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/slab.h> 24 #include <linux/sysctl.h> 25 #include <linux/workqueue.h> 26 #include <net/tcp.h> 27 #include <net/inet_common.h> 28 #include <net/xfrm.h> 29 30 int sysctl_tcp_syncookies __read_mostly = 1; 31 EXPORT_SYMBOL(sysctl_tcp_syncookies); 32 33 int sysctl_tcp_abort_on_overflow __read_mostly; 34 35 struct inet_timewait_death_row tcp_death_row = { 36 .sysctl_max_tw_buckets = NR_FILE * 2, 37 .period = TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS, 38 .death_lock = __SPIN_LOCK_UNLOCKED(tcp_death_row.death_lock), 39 .hashinfo = &tcp_hashinfo, 40 .tw_timer = TIMER_INITIALIZER(inet_twdr_hangman, 0, 41 (unsigned long)&tcp_death_row), 42 .twkill_work = __WORK_INITIALIZER(tcp_death_row.twkill_work, 43 inet_twdr_twkill_work), 44 /* Short-time timewait calendar */ 45 46 .twcal_hand = -1, 47 .twcal_timer = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0, 48 (unsigned long)&tcp_death_row), 49 }; 50 EXPORT_SYMBOL_GPL(tcp_death_row); 51 52 static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win) 53 { 54 if (seq == s_win) 55 return true; 56 if (after(end_seq, s_win) && before(seq, e_win)) 57 return true; 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 * We don't need to initialize tmp_out.sack_ok as we don't use the results 90 */ 91 enum tcp_tw_status 92 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb, 93 const struct tcphdr *th) 94 { 95 struct tcp_options_received tmp_opt; 96 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 97 bool paws_reject = false; 98 99 tmp_opt.saw_tstamp = 0; 100 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) { 101 tcp_parse_options(skb, &tmp_opt, 0, NULL); 102 103 if (tmp_opt.saw_tstamp) { 104 tmp_opt.rcv_tsecr -= tcptw->tw_ts_offset; 105 tmp_opt.ts_recent = tcptw->tw_ts_recent; 106 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp; 107 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 108 } 109 } 110 111 if (tw->tw_substate == TCP_FIN_WAIT2) { 112 /* Just repeat all the checks of tcp_rcv_state_process() */ 113 114 /* Out of window, send ACK */ 115 if (paws_reject || 116 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 117 tcptw->tw_rcv_nxt, 118 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd)) 119 return TCP_TW_ACK; 120 121 if (th->rst) 122 goto kill; 123 124 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt)) 125 goto kill_with_rst; 126 127 /* Dup ACK? */ 128 if (!th->ack || 129 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) || 130 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) { 131 inet_twsk_put(tw); 132 return TCP_TW_SUCCESS; 133 } 134 135 /* New data or FIN. If new data arrive after half-duplex close, 136 * reset. 137 */ 138 if (!th->fin || 139 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) { 140 kill_with_rst: 141 inet_twsk_deschedule(tw, &tcp_death_row); 142 inet_twsk_put(tw); 143 return TCP_TW_RST; 144 } 145 146 /* FIN arrived, enter true time-wait state. */ 147 tw->tw_substate = TCP_TIME_WAIT; 148 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq; 149 if (tmp_opt.saw_tstamp) { 150 tcptw->tw_ts_recent_stamp = get_seconds(); 151 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 152 } 153 154 if (tcp_death_row.sysctl_tw_recycle && 155 tcptw->tw_ts_recent_stamp && 156 tcp_tw_remember_stamp(tw)) 157 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout, 158 TCP_TIMEWAIT_LEN); 159 else 160 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 161 TCP_TIMEWAIT_LEN); 162 return TCP_TW_ACK; 163 } 164 165 /* 166 * Now real TIME-WAIT state. 167 * 168 * RFC 1122: 169 * "When a connection is [...] on TIME-WAIT state [...] 170 * [a TCP] MAY accept a new SYN from the remote TCP to 171 * reopen the connection directly, if it: 172 * 173 * (1) assigns its initial sequence number for the new 174 * connection to be larger than the largest sequence 175 * number it used on the previous connection incarnation, 176 * and 177 * 178 * (2) returns to TIME-WAIT state if the SYN turns out 179 * to be an old duplicate". 180 */ 181 182 if (!paws_reject && 183 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 184 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 185 /* In window segment, it may be only reset or bare ack. */ 186 187 if (th->rst) { 188 /* This is TIME_WAIT assassination, in two flavors. 