tcp_minisocks.c revision ec3c0982a2dd1e671bad8e9d26c28dcba0039d87
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 * Version: $Id: tcp_minisocks.c,v 1.15 2002/02/01 22:01:04 davem Exp $ 9 * 10 * Authors: Ross Biro 11 * Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG> 12 * Mark Evans, <evansmp@uhura.aston.ac.uk> 13 * Corey Minyard <wf-rch!minyard@relay.EU.net> 14 * Florian La Roche, <flla@stud.uni-sb.de> 15 * Charles Hedrick, <hedrick@klinzhai.rutgers.edu> 16 * Linus Torvalds, <torvalds@cs.helsinki.fi> 17 * Alan Cox, <gw4pts@gw4pts.ampr.org> 18 * Matthew Dillon, <dillon@apollo.west.oic.com> 19 * Arnt Gulbrandsen, <agulbra@nvg.unit.no> 20 * Jorge Cwik, <jorge@laser.satlink.net> 21 */ 22 23#include <linux/mm.h> 24#include <linux/module.h> 25#include <linux/sysctl.h> 26#include <linux/workqueue.h> 27#include <net/tcp.h> 28#include <net/inet_common.h> 29#include <net/xfrm.h> 30 31#ifdef CONFIG_SYSCTL 32#define SYNC_INIT 0 /* let the user enable it */ 33#else 34#define SYNC_INIT 1 35#endif 36 37int sysctl_tcp_syncookies __read_mostly = SYNC_INIT; 38EXPORT_SYMBOL(sysctl_tcp_syncookies); 39 40int sysctl_tcp_abort_on_overflow __read_mostly; 41 42struct inet_timewait_death_row tcp_death_row = { 43 .sysctl_max_tw_buckets = NR_FILE * 2, 44 .period = TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS, 45 .death_lock = __SPIN_LOCK_UNLOCKED(tcp_death_row.death_lock), 46 .hashinfo = &tcp_hashinfo, 47 .tw_timer = TIMER_INITIALIZER(inet_twdr_hangman, 0, 48 (unsigned long)&tcp_death_row), 49 .twkill_work = __WORK_INITIALIZER(tcp_death_row.twkill_work, 50 inet_twdr_twkill_work), 51/* Short-time timewait calendar */ 52 53 .twcal_hand = -1, 54 .twcal_timer = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0, 55 (unsigned long)&tcp_death_row), 56}; 57 58EXPORT_SYMBOL_GPL(tcp_death_row); 59 60static __inline__ int tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win) 61{ 62 if (seq == s_win) 63 return 1; 64 if (after(end_seq, s_win) && before(seq, e_win)) 65 return 1; 66 return (seq == e_win && seq == end_seq); 67} 68 69/* 70 * * Main purpose of TIME-WAIT state is to close connection gracefully, 71 * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN 72 * (and, probably, tail of data) and one or more our ACKs are lost. 73 * * What is TIME-WAIT timeout? It is associated with maximal packet 74 * lifetime in the internet, which results in wrong conclusion, that 75 * it is set to catch "old duplicate segments" wandering out of their path. 76 * It is not quite correct. This timeout is calculated so that it exceeds 77 * maximal retransmission timeout enough to allow to lose one (or more) 78 * segments sent by peer and our ACKs. This time may be calculated from RTO. 79 * * When TIME-WAIT socket receives RST, it means that another end 80 * finally closed and we are allowed to kill TIME-WAIT too. 81 * * Second purpose of TIME-WAIT is catching old duplicate segments. 82 * Well, certainly it is pure paranoia, but if we load TIME-WAIT 83 * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs. 84 * * If we invented some more clever way to catch duplicates 85 * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs. 86 * 87 * The algorithm below is based on FORMAL INTERPRETATION of RFCs. 88 * When you compare it to RFCs, please, read section SEGMENT ARRIVES 89 * from the very beginning. 90 * 91 * NOTE. With recycling (and later with fin-wait-2) TW bucket 92 * is _not_ stateless. It means, that strictly speaking we must 93 * spinlock it. I do not want! Well, probability of misbehaviour 94 * is ridiculously low and, seems, we could use some mb() tricks 95 * to avoid misread sequence numbers, states etc. --ANK 96 */ 97enum tcp_tw_status 98tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb, 99 const struct tcphdr *th) 100{ 101 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 102 struct tcp_options_received tmp_opt; 103 int paws_reject = 0; 104 105 tmp_opt.saw_tstamp = 0; 106 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) { 107 tcp_parse_options(skb, &tmp_opt, 0); 108 109 if (tmp_opt.saw_tstamp) { 110 tmp_opt.ts_recent = tcptw->tw_ts_recent; 111 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp; 112 paws_reject = tcp_paws_check(&tmp_opt, th->rst); 113 } 114 } 115 116 