2 * INET An implementation of the TCP/IP protocol suite for the LINUX
3 * operating system. INET is implemented using the BSD Socket
4 * interface as the means of communication with the user level.
6 * Implementation of the Transmission Control Protocol(TCP).
9 * Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
10 * Mark Evans, <evansmp@uhura.aston.ac.uk>
11 * Corey Minyard <wf-rch!minyard@relay.EU.net>
12 * Florian La Roche, <flla@stud.uni-sb.de>
13 * Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
14 * Linus Torvalds, <torvalds@cs.helsinki.fi>
15 * Alan Cox, <gw4pts@gw4pts.ampr.org>
16 * Matthew Dillon, <dillon@apollo.west.oic.com>
17 * Arnt Gulbrandsen, <agulbra@nvg.unit.no>
18 * Jorge Cwik, <jorge@laser.satlink.net>
22 #include <linux/module.h>
23 #include <linux/slab.h>
24 #include <linux/sysctl.h>
25 #include <linux/workqueue.h>
27 #include <net/inet_common.h>
30 int sysctl_tcp_abort_on_overflow __read_mostly
;
32 struct inet_timewait_death_row tcp_death_row
= {
33 .sysctl_max_tw_buckets
= NR_FILE
* 2,
34 .hashinfo
= &tcp_hashinfo
,
36 EXPORT_SYMBOL_GPL(tcp_death_row
);
38 static bool tcp_in_window(u32 seq
, u32 end_seq
, u32 s_win
, u32 e_win
)
42 if (after(end_seq
, s_win
) && before(seq
, e_win
))
44 return seq
== e_win
&& seq
== end_seq
;
47 static enum tcp_tw_status
48 tcp_timewait_check_oow_rate_limit(struct inet_timewait_sock
*tw
,
49 const struct sk_buff
*skb
, int mib_idx
)
51 struct tcp_timewait_sock
*tcptw
= tcp_twsk((struct sock
*)tw
);
53 if (!tcp_oow_rate_limited(twsk_net(tw
), skb
, mib_idx
,
54 &tcptw
->tw_last_oow_ack_time
)) {
55 /* Send ACK. Note, we do not put the bucket,
56 * it will be released by caller.
61 /* We are rate-limiting, so just release the tw sock and drop skb. */
63 return TCP_TW_SUCCESS
;
67 * * Main purpose of TIME-WAIT state is to close connection gracefully,
68 * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
69 * (and, probably, tail of data) and one or more our ACKs are lost.
70 * * What is TIME-WAIT timeout? It is associated with maximal packet
71 * lifetime in the internet, which results in wrong conclusion, that
72 * it is set to catch "old duplicate segments" wandering out of their path.
73 * It is not quite correct. This timeout is calculated so that it exceeds
74 * maximal retransmission timeout enough to allow to lose one (or more)
75 * segments sent by peer and our ACKs. This time may be calculated from RTO.
76 * * When TIME-WAIT socket receives RST, it means that another end
77 * finally closed and we are allowed to kill TIME-WAIT too.
78 * * Second purpose of TIME-WAIT is catching old duplicate segments.
79 * Well, certainly it is pure paranoia, but if we load TIME-WAIT
80 * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
81 * * If we invented some more clever way to catch duplicates
82 * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
84 * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
85 * When you compare it to RFCs, please, read section SEGMENT ARRIVES
86 * from the very beginning.
