tcp_minisocks.c revision e6b4d11367519bc71729c09d05a126b133c755be
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 29int sysctl_tcp_syncookies __read_mostly = 1; 30EXPORT_SYMBOL(sysctl_tcp_syncookies); 31 32int sysctl_tcp_abort_on_overflow __read_mostly; 33 34struct inet_timewait_death_row tcp_death_row = { 35 .sysctl_max_tw_buckets = NR_FILE * 2, 36 .period = TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS, 37 .death_lock = __SPIN_LOCK_UNLOCKED(tcp_death_row.death_lock), 38 .hashinfo = &tcp_hashinfo, 39 .tw_timer = TIMER_INITIALIZER(inet_twdr_hangman, 0, 40 (unsigned long)&tcp_death_row), 41 .twkill_work = __WORK_INITIALIZER(tcp_death_row.twkill_work, 42 inet_twdr_twkill_work), 43/* Short-time timewait calendar */ 44 45 .twcal_hand = -1, 46 .twcal_timer = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0, 47 (unsigned long)&tcp_death_row), 48}; 49 50EXPORT_SYMBOL_GPL(tcp_death_row); 51 52static __inline__ int tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win) 53{ 54 if (seq == s_win) 55 return 1; 56 if (after(end_seq, s_win) && before(seq, e_win)) 57 return 1; 58 return (seq == e_win && seq == end_seq); 59} 60 61/* 62 * * Main purpose of TIME-WAIT state is to close connection gracefully, 63 * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN 64 * (and, probably, tail of data) and one or more our ACKs are lost. 65 * * What is TIME-WAIT timeout? It is associated with maximal packet 66 * lifetime in the internet, which results in wrong conclusion, that 67 * it is set to catch "old duplicate segments" wandering out of their path. 68 * It is not quite correct. This timeout is calculated so that it exceeds 69 * maximal retransmission timeout enough to allow to lose one (or more) 70 * segments sent by peer and our ACKs. This time may be calculated from RTO. 71 * * When TIME-WAIT socket receives RST, it means that another end 72 * finally closed and we are allowed to kill TIME-WAIT too. 73 * * Second purpose of TIME-WAIT is catching old duplicate segments. 74 * Well, certainly it is pure paranoia, but if we load TIME-WAIT 75 * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs. 76 * * If we invented some more clever way to catch duplicates 77 * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs. 78 * 79 * The algorithm below is based on FORMAL INTERPRETATION of RFCs. 80 * When you compare it to RFCs, please, read section SEGMENT ARRIVES 81 * from the very beginning. 82 * 83 * NOTE. With recycling (and later with fin-wait-2) TW bucket 84 * is _not_ stateless. It means, that strictly speaking we must 85 * spinlock it. I do not want! Well, probability of misbehaviour 86 * is ridiculously low and, seems, we could use some mb() tricks 87 * to avoid misread sequence numbers, states etc. --ANK 88 */ 89enum tcp_tw_status 90tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb, 91 const struct tcphdr *th) 92{ 93 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 94 struct tcp_options_received tmp_opt; 95 int paws_reject = 0; 96 97 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) { 98 tmp_opt.tstamp_ok = 1; 99 tcp_parse_options(skb, &tmp_opt, 1, NULL); 100 101 if (tmp_opt.saw_tstamp) { 102 tmp_opt.ts_recent = tcptw->tw_ts_recent; 103 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp; 104 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 105 } 106 } 107 108 if (tw->tw_substate == TCP_FIN_WAIT2) { 109 /* Just repeat all the checks of tcp_rcv_state_process() */ 110 111 /* Out of window, send ACK */ 112 if (paws_reject || 113 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 114 tcptw->tw_rcv_nxt, 115 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd)) 116 return TCP_TW_ACK; 117 118 if (th->rst) 119 goto kill; 120 121 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt)) 122 goto kill_with_rst; 123 124 /* Dup ACK? */ 125 if (!th->ack || 126 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) || 127 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) { 128 inet_twsk_put(tw); 129 return TCP_TW_SUCCESS; 130 } 131 132 /* New data or FIN. If new data arrive after half-duplex close, 133 * reset. 