tcp_minisocks.c revision 63231bddf6514778792d3784f63822473d250fc0
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 35int sysctl_tcp_syncookies __read_mostly = SYNC_INIT; 36EXPORT_SYMBOL(sysctl_tcp_syncookies); 37 38int sysctl_tcp_abort_on_overflow __read_mostly; 39 40struct 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 56EXPORT_SYMBOL_GPL(tcp_death_row); 57 58static __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 */ 95enum tcp_tw_status 96tcp_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) { 142kill_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) { 200kill: 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(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 */ 272void 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 360void 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 369EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 370 371static 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 */ 383struct 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; 398 399 tcp_prequeue_init(newtp); 400 401 tcp_init_wl(newtp, treq->snt_isn, treq->rcv_isn); 402 403 newtp->srtt = 0; 404 newtp->mdev = TCP_TIMEOUT_INIT; 405 newicsk->icsk_rto = TCP_TIMEOUT_INIT; 406 407 newtp->packets_out = 0; 408 newtp->retrans_out = 0; 409 newtp->sacked_out = 0; 410 newtp->fackets_out = 0; 411 newtp->snd_ssthresh = 0x7fffffff; 412 413 /* So many TCP implementations out there (incorrectly) count the 414 * initial SYN frame in their delayed-ACK and congestion control 415 * algorithms that we must have the following bandaid to talk 416 * efficiently to them. -DaveM 417 */ 418 newtp->snd_cwnd = 2; 419 newtp->snd_cwnd_cnt = 0; 420 newtp->bytes_acked = 0; 421 422 newtp->frto_counter = 0; 423 newtp->frto_highmark = 0; 424 425 newicsk->icsk_ca_ops = &tcp_init_congestion_ops; 426 427 tcp_set_ca_state(newsk, TCP_CA_Open); 428 tcp_init_xmit_timers(newsk); 429 skb_queue_head_init(&newtp->out_of_order_queue); 430 newtp->write_seq = treq->snt_isn + 1; 431 newtp->pushed_seq = newtp->write_seq; 432 433 newtp->rx_opt.saw_tstamp = 0; 434 435 newtp->rx_opt.dsack = 0; 436 newtp->rx_opt.eff_sacks = 0; 437 438 newtp->rx_opt.num_sacks = 0; 439 newtp->urg_data = 0; 440 441 if (sock_flag(newsk, SOCK_KEEPOPEN)) 442 inet_csk_reset_keepalive_timer(newsk, 443 keepalive_time_when(newtp)); 444 445 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 446 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 447 if (sysctl_tcp_fack) 448 tcp_enable_fack(newtp); 449 } 450 newtp->window_clamp = req->window_clamp; 451 newtp->rcv_ssthresh = req->rcv_wnd; 452 newtp->rcv_wnd = req->rcv_wnd; 453 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 454 if (newtp->rx_opt.wscale_ok) { 455 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 456 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 457 } else { 458 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 459 newtp->window_clamp = min(newtp->window_clamp, 65535U); 460 } 461 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 462 newtp->rx_opt.snd_wscale); 463 newtp->max_window = newtp->snd_wnd; 464 465 if (newtp->rx_opt.tstamp_ok) { 466 newtp->rx_opt.ts_recent = req->ts_recent; 467 newtp->rx_opt.ts_recent_stamp = get_seconds(); 468 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 469 } else { 470 newtp->rx_opt.ts_recent_stamp = 0; 471 newtp->tcp_header_len = sizeof(struct tcphdr); 472 } 473#ifdef CONFIG_TCP_MD5SIG 474 newtp->md5sig_info = NULL; /*XXX*/ 475 if (newtp->af_specific->md5_lookup(sk, newsk)) 476 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 477#endif 478 if (skb->len >= TCP_MIN_RCVMSS+newtp->tcp_header_len) 479 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 480 newtp->rx_opt.mss_clamp = req->mss; 481 TCP_ECN_openreq_child(newtp, req); 482 483 