88 * NOTE. With recycling (and later with fin-wait-2) TW bucket
89 * is _not_ stateless. It means, that strictly speaking we must
90 * spinlock it. I do not want! Well, probability of misbehaviour
91 * is ridiculously low and, seems, we could use some mb() tricks
92 * to avoid misread sequence numbers, states etc. --ANK
94 * We don't need to initialize tmp_out.sack_ok as we don't use the results
97 tcp_timewait_state_process(struct inet_timewait_sock
*tw
, struct sk_buff
*skb
,
98 const struct tcphdr
*th
)
100 struct tcp_options_received tmp_opt
;
101 struct tcp_timewait_sock
*tcptw
= tcp_twsk((struct sock
*)tw
);
102 bool paws_reject
= false;
104 tmp_opt
.saw_tstamp
= 0;
105 if (th
->doff
> (sizeof(*th
) >> 2) && tcptw
->tw_ts_recent_stamp
) {
106 tcp_parse_options(skb
, &tmp_opt
, 0, NULL
);
108 if (tmp_opt
.saw_tstamp
) {
109 tmp_opt
.rcv_tsecr
-= tcptw
->tw_ts_offset
;
110 tmp_opt
.ts_recent
= tcptw
->tw_ts_recent
;
111 tmp_opt
.ts_recent_stamp
= tcptw
->tw_ts_recent_stamp
;
112 paws_reject
= tcp_paws_reject(&tmp_opt
, th
->rst
);
116 if (tw
->tw_substate
== TCP_FIN_WAIT2
) {
117 /* Just repeat all the checks of tcp_rcv_state_process() */
119 /* Out of window, send ACK */
121 !tcp_in_window(TCP_SKB_CB(skb
)->seq
, TCP_SKB_CB(skb
)->end_seq
,
123 tcptw
->tw_rcv_nxt
+ tcptw
->tw_rcv_wnd
))
124 return tcp_timewait_check_oow_rate_limit(
125 tw
, skb
, LINUX_MIB_TCPACKSKIPPEDFINWAIT2
);
130 if (th
->syn
&& !before(TCP_SKB_CB(skb
)->seq
, tcptw
->tw_rcv_nxt
))
135 !after(TCP_SKB_CB(skb
)->end_seq
, tcptw
->tw_rcv_nxt
) ||
136 TCP_SKB_CB(skb
)->end_seq
== TCP_SKB_CB(skb
)->seq
) {
138 return TCP_TW_SUCCESS
;
141 /* New data or FIN. If new data arrive after half-duplex close,
145 TCP_SKB_CB(skb
)->end_seq
!= tcptw
->tw_rcv_nxt
+ 1)
148 /* FIN arrived, enter true time-wait state. */
149 tw
->tw_substate
= TCP_TIME_WAIT
;
150 tcptw
->tw_rcv_nxt
= TCP_SKB_CB(skb
)->end_seq
;
151 if (tmp_opt
.saw_tstamp
) {
152 tcptw
->tw_ts_recent_stamp
= get_seconds();
153 tcptw
->tw_ts_recent
= tmp_opt
.rcv_tsval
;
156 if (tcp_death_row
.sysctl_tw_recycle
&&
157 tcptw
->tw_ts_recent_stamp
&&
158 tcp_tw_remember_stamp(tw
))
159 inet_twsk_reschedule(tw
, tw
->tw_timeout
);
161 inet_twsk_reschedule(tw
, 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_put(tw
);
195 return TCP_TW_SUCCESS
;
198 inet_twsk_reschedule(tw
, TCP_TIMEWAIT_LEN
);
200 if (tmp_opt
.saw_tstamp
) {
201 tcptw
->tw_ts_recent
= tmp_opt
.rcv_tsval
;
202 tcptw
->tw_ts_recent_stamp
= get_seconds();
206 return TCP_TW_SUCCESS
;
209 /* Out of window segment.
211 All the segments are ACKed immediately.
213 The only exception is new SYN. We accept it, if it is
214 not old duplicate and we are not in danger to be killed
215 by delayed old duplicates. RFC check is that it has
216 newer sequence number works at rates <40Mbit/sec.
217 However, if paws works, it is reliable AND even more,
218 we even may relax silly seq space cutoff.
220 RED-PEN: we violate main RFC requirement, if this SYN will appear
221 old duplicate (i.e. we receive RST in reply to SYN-ACK),
222 we must return socket to time-wait state. It is not good,
226 if (th
->syn
&& !th
->rst
&& !th
->ack
&& !paws_reject
&&
227 (after(TCP_SKB_CB(skb
)->seq
, tcptw
->tw_rcv_nxt
) ||
228 (tmp_opt
.saw_tstamp
&&
229 (s32
)(tcptw
->tw_ts_recent
- tmp_opt
.rcv_tsval
) < 0))) {
230 u32 isn
= tcptw
->tw_snd_nxt
+ 65535 + 2;
233 TCP_SKB_CB(skb
)->tcp_tw_isn
= isn
;
238 __NET_INC_STATS(twsk_net(tw
), LINUX_MIB_PAWSESTABREJECTED
);
241 /* In this case we must reset the TIMEWAIT timer.
