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/sysctl.h>
24 #include <linux/workqueue.h>
25 #include <net/tcp.h>
26 #include <net/inet_common.h>
27 #include <net/xfrm.h>
28
29 #ifdef CONFIG_SYSCTL
30 #define SYNC_INIT 0 /* let the user enable it */
31 #else
32 #define SYNC_INIT 1
33 #endif
34
35 int sysctl_tcp_syncookies __read_mostly = SYNC_INIT;
36 EXPORT_SYMBOL(sysctl_tcp_syncookies);
37
38 int sysctl_tcp_abort_on_overflow __read_mostly;
39
40 struct inet_timewait_death_row tcp_death_row = {
41 .sysctl_max_tw_buckets = NR_FILE * 2,
42 .period = TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS,
43 .death_lock = __SPIN_LOCK_UNLOCKED(tcp_death_row.death_lock),
44 .hashinfo = &tcp_hashinfo,
45 .tw_timer = TIMER_INITIALIZER(inet_twdr_hangman, 0,
46 (unsigned long)&tcp_death_row),
47 .twkill_work = __WORK_INITIALIZER(tcp_death_row.twkill_work,
48 inet_twdr_twkill_work),
49 /* Short-time timewait calendar */
50
51 .twcal_hand = -1,
52 .twcal_timer = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0,
53 (unsigned long)&tcp_death_row),
54 };
55
56 EXPORT_SYMBOL_GPL(tcp_death_row);
57
tcp_in_window(u32 seq,u32 end_seq,u32 s_win,u32 e_win)58 static __inline__ int tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
59 {
60 if (seq == s_win)
61 return 1;
62 if (after(end_seq, s_win) && before(seq, e_win))
63 return 1;
64 return (seq == e_win && seq == end_seq);
65 }
66
67 /*
68 * * Main purpose of TIME-WAIT state is to close connection gracefully,
69 * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
70 * (and, probably, tail of data) and one or more our ACKs are lost.
71 * * What is TIME-WAIT timeout? It is associated with maximal packet
72 * lifetime in the internet, which results in wrong conclusion, that
73 * it is set to catch "old duplicate segments" wandering out of their path.
74 * It is not quite correct. This timeout is calculated so that it exceeds
75 * maximal retransmission timeout enough to allow to lose one (or more)
76 * segments sent by peer and our ACKs. This time may be calculated from RTO.
77 * * When TIME-WAIT socket receives RST, it means that another end
78 * finally closed and we are allowed to kill TIME-WAIT too.
79 * * Second purpose of TIME-WAIT is catching old duplicate segments.
80 * Well, certainly it is pure paranoia, but if we load TIME-WAIT
81 * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
82 * * If we invented some more clever way to catch duplicates
83 * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
84 *
85 * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
86 * When you compare it to RFCs, please, read section SEGMENT ARRIVES
87 * from the very beginning.
88 *
89 * NOTE. With recycling (and later with fin-wait-2) TW bucket
90 * is _not_ stateless. It means, that strictly speaking we must
91 * spinlock it. I do not want! Well, probability of misbehaviour
92 * is ridiculously low and, seems, we could use some mb() tricks
93 * to avoid misread sequence numbers, states etc. --ANK
94 */
95 enum tcp_tw_status
tcp_timewait_state_process(struct inet_timewait_sock * tw,struct sk_buff * skb,const struct tcphdr * th)96 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
97 const struct tcphdr *th)
98 {
99 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
100 struct tcp_options_received tmp_opt;
101 int paws_reject = 0;
102
103 tmp_opt.saw_tstamp = 0;
104 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
105 tcp_parse_options(skb, &tmp_opt, 0);
106
107 if (tmp_opt.saw_tstamp) {
108 tmp_opt.ts_recent = tcptw->tw_ts_recent;
109 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
110 paws_reject = tcp_paws_check(&tmp_opt, th->rst);
111 }
112 }
113
114 if (tw->tw_substate == TCP_FIN_WAIT2) {
115 /* Just repeat all the checks of tcp_rcv_state_process() */
116
117 /* Out of window, send ACK */
118 if (paws_reject ||
119 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
120 tcptw->tw_rcv_nxt,
121 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
122 return TCP_TW_ACK;
123
124 if (th->rst)
125 goto kill;
126
127 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
128 goto kill_with_rst;
129
130 /* Dup ACK? */
131 if (!after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
132 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
133 inet_twsk_put(tw);
134 return TCP_TW_SUCCESS;
135 }
136
137 /* New data or FIN. If new data arrive after half-duplex close,
138 * reset.
