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>
26 #include <linux/static_key.h>
28 #include <net/inet_common.h>
30 #include <net/busy_poll.h>
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;
98 tmp_opt.saw_tstamp = 0;
99 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
100 tcp_parse_options(twsk_net(tw), skb, &tmp_opt, 0, NULL);
102 if (tmp_opt.saw_tstamp) {
103 if (tmp_opt.rcv_tsecr)
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 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
156 * Now real TIME-WAIT state.
159 * "When a connection is [...] on TIME-WAIT state [...]
160 * [a TCP] MAY accept a new SYN from the remote TCP to
161 * reopen the connection directly, if it:
163 * (1) assigns its initial sequence number for the new
164 * connection to be larger than the largest sequence
165 * number it used on the previous connection incarnation,
168 * (2) returns to TIME-WAIT state if the SYN turns out
169 * to be an old duplicate".
173 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
174 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
175 /* In window segment, it may be only reset or bare ack. */
178 /* This is TIME_WAIT assassination, in two flavors.
179 * Oh well... nobody has a sufficient solution to this
182 if (twsk_net(tw)->ipv4.sysctl_tcp_rfc1337 == 0) {
184 inet_twsk_deschedule_put(tw);
185 return TCP_TW_SUCCESS;
188 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
190 if (tmp_opt.saw_tstamp) {
191 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
192 tcptw->tw_ts_recent_stamp = get_seconds();
196 return TCP_TW_SUCCESS;
199 /* Out of window segment.
201 All the segments are ACKed immediately.
203 The only exception is new SYN. We accept it, if it is
204 not old duplicate and we are not in danger to be killed
205 by delayed old duplicates. RFC check is that it has
206 newer sequence number works at rates <40Mbit/sec.
207 However, if paws works, it is reliable AND even more,
208 we even may relax silly seq space cutoff.
210 RED-PEN: we violate main RFC requirement, if this SYN will appear
211 old duplicate (i.e. we receive RST in reply to SYN-ACK),
212 we must return socket to time-wait state. It is not good,
216 if (th->syn && !th->rst && !th->ack && !paws_reject &&
217 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
218 (tmp_opt.saw_tstamp &&
219 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
220 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
223 TCP_SKB_CB(skb)->tcp_tw_isn = isn;
228 __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
231 /* In this case we must reset the TIMEWAIT timer.
233 * If it is ACKless SYN it may be both old duplicate
234 * and new good SYN with random sequence number <rcv_nxt.
235 * Do not reschedule in the last case.
237 if (paws_reject || th->ack)
238 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
240 return tcp_timewait_check_oow_rate_limit(
241 tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT);
244 return TCP_TW_SUCCESS;
246 EXPORT_SYMBOL(tcp_timewait_state_process);
249 * Move a socket to time-wait or dead fin-wait-2 state.
251 void tcp_time_wait(struct sock *sk, int state, int timeo)
253 const struct inet_connection_sock *icsk = inet_csk(sk);
254 const struct tcp_sock *tp = tcp_sk(sk);
255 struct inet_timewait_sock *tw;
256 struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row;
258 tw = inet_twsk_alloc(sk, tcp_death_row, state);
261 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
262 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
263 struct inet_sock *inet = inet_sk(sk);
265 tw->tw_transparent = inet->transparent;
266 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale;
267 tcptw->tw_rcv_nxt = tp->rcv_nxt;
268 tcptw->tw_snd_nxt = tp->snd_nxt;
269 tcptw->tw_rcv_wnd = tcp_receive_window(tp);
270 tcptw->tw_ts_recent = tp->rx_opt.ts_recent;
271 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
272 tcptw->tw_ts_offset = tp->tsoffset;
273 tcptw->tw_last_oow_ack_time = 0;
275 #if IS_ENABLED(CONFIG_IPV6)
276 if (tw->tw_family == PF_INET6) {
277 struct ipv6_pinfo *np = inet6_sk(sk);
279 tw->tw_v6_daddr = sk->sk_v6_daddr;
280 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
281 tw->tw_tclass = np->tclass;
282 tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK);
283 tw->tw_ipv6only = sk->sk_ipv6only;
287 #ifdef CONFIG_TCP_MD5SIG
289 * The timewait bucket does not have the key DB from the
290 * sock structure. We just make a quick copy of the
291 * md5 key being used (if indeed we are using one)
292 * so the timewait ack generating code has the key.
