1/*
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.
5 *
6 * Implementation of the Transmission Control Protocol(TCP).
7 *
8 * Authors: Ross Biro
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>
19 */
20
21#include <linux/mm.h>
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>
27#include <net/tcp.h>
28#include <net/inet_common.h>
29#include <net/xfrm.h>
30#include <net/busy_poll.h>
31
32static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
33{
34 if (seq == s_win)
35 return true;
36 if (after(end_seq, s_win) && before(seq, e_win))
37 return true;
38 return seq == e_win && seq == end_seq;
39}
40
41static enum tcp_tw_status
42tcp_timewait_check_oow_rate_limit(struct inet_timewait_sock *tw,
43 const struct sk_buff *skb, int mib_idx)
44{
45 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
46
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.
51 */
52 return TCP_TW_ACK;
53 }
54
55 /* We are rate-limiting, so just release the tw sock and drop skb. */
56 inet_twsk_put(tw);
57 return TCP_TW_SUCCESS;
58}
59
60/*
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.
77 *
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.
81 *
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
87 *
88 * We don't need to initialize tmp_out.sack_ok as we don't use the results
89 */
90enum tcp_tw_status
91tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
92 const struct tcphdr *th)
93{
94 struct tcp_options_received tmp_opt;
95 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
96 bool paws_reject = false;
97
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);
101
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);
108 }
109 }
110
111 if (tw->tw_substate == TCP_FIN_WAIT2) {
112 /* Just repeat all the checks of tcp_rcv_state_process() */
113
114 /* Out of window, send ACK */
115 if (paws_reject ||
116 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
117 tcptw->tw_rcv_nxt,
118 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
119 return tcp_timewait_check_oow_rate_limit(
120 tw, skb, LINUX_MIB_TCPACKSKIPPEDFINWAIT2);
121
122 if (th->rst)
123 goto kill;
124
125 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
126 return TCP_TW_RST;
127
128 /* Dup ACK? */
129 if (!th->ack ||
130 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
131 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
132 inet_twsk_put(tw);
133 return TCP_TW_SUCCESS;
134 }
135
136 /* New data or FIN. If new data arrive after half-duplex close,
137 * reset.
138 */
139 if (!th->fin ||
140 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1)
141 return TCP_TW_RST;
142
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 = ktime_get_seconds();
148 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
149 }
150
151 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
152 return TCP_TW_ACK;
153 }
154
155 /*
156 * Now real TIME-WAIT state.
157 *
158 * RFC 1122:
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:
162 *
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,
166 * and
167 *
168 * (2) returns to TIME-WAIT state if the SYN turns out
169 * to be an old duplicate".
170 */
171
172 if (!paws_reject &&
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. */
176
177 if (th->rst) {
178 /* This is TIME_WAIT assassination, in two flavors.
179 * Oh well... nobody has a sufficient solution to this
180 * protocol bug yet.
181 */
182 if (twsk_net(tw)->ipv4.sysctl_tcp_rfc1337 == 0) {
183kill:
184 inet_twsk_deschedule_put(tw);
185 return TCP_TW_SUCCESS;
186 }
187 } else {
188 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
189 }
190
191 if (tmp_opt.saw_tstamp) {
192 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
193 tcptw->tw_ts_recent_stamp = ktime_get_seconds();
194 }
195
196 inet_twsk_put(tw);
197 return TCP_TW_SUCCESS;
198 }
199
200 /* Out of window segment.
201
202 All the segments are ACKed immediately.
203
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.
210
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,
214 but not fatal yet.
215 */
216
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;
222 if (isn == 0)
223 isn++;
224 TCP_SKB_CB(skb)->tcp_tw_isn = isn;
225 return TCP_TW_SYN;
226 }
227
228 if (paws_reject)
229 __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
230
231 if (!th->rst) {
232 /* In this case we must reset the TIMEWAIT timer.
233 *
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.
237 */
238 if (paws_reject || th->ack)
239 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
240
241 return tcp_timewait_check_oow_rate_limit(
242 tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT);
243 }
244 inet_twsk_put(tw);
245 return TCP_TW_SUCCESS;
246}
247EXPORT_SYMBOL(tcp_timewait_state_process);
248
249/*
250 * Move a socket to time-wait or dead fin-wait-2 state.
