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_syncookies __read_mostly = 1;
31 EXPORT_SYMBOL(sysctl_tcp_syncookies);
33 int sysctl_tcp_abort_on_overflow __read_mostly;
35 struct inet_timewait_death_row tcp_death_row = {
36 .sysctl_max_tw_buckets = NR_FILE * 2,
37 .period = TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS,
38 .death_lock = __SPIN_LOCK_UNLOCKED(tcp_death_row.death_lock),
39 .hashinfo = &tcp_hashinfo,
40 .tw_timer = TIMER_INITIALIZER(inet_twdr_hangman, 0,
41 (unsigned long)&tcp_death_row),
42 .twkill_work = __WORK_INITIALIZER(tcp_death_row.twkill_work,
43 inet_twdr_twkill_work),
44 /* Short-time timewait calendar */
47 .twcal_timer = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0,
48 (unsigned long)&tcp_death_row),
50 EXPORT_SYMBOL_GPL(tcp_death_row);
52 static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
56 if (after(end_seq, s_win) && before(seq, e_win))
58 return seq == e_win && seq == end_seq;
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(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))
124 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
129 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
130 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
132 return TCP_TW_SUCCESS;
135 /* New data or FIN. If new data arrive after half-duplex close,
139 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) {
141 inet_twsk_deschedule(tw, &tcp_death_row);
146 /* FIN arrived, enter true time-wait state. */
147 tw->tw_substate = TCP_TIME_WAIT;
148 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
149 if (tmp_opt.saw_tstamp) {
150 tcptw->tw_ts_recent_stamp = get_seconds();
151 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
154 if (tcp_death_row.sysctl_tw_recycle &&
155 tcptw->tw_ts_recent_stamp &&
156 tcp_tw_remember_stamp(tw))
157 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout,
160 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
166 * Now real TIME-WAIT state.
169 * "When a connection is [...] on TIME-WAIT state [...]
170 * [a TCP] MAY accept a new SYN from the remote TCP to
171 * reopen the connection directly, if it:
173 * (1) assigns its initial sequence number for the new
174 * connection to be larger than the largest sequence
175 * number it used on the previous connection incarnation,
178 * (2) returns to TIME-WAIT state if the SYN turns out
179 * to be an old duplicate".
183 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
184 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
185 /* In window segment, it may be only reset or bare ack. */
188 /* This is TIME_WAIT assassination, in two flavors.
189 * Oh well... nobody has a sufficient solution to this
192 if (sysctl_tcp_rfc1337 == 0) {
194 inet_twsk_deschedule(tw, &tcp_death_row);
196 return TCP_TW_SUCCESS;
199 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
202 if (tmp_opt.saw_tstamp) {
203 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
204 tcptw->tw_ts_recent_stamp = get_seconds();
208 return TCP_TW_SUCCESS;
211 /* Out of window segment.
213 All the segments are ACKed immediately.
215 The only exception is new SYN. We accept it, if it is
216 not old duplicate and we are not in danger to be killed
217 by delayed old duplicates. RFC check is that it has
218 newer sequence number works at rates <40Mbit/sec.
219 However, if paws works, it is reliable AND even more,
220 we even may relax silly seq space cutoff.
222 RED-PEN: we violate main RFC requirement, if this SYN will appear
223 old duplicate (i.e. we receive RST in reply to SYN-ACK),
224 we must return socket to time-wait state. It is not good,
228 if (th->syn && !th->rst && !th->ack && !paws_reject &&
229 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
230 (tmp_opt.saw_tstamp &&
231 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
232 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
235 TCP_SKB_CB(skb)->when = isn;
240 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
243 /* In this case we must reset the TIMEWAIT timer.
245 * If it is ACKless SYN it may be both old duplicate
246 * and new good SYN with random sequence number <rcv_nxt.
247 * Do not reschedule in the last case.