295 struct tcp_md5sig_key *key;
296 tcptw->tw_md5_key = NULL;
297 key = tp->af_specific->md5_lookup(sk, sk);
299 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
300 BUG_ON(tcptw->tw_md5_key && !tcp_alloc_md5sig_pool());
305 /* Get the TIME_WAIT timeout firing. */
309 tw->tw_timeout = TCP_TIMEWAIT_LEN;
310 if (state == TCP_TIME_WAIT)
311 timeo = TCP_TIMEWAIT_LEN;
313 /* tw_timer is pinned, so we need to make sure BH are disabled
314 * in following section, otherwise timer handler could run before
315 * we complete the initialization.
318 inet_twsk_schedule(tw, timeo);
319 /* Linkage updates. */
320 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
324 /* Sorry, if we're out of memory, just CLOSE this
325 * socket up. We've got bigger problems than
326 * non-graceful socket closings.
328 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW);
331 tcp_update_metrics(sk);
335 void tcp_twsk_destructor(struct sock *sk)
337 #ifdef CONFIG_TCP_MD5SIG
338 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
340 if (twsk->tw_md5_key)
341 kfree_rcu(twsk->tw_md5_key, rcu);
344 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
346 /* Warning : This function is called without sk_listener being locked.
347 * Be sure to read socket fields once, as their value could change under us.
349 void tcp_openreq_init_rwin(struct request_sock *req,
350 const struct sock *sk_listener,
351 const struct dst_entry *dst)
353 struct inet_request_sock *ireq = inet_rsk(req);
354 const struct tcp_sock *tp = tcp_sk(sk_listener);
355 int full_space = tcp_full_space(sk_listener);
361 mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
362 window_clamp = READ_ONCE(tp->window_clamp);
363 /* Set this up on the first call only */
364 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);
366 /* limit the window selection if the user enforce a smaller rx buffer */
367 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK &&
368 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
369 req->rsk_window_clamp = full_space;
371 rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req);
373 rcv_wnd = dst_metric(dst, RTAX_INITRWND);
374 else if (full_space < rcv_wnd * mss)
375 full_space = rcv_wnd * mss;
377 /* tcp_full_space because it is guaranteed to be the first packet */
378 tcp_select_initial_window(sk_listener, full_space,
379 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
381 &req->rsk_window_clamp,
385 ireq->rcv_wscale = rcv_wscale;
387 EXPORT_SYMBOL(tcp_openreq_init_rwin);
389 static void tcp_ecn_openreq_child(struct tcp_sock *tp,
390 const struct request_sock *req)
392 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
395 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
397 struct inet_connection_sock *icsk = inet_csk(sk);
398 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
399 bool ca_got_dst = false;
401 if (ca_key != TCP_CA_UNSPEC) {
402 const struct tcp_congestion_ops *ca;
405 ca = tcp_ca_find_key(ca_key);
406 if (likely(ca && try_module_get(ca->owner))) {
407 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
408 icsk->icsk_ca_ops = ca;
414 /* If no valid choice made yet, assign current system default ca. */
416 (!icsk->icsk_ca_setsockopt ||
417 !try_module_get(icsk->icsk_ca_ops->owner)))
418 tcp_assign_congestion_control(sk);
420 tcp_set_ca_state(sk, TCP_CA_Open);
422 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child);
424 static void smc_check_reset_syn_req(struct tcp_sock *oldtp,
425 struct request_sock *req,
426 struct tcp_sock *newtp)
428 #if IS_ENABLED(CONFIG_SMC)
429 struct inet_request_sock *ireq;
431 if (static_branch_unlikely(&tcp_have_smc)) {
432 ireq = inet_rsk(req);
433 if (oldtp->syn_smc && !ireq->smc_ok)
439 /* This is not only more efficient than what we used to do, it eliminates
440 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
442 * Actually, we could lots of memory writes here. tp of listening
443 * socket contains all necessary default parameters.