251 */
252void tcp_time_wait(struct sock *sk, int state, int timeo)
253{
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;
258
259 tw = inet_twsk_alloc(sk, tcp_death_row, state);
260
261 if (tw) {
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);
265
266 tw->tw_transparent = inet->transparent;
267 tw->tw_mark = sk->sk_mark;
268 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale;
269 tcptw->tw_rcv_nxt = tp->rcv_nxt;
270 tcptw->tw_snd_nxt = tp->snd_nxt;
271 tcptw->tw_rcv_wnd = tcp_receive_window(tp);
272 tcptw->tw_ts_recent = tp->rx_opt.ts_recent;
273 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
274 tcptw->tw_ts_offset = tp->tsoffset;
275 tcptw->tw_last_oow_ack_time = 0;
276
277#if IS_ENABLED(CONFIG_IPV6)
278 if (tw->tw_family == PF_INET6) {
279 struct ipv6_pinfo *np = inet6_sk(sk);
280
281 tw->tw_v6_daddr = sk->sk_v6_daddr;
282 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
283 tw->tw_tclass = np->tclass;
284 tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK);
285 tw->tw_ipv6only = sk->sk_ipv6only;
286 }
287#endif
288
289#ifdef CONFIG_TCP_MD5SIG
290 /*
291 * The timewait bucket does not have the key DB from the
292 * sock structure. We just make a quick copy of the
293 * md5 key being used (if indeed we are using one)
294 * so the timewait ack generating code has the key.
295 */
296 do {
297 tcptw->tw_md5_key = NULL;
298 if (static_branch_unlikely(&tcp_md5_needed)) {
299 struct tcp_md5sig_key *key;
300
301 key = tp->af_specific->md5_lookup(sk, sk);
302 if (key) {
303 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
304 BUG_ON(tcptw->tw_md5_key && !tcp_alloc_md5sig_pool());
305 }
306 }
307 } while (0);
308#endif
309
310 /* Get the TIME_WAIT timeout firing. */
311 if (timeo < rto)
312 timeo = rto;
313
314 if (state == TCP_TIME_WAIT)
315 timeo = TCP_TIMEWAIT_LEN;
316
317 /* tw_timer is pinned, so we need to make sure BH are disabled
318 * in following section, otherwise timer handler could run before
319 * we complete the initialization.
320 */
321 local_bh_disable();
322 inet_twsk_schedule(tw, timeo);
323 /* Linkage updates.
324 * Note that access to tw after this point is illegal.
325 */
326 inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
327 local_bh_enable();
328 } else {
329 /* Sorry, if we're out of memory, just CLOSE this
330 * socket up. We've got bigger problems than
331 * non-graceful socket closings.
332 */
333 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW);
334 }
335
336 tcp_update_metrics(sk);
337 tcp_done(sk);
338}
339EXPORT_SYMBOL(tcp_time_wait);
340
341void tcp_twsk_destructor(struct sock *sk)
342{
343#ifdef CONFIG_TCP_MD5SIG
344 if (static_branch_unlikely(&tcp_md5_needed)) {
345 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
346
347 if (twsk->tw_md5_key)
348 kfree_rcu(twsk->tw_md5_key, rcu);
349 }
350#endif
351}
352EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
353
354/* Warning : This function is called without sk_listener being locked.
355 * Be sure to read socket fields once, as their value could change under us.
356 */
357void tcp_openreq_init_rwin(struct request_sock *req,
358 const struct sock *sk_listener,
359 const struct dst_entry *dst)
360{
361 struct inet_request_sock *ireq = inet_rsk(req);
362 const struct tcp_sock *tp = tcp_sk(sk_listener);
363 int full_space = tcp_full_space(sk_listener);
364 u32 window_clamp;
365 __u8 rcv_wscale;
366 u32 rcv_wnd;
367 int mss;
368
369 mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
370 window_clamp = READ_ONCE(tp->window_clamp);
371 /* Set this up on the first call only */
372 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);
373
374 /* limit the window selection if the user enforce a smaller rx buffer */
375 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK &&
376 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
377 req->rsk_window_clamp = full_space;
378
379 rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req);
380 if (rcv_wnd == 0)
381 rcv_wnd = dst_metric(dst, RTAX_INITRWND);
382 else if (full_space < rcv_wnd * mss)
383 full_space = rcv_wnd * mss;
384
385 /* tcp_full_space because it is guaranteed to be the first packet */
386 tcp_select_initial_window(sk_listener, full_space,
387 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
388 &req->rsk_rcv_wnd,
389 &req->rsk_window_clamp,
390 ireq->wscale_ok,
391 &rcv_wscale,
392 rcv_wnd);
393 ireq->rcv_wscale = rcv_wscale;
394}
395EXPORT_SYMBOL(tcp_openreq_init_rwin);
396
397static void tcp_ecn_openreq_child(struct tcp_sock *tp,
398 const struct request_sock *req)
399{