243 * If it is ACKless SYN it may be both old duplicate
244 * and new good SYN with random sequence number <rcv_nxt.
245 * Do not reschedule in the last case.
247 if (paws_reject
|| th
->ack
)
248 inet_twsk_reschedule(tw
, TCP_TIMEWAIT_LEN
);
250 return tcp_timewait_check_oow_rate_limit(
251 tw
, skb
, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT
);
254 return TCP_TW_SUCCESS
;
256 EXPORT_SYMBOL(tcp_timewait_state_process
);
259 * Move a socket to time-wait or dead fin-wait-2 state.
261 void tcp_time_wait(struct sock
*sk
, int state
, int timeo
)
263 const struct inet_connection_sock
*icsk
= inet_csk(sk
);
264 const struct tcp_sock
*tp
= tcp_sk(sk
);
265 struct inet_timewait_sock
*tw
;
266 bool recycle_ok
= false;
268 if (tcp_death_row
.sysctl_tw_recycle
&& tp
->rx_opt
.ts_recent_stamp
)
269 recycle_ok
= tcp_remember_stamp(sk
);
271 tw
= inet_twsk_alloc(sk
, &tcp_death_row
, state
);
274 struct tcp_timewait_sock
*tcptw
= tcp_twsk((struct sock
*)tw
);
275 const int rto
= (icsk
->icsk_rto
<< 2) - (icsk
->icsk_rto
>> 1);
276 struct inet_sock
*inet
= inet_sk(sk
);
278 tw
->tw_transparent
= inet
->transparent
;
279 tw
->tw_rcv_wscale
= tp
->rx_opt
.rcv_wscale
;
280 tcptw
->tw_rcv_nxt
= tp
->rcv_nxt
;
281 tcptw
->tw_snd_nxt
= tp
->snd_nxt
;
282 tcptw
->tw_rcv_wnd
= tcp_receive_window(tp
);
283 tcptw
->tw_ts_recent
= tp
->rx_opt
.ts_recent
;
284 tcptw
->tw_ts_recent_stamp
= tp
->rx_opt
.ts_recent_stamp
;
285 tcptw
->tw_ts_offset
= tp
->tsoffset
;
286 tcptw
->tw_last_oow_ack_time
= 0;
288 #if IS_ENABLED(CONFIG_IPV6)
289 if (tw
->tw_family
== PF_INET6
) {
290 struct ipv6_pinfo
*np
= inet6_sk(sk
);
292 tw
->tw_v6_daddr
= sk
->sk_v6_daddr
;
293 tw
->tw_v6_rcv_saddr
= sk
->sk_v6_rcv_saddr
;
294 tw
->tw_tclass
= np
->tclass
;
295 tw
->tw_flowlabel
= be32_to_cpu(np
->flow_label
& IPV6_FLOWLABEL_MASK
);
296 tw
->tw_ipv6only
= sk
->sk_ipv6only
;
300 #ifdef CONFIG_TCP_MD5SIG
302 * The timewait bucket does not have the key DB from the
303 * sock structure. We just make a quick copy of the
304 * md5 key being used (if indeed we are using one)
305 * so the timewait ack generating code has the key.
308 struct tcp_md5sig_key
*key
;
309 tcptw
->tw_md5_key
= NULL
;
310 key
= tp
->af_specific
->md5_lookup(sk
, sk
);
312 tcptw
->tw_md5_key
= kmemdup(key
, sizeof(*key
), GFP_ATOMIC
);
313 if (tcptw
->tw_md5_key
&& !tcp_alloc_md5sig_pool())
319 /* Get the TIME_WAIT timeout firing. */
324 tw
->tw_timeout
= rto
;
326 tw
->tw_timeout
= TCP_TIMEWAIT_LEN
;
327 if (state
== TCP_TIME_WAIT
)
328 timeo
= TCP_TIMEWAIT_LEN
;
331 inet_twsk_schedule(tw
, timeo
);
332 /* Linkage updates. */
333 __inet_twsk_hashdance(tw
, sk
, &tcp_hashinfo
);
336 /* Sorry, if we're out of memory, just CLOSE this
337 * socket up. We've got bigger problems than
338 * non-graceful socket closings.
