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 int sysctl_tcp_syncookies __read_mostly = 1; 30 EXPORT_SYMBOL(sysctl_tcp_syncookies); 31 32 int sysctl_tcp_abort_on_overflow __read_mostly; 33 34 struct 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 50 EXPORT_SYMBOL_GPL(tcp_death_row); 51 52 static __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 */ 89 enum tcp_tw_status 90 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb, 91 const struct tcphdr *th) 92 { 93 struct tcp_options_received tmp_opt; 94 u8 *hash_location; 95 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 96 int paws_reject = 0; 97 98 tmp_opt.saw_tstamp = 0; 99 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) { 100 tcp_parse_options(skb, &tmp_opt, &hash_location, 0); 101 102 if (tmp_opt.saw_tstamp) { 103 tmp_opt.ts_recent = tcptw->tw_ts_recent; 104 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp; 105 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 106 } 107 } 108 109 if (tw->tw_substate == TCP_FIN_WAIT2) { 110 /* Just repeat all the checks of tcp_rcv_state_process() */ 111 112 /* Out of window, send ACK */ 113 if (paws_reject || 114 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 115 tcptw->tw_rcv_nxt, 116 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd)) 117 return TCP_TW_ACK; 118 119 if (th->rst) 120 goto kill; 121 122 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt)) 123 goto kill_with_rst; 124 125 /* Dup ACK? */ 126 if (!th->ack || 127 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) || 128 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) { 129 inet_twsk_put(tw); 130 return TCP_TW_SUCCESS; 131 } 132 133 /* New data or FIN. If new data arrive after half-duplex close, 134 * reset. 135 */ 136 if (!th->fin || 137 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) { 138 kill_with_rst: 139 inet_twsk_deschedule(tw, &tcp_death_row); 140 inet_twsk_put(tw); 141 return TCP_TW_RST; 142 } 143 144 /* FIN arrived, enter true time-wait state. */ 145 tw->tw_substate = TCP_TIME_WAIT; 146 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq; 147 if (tmp_opt.saw_tstamp) { 148 tcptw->tw_ts_recent_stamp = get_seconds(); 149 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 150 } 151 152 /* I am shamed, but failed to make it more elegant. 153 * Yes, it is direct reference to IP, which is impossible 154 * to generalize to IPv6. Taking into account that IPv6 155 * do not understand recycling in any case, it not 156 * a big problem in practice. --ANK */ 157 if (tw->tw_family == AF_INET && 158 tcp_death_row.sysctl_tw_recycle && tcptw->tw_ts_recent_stamp && 159 tcp_v4_tw_remember_stamp(tw)) 160 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout, 161 TCP_TIMEWAIT_LEN); 162 else 163 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 164 TCP_TIMEWAIT_LEN); 165 return TCP_TW_ACK; 166 } 167 168 /* 169 * Now real TIME-WAIT state. 170 * 171 * RFC 1122: 172 * "When a connection is [...] on TIME-WAIT state [...] 173 * [a TCP] MAY accept a new SYN from the remote TCP to 174 * reopen the connection directly, if it: 175 * 176 * (1) assigns its initial sequence number for the new 177 * connection to be larger than the largest sequence 178 * number it used on the previous connection incarnation, 179 * and 180 * 181 * (2) returns to TIME-WAIT state if the SYN turns out 182 * to be an old duplicate". 183 */ 184 185 if (!paws_reject && 186 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 187 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 188 /* In window segment, it may be only reset or bare ack. */ 189 190 if (th->rst) { 191 /* This is TIME_WAIT assassination, in two flavors. 192 * Oh well... nobody has a sufficient solution to this 193 * protocol bug yet. 194 */ 195 if (sysctl_tcp_rfc1337 == 0) { 196 kill: 197 inet_twsk_deschedule(tw, &tcp_death_row); 198 inet_twsk_put(tw); 199 return TCP_TW_SUCCESS; 200 } 201 } 202 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 203 TCP_TIMEWAIT_LEN); 204 205 if (tmp_opt.saw_tstamp) { 206 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 207 tcptw->tw_ts_recent_stamp = get_seconds(); 208 } 209 210 inet_twsk_put(tw); 211 return TCP_TW_SUCCESS; 212 } 213 214 /* Out of window segment. 