139 */
140 if (!th->fin ||
141 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) {
142 kill_with_rst:
143 inet_twsk_deschedule(tw, &tcp_death_row);
144 inet_twsk_put(tw);
145 return TCP_TW_RST;
146 }
147
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;
154 }
155
156 /* I am shamed, but failed to make it more elegant.
157 * Yes, it is direct reference to IP, which is impossible
158 * to generalize to IPv6. Taking into account that IPv6
159 * do not understand recycling in any case, it not
160 * a big problem in practice. --ANK */
161 if (tw->tw_family == AF_INET &&
162 tcp_death_row.sysctl_tw_recycle && tcptw->tw_ts_recent_stamp &&
163 tcp_v4_tw_remember_stamp(tw))
164 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout,
165 TCP_TIMEWAIT_LEN);
166 else
167 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
168 TCP_TIMEWAIT_LEN);
169 return TCP_TW_ACK;
170 }
171
172 /*
173 * Now real TIME-WAIT state.
174 *
175 * RFC 1122:
176 * "When a connection is [...] on TIME-WAIT state [...]
177 * [a TCP] MAY accept a new SYN from the remote TCP to
178 * reopen the connection directly, if it:
179 *
180 * (1) assigns its initial sequence number for the new
181 * connection to be larger than the largest sequence
182 * number it used on the previous connection incarnation,
183 * and
184 *
185 * (2) returns to TIME-WAIT state if the SYN turns out
186 * to be an old duplicate".
187 */
188
189 if (!paws_reject &&
190 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
191 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
192 /* In window segment, it may be only reset or bare ack. */
193
194 if (th->rst) {
195 /* This is TIME_WAIT assassination, in two flavors.
196 * Oh well... nobody has a sufficient solution to this
197 * protocol bug yet.
198 */
199 if (sysctl_tcp_rfc1337 == 0) {
200 kill:
201 inet_twsk_deschedule(tw, &tcp_death_row);
202 inet_twsk_put(tw);
203 return TCP_TW_SUCCESS;
204 }
205 }
206 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
207 TCP_TIMEWAIT_LEN);
208
209 if (tmp_opt.saw_tstamp) {
210 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
211 tcptw->tw_ts_recent_stamp = get_seconds();
212 }
213
214 inet_twsk_put(tw);
215 return TCP_TW_SUCCESS;
216 }
217
218 /* Out of window segment.
219
220 All the segments are ACKed immediately.
221
222 The only exception is new SYN. We accept it, if it is
223 not old duplicate and we are not in danger to be killed
224 by delayed old duplicates. RFC check is that it has
225 newer sequence number works at rates <40Mbit/sec.
226 However, if paws works, it is reliable AND even more,
227 we even may relax silly seq space cutoff.
228
229 RED-PEN: we violate main RFC requirement, if this SYN will appear
230 old duplicate (i.e. we receive RST in reply to SYN-ACK),
231 we must return socket to time-wait state. It is not good,
232 but not fatal yet.
233 */
234
235 if (th->syn && !th->rst && !th->ack && !paws_reject &&
236 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
237 (tmp_opt.saw_tstamp &&
238 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
239 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
240 if (isn == 0)
241 isn++;
242 TCP_SKB_CB(skb)->when = isn;
243 return TCP_TW_SYN;
244 }
245
246 if (paws_reject)
247 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
248
249 if (!th->rst) {
250 /* In this case we must reset the TIMEWAIT timer.
251 *
252 * If it is ACKless SYN it may be both old duplicate
253 * and new good SYN with random sequence number <rcv_nxt.
254 * Do not reschedule in the last case.
255 */
256 if (paws_reject || th->ack)
257 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
258 TCP_TIMEWAIT_LEN);
259
260 /* Send ACK. Note, we do not put the bucket,
261 * it will be released by caller.
262 */
263 return TCP_TW_ACK;
264 }
265 inet_twsk_put(tw);
266 return TCP_TW_SUCCESS;
267 }
268
269 /*
270 * Move a socket to time-wait or dead fin-wait-2 state.