445 struct sock *tcp_create_openreq_child(const struct sock *sk,
446 struct request_sock *req,
449 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
452 const struct inet_request_sock *ireq = inet_rsk(req);
453 struct tcp_request_sock *treq = tcp_rsk(req);
454 struct inet_connection_sock *newicsk = inet_csk(newsk);
455 struct tcp_sock *newtp = tcp_sk(newsk);
456 struct tcp_sock *oldtp = tcp_sk(sk);
458 smc_check_reset_syn_req(oldtp, req, newtp);
460 /* Now setup tcp_sock */
461 newtp->pred_flags = 0;
463 newtp->rcv_wup = newtp->copied_seq =
464 newtp->rcv_nxt = treq->rcv_isn + 1;
467 newtp->snd_sml = newtp->snd_una =
468 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;
470 INIT_LIST_HEAD(&newtp->tsq_node);
471 INIT_LIST_HEAD(&newtp->tsorted_sent_queue);
473 tcp_init_wl(newtp, treq->rcv_isn);
476 newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT);
477 minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U);
478 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
479 newicsk->icsk_ack.lrcvtime = tcp_jiffies32;
481 newtp->packets_out = 0;
482 newtp->retrans_out = 0;
483 newtp->sacked_out = 0;
484 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
485 newtp->tlp_high_seq = 0;
486 newtp->lsndtime = tcp_jiffies32;
487 newsk->sk_txhash = treq->txhash;
488 newtp->last_oow_ack_time = 0;
489 newtp->total_retrans = req->num_retrans;
491 /* So many TCP implementations out there (incorrectly) count the
492 * initial SYN frame in their delayed-ACK and congestion control
493 * algorithms that we must have the following bandaid to talk
494 * efficiently to them. -DaveM
496 newtp->snd_cwnd = TCP_INIT_CWND;
497 newtp->snd_cwnd_cnt = 0;
499 /* There's a bubble in the pipe until at least the first ACK. */
500 newtp->app_limited = ~0U;
502 tcp_init_xmit_timers(newsk);
503 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
505 newtp->rx_opt.saw_tstamp = 0;
507 newtp->rx_opt.dsack = 0;
508 newtp->rx_opt.num_sacks = 0;
512 if (sock_flag(newsk, SOCK_KEEPOPEN))
513 inet_csk_reset_keepalive_timer(newsk,
514 keepalive_time_when(newtp));
516 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
517 newtp->rx_opt.sack_ok = ireq->sack_ok;
518 newtp->window_clamp = req->rsk_window_clamp;
519 newtp->rcv_ssthresh = req->rsk_rcv_wnd;
520 newtp->rcv_wnd = req->rsk_rcv_wnd;
521 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
522 if (newtp->rx_opt.wscale_ok) {
523 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
524 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
526 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
527 newtp->window_clamp = min(newtp->window_clamp, 65535U);
529 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
530 newtp->rx_opt.snd_wscale);
531 newtp->max_window = newtp->snd_wnd;
533 if (newtp->rx_opt.tstamp_ok) {
534 newtp->rx_opt.ts_recent = req->ts_recent;
535 newtp->rx_opt.ts_recent_stamp = get_seconds();
536 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
538 newtp->rx_opt.ts_recent_stamp = 0;
539 newtp->tcp_header_len = sizeof(struct tcphdr);
541 newtp->tsoffset = treq->ts_off;
542 #ifdef CONFIG_TCP_MD5SIG
543 newtp->md5sig_info = NULL; /*XXX*/
544 if (newtp->af_specific->md5_lookup(sk, newsk))
545 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
547 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
548 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
549 newtp->rx_opt.mss_clamp = req->mss;
550 tcp_ecn_openreq_child(newtp, req);
551 newtp->fastopen_req = NULL;
552 newtp->fastopen_rsk = NULL;
553 newtp->syn_data_acked = 0;
554 newtp->rack.mstamp = 0;
555 newtp->rack.advanced = 0;
556 newtp->rack.reo_wnd_steps = 1;
557 newtp->rack.last_delivered = 0;
558 newtp->rack.reo_wnd_persist = 0;
559 newtp->rack.dsack_seen = 0;
561 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS);
565 EXPORT_SYMBOL(tcp_create_openreq_child);
568 * Process an incoming packet for SYN_RECV sockets represented as a
569 * request_sock. Normally sk is the listener socket but for TFO it
570 * points to the child socket.