215 216 All the segments are ACKed immediately. 217 218 The only exception is new SYN. We accept it, if it is 219 not old duplicate and we are not in danger to be killed 220 by delayed old duplicates. RFC check is that it has 221 newer sequence number works at rates <40Mbit/sec. 222 However, if paws works, it is reliable AND even more, 223 we even may relax silly seq space cutoff. 224 225 RED-PEN: we violate main RFC requirement, if this SYN will appear 226 old duplicate (i.e. we receive RST in reply to SYN-ACK), 227 we must return socket to time-wait state. It is not good, 228 but not fatal yet. 229 */ 230 231 if (th->syn && !th->rst && !th->ack && !paws_reject && 232 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 233 (tmp_opt.saw_tstamp && 234 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 235 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 236 if (isn == 0) 237 isn++; 238 TCP_SKB_CB(skb)->when = isn; 239 return TCP_TW_SYN; 240 } 241 242 if (paws_reject) 243 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 244 245 if (!th->rst) { 246 /* In this case we must reset the TIMEWAIT timer. 247 * 248 * If it is ACKless SYN it may be both old duplicate 249 * and new good SYN with random sequence number <rcv_nxt. 250 * Do not reschedule in the last case. 251 */ 252 if (paws_reject || th->ack) 253 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 254 TCP_TIMEWAIT_LEN); 255 256 /* Send ACK. Note, we do not put the bucket, 257 * it will be released by caller. 258 */ 259 return TCP_TW_ACK; 260 } 261 inet_twsk_put(tw); 262 return TCP_TW_SUCCESS; 263 } 264 265 /* 266 * Move a socket to time-wait or dead fin-wait-2 state. 267 */ 268 void tcp_time_wait(struct sock *sk, int state, int timeo) 269 { 270 struct inet_timewait_sock *tw = NULL; 271 const struct inet_connection_sock *icsk = inet_csk(sk); 272 const struct tcp_sock *tp = tcp_sk(sk); 273 int recycle_ok = 0; 274 275 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp) 276 recycle_ok = icsk->icsk_af_ops->remember_stamp(sk); 277 278 if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets) 279 tw = inet_twsk_alloc(sk, state); 280 281 if (tw != NULL) { 282 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 283 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 284 285 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 286 tcptw->tw_rcv_nxt = tp->rcv_nxt; 287 tcptw->tw_snd_nxt = tp->snd_nxt; 288 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 289 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 290 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 291 292 #if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE) 293 if (tw->tw_family == PF_INET6) { 294 struct ipv6_pinfo *np = inet6_sk(sk); 295 struct inet6_timewait_sock *tw6; 296 297 tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot); 298 tw6 = inet6_twsk((struct sock *)tw); 299 ipv6_addr_copy(&tw6->tw_v6_daddr, &np->daddr); 300 ipv6_addr_copy(&tw6->tw_v6_rcv_saddr, &np->rcv_saddr); 301 tw->tw_ipv6only = np->ipv6only; 302 } 303 #endif 304 305 #ifdef CONFIG_TCP_MD5SIG 306 /* 307 * The timewait bucket does not have the key DB from the 308 * sock structure. We just make a quick copy of the 309 * md5 key being used (if indeed we are using one) 310 * so the timewait ack generating code has the key. 311 */ 312 do { 313 struct tcp_md5sig_key *key; 314 memset(tcptw->tw_md5_key, 0, sizeof(tcptw->tw_md5_key)); 315 tcptw->tw_md5_keylen = 0; 316 key = tp->af_specific->md5_lookup(sk, sk); 317 if (key != NULL) { 318 memcpy(&tcptw->tw_md5_key, key->key, key->keylen); 319 tcptw->tw_md5_keylen = key->keylen; 320 if (tcp_alloc_md5sig_pool(sk) == NULL) 321 BUG(); 322 } 323 } while (0); 324 #endif 325 326 /* Linkage updates. */ 327 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 328 329 /* Get the TIME_WAIT timeout firing. */ 330 if (timeo < rto) 331 timeo = rto; 332 333 if (recycle_ok) { 334 tw->tw_timeout = rto; 335 } else { 336 tw->tw_timeout = TCP_TIMEWAIT_LEN; 337 if (state == TCP_TIME_WAIT) 338 timeo = TCP_TIMEWAIT_LEN; 339 } 340 341 inet_twsk_schedule(tw, &tcp_death_row, timeo, 342 TCP_TIMEWAIT_LEN); 343 inet_twsk_put(tw); 344 } else { 345 /* Sorry, if we're out of memory, just CLOSE this 346 * socket up. We've got bigger problems than 347 * non-graceful socket closings. 