249 if (paws_reject || th->ack)
250 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
253 /* Send ACK. Note, we do not put the bucket,
254 * it will be released by caller.
259 return TCP_TW_SUCCESS;
261 EXPORT_SYMBOL(tcp_timewait_state_process);
264 * Move a socket to time-wait or dead fin-wait-2 state.
266 void tcp_time_wait(struct sock *sk, int state, int timeo)
268 struct inet_timewait_sock *tw = NULL;
269 const struct inet_connection_sock *icsk = inet_csk(sk);
270 const struct tcp_sock *tp = tcp_sk(sk);
271 bool recycle_ok = false;
273 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp)
274 recycle_ok = tcp_remember_stamp(sk);
276 if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets)
277 tw = inet_twsk_alloc(sk, state);
280 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
281 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
282 struct inet_sock *inet = inet_sk(sk);
284 tw->tw_transparent = inet->transparent;
285 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale;
286 tcptw->tw_rcv_nxt = tp->rcv_nxt;
287 tcptw->tw_snd_nxt = tp->snd_nxt;
288 tcptw->tw_rcv_wnd = tcp_receive_window(tp);
289 tcptw->tw_ts_recent = tp->rx_opt.ts_recent;
290 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
291 tcptw->tw_ts_offset = tp->tsoffset;
293 #if IS_ENABLED(CONFIG_IPV6)
294 if (tw->tw_family == PF_INET6) {
295 struct ipv6_pinfo *np = inet6_sk(sk);
297 tw->tw_v6_daddr = sk->sk_v6_daddr;
298 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
299 tw->tw_tclass = np->tclass;
300 tw->tw_flowlabel = np->flow_label >> 12;
301 tw->tw_ipv6only = sk->sk_ipv6only;
305 #ifdef CONFIG_TCP_MD5SIG
307 * The timewait bucket does not have the key DB from the
308 * sock structure. We just make a quick copy of the
309 * md5 key being used (if indeed we are using one)
310 * so the timewait ack generating code has the key.
313 struct tcp_md5sig_key *key;
314 tcptw->tw_md5_key = NULL;
315 key = tp->af_specific->md5_lookup(sk, sk);
317 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
318 if (tcptw->tw_md5_key && !tcp_alloc_md5sig_pool())
324 /* Linkage updates. */
325 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
327 /* Get the TIME_WAIT timeout firing. */
332 tw->tw_timeout = rto;
334 tw->tw_timeout = TCP_TIMEWAIT_LEN;
335 if (state == TCP_TIME_WAIT)
336 timeo = TCP_TIMEWAIT_LEN;
339 inet_twsk_schedule(tw, &tcp_death_row, timeo,
343 /* Sorry, if we're out of memory, just CLOSE this
344 * socket up. We've got bigger problems than
345 * non-graceful socket closings.
347 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW);
350 tcp_update_metrics(sk);
354 void tcp_twsk_destructor(struct sock *sk)
356 #ifdef CONFIG_TCP_MD5SIG
357 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
359 if (twsk->tw_md5_key)
360 kfree_rcu(twsk->tw_md5_key, rcu);
363 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
365 void tcp_openreq_init_rwin(struct request_sock *req,
366 struct sock *sk, struct dst_entry *dst)
368 struct inet_request_sock *ireq = inet_rsk(req);
369 struct tcp_sock *tp = tcp_sk(sk);
371 int mss = dst_metric_advmss(dst);
373 if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss)
374 mss = tp->rx_opt.user_mss;
376 /* Set this up on the first call only */
377 req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW);
379 /* limit the window selection if the user enforce a smaller rx buffer */
380 if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
381 (req->window_clamp > tcp_full_space(sk) || req->window_clamp == 0))
382 req->window_clamp = tcp_full_space(sk);
384 /* tcp_full_space because it is guaranteed to be the first packet */
385 tcp_select_initial_window(tcp_full_space(sk),
386 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
391 dst_metric(dst, RTAX_INITRWND));
392 ireq->rcv_wscale = rcv_wscale;
394 EXPORT_SYMBOL(tcp_openreq_init_rwin);
396 static inline void TCP_ECN_openreq_child(struct tcp_sock *tp,
397 struct request_sock *req)
399 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
402 /* This is not only more efficient than what we used to do, it eliminates
403 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
405 * Actually, we could lots of memory writes here. tp of listening
406 * socket contains all necessary default parameters.