400 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
401}
402
403void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
404{
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;
408
409 if (ca_key != TCP_CA_UNSPEC) {
410 const struct tcp_congestion_ops *ca;
411
412 rcu_read_lock();
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;
417 ca_got_dst = true;
418 }
419 rcu_read_unlock();
420 }
421
422 /* If no valid choice made yet, assign current system default ca. */
423 if (!ca_got_dst &&
424 (!icsk->icsk_ca_setsockopt ||
425 !try_module_get(icsk->icsk_ca_ops->owner)))
426 tcp_assign_congestion_control(sk);
427
428 tcp_set_ca_state(sk, TCP_CA_Open);
429}
430EXPORT_SYMBOL_GPL(tcp_ca_openreq_child);
431
432static void smc_check_reset_syn_req(struct tcp_sock *oldtp,
433 struct request_sock *req,
434 struct tcp_sock *newtp)
435{
436#if IS_ENABLED(CONFIG_SMC)
437 struct inet_request_sock *ireq;
438
439 if (static_branch_unlikely(&tcp_have_smc)) {
440 ireq = inet_rsk(req);
441 if (oldtp->syn_smc && !ireq->smc_ok)
442 newtp->syn_smc = 0;
443 }
444#endif
445}
446
447/* This is not only more efficient than what we used to do, it eliminates
448 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
449 *
450 * Actually, we could lots of memory writes here. tp of listening
451 * socket contains all necessary default parameters.
452 */
453struct sock *tcp_create_openreq_child(const struct sock *sk,
454 struct request_sock *req,
455 struct sk_buff *skb)
456{
457 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
458 const struct inet_request_sock *ireq = inet_rsk(req);
459 struct tcp_request_sock *treq = tcp_rsk(req);
460 struct inet_connection_sock *newicsk;
461 struct tcp_sock *oldtp, *newtp;
462
463 if (!newsk)
464 return NULL;
465
466 newicsk = inet_csk(newsk);
467 newtp = tcp_sk(newsk);
468 oldtp = tcp_sk(sk);
469
470 smc_check_reset_syn_req(oldtp, req, newtp);
471
472 /* Now setup tcp_sock */
473 newtp->pred_flags = 0;
474
475 newtp->rcv_wup = newtp->copied_seq =
476 newtp->rcv_nxt = treq->rcv_isn + 1;
477 newtp->segs_in = 1;
478
479 newtp->snd_sml = newtp->snd_una =
480 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;
481
482 INIT_LIST_HEAD(&newtp->tsq_node);
483 INIT_LIST_HEAD(&newtp->tsorted_sent_queue);
484
485 tcp_init_wl(newtp, treq->rcv_isn);
486
487 minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U);
488 newicsk->icsk_ack.lrcvtime = tcp_jiffies32;
489
490 newtp->lsndtime = tcp_jiffies32;
491 newsk->sk_txhash = treq->txhash;
492 newtp->total_retrans = req->num_retrans;
493
494 tcp_init_xmit_timers(newsk);
495 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
496
497 if (sock_flag(newsk, SOCK_KEEPOPEN))
498 inet_csk_reset_keepalive_timer(newsk,
499 keepalive_time_when(newtp));
500
501 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
502 newtp->rx_opt.sack_ok = ireq->sack_ok;
503 newtp->window_clamp = req->rsk_window_clamp;
504 newtp->rcv_ssthresh = req->rsk_rcv_wnd;
505 newtp->rcv_wnd = req->rsk_rcv_wnd;
506 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
507 if (newtp->rx_opt.wscale_ok) {
508 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
509 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
510 } else {
511 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
512 newtp->window_clamp = min(newtp->window_clamp, 65535U);
513 }
514 newtp->snd_wnd = ntohs(tcp_hdr(skb)->window) << newtp->rx_opt.snd_wscale;
515 newtp->max_window = newtp->snd_wnd;
516
517 if (newtp->rx_opt.tstamp_ok) {
518 newtp->rx_opt.ts_recent = req->ts_recent;
519 newtp->rx_opt.ts_recent_stamp = ktime_get_seconds();
520 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
521 } else {
522 newtp->rx_opt.ts_recent_stamp = 0;
523 newtp->tcp_header_len = sizeof(struct tcphdr);
524 }
525 newtp->tsoffset = treq->ts_off;
526#ifdef CONFIG_TCP_MD5SIG
527 newtp->md5sig_info = NULL; /*XXX*/
528 if (newtp->af_specific->md5_lookup(sk, newsk))
529 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
530#endif
531 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
532 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
533 newtp->rx_opt.mss_clamp = req->mss;
534 tcp_ecn_openreq_child(newtp, req);
535 newtp->fastopen_req = NULL;
536 newtp->fastopen_rsk = NULL;
537
538 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS);
539
540 return newsk;
541}
542EXPORT_SYMBOL(tcp_create_openreq_child);
543
544/*
545 * Process an incoming packet for SYN_RECV sockets represented as a
546 * request_sock. Normally sk is the listener socket but for TFO it
547 * points to the child socket.