348 */ 349 LIMIT_NETDEBUG(KERN_INFO "TCP: time wait bucket table overflow\n"); 350 } 351 352 tcp_update_metrics(sk); 353 tcp_done(sk); 354 } 355 356 void tcp_twsk_destructor(struct sock *sk) 357 { 358 #ifdef CONFIG_TCP_MD5SIG 359 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 360 if (twsk->tw_md5_keylen) 361 tcp_free_md5sig_pool(); 362 #endif 363 } 364 365 EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 366 367 static inline void TCP_ECN_openreq_child(struct tcp_sock *tp, 368 struct request_sock *req) 369 { 370 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 371 } 372 373 /* This is not only more efficient than what we used to do, it eliminates 374 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 375 * 376 * Actually, we could lots of memory writes here. tp of listening 377 * socket contains all necessary default parameters. 378 */ 379 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb) 380 { 381 struct sock *newsk = inet_csk_clone(sk, req, GFP_ATOMIC); 382 383 if (newsk != NULL) { 384 const struct inet_request_sock *ireq = inet_rsk(req); 385 struct tcp_request_sock *treq = tcp_rsk(req); 386 struct inet_connection_sock *newicsk = inet_csk(newsk); 387 struct tcp_sock *newtp = tcp_sk(newsk); 388 struct tcp_sock *oldtp = tcp_sk(sk); 389 struct tcp_cookie_values *oldcvp = oldtp->cookie_values; 390 391 /* TCP Cookie Transactions require space for the cookie pair, 392 * as it differs for each connection. There is no need to 393 * copy any s_data_payload stored at the original socket. 394 * Failure will prevent resuming the connection. 395 * 396 * Presumed copied, in order of appearance: 397 * cookie_in_always, cookie_out_never 398 */ 399 if (oldcvp != NULL) { 400 struct tcp_cookie_values *newcvp = 401 kzalloc(sizeof(*newtp->cookie_values), 402 GFP_ATOMIC); 403 404 if (newcvp != NULL) { 405 kref_init(&newcvp->kref); 406 newcvp->cookie_desired = 407 oldcvp->cookie_desired; 408 newtp->cookie_values = newcvp; 409 } else { 410 /* Not Yet Implemented */ 411 newtp->cookie_values = NULL; 412 } 413 } 414 415 /* Now setup tcp_sock */ 416 newtp->pred_flags = 0; 417 418 newtp->rcv_wup = newtp->copied_seq = 419 newtp->rcv_nxt = treq->rcv_isn + 1; 420 421 newtp->snd_sml = newtp->snd_una = 422 newtp->snd_nxt = newtp->snd_up = 423 treq->snt_isn + 1 + tcp_s_data_size(oldtp); 424 425 tcp_prequeue_init(newtp); 426 427 tcp_init_wl(newtp, treq->rcv_isn); 428 429 newtp->srtt = 0; 430 newtp->mdev = TCP_TIMEOUT_INIT; 431 newicsk->icsk_rto = TCP_TIMEOUT_INIT; 432 433 newtp->packets_out = 0; 434 newtp->retrans_out = 0; 435 newtp->sacked_out = 0; 436 newtp->fackets_out = 0; 437 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH; 438 439 /* So many TCP implementations out there (incorrectly) count the 440 * initial SYN frame in their delayed-ACK and congestion control 441 * algorithms that we must have the following bandaid to talk 442 * efficiently to them. -DaveM 443 */ 444 newtp->snd_cwnd = 2; 445 newtp->snd_cwnd_cnt = 0; 446 newtp->bytes_acked = 0; 447 448 newtp->frto_counter = 0; 449 newtp->frto_highmark = 0; 450 451 newicsk->icsk_ca_ops = &tcp_init_congestion_ops; 452 453 tcp_set_ca_state(newsk, TCP_CA_Open); 454 tcp_init_xmit_timers(newsk); 455 skb_queue_head_init(&newtp->out_of_order_queue); 456 newtp->write_seq = newtp->pushed_seq = 457 treq->snt_isn + 1 + tcp_s_data_size(oldtp); 458 459 newtp->rx_opt.saw_tstamp = 0; 460 461 newtp->rx_opt.dsack = 0; 462 newtp->rx_opt.num_sacks = 0; 463 464 newtp->urg_data = 0; 465 466 if (sock_flag(newsk, SOCK_KEEPOPEN)) 467 inet_csk_reset_keepalive_timer(newsk, 468 keepalive_time_when(newtp)); 469 470 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 471 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 472 if (sysctl_tcp_fack) 473 tcp_enable_fack(newtp); 474 } 475 newtp->window_clamp = req->window_clamp; 476 newtp->rcv_ssthresh = req->rcv_wnd; 477 newtp->rcv_wnd = req->rcv_wnd; 478 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 479 if (newtp->rx_opt.wscale_ok) { 480 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 481 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 482 } else { 483 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 484 newtp->window_clamp = min(newtp->window_clamp, 65535U); 485 } 486 