572 * XXX (TFO) - The current impl contains a special check for ack
573 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
575 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
578 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
579 struct request_sock *req,
582 struct tcp_options_received tmp_opt;
584 const struct tcphdr *th = tcp_hdr(skb);
585 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
586 bool paws_reject = false;
589 tmp_opt.saw_tstamp = 0;
590 if (th->doff > (sizeof(struct tcphdr)>>2)) {
591 tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL);
593 if (tmp_opt.saw_tstamp) {
594 tmp_opt.ts_recent = req->ts_recent;
595 if (tmp_opt.rcv_tsecr)
596 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off;
597 /* We do not store true stamp, but it is not required,
598 * it can be estimated (approximately)
601 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
602 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
606 /* Check for pure retransmitted SYN. */
607 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
608 flg == TCP_FLAG_SYN &&
611 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
612 * this case on figure 6 and figure 8, but formal
613 * protocol description says NOTHING.
614 * To be more exact, it says that we should send ACK,
615 * because this segment (at least, if it has no data)
618 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
619 * describe SYN-RECV state. All the description
620 * is wrong, we cannot believe to it and should
621 * rely only on common sense and implementation
624 * Enforce "SYN-ACK" according to figure 8, figure 6
625 * of RFC793, fixed by RFC1122.
627 * Note that even if there is new data in the SYN packet
628 * they will be thrown away too.
630 * Reset timer after retransmitting SYNACK, similar to
631 * the idea of fast retransmit in recovery.
633 if (!tcp_oow_rate_limited(sock_net(sk), skb,
634 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
635 &tcp_rsk(req)->last_oow_ack_time) &&
637 !inet_rtx_syn_ack(sk, req)) {
638 unsigned long expires = jiffies;
640 expires += min(TCP_TIMEOUT_INIT << req->num_timeout,
643 mod_timer_pending(&req->rsk_timer, expires);
645 req->rsk_timer.expires = expires;
650 /* Further reproduces section "SEGMENT ARRIVES"
651 for state SYN-RECEIVED of RFC793.
652 It is broken, however, it does not work only
653 when SYNs are crossed.
655 You would think that SYN crossing is impossible here, since
656 we should have a SYN_SENT socket (from connect()) on our end,
657 but this is not true if the crossed SYNs were sent to both
658 ends by a malicious third party. We must defend against this,
659 and to do that we first verify the ACK (as per RFC793, page
660 36) and reset if it is invalid. Is this a true full defense?
661 To convince ourselves, let us consider a way in which the ACK
662 test can still pass in this 'malicious crossed SYNs' case.
663 Malicious sender sends identical SYNs (and thus identical sequence
664 numbers) to both A and B:
669 By our good fortune, both A and B select the same initial
670 send sequence number of seven :-)
672 A: sends SYN|ACK, seq=7, ack_seq=8
673 B: sends SYN|ACK, seq=7, ack_seq=8
675 So we are now A eating this SYN|ACK, ACK test passes. So
676 does sequence test, SYN is truncated, and thus we consider
679 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
680 bare ACK. Otherwise, we create an established connection. Both
681 ends (listening sockets) accept the new incoming connection and try
682 to talk to each other. 8-)
684 Note: This case is both harmless, and rare. Possibility is about the
685 same as us discovering intelligent life on another plant tomorrow.
687 But generally, we should (RFC lies!) to accept ACK
688 from SYNACK both here and in tcp_rcv_state_process().
689 tcp_rcv_state_process() does not, hence, we do not too.
691 Note that the case is absolutely generic:
692 we cannot optimize anything here without
693 violating protocol. All the checks must be made
694 before attempt to create socket.
697 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
698 * and the incoming segment acknowledges something not yet
699 * sent (the segment carries an unacceptable ACK) ...