271 */
tcp_time_wait(struct sock * sk,int state,int timeo)272 void tcp_time_wait(struct sock *sk, int state, int timeo)
273 {
274 struct inet_timewait_sock *tw = NULL;
275 const struct inet_connection_sock *icsk = inet_csk(sk);
276 const struct tcp_sock *tp = tcp_sk(sk);
277 int recycle_ok = 0;
278
279 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp)
280 recycle_ok = icsk->icsk_af_ops->remember_stamp(sk);
281
282 if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets)
283 tw = inet_twsk_alloc(sk, state);
284
285 if (tw != NULL) {
286 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
287 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
288
289 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale;
290 tcptw->tw_rcv_nxt = tp->rcv_nxt;
291 tcptw->tw_snd_nxt = tp->snd_nxt;
292 tcptw->tw_rcv_wnd = tcp_receive_window(tp);
293 tcptw->tw_ts_recent = tp->rx_opt.ts_recent;
294 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
295
296 #if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
297 if (tw->tw_family == PF_INET6) {
298 struct ipv6_pinfo *np = inet6_sk(sk);
299 struct inet6_timewait_sock *tw6;
300
301 tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot);
302 tw6 = inet6_twsk((struct sock *)tw);
303 ipv6_addr_copy(&tw6->tw_v6_daddr, &np->daddr);
304 ipv6_addr_copy(&tw6->tw_v6_rcv_saddr, &np->rcv_saddr);
305 tw->tw_ipv6only = np->ipv6only;
306 }
307 #endif
308
309 #ifdef CONFIG_TCP_MD5SIG
310 /*
311 * The timewait bucket does not have the key DB from the
312 * sock structure. We just make a quick copy of the
313 * md5 key being used (if indeed we are using one)
314 * so the timewait ack generating code has the key.
315 */
316 do {
317 struct tcp_md5sig_key *key;
318 memset(tcptw->tw_md5_key, 0, sizeof(tcptw->tw_md5_key));
319 tcptw->tw_md5_keylen = 0;
320 key = tp->af_specific->md5_lookup(sk, sk);
321 if (key != NULL) {
322 memcpy(&tcptw->tw_md5_key, key->key, key->keylen);
323 tcptw->tw_md5_keylen = key->keylen;
324 if (tcp_alloc_md5sig_pool() == NULL)
325 BUG();
326 }
327 } while (0);
328 #endif
329
330 /* Linkage updates. */
331 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
332
333 /* Get the TIME_WAIT timeout firing. */
334 if (timeo < rto)
335 timeo = rto;
336
337 if (recycle_ok) {
338 tw->tw_timeout = rto;
339 } else {
340 tw->tw_timeout = TCP_TIMEWAIT_LEN;
341 if (state == TCP_TIME_WAIT)
342 timeo = TCP_TIMEWAIT_LEN;
343 }
344
345 inet_twsk_schedule(tw, &tcp_death_row, timeo,
346 TCP_TIMEWAIT_LEN);
347 inet_twsk_put(tw);
348 } else {
349 /* Sorry, if we're out of memory, just CLOSE this
350 * socket up. We've got bigger problems than
351 * non-graceful socket closings.
352 */
353 LIMIT_NETDEBUG(KERN_INFO "TCP: time wait bucket table overflow\n");
354 }
355
356 tcp_update_metrics(sk);
357 tcp_done(sk);
358 }
359
tcp_twsk_destructor(struct sock * sk)360 void tcp_twsk_destructor(struct sock *sk)
361 {
362 #ifdef CONFIG_TCP_MD5SIG
363 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
364 if (twsk->tw_md5_keylen)
365 tcp_put_md5sig_pool();
366 #endif
367 }
368
369 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
370
TCP_ECN_openreq_child(struct tcp_sock * tp,struct request_sock * req)371 static inline void TCP_ECN_openreq_child(struct tcp_sock *tp,
372 struct request_sock *req)
373 {
374 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
375 }
376
377 /* This is not only more efficient than what we used to do, it eliminates
378 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
379 *
380 * Actually, we could lots of memory writes here. tp of listening
381 * socket contains all necessary default parameters.