if (tw->tw_substate == TCP_FIN_WAIT2) { 117 /* Just repeat all the checks of tcp_rcv_state_process() */ 118 119 /* Out of window, send ACK */ 120 if (paws_reject || 121 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 122 tcptw->tw_rcv_nxt, 123 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd)) 124 return TCP_TW_ACK; 125 126 if (th->rst) 127 goto kill; 128 129 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt)) 130 goto kill_with_rst; 131 132 /* Dup ACK? */ 133 if (!after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) || 134 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) { 135 inet_twsk_put(tw); 136 return TCP_TW_SUCCESS; 137 } 138 139 /* New data or FIN. If new data arrive after half-duplex close, 140 * reset. 141 */ 142 if (!th->fin || 143 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) { 144kill_with_rst: 145 inet_twsk_deschedule(tw, &tcp_death_row); 146 inet_twsk_put(tw); 147 return TCP_TW_RST; 148 } 149 150 /* FIN arrived, enter true time-wait state. */ 151 tw->tw_substate = TCP_TIME_WAIT; 152 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq; 153 if (tmp_opt.saw_tstamp) { 154 tcptw->tw_ts_recent_stamp = get_seconds(); 155 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 156 } 157 158 /* I am shamed, but failed to make it more elegant. 159 * Yes, it is direct reference to IP, which is impossible 160 * to generalize to IPv6. Taking into account that IPv6 161 * do not understand recycling in any case, it not 162 * a big problem in practice. --ANK */ 163 if (tw->tw_family == AF_INET && 164 tcp_death_row.sysctl_tw_recycle && tcptw->tw_ts_recent_stamp && 165 tcp_v4_tw_remember_stamp(tw)) 166 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout, 167 TCP_TIMEWAIT_LEN); 168 else 169 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 170 TCP_TIMEWAIT_LEN); 171 return TCP_TW_ACK; 172 } 173 174 /* 175 * Now real TIME-WAIT state. 176 * 177 * RFC 1122: 178 * "When a connection is [...] on TIME-WAIT state [...] 179 * [a TCP] MAY accept a new SYN from the remote TCP to 180 * reopen the connection directly, if it: 181 * 182 * (1) assigns its initial sequence number for the new 183 * connection to be larger than the largest sequence 184 * number it used on the previous connection incarnation, 185 * and 186 * 187 * (2) returns to TIME-WAIT state if the SYN turns out 188 * to be an old duplicate". 189 */ 190 191 if (!paws_reject && 192 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 193 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 194 /* In window segment, it may be only reset or bare ack. */ 195 196 if (th->rst) { 197 /* This is TIME_WAIT assassination, in two flavors. 198 * Oh well... nobody has a sufficient solution to this 199 * protocol bug yet. 200 */ 201 if (sysctl_tcp_rfc1337 == 0) { 202kill: 203 inet_twsk_deschedule(tw, &tcp_death_row); 204 inet_twsk_put(tw); 205 return TCP_TW_SUCCESS; 206 } 207 } 208 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 209 TCP_TIMEWAIT_LEN); 210 211 if (tmp_opt.saw_tstamp) { 212 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 213 tcptw->tw_ts_recent_stamp = get_seconds(); 214 } 215 216 inet_twsk_put(tw); 217 return TCP_TW_SUCCESS; 218 } 219 220 /* Out of window segment. 221 222 All the segments are ACKed immediately. 223 224 The only exception is new SYN. We accept it, if it is 225 not old duplicate and we are not in danger to be killed 226 by delayed old duplicates. RFC check is that it has 227 newer sequence number works at rates <40Mbit/sec. 228 However, if paws works, it is reliable AND even more, 229 we even may relax silly seq space cutoff. 230 231 RED-PEN: we violate main RFC requirement, if this SYN will appear 232 old duplicate (i.e. we receive RST in reply to SYN-ACK), 233 we must return socket to time-wait state. It is not good, 234 but not fatal yet. 235 */ 236 237 if (th->syn && !th->rst && !th->ack && !paws_reject && 238 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 239 (tmp_opt.saw_tstamp && 240 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 241 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 242 if (isn == 0) 243 isn++; 244 TCP_SKB_CB(skb)->when = isn; 245 return TCP_TW_SYN; 246 } 247 248 if (paws_reject) 249 NET_INC_STATS_BH(LINUX_MIB_PAWSESTABREJECTED); 250 251 if (!th->rst) { 252 /* In this case we must reset the TIMEWAIT timer. 