, skb, req);
706 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
710 /* In sequence, PAWS is OK. */
712 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
713 req->ts_recent = tmp_opt.rcv_tsval;
715 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
716 /* Truncate SYN, it is out of window starting
717 at tcp_rsk(req)->rcv_isn + 1. */
718 flg &= ~TCP_FLAG_SYN;
721 /* RFC793: "second check the RST bit" and
722 * "fourth, check the SYN bit"
724 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
725 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
726 goto embryonic_reset;
729 /* ACK sequence verified above, just make sure ACK is
730 * set. If ACK not set, just silently drop the packet.
732 * XXX (TFO) - if we ever allow "data after SYN", the
733 * following check needs to be removed.
735 if (!(flg & TCP_FLAG_ACK))
738 /* For Fast Open no more processing is needed (sk is the
744 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
745 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
746 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
747 inet_rsk(req)->acked = 1;
748 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
752 /* OK, ACK is valid, create big socket and
753 * feed this segment to it. It will repeat all
754 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
755 * ESTABLISHED STATE. If it will be dropped after
756 * socket is created, wait for troubles.
758 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
761 goto listen_overflow;
763 sock_rps_save_rxhash(child, skb);
764 tcp_synack_rtt_meas(child, req);
765 return inet_csk_complete_hashdance(sk, child, req, own_req);
768 if (!sysctl_tcp_abort_on_overflow) {
769 inet_rsk(req)->acked = 1;
774 if (!(flg & TCP_FLAG_RST)) {
775 /* Received a bad SYN pkt - for TFO We try not to reset
776 * the local connection unless it's really necessary to
777 * avoid becoming vulnerable to outside attack aiming at
778 * resetting legit local connections.
780 req->rsk_ops->send_reset(sk, skb);
781 } else if (fastopen) { /* received a valid RST pkt */
782 reqsk_fastopen_remove(sk, req, true);
786 inet_csk_reqsk_queue_drop(sk, req);
787 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
791 EXPORT_SYMBOL(tcp_check_req);
794 * Queue segment on the new socket if the new socket is active,
795 * otherwise we just shortcircuit this and continue with
798 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
799 * when entering. But other states are possible due to a race condition
800 * where after __inet_lookup_established() fails but before the listener
801 * locked is obtained, other packets cause the same connection to
805 int tcp_child_process(struct sock *parent, struct sock *child,
809 int state = child->sk_state;
811 /* record NAPI ID of child */
812 sk_mark_napi_id(child, skb);
814 tcp_segs_in(tcp_sk(child), skb);
815 if (!sock_owned_by_user(child)) {
816 ret = tcp_rcv_state_process(child, skb);
817 /* Wakeup parent, send SIGIO */
818 if (state == TCP_SYN_RECV && child->sk_state != state)
819 parent->sk_data_ready(parent);
821 /* Alas, it is possible again, because we do lookup
822 * in main socket hash table and lock on listening
823 * socket does not protect us more.
825 __sk_add_backlog(child, skb);
828 bh_unlock_sock(child);
832 EXPORT_SYMBOL(tcp_child_process);