340 NET_INC_STATS(sock_net(sk
), LINUX_MIB_TCPTIMEWAITOVERFLOW
);
343 tcp_update_metrics(sk
);
347 void tcp_twsk_destructor(struct sock
*sk
)
349 #ifdef CONFIG_TCP_MD5SIG
350 struct tcp_timewait_sock
*twsk
= tcp_twsk(sk
);
352 if (twsk
->tw_md5_key
)
353 kfree_rcu(twsk
->tw_md5_key
, rcu
);
356 EXPORT_SYMBOL_GPL(tcp_twsk_destructor
);
358 /* Warning : This function is called without sk_listener being locked.
359 * Be sure to read socket fields once, as their value could change under us.
361 void tcp_openreq_init_rwin(struct request_sock
*req
,
362 const struct sock
*sk_listener
,
363 const struct dst_entry
*dst
)
365 struct inet_request_sock
*ireq
= inet_rsk(req
);
366 const struct tcp_sock
*tp
= tcp_sk(sk_listener
);
367 u16 user_mss
= READ_ONCE(tp
->rx_opt
.user_mss
);
368 int full_space
= tcp_full_space(sk_listener
);
369 int mss
= dst_metric_advmss(dst
);
373 if (user_mss
&& user_mss
< mss
)
376 window_clamp
= READ_ONCE(tp
->window_clamp
);
377 /* Set this up on the first call only */
378 req
->rsk_window_clamp
= window_clamp
? : dst_metric(dst
, RTAX_WINDOW
);
380 /* limit the window selection if the user enforce a smaller rx buffer */
381 if (sk_listener
->sk_userlocks
& SOCK_RCVBUF_LOCK
&&
382 (req
->rsk_window_clamp
> full_space
|| req
->rsk_window_clamp
== 0))
383 req
->rsk_window_clamp
= full_space
;
385 /* tcp_full_space because it is guaranteed to be the first packet */
386 tcp_select_initial_window(full_space
,
387 mss
- (ireq
->tstamp_ok
? TCPOLEN_TSTAMP_ALIGNED
: 0),
389 &req
->rsk_window_clamp
,
392 dst_metric(dst
, RTAX_INITRWND
));
393 ireq
->rcv_wscale
= rcv_wscale
;
395 EXPORT_SYMBOL(tcp_openreq_init_rwin
);
397 static void tcp_ecn_openreq_child(struct tcp_sock
*tp
,
398 const struct request_sock
*req
)
400 tp
->ecn_flags
= inet_rsk(req
)->ecn_ok
? TCP_ECN_OK
: 0;
403 void tcp_ca_openreq_child(struct sock
*sk
, const struct dst_entry
*dst
)
405 struct inet_connection_sock
*icsk
= inet_csk(sk
);
406 u32 ca_key
= dst_metric(dst
, RTAX_CC_ALGO
);
407 bool ca_got_dst
= false;
409 if (ca_key
!= TCP_CA_UNSPEC
) {
410 const struct tcp_congestion_ops
*ca
;
413 ca
= tcp_ca_find_key(ca_key
);
414 if (likely(ca
&& try_module_get(ca
->owner
))) {
415 icsk
->icsk_ca_dst_locked
= tcp_ca_dst_locked(dst
);
416 icsk
->icsk_ca_ops
= ca
;
422 /* If no valid choice made yet, assign current system default ca. */
424 (!icsk
->icsk_ca_setsockopt
||
425 !try_module_get(icsk
->icsk_ca_ops
->owner
)))
426 tcp_assign_congestion_control(sk
);
428 tcp_set_ca_state(sk
, TCP_CA_Open
);
430 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child
);
432 /* This is not only more efficient than what we used to do, it eliminates
433 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
435 * Actually, we could lots of memory writes here. tp of listening
436 * socket contains all necessary default parameters.