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS); 484 } 485 return newsk; 486} 487 488/* 489 * Process an incoming packet for SYN_RECV sockets represented 490 * as a request_sock. 491 */ 492 493struct sock *tcp_check_req(struct sock *sk,struct sk_buff *skb, 494 struct request_sock *req, 495 struct request_sock **prev) 496{ 497 const struct tcphdr *th = tcp_hdr(skb); 498 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 499 int paws_reject = 0; 500 struct tcp_options_received tmp_opt; 501 struct sock *child; 502 503 tmp_opt.saw_tstamp = 0; 504 if (th->doff > (sizeof(struct tcphdr)>>2)) { 505 tcp_parse_options(skb, &tmp_opt, 0); 506 507 if (tmp_opt.saw_tstamp) { 508 tmp_opt.ts_recent = req->ts_recent; 509 /* We do not store true stamp, but it is not required, 510 * it can be estimated (approximately) 511 * from another data. 512 */ 513 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans); 514 paws_reject = tcp_paws_check(&tmp_opt, th->rst); 515 } 516 } 517 518 /* Check for pure retransmitted SYN. */ 519 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 520 flg == TCP_FLAG_SYN && 521 !paws_reject) { 522 /* 523 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 524 * this case on figure 6 and figure 8, but formal 525 * protocol description says NOTHING. 526 * To be more exact, it says that we should send ACK, 527 * because this segment (at least, if it has no data) 528 * is out of window. 529 * 530 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 531 * describe SYN-RECV state. All the description 532 * is wrong, we cannot believe to it and should 533 * rely only on common sense and implementation 534 * experience. 535 * 536 * Enforce "SYN-ACK" according to figure 8, figure 6 537 * of RFC793, fixed by RFC1122. 538 */ 539 req->rsk_ops->rtx_syn_ack(sk, req); 540 return NULL; 541 } 542 543 /* Further reproduces section "SEGMENT ARRIVES" 544 for state SYN-RECEIVED of RFC793. 545 It is broken, however, it does not work only 546 when SYNs are crossed. 547 548 You would think that SYN crossing is impossible here, since 549 we should have a SYN_SENT socket (from connect()) on our end, 550 but this is not true if the crossed SYNs were sent to both 551 ends by a malicious third party. We must defend against this, 552 and to do that we first verify the ACK (as per RFC793, page 553 36) and reset if it is invalid. Is this a true full defense? 554 To convince ourselves, let us consider a way in which the ACK 555 test can still pass in this 'malicious crossed SYNs' case. 556 Malicious sender sends identical SYNs (and thus identical sequence 557 numbers) to both A and B: 558 559 A: gets SYN, seq=7 560 B: gets SYN, seq=7 561 562 By our good fortune, both A and B select the same initial 563 send sequence number of seven :-) 564 565 A: sends SYN|ACK, seq=7, ack_seq=8 566 B: sends SYN|ACK, seq=7, ack_seq=8 567 568 So we are now A eating this SYN|ACK, ACK test passes. So 569 does sequence test, SYN is truncated, and thus we consider 570 it a bare ACK. 571 572 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 573 bare ACK. Otherwise, we create an established connection. Both 574 ends (listening sockets) accept the new incoming connection and try 575 to talk to each other. 8-) 576 577 Note: This case is both harmless, and rare. Possibility is about the 578 same as us discovering intelligent life on another plant tomorrow. 579 580 But generally, we should (RFC lies!) to accept ACK 581 from SYNACK both here and in tcp_rcv_state_process(). 582 tcp_rcv_state_process() does not, hence, we do not too. 583 584 Note that the case is absolutely generic: 585 we cannot optimize anything here without 586 violating protocol. All the checks must be made 587 before attempt to create socket. 