408 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
410 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
413 const struct inet_request_sock *ireq = inet_rsk(req);
414 struct tcp_request_sock *treq = tcp_rsk(req);
415 struct inet_connection_sock *newicsk = inet_csk(newsk);
416 struct tcp_sock *newtp = tcp_sk(newsk);
418 /* Now setup tcp_sock */
419 newtp->pred_flags = 0;
421 newtp->rcv_wup = newtp->copied_seq =
422 newtp->rcv_nxt = treq->rcv_isn + 1;
424 newtp->snd_sml = newtp->snd_una =
425 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;
427 tcp_prequeue_init(newtp);
428 INIT_LIST_HEAD(&newtp->tsq_node);
430 tcp_init_wl(newtp, treq->rcv_isn);
433 newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT);
434 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
436 newtp->packets_out = 0;
437 newtp->retrans_out = 0;
438 newtp->sacked_out = 0;
439 newtp->fackets_out = 0;
440 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
441 tcp_enable_early_retrans(newtp);
442 newtp->tlp_high_seq = 0;
443 newtp->lsndtime = treq->snt_synack;
444 newtp->total_retrans = req->num_retrans;
446 /* So many TCP implementations out there (incorrectly) count the
447 * initial SYN frame in their delayed-ACK and congestion control
448 * algorithms that we must have the following bandaid to talk
449 * efficiently to them. -DaveM
451 newtp->snd_cwnd = TCP_INIT_CWND;
452 newtp->snd_cwnd_cnt = 0;
454 if (newicsk->icsk_ca_ops != &tcp_init_congestion_ops &&
455 !try_module_get(newicsk->icsk_ca_ops->owner))
456 newicsk->icsk_ca_ops = &tcp_init_congestion_ops;
458 tcp_set_ca_state(newsk, TCP_CA_Open);
459 tcp_init_xmit_timers(newsk);
460 __skb_queue_head_init(&newtp->out_of_order_queue);
461 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
463 newtp->rx_opt.saw_tstamp = 0;
465 newtp->rx_opt.dsack = 0;
466 newtp->rx_opt.num_sacks = 0;
470 if (sock_flag(newsk, SOCK_KEEPOPEN))
471 inet_csk_reset_keepalive_timer(newsk,
472 keepalive_time_when(newtp));
474 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
475 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
477 tcp_enable_fack(newtp);
479 newtp->window_clamp = req->window_clamp;
480 newtp->rcv_ssthresh = req->rcv_wnd;
481 newtp->rcv_wnd = req->rcv_wnd;
482 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
483 if (newtp->rx_opt.wscale_ok) {
484 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
485 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
487 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
488 newtp->window_clamp = min(newtp->window_clamp, 65535U);
490 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
491 newtp->rx_opt.snd_wscale);
492 newtp->max_window = newtp->snd_wnd;
494 if (newtp->rx_opt.tstamp_ok) {
495 newtp->rx_opt.ts_recent = req->ts_recent;
496 newtp->rx_opt.ts_recent_stamp = get_seconds();
497 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
499 newtp->rx_opt.ts_recent_stamp = 0;
500 newtp->tcp_header_len = sizeof(struct tcphdr);
503 #ifdef CONFIG_TCP_MD5SIG
504 newtp->md5sig_info = NULL; /*XXX*/
505 if (newtp->af_specific->md5_lookup(sk, newsk))
506 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
508 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
509 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
510 newtp->rx_opt.mss_clamp = req->mss;
511 TCP_ECN_openreq_child(newtp, req);
512 newtp->fastopen_rsk = NULL;
513 newtp->syn_data_acked = 0;
515 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS);
519 EXPORT_SYMBOL(tcp_create_openreq_child);
522 * Process an incoming packet for SYN_RECV sockets represented as a
523 * request_sock. Normally sk is the listener socket but for TFO it
524 * points to the child socket.