134 */ 135 if (!th->fin || 136 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) { 137kill_with_rst: 138 inet_twsk_deschedule(tw, &tcp_death_row); 139 inet_twsk_put(tw); 140 return TCP_TW_RST; 141 } 142 143 /* FIN arrived, enter true time-wait state. */ 144 tw->tw_substate = TCP_TIME_WAIT; 145 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq; 146 if (tmp_opt.saw_tstamp) { 147 tcptw->tw_ts_recent_stamp = get_seconds(); 148 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 149 } 150 151 /* I am shamed, but failed to make it more elegant. 152 * Yes, it is direct reference to IP, which is impossible 153 * to generalize to IPv6. Taking into account that IPv6 154 * do not understand recycling in any case, it not 155 * a big problem in practice. --ANK */ 156 if (tw->tw_family == AF_INET && 157 tcp_death_row.sysctl_tw_recycle && tcptw->tw_ts_recent_stamp && 158 tcp_v4_tw_remember_stamp(tw)) 159 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout, 160 TCP_TIMEWAIT_LEN); 161 else 162 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 163 TCP_TIMEWAIT_LEN); 164 return TCP_TW_ACK; 165 } 166 167 /* 168 * Now real TIME-WAIT state. 169 * 170 * RFC 1122: 171 * "When a connection is [...] on TIME-WAIT state [...] 172 * [a TCP] MAY accept a new SYN from the remote TCP to 173 * reopen the connection directly, if it: 174 * 175 * (1) assigns its initial sequence number for the new 176 * connection to be larger than the largest sequence 177 * number it used on the previous connection incarnation, 178 * and 179 * 180 * (2) returns to TIME-WAIT state if the SYN turns out 181 * to be an old duplicate". 182 */ 183 184 if (!paws_reject && 185 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 186 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 187 /* In window segment, it may be only reset or bare ack. */ 188 189 if (th->rst) { 190 /* This is TIME_WAIT assassination, in two flavors. 191 * Oh well... nobody has a sufficient solution to this 192 * protocol bug yet. 193 */ 194 if (sysctl_tcp_rfc1337 == 0) { 195kill: 196 inet_twsk_deschedule(tw, &tcp_death_row); 197 inet_twsk_put(tw); 198 return TCP_TW_SUCCESS; 199 } 200 } 201 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 202 TCP_TIMEWAIT_LEN); 203 204 if (tmp_opt.saw_tstamp) { 205 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 206 tcptw->tw_ts_recent_stamp = get_seconds(); 207 } 208 209 inet_twsk_put(tw); 210 return TCP_TW_SUCCESS; 211 } 212 213 /* Out of window segment. 214 215 All the segments are ACKed immediately. 216 217 The only exception is new SYN. We accept it, if it is 218 not old duplicate and we are not in danger to be killed 219 by delayed old duplicates. RFC check is that it has 220 newer sequence number works at rates <40Mbit/sec. 221 However, if paws works, it is reliable AND even more, 222 we even may relax silly seq space cutoff. 223 224 RED-PEN: we violate main RFC requirement, if this SYN will appear 225 old duplicate (i.e. we receive RST in reply to SYN-ACK), 226 we must return socket to time-wait state. It is not good, 227 but not fatal yet. 228 */ 229 230 if (th->syn && !th->rst && !th->ack && !paws_reject && 231 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 232 (tmp_opt.saw_tstamp && 233 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 234 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 235 if (isn == 0) 236 isn++; 237 TCP_SKB_CB(skb)->when = isn; 238 return TCP_TW_SYN; 239 } 240 241 if (paws_reject) 242 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 243 244 if (!th->rst) { 245 /* In this case we must reset the TIMEWAIT timer. 