382 */
tcp_create_openreq_child(struct sock * sk,struct request_sock * req,struct sk_buff * skb)383 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
384 {
385 struct sock *newsk = inet_csk_clone(sk, req, GFP_ATOMIC);
386
387 if (newsk != NULL) {
388 const struct inet_request_sock *ireq = inet_rsk(req);
389 struct tcp_request_sock *treq = tcp_rsk(req);
390 struct inet_connection_sock *newicsk = inet_csk(newsk);
391 struct tcp_sock *newtp;
392
393 /* Now setup tcp_sock */
394 newtp = tcp_sk(newsk);
395 newtp->pred_flags = 0;
396 newtp->rcv_wup = newtp->copied_seq = newtp->rcv_nxt = treq->rcv_isn + 1;
397 newtp->snd_sml = newtp->snd_una = newtp->snd_nxt = treq->snt_isn + 1;
398 newtp->snd_up = treq->snt_isn + 1;
399
400 tcp_prequeue_init(newtp);
401
402 tcp_init_wl(newtp, treq->snt_isn, treq->rcv_isn);
403
404 newtp->srtt = 0;
405 newtp->mdev = TCP_TIMEOUT_INIT;
406 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
407
408 newtp->packets_out = 0;
409 newtp->retrans_out = 0;
410 newtp->sacked_out = 0;
411 newtp->fackets_out = 0;
412 newtp->snd_ssthresh = 0x7fffffff;
413
414 /* So many TCP implementations out there (incorrectly) count the
415 * initial SYN frame in their delayed-ACK and congestion control
416 * algorithms that we must have the following bandaid to talk
417 * efficiently to them. -DaveM
418 */
419 newtp->snd_cwnd = 2;
420 newtp->snd_cwnd_cnt = 0;
421 newtp->bytes_acked = 0;
422
423 newtp->frto_counter = 0;
424 newtp->frto_highmark = 0;
425
426 newicsk->icsk_ca_ops = &tcp_init_congestion_ops;
427
428 tcp_set_ca_state(newsk, TCP_CA_Open);
429 tcp_init_xmit_timers(newsk);
430 skb_queue_head_init(&newtp->out_of_order_queue);
431 newtp->write_seq = treq->snt_isn + 1;
432 newtp->pushed_seq = newtp->write_seq;
433
434 newtp->rx_opt.saw_tstamp = 0;
435
436 newtp->rx_opt.dsack = 0;
437 newtp->rx_opt.eff_sacks = 0;
438
439 newtp->rx_opt.num_sacks = 0;
440 newtp->urg_data = 0;
441
442 if (sock_flag(newsk, SOCK_KEEPOPEN))
443 inet_csk_reset_keepalive_timer(newsk,
444 keepalive_time_when(newtp));
445
446 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
447 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
448 if (sysctl_tcp_fack)
449 tcp_enable_fack(newtp);
450 }
451 newtp->window_clamp = req->window_clamp;
452 newtp->rcv_ssthresh = req->rcv_wnd;
453 newtp->rcv_wnd = req->rcv_wnd;
454 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
455 if (newtp->rx_opt.wscale_ok) {
456 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
457 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
458 } else {
459 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
460 newtp->window_clamp = min(newtp->window_clamp, 65535U);
461 }
462 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
463 newtp->rx_opt.snd_wscale);
464 newtp->max_window = newtp->snd_wnd;
465
466 if (newtp->rx_opt.tstamp_ok) {
467 newtp->rx_opt.ts_recent = req->ts_recent;
468 newtp->rx_opt.ts_recent_stamp = get_seconds();
469 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
470 } else {
471 newtp->rx_opt.ts_recent_stamp = 0;
472 newtp->tcp_header_len = sizeof(struct tcphdr);
473 }
474 #ifdef CONFIG_TCP_MD5SIG
475 newtp->md5sig_info = NULL; /*XXX*/
476 if (newtp->af_specific->md5_lookup(sk, newsk))
477 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
478 #endif
479 if (skb->len >= TCP_MIN_RCVMSS+newtp->tcp_header_len)
480 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
481 newtp->rx_opt.mss_clamp = req->mss;
482 TCP_ECN_openreq_child(newtp, req);
483
484 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS);
485 }
486 return newsk;
487 }
488
489 /*
490 * Process an incoming packet for SYN_RECV sockets represented
491 * as a request_sock.