189 * Oh well... nobody has a sufficient solution to this 190 * protocol bug yet. 191 */ 192 if (sysctl_tcp_rfc1337 == 0) { 193 kill: 194 inet_twsk_deschedule(tw, &tcp_death_row); 195 inet_twsk_put(tw); 196 return TCP_TW_SUCCESS; 197 } 198 } 199 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 200 TCP_TIMEWAIT_LEN); 201 202 if (tmp_opt.saw_tstamp) { 203 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 204 tcptw->tw_ts_recent_stamp = get_seconds(); 205 } 206 207 inet_twsk_put(tw); 208 return TCP_TW_SUCCESS; 209 } 210 211 /* Out of window segment. 212 213 All the segments are ACKed immediately. 214 215 The only exception is new SYN. We accept it, if it is 216 not old duplicate and we are not in danger to be killed 217 by delayed old duplicates. RFC check is that it has 218 newer sequence number works at rates <40Mbit/sec. 219 However, if paws works, it is reliable AND even more, 220 we even may relax silly seq space cutoff. 221 222 RED-PEN: we violate main RFC requirement, if this SYN will appear 223 old duplicate (i.e. we receive RST in reply to SYN-ACK), 224 we must return socket to time-wait state. It is not good, 225 but not fatal yet. 226 */ 227 228 if (th->syn && !th->rst && !th->ack && !paws_reject && 229 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 230 (tmp_opt.saw_tstamp && 231 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 232 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 233 if (isn == 0) 234 isn++; 235 TCP_SKB_CB(skb)->when = isn; 236 return TCP_TW_SYN; 237 } 238 239 if (paws_reject) 240 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 241 242 if (!th->rst) { 243 /* In this case we must reset the TIMEWAIT timer. 244 * 245 * If it is ACKless SYN it may be both old duplicate 246 * and new good SYN with random sequence number <rcv_nxt. 247 * Do not reschedule in the last case. 248 */ 249 if (paws_reject || th->ack) 250 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 251 TCP_TIMEWAIT_LEN); 252 253 /* Send ACK. Note, we do not put the bucket, 254 * it will be released by caller. 255 */ 256 return TCP_TW_ACK; 257 } 258 inet_twsk_put(tw); 259 return TCP_TW_SUCCESS; 260 } 261 EXPORT_SYMBOL(tcp_timewait_state_process); 262 263 /* 264 * Move a socket to time-wait or dead fin-wait-2 state. 265 */ 266 void tcp_time_wait(struct sock *sk, int state, int timeo) 267 { 268 struct inet_timewait_sock *tw = NULL; 269 const struct inet_connection_sock *icsk = inet_csk(sk); 270 const struct tcp_sock *tp = tcp_sk(sk); 271 bool recycle_ok = false; 272 273 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp) 274 recycle_ok = tcp_remember_stamp(sk); 275 276 if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets) 277 tw = inet_twsk_alloc(sk, state); 278 279 if (tw != NULL) { 280 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 281 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 282 struct inet_sock *inet = inet_sk(sk); 283 284 tw->tw_transparent = inet->transparent; 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 tcptw->tw_ts_offset = tp->tsoffset; 292 293 #if IS_ENABLED(CONFIG_IPV6) 294 if (tw->tw_family == PF_INET6) { 295 struct ipv6_pinfo *np = inet6_sk(sk); 296 297 tw->tw_v6_daddr = sk->sk_v6_daddr; 298 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr; 299 tw->tw_tclass = np->tclass; 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 tcptw->tw_md5_key = NULL; 314 key = tp->af_specific->md5_lookup(sk, sk); 315 if (key != NULL) { 316 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC); 317 if (tcptw->tw_md5_key && !tcp_alloc_md5sig_pool()) 318 BUG(); 319 } 320 } while (0); 321 #endif 322 323 /* Linkage updates. */ 324 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 325 326 /* Get the TIME_WAIT timeout firing. */ 327 if (timeo < rto) 328 timeo = rto; 329 330 if (recycle_ok) { 331 tw->tw_timeout = rto; 332 } else { 333 tw->tw_timeout = TCP_TIMEWAIT_LEN; 334 if (state == TCP_TIME_WAIT) 335 timeo = TCP_TIMEWAIT_LEN; 336 } 337 338 inet_twsk_schedule(tw, &tcp_death_row, timeo, 339 TCP_TIMEWAIT_LEN); 340 inet_twsk_put(tw); 341 } else { 342 /* Sorry, if we're out of memory, just CLOSE this 343 * socket up. We've got bigger problems than 344 * non-graceful socket closings. 