246 * 247 * If it is ACKless SYN it may be both old duplicate 248 * and new good SYN with random sequence number <rcv_nxt. 249 * Do not reschedule in the last case. 250 */ 251 if (paws_reject || th->ack) 252 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 253 TCP_TIMEWAIT_LEN); 254 255 /* Send ACK. Note, we do not put the bucket, 256 * it will be released by caller. 257 */ 258 return TCP_TW_ACK; 259 } 260 inet_twsk_put(tw); 261 return TCP_TW_SUCCESS; 262} 263 264/* 265 * Move a socket to time-wait or dead fin-wait-2 state. 266 */ 267void tcp_time_wait(struct sock *sk, int state, int timeo) 268{ 269 struct inet_timewait_sock *tw = NULL; 270 const struct inet_connection_sock *icsk = inet_csk(sk); 271 const struct tcp_sock *tp = tcp_sk(sk); 272 int recycle_ok = 0; 273 274 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp) 275 recycle_ok = icsk->icsk_af_ops->remember_stamp(sk); 276 277 if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets) 278 tw = inet_twsk_alloc(sk, state); 279 280 if (tw != NULL) { 281 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 282 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 283 284 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 285 tcptw->tw_rcv_nxt = tp->rcv_nxt; 286 tcptw->tw_snd_nxt = tp->snd_nxt; 287 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 288 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 289 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 290 291#if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE) 292 if (tw->tw_family == PF_INET6) { 293 struct ipv6_pinfo *np = inet6_sk(sk); 294 struct inet6_timewait_sock *tw6; 295 296 tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot); 297 tw6 = inet6_twsk((struct sock *)tw); 298 ipv6_addr_copy(&tw6->tw_v6_daddr, &np->daddr); 299 ipv6_addr_copy(&tw6->tw_v6_rcv_saddr, &np->rcv_saddr); 300 tw->tw_ipv6only = np->ipv6only; 301 } 302#endif 303 304#ifdef CONFIG_TCP_MD5SIG 305 /* 306 * The timewait bucket does not have the key DB from the 307 * sock structure. We just make a quick copy of the 308 * md5 key being used (if indeed we are using one) 309 * so the timewait ack generating code has the key. 310 */ 311 do { 312 struct tcp_md5sig_key *key; 313 memset(tcptw->tw_md5_key, 0, sizeof(tcptw->tw_md5_key)); 314 tcptw->tw_md5_keylen = 0; 315 key = tp->af_specific->md5_lookup(sk, sk); 316 if (key != NULL) { 317 memcpy(&tcptw->tw_md5_key, key->key, key->keylen); 318 tcptw->tw_md5_keylen = key->keylen; 319 if (tcp_alloc_md5sig_pool(sk) == NULL) 320 BUG(); 321 } 322 } while (0); 323#endif 324 325 /* Linkage updates. */ 326 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 327 328 /* Get the TIME_WAIT timeout firing. */ 329 if (timeo < rto) 330 timeo = rto; 331 332 if (recycle_ok) { 333 tw->tw_timeout = rto; 334 } else { 335 tw->tw_timeout = TCP_TIMEWAIT_LEN; 336 if (state == TCP_TIME_WAIT) 337 timeo = TCP_TIMEWAIT_LEN; 338 } 339 340 inet_twsk_schedule(tw, &tcp_death_row, timeo, 341 TCP_TIMEWAIT_LEN); 342 inet_twsk_put(tw); 343 } else { 344 /* Sorry, if we're out of memory, just CLOSE this 345 * socket up. We've got bigger problems than 346 * non-graceful socket closings. 347 */ 348 LIMIT_NETDEBUG(KERN_INFO "TCP: time wait bucket table overflow\n"); 349 } 350 351 tcp_update_metrics(sk); 352 tcp_done(sk); 353} 354 355void tcp_twsk_destructor(struct sock *sk) 356{ 357#ifdef CONFIG_TCP_MD5SIG 358 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 359 if (twsk->tw_md5_keylen) 360 tcp_free_md5sig_pool(); 361#endif 362} 363 364EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 365 366static inline void TCP_ECN_openreq_child(struct tcp_sock *tp, 367 struct request_sock *req) 368{ 369 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 370} 371 372/* This is not only more efficient than what we used to do, it eliminates 373 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 374 * 375 * Actually, we could lots of memory writes here. tp of listening 376 * socket contains all necessary default parameters. 