492 */
493
tcp_check_req(struct sock * sk,struct sk_buff * skb,struct request_sock * req,struct request_sock ** prev)494 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
495 struct request_sock *req,
496 struct request_sock **prev)
497 {
498 const struct tcphdr *th = tcp_hdr(skb);
499 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
500 int paws_reject = 0;
501 struct tcp_options_received tmp_opt;
502 struct sock *child;
503
504 tmp_opt.saw_tstamp = 0;
505 if (th->doff > (sizeof(struct tcphdr)>>2)) {
506 tcp_parse_options(skb, &tmp_opt, 0);
507
508 if (tmp_opt.saw_tstamp) {
509 tmp_opt.ts_recent = req->ts_recent;
510 /* We do not store true stamp, but it is not required,
511 * it can be estimated (approximately)
512 * from another data.
513 */
514 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans);
515 paws_reject = tcp_paws_check(&tmp_opt, th->rst);
516 }
517 }
518
519 /* Check for pure retransmitted SYN. */
520 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
521 flg == TCP_FLAG_SYN &&
522 !paws_reject) {
523 /*
524 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
525 * this case on figure 6 and figure 8, but formal
526 * protocol description says NOTHING.
527 * To be more exact, it says that we should send ACK,
528 * because this segment (at least, if it has no data)
529 * is out of window.
530 *
531 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
532 * describe SYN-RECV state. All the description
533 * is wrong, we cannot believe to it and should
534 * rely only on common sense and implementation
535 * experience.
536 *
537 * Enforce "SYN-ACK" according to figure 8, figure 6
538 * of RFC793, fixed by RFC1122.
539 */
540 req->rsk_ops->rtx_syn_ack(sk, req);
541 return NULL;
542 }
543
544 /* Further reproduces section "SEGMENT ARRIVES"
545 for state SYN-RECEIVED of RFC793.
546 It is broken, however, it does not work only
547 when SYNs are crossed.
548
549 You would think that SYN crossing is impossible here, since
550 we should have a SYN_SENT socket (from connect()) on our end,
551 but this is not true if the crossed SYNs were sent to both
552 ends by a malicious third party. We must defend against this,
553 and to do that we first verify the ACK (as per RFC793, page
554 36) and reset if it is invalid. Is this a true full defense?
555 To convince ourselves, let us consider a way in which the ACK
556 test can still pass in this 'malicious crossed SYNs' case.
557 Malicious sender sends identical SYNs (and thus identical sequence
558 numbers) to both A and B:
559
560 A: gets SYN, seq=7
561 B: gets SYN, seq=7
562
563 By our good fortune, both A and B select the same initial
564 send sequence number of seven :-)
565
566 A: sends SYN|ACK, seq=7, ack_seq=8
567 B: sends SYN|ACK, seq=7, ack_seq=8
568
569 So we are now A eating this SYN|ACK, ACK test passes. So
570 does sequence test, SYN is truncated, and thus we consider
571 it a bare ACK.
572
573 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
574 bare ACK. Otherwise, we create an established connection. Both
575 ends (listening sockets) accept the new incoming connection and try
576 to talk to each other. 8-)
577
578 Note: This case is both harmless, and rare. Possibility is about the
579 same as us discovering intelligent life on another plant tomorrow.
580
581 But generally, we should (RFC lies!) to accept ACK
582 from SYNACK both here and in tcp_rcv_state_process().
583 tcp_rcv_state_process() does not, hence, we do not too.
584
585 Note that the case is absolutely generic:
586 we cannot optimize anything here without
587 violating protocol. All the checks must be made
588 before attempt to create socket.
589 */
590
591 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
592 * and the incoming segment acknowledges something not yet
593 * sent (the segment carries an unacceptable ACK) ...
594 * a reset is sent."
595 *
596 * Invalid ACK: reset will be sent by listening socket
597 */
598 if ((flg & TCP_FLAG_ACK) &&
599 (TCP_SKB_CB(skb)->ack_seq != tcp_rsk(req)->snt_isn + 1))
600 return sk;
601
602 /* Also, it would be not so bad idea to check rcv_tsecr, which
603 * is essentially ACK extension and too early or too late values
604 * should cause reset in unsynchronized states.