253 * 254 * If it is ACKless SYN it may be both old duplicate 255 * and new good SYN with random sequence number <rcv_nxt. 256 * Do not reschedule in the last case. 257 */ 258 if (paws_reject || th->ack) 259 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 260 TCP_TIMEWAIT_LEN); 261 262 /* Send ACK. Note, we do not put the bucket, 263 * it will be released by caller. 264 */ 265 return TCP_TW_ACK; 266 } 267 inet_twsk_put(tw); 268 return TCP_TW_SUCCESS; 269} 270 271/* 272 * Move a socket to time-wait or dead fin-wait-2 state. 273 */ 274void tcp_time_wait(struct sock *sk, int state, int timeo) 275{ 276 struct inet_timewait_sock *tw = NULL; 277 const struct inet_connection_sock *icsk = inet_csk(sk); 278 const struct tcp_sock *tp = tcp_sk(sk); 279 int recycle_ok = 0; 280 281 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp) 282 recycle_ok = icsk->icsk_af_ops->remember_stamp(sk); 283 284 if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets) 285 tw = inet_twsk_alloc(sk, state); 286 287 if (tw != NULL) { 288 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 289 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 290 291 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 292 tcptw->tw_rcv_nxt = tp->rcv_nxt; 293 tcptw->tw_snd_nxt = tp->snd_nxt; 294 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 295 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 296 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 297 298#if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE) 299 if (tw->tw_family == PF_INET6) { 300 struct ipv6_pinfo *np = inet6_sk(sk); 301 struct inet6_timewait_sock *tw6; 302 303 tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot); 304 tw6 = inet6_twsk((struct sock *)tw); 305 ipv6_addr_copy(&tw6->tw_v6_daddr, &np->daddr); 306 ipv6_addr_copy(&tw6->tw_v6_rcv_saddr, &np->rcv_saddr); 307 tw->tw_ipv6only = np->ipv6only; 308 } 309#endif 310 311#ifdef CONFIG_TCP_MD5SIG 312 /* 313 * The timewait bucket does not have the key DB from the 314 * sock structure. We just make a quick copy of the 315 * md5 key being used (if indeed we are using one) 316 * so the timewait ack generating code has the key. 317 */ 318 do { 319 struct tcp_md5sig_key *key; 320 memset(tcptw->tw_md5_key, 0, sizeof(tcptw->tw_md5_key)); 321 tcptw->tw_md5_keylen = 0; 322 key = tp->af_specific->md5_lookup(sk, sk); 323 if (key != NULL) { 324 memcpy(&tcptw->tw_md5_key, key->key, key->keylen); 325 tcptw->tw_md5_keylen = key->keylen; 326 if (tcp_alloc_md5sig_pool() == NULL) 327 BUG(); 328 } 329 } while (0); 330#endif 331 332 /* Linkage updates. */ 333 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 334 335 /* Get the TIME_WAIT timeout firing. */ 336 if (timeo < rto) 337 timeo = rto; 338 339 if (recycle_ok) { 340 tw->tw_timeout = rto; 341 } else { 342 tw->tw_timeout = TCP_TIMEWAIT_LEN; 343 if (state == TCP_TIME_WAIT) 344 timeo = TCP_TIMEWAIT_LEN; 345 } 346 347 inet_twsk_schedule(tw, &tcp_death_row, timeo, 348 TCP_TIMEWAIT_LEN); 349 inet_twsk_put(tw); 350 } else { 351 /* Sorry, if we're out of memory, just CLOSE this 352 * socket up. We've got bigger problems than 353 * non-graceful socket closings. 354 */ 355 LIMIT_NETDEBUG(KERN_INFO "TCP: time wait bucket table overflow\n"); 356 } 357 358 tcp_update_metrics(sk); 359 tcp_done(sk); 360} 361 362void tcp_twsk_destructor(struct sock *sk) 363{ 364#ifdef CONFIG_TCP_MD5SIG 365 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 366 if (twsk->tw_md5_keylen) 367 tcp_put_md5sig_pool(); 368#endif 369} 370 371EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 372 373static inline void TCP_ECN_openreq_child(struct tcp_sock *tp, 374 struct request_sock *req) 375{ 376 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 377} 378 379/* This is not only more efficient than what we used to do, it eliminates 380 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 381 * 382 * Actually, we could lots of memory writes here. tp of listening 383 * socket contains all necessary default parameters. 