548 *
549 * XXX (TFO) - The current impl contains a special check for ack
550 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
551 *
552 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
553 */
554
555struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
556 struct request_sock *req,
557 bool fastopen, bool *req_stolen)
558{
559 struct tcp_options_received tmp_opt;
560 struct sock *child;
561 const struct tcphdr *th = tcp_hdr(skb);
562 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
563 bool paws_reject = false;
564 bool own_req;
565
566 tmp_opt.saw_tstamp = 0;
567 if (th->doff > (sizeof(struct tcphdr)>>2)) {
568 tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL);
569
570 if (tmp_opt.saw_tstamp) {
571 tmp_opt.ts_recent = req->ts_recent;
572 if (tmp_opt.rcv_tsecr)
573 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off;
574 /* We do not store true stamp, but it is not required,
575 * it can be estimated (approximately)
576 * from another data.
577 */
578 tmp_opt.ts_recent_stamp = ktime_get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
579 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
580 }
581 }
582
583 /* Check for pure retransmitted SYN. */
584 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
585 flg == TCP_FLAG_SYN &&
586 !paws_reject) {
587 /*
588 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
589 * this case on figure 6 and figure 8, but formal
590 * protocol description says NOTHING.
591 * To be more exact, it says that we should send ACK,
592 * because this segment (at least, if it has no data)
593 * is out of window.
594 *
595 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
596 * describe SYN-RECV state. All the description
597 * is wrong, we cannot believe to it and should
598 * rely only on common sense and implementation
599 * experience.
600 *
601 * Enforce "SYN-ACK" according to figure 8, figure 6
602 * of RFC793, fixed by RFC1122.
603 *
604 * Note that even if there is new data in the SYN packet
605 * they will be thrown away too.
606 *
607 * Reset timer after retransmitting SYNACK, similar to
608 * the idea of fast retransmit in recovery.
609 */
610 if (!tcp_oow_rate_limited(sock_net(sk), skb,
611 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
612 &tcp_rsk(req)->last_oow_ack_time) &&
613
614 !inet_rtx_syn_ack(sk, req)) {
615 unsigned long expires = jiffies;
616
617 expires += min(TCP_TIMEOUT_INIT << req->num_timeout,
618 TCP_RTO_MAX);
619 if (!fastopen)
620 mod_timer_pending(&req->rsk_timer, expires);
621 else
622 req->rsk_timer.expires = expires;
623 }
624 return NULL;
625 }
626
627 /* Further reproduces section "SEGMENT ARRIVES"
628 for state SYN-RECEIVED of RFC793.
629 It is broken, however, it does not work only
630 when SYNs are crossed.
631
632 You would think that SYN crossing is impossible here, since
633 we should have a SYN_SENT socket (from connect()) on our end,
634 but this is not true if the crossed SYNs were sent to both
635 ends by a malicious third party. We must defend against this,
636 and to do that we first verify the ACK (as per RFC793, page
637 36) and reset if it is invalid. Is this a true full defense?
638 To convince ourselves, let us consider a way in which the ACK
639 test can still pass in this 'malicious crossed SYNs' case.
640 Malicious sender sends identical SYNs (and thus identical sequence
641 numbers) to both A and B:
642
643 A: gets SYN, seq=7
644 B: gets SYN, seq=7
645
646 By our good fortune, both A and B select the same initial
647 send sequence number of seven :-)
648
649 A: sends SYN|ACK, seq=7, ack_seq=8
650 B: sends SYN|ACK, seq=7, ack_seq=8
651
652 So we are now A eating this SYN|ACK, ACK test passes. So
653 does sequence test, SYN is truncated, and thus we consider
654 it a bare ACK.
655
656 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
657 bare ACK. Otherwise, we create an established connection. Both
658 ends (listening sockets) accept the new incoming connection and try
659 to talk to each other. 8-)
660
661 Note: This case is both harmless, and rare. Possibility is about the
662 same as us discovering intelligent life on another plant tomorrow.
663
664 But generally, we should (RFC lies!) to accept ACK
665 from SYNACK both here and in tcp_rcv_state_process().