702 * Invalid ACK: reset will be sent by listening socket.
703 * Note that the ACK validity check for a Fast Open socket is done
704 * elsewhere and is checked directly against the child socket rather
705 * than req because user data may have been sent out.
707 if ((flg & TCP_FLAG_ACK) && !fastopen &&
708 (TCP_SKB_CB(skb)->ack_seq !=
709 tcp_rsk(req)->snt_isn + 1))
712 /* Also, it would be not so bad idea to check rcv_tsecr, which
713 * is essentially ACK extension and too early or too late values
714 * should cause reset in unsynchronized states.
717 /* RFC793: "first check sequence number". */
719 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
720 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) {
721 /* Out of window: send ACK and drop. */
722 if (!(flg & TCP_FLAG_RST) &&
723 !tcp_oow_rate_limited(sock_net(sk), skb,
724 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
725 &tcp_rsk(req)->last_oow_ack_time))
726 req->rsk_ops->send_ack(sk, skb, req);
728 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
732 /* In sequence, PAWS is OK. */
734 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
735 req->ts_recent = tmp_opt.rcv_tsval;
737 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
738 /* Truncate SYN, it is out of window starting
739 at tcp_rsk(req)->rcv_isn + 1. */
740 flg &= ~TCP_FLAG_SYN;
743 /* RFC793: "second check the RST bit" and
744 * "fourth, check the SYN bit"
746 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
747 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
748 goto embryonic_reset;
751 /* ACK sequence verified above, just make sure ACK is
752 * set. If ACK not set, just silently drop the packet.
754 * XXX (TFO) - if we ever allow "data after SYN", the
755 * following check needs to be removed.
757 if (!(flg & TCP_FLAG_ACK))
760 /* For Fast Open no more processing is needed (sk is the
766 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
767 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
768 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
769 inet_rsk(req)->acked = 1;
770 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
774 /* OK, ACK is valid, create big socket and
775 * feed this segment to it. It will repeat all
776 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
777 * ESTABLISHED STATE. If it will be dropped after
778 * socket is created, wait for troubles.
780 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
783 goto listen_overflow;
785 sock_rps_save_rxhash(child, skb);
786 tcp_synack_rtt_meas(child, req);
787 return inet_csk_complete_hashdance(sk, child, req, own_req);
790 if (!sock_net(sk)->ipv4.sysctl_tcp_abort_on_overflow) {
791 inet_rsk(req)->acked = 1;
796 if (!(flg & TCP_FLAG_RST)) {
797 /* Received a bad SYN pkt - for TFO We try not to reset
798 * the local connection unless it's really necessary to
799 * avoid becoming vulnerable to outside attack aiming at
800 * resetting legit local connections.
802 req->rsk_ops->send_reset(sk, skb);
803 } else if (fastopen) { /* received a valid RST pkt */
804 reqsk_fastopen_remove(sk, req, true);
808 inet_csk_reqsk_queue_drop(sk, req);
809 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
813 EXPORT_SYMBOL(tcp_check_req);
816 * Queue segment on the new socket if the new socket is active,
817 * otherwise we just shortcircuit this and continue with
820 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
821 * when entering. But other states are possible due to a race condition
822 * where after __inet_lookup_established() fails but before the listener
823 * locked is obtained, other packets cause the same connection to
827 int tcp_child_process(struct sock *parent, struct sock *child,
831 int state = child->sk_state;
833 /* record NAPI ID of child */
834 sk_mark_napi_id(child, skb);
836 tcp_segs_in(tcp_sk(child), skb);
837 if (!sock_owned_by_user(child)) {
838 ret = tcp_rcv_state_process(child, skb);
839 /* Wakeup parent, send SIGIO */
840 if (state == TCP_SYN_RECV && child->sk_state != state)
841 parent->sk_data_ready(parent);
843 /* Alas, it is possible again, because we do lookup
844 * in main socket hash table and lock on listening
845 * socket does not protect us more.
847 __sk_add_backlog(child, skb);
850 bh_unlock_sock(child);
854 EXPORT_SYMBOL(tcp_child_process);