377 */ 378struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb) 379{ 380 struct sock *newsk = inet_csk_clone(sk, req, GFP_ATOMIC); 381 382 if (newsk != NULL) { 383 const struct inet_request_sock *ireq = inet_rsk(req); 384 struct tcp_request_sock *treq = tcp_rsk(req); 385 struct inet_connection_sock *newicsk = inet_csk(newsk); 386 struct tcp_sock *newtp; 387 388 /* Now setup tcp_sock */ 389 newtp = tcp_sk(newsk); 390 newtp->pred_flags = 0; 391 newtp->rcv_wup = newtp->copied_seq = newtp->rcv_nxt = treq->rcv_isn + 1; 392 newtp->snd_sml = newtp->snd_una = newtp->snd_nxt = treq->snt_isn + 1; 393 newtp->snd_up = treq->snt_isn + 1; 394 395 tcp_prequeue_init(newtp); 396 397 tcp_init_wl(newtp, treq->rcv_isn); 398 399 newtp->srtt = 0; 400 newtp->mdev = TCP_TIMEOUT_INIT; 401 newicsk->icsk_rto = TCP_TIMEOUT_INIT; 402 403 newtp->packets_out = 0; 404 newtp->retrans_out = 0; 405 newtp->sacked_out = 0; 406 newtp->fackets_out = 0; 407 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH; 408 409 /* So many TCP implementations out there (incorrectly) count the 410 * initial SYN frame in their delayed-ACK and congestion control 411 * algorithms that we must have the following bandaid to talk 412 * efficiently to them. -DaveM 413 */ 414 newtp->snd_cwnd = 2; 415 newtp->snd_cwnd_cnt = 0; 416 newtp->bytes_acked = 0; 417 418 newtp->frto_counter = 0; 419 newtp->frto_highmark = 0; 420 421 newicsk->icsk_ca_ops = &tcp_init_congestion_ops; 422 423 tcp_set_ca_state(newsk, TCP_CA_Open); 424 tcp_init_xmit_timers(newsk); 425 skb_queue_head_init(&newtp->out_of_order_queue); 426 newtp->write_seq = treq->snt_isn + 1; 427 newtp->pushed_seq = newtp->write_seq; 428 429 newtp->rx_opt.saw_tstamp = 0; 430 431 newtp->rx_opt.dsack = 0; 432 newtp->rx_opt.num_sacks = 0; 433 434 newtp->urg_data = 0; 435 436 if (sock_flag(newsk, SOCK_KEEPOPEN)) 437 inet_csk_reset_keepalive_timer(newsk, 438 keepalive_time_when(newtp)); 439 440 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 441 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 442 if (sysctl_tcp_fack) 443 tcp_enable_fack(newtp); 444 } 445 newtp->window_clamp = req->window_clamp; 446 newtp->rcv_ssthresh = req->rcv_wnd; 447 newtp->rcv_wnd = req->rcv_wnd; 448 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 449 if (newtp->rx_opt.wscale_ok) { 450 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 451 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 452 } else { 453 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 454 newtp->window_clamp = min(newtp->window_clamp, 65535U); 455 } 456 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 457 newtp->rx_opt.snd_wscale); 458 newtp->max_window = newtp->snd_wnd; 459 460 if (newtp->rx_opt.tstamp_ok) { 461 newtp->rx_opt.ts_recent = req->ts_recent; 462 newtp->rx_opt.ts_recent_stamp = get_seconds(); 463 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 464 } else { 465 newtp->rx_opt.ts_recent_stamp = 0; 466 newtp->tcp_header_len = sizeof(struct tcphdr); 467 } 468#ifdef CONFIG_TCP_MD5SIG 469 newtp->md5sig_info = NULL; /*XXX*/ 470 if (newtp->af_specific->md5_lookup(sk, newsk)) 471 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 472#endif 473 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) 474 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 475 newtp->rx_opt.mss_clamp = req->mss; 476 TCP_ECN_openreq_child(newtp, req); 477 478 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS); 