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 487 newtp->rx_opt.snd_wscale); 488 newtp->max_window = newtp->snd_wnd; 489 490 if (newtp->rx_opt.tstamp_ok) { 491 newtp->rx_opt.ts_recent = req->ts_recent; 492 newtp->rx_opt.ts_recent_stamp = get_seconds(); 493 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 494 } else { 495 newtp->rx_opt.ts_recent_stamp = 0; 496 newtp->tcp_header_len = sizeof(struct tcphdr); 497 } 498 #ifdef CONFIG_TCP_MD5SIG 499 newtp->md5sig_info = NULL; /*XXX*/ 500 if (newtp->af_specific->md5_lookup(sk, newsk)) 501 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 502 #endif 503 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) 504 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 505 newtp->rx_opt.mss_clamp = req->mss; 506 TCP_ECN_openreq_child(newtp, req); 507 508 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS); 509 } 510 return newsk; 511 } 512 513 /* 514 * Process an incoming packet for SYN_RECV sockets represented 515 * as a request_sock. 516 */ 517 518 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 519 struct request_sock *req, 520 struct request_sock **prev) 521 { 522 struct tcp_options_received tmp_opt; 523 u8 *hash_location; 524 struct sock *child; 525 const struct tcphdr *th = tcp_hdr(skb); 526 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 527 int paws_reject = 0; 528 529 tmp_opt.saw_tstamp = 0; 530 if (th->doff > (sizeof(struct tcphdr)>>2)) { 531 tcp_parse_options(skb, &tmp_opt, &hash_location, 0); 532 533 if (tmp_opt.saw_tstamp) { 534 tmp_opt.ts_recent = req->ts_recent; 535 /* We do not store true stamp, but it is not required, 536 * it can be estimated (approximately) 537 * from another data. 538 */ 539 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans); 540 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 541 } 542 } 543 544 /* Check for pure retransmitted SYN. */ 545 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 546 flg == TCP_FLAG_SYN && 547 !paws_reject) { 548 /* 549 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 550 * this case on figure 6 and figure 8, but formal 551 * protocol description says NOTHING. 552 * To be more exact, it says that we should send ACK, 553 * because this segment (at least, if it has no data) 554 * is out of window. 555 * 556 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 557 * describe SYN-RECV state. All the description 558 * is wrong, we cannot believe to it and should 559 * rely only on common sense and implementation 560 * experience. 561 * 562 * Enforce "SYN-ACK" according to figure 8, figure 6 563 * of RFC793, fixed by RFC1122. 564 */ 565 req->rsk_ops->rtx_syn_ack(sk, req, NULL); 566 return NULL; 567 } 568 569 /* Further reproduces section "SEGMENT ARRIVES" 570 for state SYN-RECEIVED of RFC793. 571 It is broken, however, it does not work only 572 when SYNs are crossed. 573 574 You would think that SYN crossing is impossible here, since 575 we should have a SYN_SENT socket (from connect()) on our end, 576 but this is not true if the crossed SYNs were sent to both 577 ends by a malicious third party. We must defend against this, 578 and to do that we first verify the ACK (as per RFC793, page 579 36) and reset if it is invalid. Is this a true full defense? 580 To convince ourselves, let us consider a way in which the ACK 581 test can still pass in this 'malicious crossed SYNs' case. 582 Malicious sender sends identical SYNs (and thus identical sequence 583 numbers) to both A and B: 584 585 A: gets SYN, seq=7 586 B: gets SYN, seq=7 587 588 By our good fortune, both A and B select the same initial 589 send sequence number of seven :-) 590 591 A: sends SYN|ACK, seq=7, ack_seq=8 592 B: sends SYN|ACK, seq=7, ack_seq=8 593 594 So we are now A eating this SYN|ACK, ACK test passes. So 595 does sequence test, SYN is truncated, and thus we consider 596 it a bare ACK. 597 598 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 599 bare ACK. Otherwise, we create an established connection. Both 600 ends (listening sockets) accept the new incoming connection and try 601 to talk to each other. 8-) 602 603 Note: This case is both harmless, and rare. Possibility is about the 604 same as us discovering intelligent life on another plant tomorrow. 