438 struct sock
*tcp_create_openreq_child(const struct sock
*sk
,
439 struct request_sock
*req
,
442 struct sock
*newsk
= inet_csk_clone_lock(sk
, req
, GFP_ATOMIC
);
445 const struct inet_request_sock
*ireq
= inet_rsk(req
);
446 struct tcp_request_sock
*treq
= tcp_rsk(req
);
447 struct inet_connection_sock
*newicsk
= inet_csk(newsk
);
448 struct tcp_sock
*newtp
= tcp_sk(newsk
);
450 /* Now setup tcp_sock */
451 newtp
->pred_flags
= 0;
453 newtp
->rcv_wup
= newtp
->copied_seq
=
454 newtp
->rcv_nxt
= treq
->rcv_isn
+ 1;
457 newtp
->snd_sml
= newtp
->snd_una
=
458 newtp
->snd_nxt
= newtp
->snd_up
= treq
->snt_isn
+ 1;
460 tcp_prequeue_init(newtp
);
461 INIT_LIST_HEAD(&newtp
->tsq_node
);
463 tcp_init_wl(newtp
, treq
->rcv_isn
);
466 newtp
->mdev_us
= jiffies_to_usecs(TCP_TIMEOUT_INIT
);
467 newtp
->rtt_min
[0].rtt
= ~0U;
468 newicsk
->icsk_rto
= TCP_TIMEOUT_INIT
;
470 newtp
->packets_out
= 0;
471 newtp
->retrans_out
= 0;
472 newtp
->sacked_out
= 0;
473 newtp
->fackets_out
= 0;
474 newtp
->snd_ssthresh
= TCP_INFINITE_SSTHRESH
;
475 tcp_enable_early_retrans(newtp
);
476 newtp
->tlp_high_seq
= 0;
477 newtp
->lsndtime
= treq
->snt_synack
.stamp_jiffies
;
478 newsk
->sk_txhash
= treq
->txhash
;
479 newtp
->last_oow_ack_time
= 0;
480 newtp
->total_retrans
= req
->num_retrans
;
482 /* So many TCP implementations out there (incorrectly) count the
483 * initial SYN frame in their delayed-ACK and congestion control
484 * algorithms that we must have the following bandaid to talk
485 * efficiently to them. -DaveM
487 newtp
->snd_cwnd
= TCP_INIT_CWND
;
488 newtp
->snd_cwnd_cnt
= 0;
490 tcp_init_xmit_timers(newsk
);
491 __skb_queue_head_init(&newtp
->out_of_order_queue
);
492 newtp
->write_seq
= newtp
->pushed_seq
= treq
->snt_isn
+ 1;
494 newtp
->rx_opt
.saw_tstamp
= 0;
496 newtp
->rx_opt
.dsack
= 0;
497 newtp
->rx_opt
.num_sacks
= 0;
501 if (sock_flag(newsk
, SOCK_KEEPOPEN
))
502 inet_csk_reset_keepalive_timer(newsk
,
503 keepalive_time_when(newtp
));
505 newtp
->rx_opt
.tstamp_ok
= ireq
->tstamp_ok
;
506 if ((newtp
->rx_opt
.sack_ok
= ireq
->sack_ok
) != 0) {
508 tcp_enable_fack(newtp
);
510 newtp
->window_clamp
= req
->rsk_window_clamp
;
511 newtp
->rcv_ssthresh
= req
->rsk_rcv_wnd
;
512 newtp
->rcv_wnd
= req
->rsk_rcv_wnd
;
513 newtp
->rx_opt
.wscale_ok
= ireq
->wscale_ok
;
514 if (newtp
->rx_opt
.wscale_ok
) {
515 newtp
->rx_opt
.snd_wscale
= ireq
->snd_wscale
;
516 newtp
->rx_opt
.rcv_wscale
= ireq
->rcv_wscale
;
518 newtp
->rx_opt
.snd_wscale
= newtp
->rx_opt
.rcv_wscale
= 0;
519 newtp
->window_clamp
= min(newtp
->window_clamp
, 65535U);
521 newtp
->snd_wnd
= (ntohs(tcp_hdr(skb
)->window
) <<
522 newtp
->rx_opt
.snd_wscale
);
523 newtp
->max_window
= newtp
->snd_wnd
;
525 if (newtp
->rx_opt
.tstamp_ok
) {
526 newtp
->rx_opt
.ts_recent
= req
