tcp_minisocks.c revision 1ac530b3553e0b4dc1e18a32bed57cfa84cd57cb
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_reject(&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 (!th->ack || 132 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) || 133 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) { 134 inet_twsk_put(tw); 135 return TCP_TW_SUCCESS; 136 } 137 138 /* New data or FIN. If new data arrive after half-duplex close, 139 * reset. 140 */ 141 if (!th->fin || 142 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) { 143kill_with_rst: 144 inet_twsk_deschedule(tw, &tcp_death_row); 145 inet_twsk_put(tw); 146 return TCP_TW_RST; 147 } 148 149 /* FIN arrived, enter true time-wait state. */ 150 tw->tw_substate = TCP_TIME_WAIT; 151 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq; 152 if (tmp_opt.saw_tstamp) { 153 tcptw->tw_ts_recent_stamp = get_seconds(); 154 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 155 } 156 157 /* I am shamed, but failed to make it more elegant. 158 * Yes, it is direct reference to IP, which is impossible 159 * to generalize to IPv6. Taking into account that IPv6 160 * do not understand recycling in any case, it not 161 * a big problem in practice. --ANK */ 162 if (tw->tw_family == AF_INET && 163 tcp_death_row.sysctl_tw_recycle && tcptw->tw_ts_recent_stamp && 164 tcp_v4_tw_remember_stamp(tw)) 165 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout, 166 TCP_TIMEWAIT_LEN); 167 else 168 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 169 TCP_TIMEWAIT_LEN); 170 return TCP_TW_ACK; 171 } 172 173 /* 174 * Now real TIME-WAIT state. 175 * 176 * RFC 1122: 177 * "When a connection is [...] on TIME-WAIT state [...] 178 * [a TCP] MAY accept a new SYN from the remote TCP to 179 * reopen the connection directly, if it: 180 * 181 * (1) assigns its initial sequence number for the new 182 * connection to be larger than the largest sequence 183 * number it used on the previous connection incarnation, 184 * and 185 * 186 * (2) returns to TIME-WAIT state if the SYN turns out 187 * to be an old duplicate". 188 */ 189 190 if (!paws_reject && 191 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 192 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 193 /* In window segment, it may be only reset or bare ack. */ 194 195 if (th->rst) { 196 /* This is TIME_WAIT assassination, in two flavors. 197 * Oh well... nobody has a sufficient solution to this 198 * protocol bug yet. 199 */ 200 if (sysctl_tcp_rfc1337 == 0) { 201kill: 202 inet_twsk_deschedule(tw, &tcp_death_row); 203 inet_twsk_put(tw); 204 return TCP_TW_SUCCESS; 205 } 206 } 207 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 208 TCP_TIMEWAIT_LEN); 209 210 if (tmp_opt.saw_tstamp) { 211 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 212 tcptw->tw_ts_recent_stamp = get_seconds(); 213 } 214 215 inet_twsk_put(tw); 216 return TCP_TW_SUCCESS; 217 } 218 219 /* Out of window segment. 220 221 All the segments are ACKed immediately. 222 223 The only exception is new SYN. We accept it, if it is 224 not old duplicate and we are not in danger to be killed 225 by delayed old duplicates. RFC check is that it has 226 newer sequence number works at rates <40Mbit/sec. 227 However, if paws works, it is reliable AND even more, 228 we even may relax silly seq space cutoff. 229 230 RED-PEN: we violate main RFC requirement, if this SYN will appear 231 old duplicate (i.e. we receive RST in reply to SYN-ACK), 232 we must return socket to time-wait state. It is not good, 233 but not fatal yet. 234 */ 235 236 if (th->syn && !th->rst && !th->ack && !paws_reject && 237 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 238 (tmp_opt.saw_tstamp && 239 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 240 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 241 if (isn == 0) 242 isn++; 243 TCP_SKB_CB(skb)->when = isn; 244 return TCP_TW_SYN; 245 } 246 247 if (paws_reject) 248 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 249 250 if (!th->rst) { 251 /* In this case we must reset the TIMEWAIT timer. 252 * 253 * If it is ACKless SYN it may be both old duplicate 254 * and new good SYN with random sequence number <rcv_nxt. 255 * Do not reschedule in the last case. 256 */ 257 if (paws_reject || th->ack) 258 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 259 TCP_TIMEWAIT_LEN); 260 261 /* Send ACK. Note, we do not put the bucket, 262 * it will be released by caller. 263 */ 264 return TCP_TW_ACK; 265 } 266 inet_twsk_put(tw); 267 return TCP_TW_SUCCESS; 268} 269 270/* 271 * Move a socket to time-wait or dead fin-wait-2 state. 