605 */
606
607 /* RFC793: "first check sequence number". */
608
609 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
610 tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) {
611 /* Out of window: send ACK and drop. */
612 if (!(flg & TCP_FLAG_RST))
613 req->rsk_ops->send_ack(sk, skb, req);
614 if (paws_reject)
615 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
616 return NULL;
617 }
618
619 /* In sequence, PAWS is OK. */
620
621 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1))
622 req->ts_recent = tmp_opt.rcv_tsval;
623
624 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
625 /* Truncate SYN, it is out of window starting
626 at tcp_rsk(req)->rcv_isn + 1. */
627 flg &= ~TCP_FLAG_SYN;
628 }
629
630 /* RFC793: "second check the RST bit" and
631 * "fourth, check the SYN bit"
632 */
633 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
634 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
635 goto embryonic_reset;
636 }
637
638 /* ACK sequence verified above, just make sure ACK is
639 * set. If ACK not set, just silently drop the packet.
640 */
641 if (!(flg & TCP_FLAG_ACK))
642 return NULL;
643
644 /* If TCP_DEFER_ACCEPT is set, drop bare ACK. */
645 if (inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
646 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
647 inet_rsk(req)->acked = 1;
648 return NULL;
649 }
650
651 /* OK, ACK is valid, create big socket and
652 * feed this segment to it. It will repeat all
653 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
654 * ESTABLISHED STATE. If it will be dropped after
655 * socket is created, wait for troubles.
656 */
657 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL);
658 if (child == NULL)
659 goto listen_overflow;
660 #ifdef CONFIG_TCP_MD5SIG
661 else {
662 /* Copy over the MD5 key from the original socket */
663 struct tcp_md5sig_key *key;
664 struct tcp_sock *tp = tcp_sk(sk);
665 key = tp->af_specific->md5_lookup(sk, child);
666 if (key != NULL) {
667 /*
668 * We're using one, so create a matching key on the
669 * newsk structure. If we fail to get memory then we
670 * end up not copying the key across. Shucks.
671 */
672 char *newkey = kmemdup(key->key, key->keylen,
673 GFP_ATOMIC);
674 if (newkey) {
675 if (!tcp_alloc_md5sig_pool())
676 BUG();
677 tp->af_specific->md5_add(child, child, newkey,
678 key->keylen);
679 }
680 }
681 }
682 #endif
683
684 inet_csk_reqsk_queue_unlink(sk, req, prev);
685 inet_csk_reqsk_queue_removed(sk, req);
686
687 inet_csk_reqsk_queue_add(sk, req, child);
688 return child;
689
690 listen_overflow:
691 if (!sysctl_tcp_abort_on_overflow) {
692 inet_rsk(req)->acked = 1;
693 return NULL;
694 }
695
696 embryonic_reset:
697 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
698 if (!(flg & TCP_FLAG_RST))
699 req->rsk_ops->send_reset(sk, skb);
700
701 inet_csk_reqsk_queue_drop(sk, req, prev);
702 return NULL;
703 }
704
705 /*
706 * Queue segment on the new socket if the new socket is active,
707 * otherwise we just shortcircuit this and continue with
708 * the new socket.
709 */
710
tcp_child_process(struct sock * parent,struct sock * child,struct sk_buff * skb)711 int tcp_child_process(struct sock *parent, struct sock *child,
712 struct sk_buff *skb)
713 {
714 int ret = 0;
715 int state = child->sk_state;
716
717 if (!sock_owned_by_user(child)) {
718 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
719 skb->len);
720 /* Wakeup parent, send SIGIO */
721 if (state == TCP_SYN_RECV && child->sk_state != state)
722 parent->sk_data_ready(parent, 0);
723 } else {
724 /* Alas, it is possible again, because we do lookup
725 * in main socket hash table and lock on listening
726 * socket does not protect us more.
727 */
728 sk_add_backlog(child, skb);
729 }
730
731 bh_unlock_sock(child);
732 sock_put(child);
733 return ret;
734 }
735
736 EXPORT_SYMBOL(tcp_check_req);
737 EXPORT_SYMBOL(tcp_child_process);
738 EXPORT_SYMBOL(tcp_create_openreq_child);
739 EXPORT_SYMBOL(tcp_timewait_state_process);
740