345 */ 346 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW); 347 } 348 349 tcp_update_metrics(sk); 350 tcp_done(sk); 351 } 352 353 void tcp_twsk_destructor(struct sock *sk) 354 { 355 #ifdef CONFIG_TCP_MD5SIG 356 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 357 358 if (twsk->tw_md5_key) 359 kfree_rcu(twsk->tw_md5_key, rcu); 360 #endif 361 } 362 EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 363 364 static inline void TCP_ECN_openreq_child(struct tcp_sock *tp, 365 struct request_sock *req) 366 { 367 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 368 } 369 370 /* This is not only more efficient than what we used to do, it eliminates 371 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 372 * 373 * Actually, we could lots of memory writes here. tp of listening 374 * socket contains all necessary default parameters. 375 */ 376 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb) 377 { 378 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC); 379 380 if (newsk != NULL) { 381 const struct inet_request_sock *ireq = inet_rsk(req); 382 struct tcp_request_sock *treq = tcp_rsk(req); 383 struct inet_connection_sock *newicsk = inet_csk(newsk); 384 struct tcp_sock *newtp = tcp_sk(newsk); 385 386 /* Now setup tcp_sock */ 387 newtp->pred_flags = 0; 388 389 newtp->rcv_wup = newtp->copied_seq = 390 newtp->rcv_nxt = treq->rcv_isn + 1; 391 392 newtp->snd_sml = newtp->snd_una = 393 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1; 394 395 tcp_prequeue_init(newtp); 396 INIT_LIST_HEAD(&newtp->tsq_node); 397 398 tcp_init_wl(newtp, treq->rcv_isn); 399 400 newtp->srtt = 0; 401 newtp->mdev = TCP_TIMEOUT_INIT; 402 newicsk->icsk_rto = TCP_TIMEOUT_INIT; 403 404 newtp->packets_out = 0; 405 newtp->retrans_out = 0; 406 newtp->sacked_out = 0; 407 newtp->fackets_out = 0; 408 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH; 409 tcp_enable_early_retrans(newtp); 410 newtp->tlp_high_seq = 0; 411 newtp->lsndtime = treq->snt_synack; 412 newtp->total_retrans = req->num_retrans; 413 414 /* So many TCP implementations out there (incorrectly) count the 415 * initial SYN frame in their delayed-ACK and congestion control 416 * algorithms that we must have the following bandaid to talk 417 * efficiently to them. -DaveM 418 */ 419 newtp->snd_cwnd = TCP_INIT_CWND; 420 newtp->snd_cwnd_cnt = 0; 421 422 if (newicsk->icsk_ca_ops != &tcp_init_congestion_ops && 423 !try_module_get(newicsk->icsk_ca_ops->owner)) 424 newicsk->icsk_ca_ops = &tcp_init_congestion_ops; 425 426 tcp_set_ca_state(newsk, TCP_CA_Open); 427 tcp_init_xmit_timers(newsk); 428 skb_queue_head_init(&newtp->out_of_order_queue); 429 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1; 430 431 newtp->rx_opt.saw_tstamp = 0; 432 433 newtp->rx_opt.dsack = 0; 434 newtp->rx_opt.num_sacks = 0; 435 436 newtp->urg_data = 0; 437 438 if (sock_flag(newsk, SOCK_KEEPOPEN)) 439 inet_csk_reset_keepalive_timer(newsk, 440 keepalive_time_when(newtp)); 441 442 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 443 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 444 if (sysctl_tcp_fack) 445 tcp_enable_fack(newtp); 446 } 447 newtp->window_clamp = req->window_clamp; 448 newtp->rcv_ssthresh = req->rcv_wnd; 449 newtp->rcv_wnd = req->rcv_wnd; 450 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 451 if (newtp->rx_opt.wscale_ok) { 452 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 453 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 454 } else { 455 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 456 newtp->window_clamp = min(newtp->window_clamp, 65535U); 457 } 458 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 459 newtp->rx_opt.snd_wscale); 460 newtp->max_window = newtp->snd_wnd; 461 462 if (newtp->rx_opt.tstamp_ok) { 463 newtp->rx_opt.ts_recent = req->ts_recent; 464 newtp->rx_opt.ts_recent_stamp = get_seconds(); 465 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 466 } else { 467 newtp->rx_opt.ts_recent_stamp = 0; 468 newtp->tcp_header_len = sizeof(struct tcphdr); 469 } 470 newtp->tsoffset = 0; 471 #ifdef CONFIG_TCP_MD5SIG 472 newtp->md5sig_info = NULL; /*XXX*/ 473 if (newtp->af_specific->md5_lookup(sk, newsk)) 474 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 475 #endif 476 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) 477 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 478 newtp->rx_opt.mss_clamp = req->mss; 479 TCP_ECN_openreq_child(newtp, req); 480 newtp->fastopen_rsk = NULL; 481 newtp->syn_data_acked = 0; 482 483 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS); 484 } 485 return newsk; 486 } 487 EXPORT_SYMBOL(tcp_create_openreq_child); 488 489 /* 490 * Process an incoming packet for SYN_RECV sockets represented as a 491 * request_sock. Normally sk is the listener socket but for TFO it 492 * points to the child socket. 