272 */ 273void tcp_time_wait(struct sock *sk, int state, int timeo) 274{ 275 struct inet_timewait_sock *tw = NULL; 276 const struct inet_connection_sock *icsk = inet_csk(sk); 277 const struct tcp_sock *tp = tcp_sk(sk); 278 int recycle_ok = 0; 279 280 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp) 281 recycle_ok = icsk->icsk_af_ops->remember_stamp(sk); 282 283 if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets) 284 tw = inet_twsk_alloc(sk, state); 285 286 if (tw != NULL) { 287 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 288 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 289 290 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 291 tcptw->tw_rcv_nxt = tp->rcv_nxt; 292 tcptw->tw_snd_nxt = tp->snd_nxt; 293 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 294 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 295 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 296 297#if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE) 298 if (tw->tw_family == PF_INET6) { 299 struct ipv6_pinfo *np = inet6_sk(sk); 300 struct inet6_timewait_sock *tw6; 301 302 tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot); 303 tw6 = inet6_twsk((struct sock *)tw); 304 ipv6_addr_copy(&tw6->tw_v6_daddr, &np->daddr); 305 ipv6_addr_copy(&tw6->tw_v6_rcv_saddr, &np->rcv_saddr); 306 tw->tw_ipv6only = np->ipv6only; 307 } 308#endif 309 310#ifdef CONFIG_TCP_MD5SIG 311 /* 312 * The timewait bucket does not have the key DB from the 313 * sock structure. We just make a quick copy of the 314 * md5 key being used (if indeed we are using one) 315 * so the timewait ack generating code has the key. 316 */ 317 do { 318 struct tcp_md5sig_key *key; 319 memset(tcptw->tw_md5_key, 0, sizeof(tcptw->tw_md5_key)); 320 tcptw->tw_md5_keylen = 0; 321 key = tp->af_specific->md5_lookup(sk, sk); 322 if (key != NULL) { 323 memcpy(&tcptw->tw_md5_key, key->key, key->keylen); 324 tcptw->tw_md5_keylen = key->keylen; 325 if (tcp_alloc_md5sig_pool() == NULL) 326 BUG(); 327 } 328 } while (0); 329#endif 330 331 /* Linkage updates. */ 332 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 333 334 /* Get the TIME_WAIT timeout firing. */ 335 if (timeo < rto) 336 timeo = rto; 337 338 if (recycle_ok) { 339 tw->tw_timeout = rto; 340 } else { 341 tw->tw_timeout = TCP_TIMEWAIT_LEN; 342 if (state == TCP_TIME_WAIT) 343 timeo = TCP_TIMEWAIT_LEN; 344 } 345 346 inet_twsk_schedule(tw, &tcp_death_row, timeo, 347 TCP_TIMEWAIT_LEN); 348 inet_twsk_put(tw); 349 } else { 350 /* Sorry, if we're out of memory, just CLOSE this 351 * socket up. We've got bigger problems than 352 * non-graceful socket closings. 353 */ 354 LIMIT_NETDEBUG(KERN_INFO "TCP: time wait bucket table overflow\n"); 355 } 356 357 tcp_update_metrics(sk); 358 tcp_done(sk); 359} 360 361void tcp_twsk_destructor(struct sock *sk) 362{ 363#ifdef CONFIG_TCP_MD5SIG 364 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 365 if (twsk->tw_md5_keylen) 366 tcp_put_md5sig_pool(); 367#endif 368} 369 370EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 371 372static inline void TCP_ECN_openreq_child(struct tcp_sock *tp, 373 struct request_sock *req) 374{ 375 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 376} 377 378/* This is not only more efficient than what we used to do, it eliminates 379 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 380 * 381 * Actually, we could lots of memory writes here. tp of listening 382 * socket contains all necessary default parameters. 383 */ 384struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb) 385{ 386 struct sock *newsk = inet_csk_clone(sk, req, GFP_ATOMIC); 387 388 if (newsk != NULL) { 389 const struct inet_request_sock *ireq = inet_rsk(req); 390 struct tcp_request_sock *treq = tcp_rsk(req); 391 struct inet_connection_sock *newicsk = inet_csk(newsk); 392 struct tcp_sock *newtp; 393 394 /* Now setup tcp_sock */ 395 newtp = tcp_sk(newsk); 396 newtp->pred_flags = 0; 397 newtp->rcv_wup = newtp->copied_seq = newtp->rcv_nxt = treq->rcv_isn + 1; 398 newtp->snd_sml = newtp->snd_una = newtp->snd_nxt = treq->snt_isn + 1; 399 newtp->snd_up = treq->snt_isn + 1; 400 401 tcp_prequeue_init(newtp); 402 403 tcp_init_wl(newtp, treq->rcv_isn); 404 405 newtp->srtt = 0; 406 newtp->mdev = TCP_TIMEOUT_INIT; 407 newicsk->icsk_rto = TCP_TIMEOUT_INIT; 408 409 newtp->packets_out = 0; 410 newtp->retrans_out = 0; 411 newtp->sacked_out = 0; 412 newtp->fackets_out = 0; 413 newtp->snd_ssthresh = 0x7fffffff; 414 415 /* So many TCP implementations out there (incorrectly) count the 416 * initial SYN frame in their delayed-ACK and congestion control 417 * algorithms that we must have the following bandaid to talk 418 * efficiently to them. -DaveM 419 */ 420 newtp->snd_cwnd = 2; 421 newtp->snd_cwnd_cnt = 0; 422 newtp->bytes_acked = 0; 423 424 newtp->frto_counter = 0; 425 newtp->frto_highmark = 0; 426 427 newicsk->icsk_ca_ops = &tcp_init_congestion_ops; 428 429 tcp_set_ca_state(newsk, TCP_CA_Open); 430 tcp_init_xmit_timers(newsk); 431 skb_queue_head_init(&newtp->out_of_order_queue); 432 newtp->write_seq = treq->snt_isn + 1; 433 newtp->pushed_seq = newtp->write_seq; 434 435 newtp->rx_opt.saw_tstamp = 0; 436 437 newtp->rx_opt.dsack = 0; 438 newtp->rx_opt.num_sacks = 0; 439 440 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 494struct 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_reject(&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 690listen_overflow: 691 if (!sysctl_tcp_abort_on_overflow) { 692 inet_rsk(req)->acked = 1; 693 return NULL; 694 } 695 696embryonic_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 711int 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 736EXPORT_SYMBOL(tcp_check_req); 737EXPORT_SYMBOL(tcp_child_process); 738EXPORT_SYMBOL(tcp_create_openreq_child); 739EXPORT_SYMBOL(tcp_timewait_state_process); 740