526 * XXX (TFO) - The current impl contains a special check for ack
527 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
529 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
532 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
533 struct request_sock *req,
534 struct request_sock **prev,
537 struct tcp_options_received tmp_opt;
539 const struct tcphdr *th = tcp_hdr(skb);
540 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
541 bool paws_reject = false;
543 BUG_ON(fastopen == (sk->sk_state == TCP_LISTEN));
545 tmp_opt.saw_tstamp = 0;
546 if (th->doff > (sizeof(struct tcphdr)>>2)) {
547 tcp_parse_options(skb, &tmp_opt, 0, NULL);
549 if (tmp_opt.saw_tstamp) {
550 tmp_opt.ts_recent = req->ts_recent;
551 /* We do not store true stamp, but it is not required,
552 * it can be estimated (approximately)
555 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
556 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
560 /* Check for pure retransmitted SYN. */
561 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
562 flg == TCP_FLAG_SYN &&
565 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
566 * this case on figure 6 and figure 8, but formal
567 * protocol description says NOTHING.
568 * To be more exact, it says that we should send ACK,
569 * because this segment (at least, if it has no data)
572 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
573 * describe SYN-RECV state. All the description
574 * is wrong, we cannot believe to it and should
575 * rely only on common sense and implementation
578 * Enforce "SYN-ACK" according to figure 8, figure 6
579 * of RFC793, fixed by RFC1122.
581 * Note that even if there is new data in the SYN packet
582 * they will be thrown away too.
584 * Reset timer after retransmitting SYNACK, similar to
585 * the idea of fast retransmit in recovery.
587 if (!inet_rtx_syn_ack(sk, req))
588 req->expires = min(TCP_TIMEOUT_INIT << req->num_timeout,
589 TCP_RTO_MAX) + jiffies;
593 /* Further reproduces section "SEGMENT ARRIVES"
594 for state SYN-RECEIVED of RFC793.
595 It is broken, however, it does not work only
596 when SYNs are crossed.
598 You would think that SYN crossing is impossible here, since
599 we should have a SYN_SENT socket (from connect()) on our end,
600 but this is not true if the crossed SYNs were sent to both
601 ends by a malicious third party. We must defend against this,
602 and to do that we first verify the ACK (as per RFC793, page
603 36) and reset if it is invalid. Is this a true full defense?
604 To convince ourselves, let us consider a way in which the ACK
605 test can still pass in this 'malicious crossed SYNs' case.
606 Malicious sender sends identical SYNs (and thus identical sequence
607 numbers) to both A and B:
612 By our good fortune, both A and B select the same initial
613 send sequence number of seven :-)
615 A: sends SYN|ACK, seq=7, ack_seq=8
616 B: sends SYN|ACK, seq=7, ack_seq=8
618 So we are now A eating this SYN|ACK, ACK test passes. So
619 does sequence test, SYN is truncated, and thus we consider
622 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
623 bare ACK. Otherwise, we create an established connection. Both
624 ends (listening sockets) accept the new incoming connection and try
625 to talk to each other. 8-)
627 Note: This case is both harmless, and rare. Possibility is about the
628 same as us discovering intelligent life on another plant tomorrow.
630 But generally, we should (RFC lies!) to accept ACK
631 from SYNACK both here and in tcp_rcv_state_process().
632 tcp_rcv_state_process() does not, hence, we do not too.
634 Note that the case is absolutely generic:
635 we cannot optimize anything here without
636 violating protocol. All the checks must be made
637 before attempt to create socket.