->ts_recent
;
527 newtp
->rx_opt
.ts_recent_stamp
= get_seconds();
528 newtp
->tcp_header_len
= sizeof(struct tcphdr
) + TCPOLEN_TSTAMP_ALIGNED
;
530 newtp
->rx_opt
.ts_recent_stamp
= 0;
531 newtp
->tcp_header_len
= sizeof(struct tcphdr
);
534 #ifdef CONFIG_TCP_MD5SIG
535 newtp
->md5sig_info
= NULL
; /*XXX*/
536 if (newtp
->af_specific
->md5_lookup(sk
, newsk
))
537 newtp
->tcp_header_len
+= TCPOLEN_MD5SIG_ALIGNED
;
539 if (skb
->len
>= TCP_MSS_DEFAULT
+ newtp
->tcp_header_len
)
540 newicsk
->icsk_ack
.last_seg_size
= skb
->len
- newtp
->tcp_header_len
;
541 newtp
->rx_opt
.mss_clamp
= req
->mss
;
542 tcp_ecn_openreq_child(newtp
, req
);
543 newtp
->fastopen_rsk
= NULL
;
544 newtp
->syn_data_acked
= 0;
545 newtp
->rack
.mstamp
.v64
= 0;
546 newtp
->rack
.advanced
= 0;
548 __TCP_INC_STATS(sock_net(sk
), TCP_MIB_PASSIVEOPENS
);
552 EXPORT_SYMBOL(tcp_create_openreq_child
);
555 * Process an incoming packet for SYN_RECV sockets represented as a
556 * request_sock. Normally sk is the listener socket but for TFO it
557 * points to the child socket.
559 * XXX (TFO) - The current impl contains a special check for ack
560 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
562 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
565 struct sock
*tcp_check_req(struct sock
*sk
, struct sk_buff
*skb
,
566 struct request_sock
*req
,
569 struct tcp_options_received tmp_opt
;
571 const struct tcphdr
*th
= tcp_hdr(skb
);
572 __be32 flg
= tcp_flag_word(th
) & (TCP_FLAG_RST
|TCP_FLAG_SYN
|TCP_FLAG_ACK
);
573 bool paws_reject
= false;
576 tmp_opt
.saw_tstamp
= 0;
577 if (th
->doff
> (sizeof(struct tcphdr
)>>2)) {
578 tcp_parse_options(skb
, &tmp_opt
, 0, NULL
);
580 if (tmp_opt
.saw_tstamp
) {
581 tmp_opt
.ts_recent
= req
->ts_recent
;
582 /* We do not store true stamp, but it is not required,
583 * it can be estimated (approximately)
586 tmp_opt
.ts_recent_stamp
= get_seconds() - ((TCP_TIMEOUT_INIT
/HZ
)<<req
->num_timeout
);
587 paws_reject
= tcp_paws_reject(&tmp_opt
, th
->rst
);
591 /* Check for pure retransmitted SYN. */
592 if (TCP_SKB_CB(skb
)->seq
== tcp_rsk(req
)->rcv_isn
&&
593 flg
== TCP_FLAG_SYN
&&
596 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
597 * this case on figure 6 and figure 8, but formal
598 * protocol description says NOTHING.
599 * To be more exact, it says that we should send ACK,
600 * because this segment (at least, if it has no data)
603 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
604 * describe SYN-RECV state. All the description
605 * is wrong, we cannot believe to it and should
606 * rely only on common sense and implementation
609 * Enforce "SYN-ACK" according to figure 8, figure 6
610 * of RFC793, fixed by RFC1122.
612 * Note that even if there is new data in the SYN packet
613 * they will be thrown away too.