605 606 But generally, we should (RFC lies!) to accept ACK 607 from SYNACK both here and in tcp_rcv_state_process(). 608 tcp_rcv_state_process() does not, hence, we do not too. 609 610 Note that the case is absolutely generic: 611 we cannot optimize anything here without 612 violating protocol. All the checks must be made 613 before attempt to create socket. 614 */ 615 616 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 617 * and the incoming segment acknowledges something not yet 618 * sent (the segment carries an unacceptable ACK) ... 619 * a reset is sent." 620 * 621 * Invalid ACK: reset will be sent by listening socket 622 */ 623 if ((flg & TCP_FLAG_ACK) && 624 (TCP_SKB_CB(skb)->ack_seq != 625 tcp_rsk(req)->snt_isn + 1 + tcp_s_data_size(tcp_sk(sk)))) 626 return sk; 627 628 /* Also, it would be not so bad idea to check rcv_tsecr, which 629 * is essentially ACK extension and too early or too late values 630 * should cause reset in unsynchronized states. 631 */ 632 633 /* RFC793: "first check sequence number". */ 634 635 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 636 tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) { 637 /* Out of window: send ACK and drop. */ 638 if (!(flg & TCP_FLAG_RST)) 639 req->rsk_ops->send_ack(sk, skb, req); 640 if (paws_reject) 641 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 642 return NULL; 643 } 644 645 /* In sequence, PAWS is OK. */ 646 647 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1)) 648 req->ts_recent = tmp_opt.rcv_tsval; 649 650 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 651 /* Truncate SYN, it is out of window starting 652 at tcp_rsk(req)->rcv_isn + 1. */ 653 flg &= ~TCP_FLAG_SYN; 654 } 655 656 /* RFC793: "second check the RST bit" and 657 * "fourth, check the SYN bit" 658 */ 659 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 660 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 661 goto embryonic_reset; 662 } 663 664 /* ACK sequence verified above, just make sure ACK is 665 * set. If ACK not set, just silently drop the packet. 666 */ 667 if (!(flg & TCP_FLAG_ACK)) 668 return NULL; 669 670 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 671 if (req->retrans < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 672 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 673 inet_rsk(req)->acked = 1; 674 return NULL; 675 } 676 677 /* OK, ACK is valid, create big socket and 678 * feed this segment to it. It will repeat all 679 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 680 * ESTABLISHED STATE. If it will be dropped after 681 * socket is created, wait for troubles. 682 */ 683 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL); 684 if (child == NULL) 685 goto listen_overflow; 686 687 inet_csk_reqsk_queue_unlink(sk, req, prev); 688 inet_csk_reqsk_queue_removed(sk, req); 689 690 inet_csk_reqsk_queue_add(sk, req, child); 691 return child; 692 693 listen_overflow: 694 if (!sysctl_tcp_abort_on_overflow) { 695 inet_rsk(req)->acked = 1; 696 return NULL; 697 } 698 699 embryonic_reset: 700 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 701 if (!(flg & TCP_FLAG_RST)) 702 req->rsk_ops->send_reset(sk, skb); 703 704 inet_csk_reqsk_queue_drop(sk, req, prev); 705 return NULL; 706 } 707 708 /* 709 * Queue segment on the new socket if the new socket is active, 710 * otherwise we just shortcircuit this and continue with 711 * the new socket. 712 */ 713 714 int tcp_child_process(struct sock *parent, struct sock *child, 715 struct sk_buff *skb) 716 { 717 int ret = 0; 718 int state = child->sk_state; 719 720 if (!sock_owned_by_user(child)) { 721 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb), 722 skb->len); 723 /* Wakeup parent, send SIGIO */ 724 if (state == TCP_SYN_RECV && child->sk_state != state) 725 parent->sk_data_ready(parent, 0); 726 } else { 727 /* Alas, it is possible again, because we do lookup 728 * in main socket hash table and lock on listening 729 * socket does not protect us more. 730 */ 731 sk_add_backlog(child, skb); 732 } 733 734 bh_unlock_sock(child); 735 sock_put(child); 736 return ret; 737 } 738 739 EXPORT_SYMBOL(tcp_check_req); 740 EXPORT_SYMBOL(tcp_child_process); 741 EXPORT_SYMBOL(tcp_create_openreq_child); 742 EXPORT_SYMBOL(tcp_timewait_state_process); 743