479 } 480 return newsk; 481} 482 483/* 484 * Process an incoming packet for SYN_RECV sockets represented 485 * as a request_sock. 486 */ 487 488struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 489 struct request_sock *req, 490 struct request_sock **prev) 491{ 492 const struct tcphdr *th = tcp_hdr(skb); 493 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 494 int paws_reject = 0; 495 struct tcp_options_received tmp_opt; 496 struct sock *child; 497 498 if ((th->doff > (sizeof(struct tcphdr)>>2)) && (req->ts_recent)) { 499 tmp_opt.tstamp_ok = 1; 500 tcp_parse_options(skb, &tmp_opt, 1, NULL); 501 502 if (tmp_opt.saw_tstamp) { 503 tmp_opt.ts_recent = req->ts_recent; 504 /* We do not store true stamp, but it is not required, 505 * it can be estimated (approximately) 506 * from another data. 507 */ 508 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans); 509 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 510 } 511 } 512 513 /* Check for pure retransmitted SYN. */ 514 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 515 flg == TCP_FLAG_SYN && 516 !paws_reject) { 517 /* 518 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 519 * this case on figure 6 and figure 8, but formal 520 * protocol description says NOTHING. 521 * To be more exact, it says that we should send ACK, 522 * because this segment (at least, if it has no data) 523 * is out of window. 524 * 525 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 526 * describe SYN-RECV state. All the description 527 * is wrong, we cannot believe to it and should 528 * rely only on common sense and implementation 529 * experience. 530 * 531 * Enforce "SYN-ACK" according to figure 8, figure 6 532 * of RFC793, fixed by RFC1122. 533 */ 534 req->rsk_ops->rtx_syn_ack(sk, req, NULL); 535 return NULL; 536 } 537 538 /* Further reproduces section "SEGMENT ARRIVES" 539 for state SYN-RECEIVED of RFC793. 540 It is broken, however, it does not work only 541 when SYNs are crossed. 542 543 You would think that SYN crossing is impossible here, since 544 we should have a SYN_SENT socket (from connect()) on our end, 545 but this is not true if the crossed SYNs were sent to both 546 ends by a malicious third party. We must defend against this, 547 and to do that we first verify the ACK (as per RFC793, page 548 36) and reset if it is invalid. Is this a true full defense? 549 To convince ourselves, let us consider a way in which the ACK 550 test can still pass in this 'malicious crossed SYNs' case. 551 Malicious sender sends identical SYNs (and thus identical sequence 552 numbers) to both A and B: 553 554 A: gets SYN, seq=7 555 B: gets SYN, seq=7 556 557 By our good fortune, both A and B select the same initial 558 send sequence number of seven :-) 559 560 A: sends SYN|ACK, seq=7, ack_seq=8 561 B: sends SYN|ACK, seq=7, ack_seq=8 562 563 So we are now A eating this SYN|ACK, ACK test passes. So 564 does sequence test, SYN is truncated, and thus we consider 565 it a bare ACK. 566 567 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 568 bare ACK. Otherwise, we create an established connection. Both 569 ends (listening sockets) accept the new incoming connection and try 570 to talk to each other. 8-) 571 572 Note: This case is both harmless, and rare. Possibility is about the 573 same as us discovering intelligent life on another plant tomorrow. 574 575 But generally, we should (RFC lies!) to accept ACK 576 from SYNACK both here and in tcp_rcv_state_process(). 577 tcp_rcv_state_process() does not, hence, we do not too. 578 579 Note that the case is absolutely generic: 580 we cannot optimize anything here without 581 violating protocol. All the checks must be made 582 before attempt to create socket. 