615 * Reset timer after retransmitting SYNACK, similar to
616 * the idea of fast retransmit in recovery.
618 if (!tcp_oow_rate_limited(sock_net(sk
), skb
,
619 LINUX_MIB_TCPACKSKIPPEDSYNRECV
,
620 &tcp_rsk(req
)->last_oow_ack_time
) &&
622 !inet_rtx_syn_ack(sk
, req
)) {
623 unsigned long expires
= jiffies
;
625 expires
+= min(TCP_TIMEOUT_INIT
<< req
->num_timeout
,
628 mod_timer_pending(&req
->rsk_timer
, expires
);
630 req
->rsk_timer
.expires
= expires
;
635 /* Further reproduces section "SEGMENT ARRIVES"
636 for state SYN-RECEIVED of RFC793.
637 It is broken, however, it does not work only
638 when SYNs are crossed.
640 You would think that SYN crossing is impossible here, since
641 we should have a SYN_SENT socket (from connect()) on our end,
642 but this is not true if the crossed SYNs were sent to both
643 ends by a malicious third party. We must defend against this,
644 and to do that we first verify the ACK (as per RFC793, page
645 36) and reset if it is invalid. Is this a true full defense?
646 To convince ourselves, let us consider a way in which the ACK
647 test can still pass in this 'malicious crossed SYNs' case.
648 Malicious sender sends identical SYNs (and thus identical sequence
649 numbers) to both A and B:
654 By our good fortune, both A and B select the same initial
655 send sequence number of seven :-)
657 A: sends SYN|ACK, seq=7, ack_seq=8
658 B: sends SYN|ACK, seq=7, ack_seq=8
660 So we are now A eating this SYN|ACK, ACK test passes. So
661 does sequence test, SYN is truncated, and thus we consider
664 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
665 bare ACK. Otherwise, we create an established connection. Both
666 ends (listening sockets) accept the new incoming connection and try
667 to talk to each other. 8-)
669 Note: This case is both harmless, and rare. Possibility is about the
670 same as us discovering intelligent life on another plant tomorrow.
672 But generally, we should (RFC lies!) to accept ACK
673 from SYNACK both here and in tcp_rcv_state_process().
674 tcp_rcv_state_process() does not, hence, we do not too.
676 Note that the case is absolutely generic:
677 we cannot optimize anything here without
678 violating protocol. All the checks must be made
679 before attempt to create socket.
682 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
683 * and the incoming segment acknowledges something not yet
684 * sent (the segment carries an unacceptable ACK) ...
687 * Invalid ACK: reset will be sent by listening socket.
688 * Note that the ACK validity check for a Fast Open socket is done
689 * elsewhere and is checked directly against the child socket rather
690 * than req because user data may have been sent out.
692 if ((flg
& TCP_FLAG_ACK
) && !fastopen
&&
693 (TCP_SKB_CB(skb
)->ack_seq
!=
694 tcp_rsk(req
)->snt_isn
+ 1))
697 /* Also, it would be not so bad idea to check rcv_tsecr, which
698 * is essentially ACK extension and too early or too late values
699 * should cause reset in unsynchronized states.