588 */ 589 590 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 591 * and the incoming segment acknowledges something not yet 592 * sent (the segment carries an unacceptable ACK) ... 593 * a reset is sent." 594 * 595 * Invalid ACK: reset will be sent by listening socket 596 */ 597 if ((flg & TCP_FLAG_ACK) && 598 (TCP_SKB_CB(skb)->ack_seq != tcp_rsk(req)->snt_isn + 1)) 599 return sk; 600 601 /* Also, it would be not so bad idea to check rcv_tsecr, which 602 * is essentially ACK extension and too early or too late values 603 * should cause reset in unsynchronized states. 604 */ 605 606 /* RFC793: "first check sequence number". */ 607 608 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 609 tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) { 610 /* Out of window: send ACK and drop. */ 611 if (!(flg & TCP_FLAG_RST)) 612 req->rsk_ops->send_ack(skb, req); 613 if (paws_reject) 614 NET_INC_STATS_BH(LINUX_MIB_PAWSESTABREJECTED); 615 return NULL; 616 } 617 618 /* In sequence, PAWS is OK. */ 619 620 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1)) 621 req->ts_recent = tmp_opt.rcv_tsval; 622 623 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 624 /* Truncate SYN, it is out of window starting 625 at tcp_rsk(req)->rcv_isn + 1. */ 626 flg &= ~TCP_FLAG_SYN; 627 } 628 629 /* RFC793: "second check the RST bit" and 630 * "fourth, check the SYN bit" 631 */ 632 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 633 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 634 goto embryonic_reset; 635 } 636 637 /* ACK sequence verified above, just make sure ACK is 638 * set. If ACK not set, just silently drop the packet. 639 */ 640 if (!(flg & TCP_FLAG_ACK)) 641 return NULL; 642 643 /* If TCP_DEFER_ACCEPT is set, drop bare ACK. */ 644 if (inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 645 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 646 inet_rsk(req)->acked = 1; 647 return NULL; 648 } 649 650 /* OK, ACK is valid, create big socket and 651 * feed this segment to it. It will repeat all 652 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 653 * ESTABLISHED STATE. If it will be dropped after 654 * socket is created, wait for troubles. 655 */ 656 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, 657 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, 678 newkey, 679 key->keylen); 680 } 681 } 682 } 683#endif 684 685 inet_csk_reqsk_queue_unlink(sk, req, prev); 686 inet_csk_reqsk_queue_removed(sk, req); 687 688 inet_csk_reqsk_queue_add(sk, req, child); 689 return child; 690 691 listen_overflow: 692 if (!sysctl_tcp_abort_on_overflow) { 693 inet_rsk(req)->acked = 1; 694 return NULL; 695 } 696 697 embryonic_reset: 698 NET_INC_STATS_BH(LINUX_MIB_EMBRYONICRSTS); 699 if (!(flg & TCP_FLAG_RST)) 700 req->rsk_ops->send_reset(sk, skb); 701 702 inet_csk_reqsk_queue_drop(sk, req, prev); 703 return NULL; 704} 705 706/* 707 * Queue segment on the new socket if the new socket is active, 708 * otherwise we just shortcircuit this and continue with 709 * the new socket. 710 */ 711 712int tcp_child_process(struct sock *parent, struct sock *child, 713 struct sk_buff *skb) 714{ 715 int ret = 0; 716 int state = child->sk_state; 717 718 if (!sock_owned_by_user(child)) { 719 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb), 720 skb->len); 721 /* Wakeup parent, send SIGIO */ 722 if (state == TCP_SYN_RECV && child->sk_state != state) 723 parent->sk_data_ready(parent, 0); 724 } else { 725 /* Alas, it is possible again, because we do lookup 726 * in main socket hash table and lock on listening 727 * socket does not protect us more. 728 */ 729 sk_add_backlog(child, skb); 730 } 731 732 bh_unlock_sock(child); 733 sock_put(child); 734 return ret; 735} 736 737EXPORT_SYMBOL(tcp_check_req); 738EXPORT_SYMBOL(tcp_child_process); 739EXPORT_SYMBOL(tcp_create_openreq_child); 740EXPORT_SYMBOL(tcp_timewait_state_process); 741