384 */ 385struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb) 386{ 387 struct sock *newsk = inet_csk_clone(sk, req, GFP_ATOMIC); 388 389 if (newsk != NULL) { 390 const struct inet_request_sock *ireq = inet_rsk(req); 391 struct tcp_request_sock *treq = tcp_rsk(req); 392 struct inet_connection_sock *newicsk = inet_csk(newsk); 393 struct tcp_sock *newtp; 394 395 /* Now setup tcp_sock */ 396 newtp = tcp_sk(newsk); 397 newtp->pred_flags = 0; 398 newtp->rcv_wup = newtp->copied_seq = newtp->rcv_nxt = treq->rcv_isn + 1; 399 newtp->snd_sml = newtp->snd_una = newtp->snd_nxt = treq->snt_isn + 1; 400 401 tcp_prequeue_init(newtp); 402 403 tcp_init_wl(newtp, treq->snt_isn, 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.eff_sacks = 0; 439 440 newtp->rx_opt.num_sacks = 0; 441 newtp->urg_data = 0; 442 443 if (sock_flag(newsk, SOCK_KEEPOPEN)) 444 inet_csk_reset_keepalive_timer(newsk, 445 keepalive_time_when(newtp)); 446 447 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 448 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 449 if (sysctl_tcp_fack) 450 tcp_enable_fack(newtp); 451 } 452 newtp->window_clamp = req->window_clamp; 453 newtp->rcv_ssthresh = req->rcv_wnd; 454 newtp->rcv_wnd = req->rcv_wnd; 455 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 456 if (newtp->rx_opt.wscale_ok) { 457 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 458 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 459 } else { 460 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 461 newtp->window_clamp = min(newtp->window_clamp, 65535U); 462 } 463 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 464 newtp->rx_opt.snd_wscale); 465 newtp->max_window = newtp->snd_wnd; 466 467 if (newtp->rx_opt.tstamp_ok) { 468 newtp->rx_opt.ts_recent = req->ts_recent; 469 newtp->rx_opt.ts_recent_stamp = get_seconds(); 470 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 471 } else { 472 newtp->rx_opt.ts_recent_stamp = 0; 473 newtp->tcp_header_len = sizeof(struct tcphdr); 474 } 475#ifdef CONFIG_TCP_MD5SIG 476 newtp->md5sig_info = NULL; /*XXX*/ 477 if (newtp->af_specific->md5_lookup(sk, newsk)) 478 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 479#endif 480 if (skb->len >= TCP_MIN_RCVMSS+newtp->tcp_header_len) 481 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 482 newtp->rx_opt.mss_clamp = req->mss; 483 TCP_ECN_openreq_child(newtp, req); 484 485 TCP_INC_STATS_BH(TCP_MIB_PASSIVEOPENS); 486 } 487 return newsk; 488} 489 490/* 491 * Process an incoming packet for SYN_RECV sockets represented 492 * as a request_sock. 493 */ 494 495struct sock *tcp_check_req(struct sock *sk,struct sk_buff *skb, 496 struct request_sock *req, 497 struct request_sock **prev) 498{ 499 const struct tcphdr *th = tcp_hdr(skb); 500 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 501 int paws_reject = 0; 502 struct tcp_options_received tmp_opt; 503 struct sock *child; 504 505 tmp_opt.saw_tstamp = 0; 506 if (th->doff > (sizeof(struct tcphdr)>>2)) { 507 tcp_parse_options(skb, &tmp_opt, 0); 508 509 if (tmp_opt.saw_tstamp) { 510 tmp_opt.ts_recent = req->ts_recent; 511 /* We do not store true stamp, but it is not required, 512 * it can be estimated (approximately) 513 * from another data. 514 */ 515 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans); 516 paws_reject = tcp_paws_check(&tmp_opt, th->rst); 517 } 518 } 519 520 /* Check for pure retransmitted SYN. */ 521 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 522 flg == TCP_FLAG_SYN && 523 !paws_reject) { 524 /* 525 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 526 * this case on figure 6 and figure 8, but formal 527 * protocol description says NOTHING. 528 * To be more exact, it says that we should send ACK, 529 * because this segment (at least, if it has no data) 530 * is out of window. 531 * 532 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 533 * describe SYN-RECV state. All the description 534 * is wrong, we cannot believe to it and should 535 * rely only on common sense and implementation 536 * experience. 537 * 538 * Enforce "SYN-ACK" according to figure 8, figure 6 539 * of RFC793, fixed by RFC1122. 540 */ 541 req->rsk_ops->rtx_syn_ack(sk, req); 542 return NULL; 543 } 544 545 /* Further reproduces section "SEGMENT ARRIVES" 546 for state SYN-RECEIVED of RFC793. 547 It is broken, however, it does not work only 548 when SYNs are crossed. 