702 /* RFC793: "first check sequence number". */
704 if (paws_reject
|| !tcp_in_window(TCP_SKB_CB(skb
)->seq
, TCP_SKB_CB(skb
)->end_seq
,
705 tcp_rsk(req
)->rcv_nxt
, tcp_rsk(req
)->rcv_nxt
+ req
->rsk_rcv_wnd
)) {
706 /* Out of window: send ACK and drop. */
707 if (!(flg
& TCP_FLAG_RST
) &&
708 !tcp_oow_rate_limited(sock_net(sk
), skb
,
709 LINUX_MIB_TCPACKSKIPPEDSYNRECV
,
710 &tcp_rsk(req
)->last_oow_ack_time
))
711 req
->rsk_ops
->send_ack(sk
, skb
, req
);
713 __NET_INC_STATS(sock_net(sk
), LINUX_MIB_PAWSESTABREJECTED
);
717 /* In sequence, PAWS is OK. */
719 if (tmp_opt
.saw_tstamp
&& !after(TCP_SKB_CB(skb
)->seq
, tcp_rsk(req
)->rcv_nxt
))
720 req
->ts_recent
= tmp_opt
.rcv_tsval
;
722 if (TCP_SKB_CB(skb
)->seq
== tcp_rsk(req
)->rcv_isn
) {
723 /* Truncate SYN, it is out of window starting
724 at tcp_rsk(req)->rcv_isn + 1. */
725 flg
&= ~TCP_FLAG_SYN
;
728 /* RFC793: "second check the RST bit" and
729 * "fourth, check the SYN bit"
731 if (flg
& (TCP_FLAG_RST
|TCP_FLAG_SYN
)) {
732 __TCP_INC_STATS(sock_net(sk
), TCP_MIB_ATTEMPTFAILS
);
733 goto embryonic_reset
;
736 /* ACK sequence verified above, just make sure ACK is
737 * set. If ACK not set, just silently drop the packet.
739 * XXX (TFO) - if we ever allow "data after SYN", the
740 * following check needs to be removed.
742 if (!(flg
& TCP_FLAG_ACK
))
745 /* For Fast Open no more processing is needed (sk is the
751 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
752 if (req
->num_timeout
< inet_csk(sk
)->icsk_accept_queue
.rskq_defer_accept
&&
753 TCP_SKB_CB(skb
)->end_seq
== tcp_rsk(req
)->rcv_isn
+ 1) {
754 inet_rsk(req
)->acked
= 1;
755 __NET_INC_STATS(sock_net(sk
), LINUX_MIB_TCPDEFERACCEPTDROP
);
759 /* OK, ACK is valid, create big socket and
760 * feed this segment to it. It will repeat all
761 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
762 * ESTABLISHED STATE. If it will be dropped after
763 * socket is created, wait for troubles.
765 child
= inet_csk(sk
)->icsk_af_ops
->syn_recv_sock(sk
, skb
, req
, NULL
,
768 goto listen_overflow
;
770 sock_rps_save_rxhash(child
, skb
);
771 tcp_synack_rtt_meas(child
, req
);
772 return inet_csk_complete_hashdance(sk
, child
, req
, own_req
);
775 if (!sysctl_tcp_abort_on_overflow
) {
776 inet_rsk(req
)->acked
= 1;
781 if (!(flg
& TCP_FLAG_RST
)) {
782 /* Received a bad SYN pkt - for TFO We try not to reset
783 * the local connection unless it's really necessary to
784 * avoid becoming vulnerable to outside attack aiming at
785 * resetting legit local connections.
787 req
->rsk_ops
->send_reset(sk
, skb
);
788 } else if (fastopen
) { /* received a valid RST pkt */
789 reqsk_fastopen_remove(sk
, req
, true);
793 inet_csk_reqsk_queue_drop(sk
, req
);
794 __NET_INC_STATS(sock_net(sk
), LINUX_MIB_EMBRYONICRSTS
);
798 EXPORT_SYMBOL(tcp_check_req
);
801 * Queue segment on the new socket if the new socket is active,
802 * otherwise we just shortcircuit this and continue with
805 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
806 * when entering. But other states are possible due to a race condition
807 * where after __inet_lookup_established() fails but before the listener
808 * locked is obtained, other packets cause the same connection to
812 int tcp_child_process(struct sock
*parent
, struct sock
*child
,
816 int state
= child
->sk_state
;
818 tcp_segs_in(tcp_sk(child
), skb
);
819 if (!sock_owned_by_user(child
)) {
820 ret
= tcp_rcv_state_process(child
, skb
);
821 /* Wakeup parent, send SIGIO */
822 if (state
== TCP_SYN_RECV
&& child
->sk_state
!= state
)
823 parent
->sk_data_ready(parent
);
825 /* Alas, it is possible again, because we do lookup
826 * in main socket hash table and lock on listening
827 * socket does not protect us more.
829 __sk_add_backlog(child
, skb
);
832 bh_unlock_sock(child
);
836 EXPORT_SYMBOL(tcp_child_process
);