583 */ 584 585 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 586 * and the incoming segment acknowledges something not yet 587 * sent (the segment carries an unacceptable ACK) ... 588 * a reset is sent." 589 * 590 * Invalid ACK: reset will be sent by listening socket 591 */ 592 if ((flg & TCP_FLAG_ACK) && 593 (TCP_SKB_CB(skb)->ack_seq != tcp_rsk(req)->snt_isn + 1)) 594 return sk; 595 596 /* Also, it would be not so bad idea to check rcv_tsecr, which 597 * is essentially ACK extension and too early or too late values 598 * should cause reset in unsynchronized states. 599 */ 600 601 /* RFC793: "first check sequence number". */ 602 603 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 604 tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) { 605 /* Out of window: send ACK and drop. */ 606 if (!(flg & TCP_FLAG_RST)) 607 req->rsk_ops->send_ack(sk, skb, req); 608 if (paws_reject) 609 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 610 return NULL; 611 } 612 613 /* In sequence, PAWS is OK. */ 614 615 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1)) 616 req->ts_recent = tmp_opt.rcv_tsval; 617 618 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 619 /* Truncate SYN, it is out of window starting 620 at tcp_rsk(req)->rcv_isn + 1. */ 621 flg &= ~TCP_FLAG_SYN; 622 } 623 624 /* RFC793: "second check the RST bit" and 625 * "fourth, check the SYN bit" 626 */ 627 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 628 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 629 goto embryonic_reset; 630 } 631 632 /* ACK sequence verified above, just make sure ACK is 633 * set. If ACK not set, just silently drop the packet. 634 */ 635 if (!(flg & TCP_FLAG_ACK)) 636 return NULL; 637 638 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 639 if (req->retrans < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 640 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 641 inet_rsk(req)->acked = 1; 642 return NULL; 643 } 644 645 /* OK, ACK is valid, create big socket and 646 * feed this segment to it. It will repeat all 647 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 648 * ESTABLISHED STATE. If it will be dropped after 649 * socket is created, wait for troubles. 650 */ 651 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL); 652 if (child == NULL) 653 goto listen_overflow; 654 655 inet_csk_reqsk_queue_unlink(sk, req, prev); 656 inet_csk_reqsk_queue_removed(sk, req); 657 658 inet_csk_reqsk_queue_add(sk, req, child); 659 return child; 660 661listen_overflow: 662 if (!sysctl_tcp_abort_on_overflow) { 663 inet_rsk(req)->acked = 1; 664 return NULL; 665 } 666 667embryonic_reset: 668 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 669 if (!(flg & TCP_FLAG_RST)) 670 req->rsk_ops->send_reset(sk, skb); 671 672 inet_csk_reqsk_queue_drop(sk, req, prev); 673 return NULL; 674} 675 676/* 677 * Queue segment on the new socket if the new socket is active, 678 * otherwise we just shortcircuit this and continue with 679 * the new socket. 680 */ 681 682int tcp_child_process(struct sock *parent, struct sock *child, 683 struct sk_buff *skb) 684{ 685 int ret = 0; 686 int state = child->sk_state; 687 688 if (!sock_owned_by_user(child)) { 689 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb), 690 skb->len); 691 /* Wakeup parent, send SIGIO */ 692 if (state == TCP_SYN_RECV && child->sk_state != state) 693 parent->sk_data_ready(parent, 0); 694 } else { 695 /* Alas, it is possible again, because we do lookup 696 * in main socket hash table and lock on listening 697 * socket does not protect us more. 698 */ 699 sk_add_backlog(child, skb); 700 } 701 702 bh_unlock_sock(child); 703 sock_put(child); 704 return ret; 705} 706 707EXPORT_SYMBOL(tcp_check_req); 708EXPORT_SYMBOL(tcp_child_process); 709EXPORT_SYMBOL(tcp_create_openreq_child); 710EXPORT_SYMBOL(tcp_timewait_state_process); 711