549 550 You would think that SYN crossing is impossible here, since 551 we should have a SYN_SENT socket (from connect()) on our end, 552 but this is not true if the crossed SYNs were sent to both 553 ends by a malicious third party. We must defend against this, 554 and to do that we first verify the ACK (as per RFC793, page 555 36) and reset if it is invalid. Is this a true full defense? 556 To convince ourselves, let us consider a way in which the ACK 557 test can still pass in this 'malicious crossed SYNs' case. 558 Malicious sender sends identical SYNs (and thus identical sequence 559 numbers) to both A and B: 560 561 A: gets SYN, seq=7 562 B: gets SYN, seq=7 563 564 By our good fortune, both A and B select the same initial 565 send sequence number of seven :-) 566 567 A: sends SYN|ACK, seq=7, ack_seq=8 568 B: sends SYN|ACK, seq=7, ack_seq=8 569 570 So we are now A eating this SYN|ACK, ACK test passes. So 571 does sequence test, SYN is truncated, and thus we consider 572 it a bare ACK. 573 574 Both ends (listening sockets) accept the new incoming 575 connection and try 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(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 /* OK, ACK is valid, create big socket and 644 * feed this segment to it. It will repeat all 645 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 646 * ESTABLISHED STATE. If it will be dropped after 647 * socket is created, wait for troubles. 648 */ 649 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, 650 req, NULL); 651 if (child == NULL) 652 goto listen_overflow; 653#ifdef CONFIG_TCP_MD5SIG 654 else { 655 /* Copy over the MD5 key from the original socket */ 656 struct tcp_md5sig_key *key; 657 struct tcp_sock *tp = tcp_sk(sk); 658 key = tp->af_specific->md5_lookup(sk, child); 659 if (key != NULL) { 660 /* 661 * We're using one, so create a matching key on the 662 * newsk structure. If we fail to get memory then we 663 * end up not copying the key across. Shucks. 664 */ 665 char *newkey = kmemdup(key->key, key->keylen, 666 GFP_ATOMIC); 667 if (newkey) { 668 if (!tcp_alloc_md5sig_pool()) 669 BUG(); 670 tp->af_specific->md5_add(child, child, 671 newkey, 672 key->keylen); 673 } 674 } 675 } 676#endif 677 678 inet_csk_reqsk_queue_unlink(sk, req, prev); 679 inet_csk_reqsk_queue_removed(sk, req); 680 681 if (inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 682 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 683 684 /* the accept queue handling is done is est recv slow 685 * path so lets make sure to start there 686 */ 687 tcp_sk(child)->pred_flags = 0; 688 sock_hold(sk); 689 sock_hold(child); 690 tcp_sk(child)->defer_tcp_accept.listen_sk = sk; 691 tcp_sk(child)->defer_tcp_accept.request = req; 692 693 inet_csk_reset_keepalive_timer(child, 694 inet_csk(sk)->icsk_accept_queue.rskq_defer_accept * HZ); 695 } else { 696 inet_csk_reqsk_queue_add(sk, req, child); 697 } 698 699 return child; 700 701 listen_overflow: 702 if (!sysctl_tcp_abort_on_overflow) { 703 inet_rsk(req)->acked = 1; 704 return NULL; 705 } 706 707 embryonic_reset: 708 NET_INC_STATS_BH(LINUX_MIB_EMBRYONICRSTS); 709 if (!(flg & TCP_FLAG_RST)) 710 req->rsk_ops->send_reset(sk, skb); 711 712 inet_csk_reqsk_queue_drop(sk, req, prev); 713 return NULL; 714} 715 716/* 717 * Queue segment on the new socket if the new socket is active, 718 * otherwise we just shortcircuit this and continue with 719 * the new socket. 720 */ 721 722int tcp_child_process(struct sock *parent, struct sock *child, 723 struct sk_buff *skb) 724{ 725 int ret = 0; 726 int state = child->sk_state; 727 728 if (!sock_owned_by_user(child)) { 729 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb), 730 skb->len); 731 /* Wakeup parent, send SIGIO */ 732 if (state == TCP_SYN_RECV && child->sk_state != state) 733 parent->sk_data_ready(parent, 0); 734 } else { 735 /* Alas, it is possible again, because we do lookup 736 * in main socket hash table and lock on listening 737 * socket does not protect us more. 738 */ 739 sk_add_backlog(child, skb); 740 } 741 742 bh_unlock_sock(child); 743 sock_put(child); 744 return ret; 745} 746 747EXPORT_SYMBOL(tcp_check_req); 748EXPORT_SYMBOL(tcp_child_process); 749EXPORT_SYMBOL(tcp_create_openreq_child); 750EXPORT_SYMBOL(tcp_timewait_state_process); 751