666 tcp_rcv_state_process() does not, hence, we do not too.
667
668 Note that the case is absolutely generic:
669 we cannot optimize anything here without
670 violating protocol. All the checks must be made
671 before attempt to create socket.
672 */
673
674 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
675 * and the incoming segment acknowledges something not yet
676 * sent (the segment carries an unacceptable ACK) ...
677 * a reset is sent."
678 *
679 * Invalid ACK: reset will be sent by listening socket.
680 * Note that the ACK validity check for a Fast Open socket is done
681 * elsewhere and is checked directly against the child socket rather
682 * than req because user data may have been sent out.
683 */
684 if ((flg & TCP_FLAG_ACK) && !fastopen &&
685 (TCP_SKB_CB(skb)->ack_seq !=
686 tcp_rsk(req)->snt_isn + 1))
687 return sk;
688
689 /* Also, it would be not so bad idea to check rcv_tsecr, which
690 * is essentially ACK extension and too early or too late values
691 * should cause reset in unsynchronized states.
692 */
693
694 /* RFC793: "first check sequence number". */
695
696 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
697 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) {
698 /* Out of window: send ACK and drop. */
699 if (!(flg & TCP_FLAG_RST) &&
700 !tcp_oow_rate_limited(sock_net(sk), skb,
701 LINUX_MIB_TCPACKSKIPPEDSYNRECV,
702 &tcp_rsk(req)->last_oow_ack_time))
703 req->rsk_ops->send_ack(sk, skb, req);
704 if (paws_reject)
705 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
706 return NULL;
707 }
708
709 /* In sequence, PAWS is OK. */
710
711 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
712 req->ts_recent = tmp_opt.rcv_tsval;
713
714 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
715 /* Truncate SYN, it is out of window starting
716 at tcp_rsk(req)->rcv_isn + 1. */
717 flg &= ~TCP_FLAG_SYN;
718 }
719
720 /* RFC793: "second check the RST bit" and
721 * "fourth, check the SYN bit"
722 */
723 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
724 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
725 goto embryonic_reset;
726 }
727
728 /* ACK sequence verified above, just make sure ACK is
729 * set. If ACK not set, just silently drop the packet.
730 *
731 * XXX (TFO) - if we ever allow "data after SYN", the
732 * following check needs to be removed.
733 */
734 if (!(flg & TCP_FLAG_ACK))
735 return NULL;
736
737 /* For Fast Open no more processing is needed (sk is the
738 * child socket).
739 */
740 if (fastopen)
741 return sk;
742
743 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
744 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
745 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
746 inet_rsk(req)->acked = 1;
747 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
748 return NULL;
749 }
750
751 /* OK, ACK is valid, create big socket and
752 * feed this segment to it. It will repeat all
753 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
754 * ESTABLISHED STATE. If it will be dropped after
755 * socket is created, wait for troubles.
756 */
757 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
758 req, &own_req);
759 if (!child)
760 goto listen_overflow;
761
762 sock_rps_save_rxhash(child, skb);
763 tcp_synack_rtt_meas(child, req);
764 *req_stolen = !own_req;
765 return inet_csk_complete_hashdance(sk, child, req, own_req);
766
767listen_overflow:
768 if (!sock_net(sk)->ipv4.sysctl_tcp_abort_on_overflow) {
769 inet_rsk(req)->acked = 1;
770 return NULL;
771 }
772
773embryonic_reset:
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.
779 */
780 req->rsk_ops->send_reset(sk, skb);
781 } else if (fastopen) { /* received a valid RST pkt */
782 reqsk_fastopen_remove(sk, req, true);
783 tcp_reset(sk);
784 }
785 if (!fastopen) {
786 inet_csk_reqsk_queue_drop(sk, req);
787 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
788 }
789 return NULL;
790}
791EXPORT_SYMBOL(tcp_check_req);
792
793/*
794 * Queue segment on the new socket if the new socket is active,
795 * otherwise we just shortcircuit this and continue with
796 * the new socket.
797 *
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
802 * be created.
803 */
804
805int tcp_child_process(struct sock *parent, struct sock *child,
806 struct sk_buff *skb)
807{
808 int ret = 0;
809 int state = child->sk_state;
810
811 /* record NAPI ID of child */
812 sk_mark_napi_id(child, skb);
813
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);
820 } else {
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.
824 */
825 __sk_add_backlog(child, skb);
826 }
827
828 bh_unlock_sock(child);
829 sock_put(child);
830 return ret;
831}
832EXPORT_SYMBOL(tcp_child_process);
833