640 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
641 * and the incoming segment acknowledges something not yet
642 * sent (the segment carries an unacceptable ACK) ...
645 * Invalid ACK: reset will be sent by listening socket.
646 * Note that the ACK validity check for a Fast Open socket is done
647 * elsewhere and is checked directly against the child socket rather
648 * than req because user data may have been sent out.
650 if ((flg & TCP_FLAG_ACK) && !fastopen &&
651 (TCP_SKB_CB(skb)->ack_seq !=
652 tcp_rsk(req)->snt_isn + 1))
655 /* Also, it would be not so bad idea to check rcv_tsecr, which
656 * is essentially ACK extension and too early or too late values
657 * should cause reset in unsynchronized states.
660 /* RFC793: "first check sequence number". */
662 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
663 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rcv_wnd)) {
664 /* Out of window: send ACK and drop. */
665 if (!(flg & TCP_FLAG_RST))
666 req->rsk_ops->send_ack(sk, skb, req);
668 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
672 /* In sequence, PAWS is OK. */
674 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
675 req->ts_recent = tmp_opt.rcv_tsval;
677 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
678 /* Truncate SYN, it is out of window starting
679 at tcp_rsk(req)->rcv_isn + 1. */
680 flg &= ~TCP_FLAG_SYN;
683 /* RFC793: "second check the RST bit" and
684 * "fourth, check the SYN bit"
686 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
687 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
688 goto embryonic_reset;
691 /* ACK sequence verified above, just make sure ACK is
692 * set. If ACK not set, just silently drop the packet.
694 * XXX (TFO) - if we ever allow "data after SYN", the
695 * following check needs to be removed.
697 if (!(flg & TCP_FLAG_ACK))
700 /* For Fast Open no more processing is needed (sk is the
706 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
707 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
708 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
709 inet_rsk(req)->acked = 1;
710 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
714 /* OK, ACK is valid, create big socket and
715 * feed this segment to it. It will repeat all
716 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
717 * ESTABLISHED STATE. If it will be dropped after
718 * socket is created, wait for troubles.
720 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL);
722 goto listen_overflow;
724 inet_csk_reqsk_queue_unlink(sk, req, prev);
725 inet_csk_reqsk_queue_removed(sk, req);
727 inet_csk_reqsk_queue_add(sk, req, child);
731 if (!sysctl_tcp_abort_on_overflow) {
732 inet_rsk(req)->acked = 1;
737 if (!(flg & TCP_FLAG_RST)) {
738 /* Received a bad SYN pkt - for TFO We try not to reset
739 * the local connection unless it's really necessary to
740 * avoid becoming vulnerable to outside attack aiming at
741 * resetting legit local connections.
743 req->rsk_ops->send_reset(sk, skb);
744 } else if (fastopen) { /* received a valid RST pkt */
745 reqsk_fastopen_remove(sk, req, true);
749 inet_csk_reqsk_queue_drop(sk, req, prev);
750 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
754 EXPORT_SYMBOL(tcp_check_req);
757 * Queue segment on the new socket if the new socket is active,
758 * otherwise we just shortcircuit this and continue with
761 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
762 * when entering. But other states are possible due to a race condition
763 * where after __inet_lookup_established() fails but before the listener
764 * locked is obtained, other packets cause the same connection to
768 int tcp_child_process(struct sock *parent, struct sock *child,
772 int state = child->sk_state;
774 if (!sock_owned_by_user(child)) {
775 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
777 /* Wakeup parent, send SIGIO */
778 if (state == TCP_SYN_RECV && child->sk_state != state)
779 parent->sk_data_ready(parent);
781 /* Alas, it is possible again, because we do lookup
782 * in main socket hash table and lock on listening
783 * socket does not protect us more.
785 __sk_add_backlog(child, skb);
788 bh_unlock_sock(child);
792 EXPORT_SYMBOL(tcp_child_process);