493 * 494 * XXX (TFO) - The current impl contains a special check for ack 495 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better? 496 * 497 * We don't need to initialize tmp_opt.sack_ok as we don't use the results 498 */ 499 500 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 501 struct request_sock *req, 502 struct request_sock **prev, 503 bool fastopen) 504 { 505 struct tcp_options_received tmp_opt; 506 struct sock *child; 507 const struct tcphdr *th = tcp_hdr(skb); 508 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 509 bool paws_reject = false; 510 511 BUG_ON(fastopen == (sk->sk_state == TCP_LISTEN)); 512 513 tmp_opt.saw_tstamp = 0; 514 if (th->doff > (sizeof(struct tcphdr)>>2)) { 515 tcp_parse_options(skb, &tmp_opt, 0, NULL); 516 517 if (tmp_opt.saw_tstamp) { 518 tmp_opt.ts_recent = req->ts_recent; 519 /* We do not store true stamp, but it is not required, 520 * it can be estimated (approximately) 521 * from another data. 522 */ 523 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout); 524 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 525 } 526 } 527 528 /* Check for pure retransmitted SYN. */ 529 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 530 flg == TCP_FLAG_SYN && 531 !paws_reject) { 532 /* 533 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 534 * this case on figure 6 and figure 8, but formal 535 * protocol description says NOTHING. 536 * To be more exact, it says that we should send ACK, 537 * because this segment (at least, if it has no data) 538 * is out of window. 539 * 540 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 541 * describe SYN-RECV state. All the description 542 * is wrong, we cannot believe to it and should 543 * rely only on common sense and implementation 544 * experience. 545 * 546 * Enforce "SYN-ACK" according to figure 8, figure 6 547 * of RFC793, fixed by RFC1122. 548 * 549 * Note that even if there is new data in the SYN packet 550 * they will be thrown away too. 551 * 552 * Reset timer after retransmitting SYNACK, similar to 553 * the idea of fast retransmit in recovery. 554 */ 555 if (!inet_rtx_syn_ack(sk, req)) 556 req->expires = min(TCP_TIMEOUT_INIT << req->num_timeout, 557 TCP_RTO_MAX) + jiffies; 558 return NULL; 559 } 560 561 /* Further reproduces section "SEGMENT ARRIVES" 562 for state SYN-RECEIVED of RFC793. 563 It is broken, however, it does not work only 564 when SYNs are crossed. 565 566 You would think that SYN crossing is impossible here, since 567 we should have a SYN_SENT socket (from connect()) on our end, 568 but this is not true if the crossed SYNs were sent to both 569 ends by a malicious third party. We must defend against this, 570 and to do that we first verify the ACK (as per RFC793, page 571 36) and reset if it is invalid. Is this a true full defense? 572 To convince ourselves, let us consider a way in which the ACK 573 test can still pass in this 'malicious crossed SYNs' case. 574 Malicious sender sends identical SYNs (and thus identical sequence 575 numbers) to both A and B: 576 577 A: gets SYN, seq=7 578 B: gets SYN, seq=7 579 580 By our good fortune, both A and B select the same initial 581 send sequence number of seven :-) 582 583 A: sends SYN|ACK, seq=7, ack_seq=8 584 B: sends SYN|ACK, seq=7, ack_seq=8 585 586 So we are now A eating this SYN|ACK, ACK test passes. So 587 does sequence test, SYN is truncated, and thus we consider 588 it a bare ACK. 589 590 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 591 bare ACK. Otherwise, we create an established connection. Both 592 ends (listening sockets) accept the new incoming connection and try 593 to talk to each other. 8-) 594 595 Note: This case is both harmless, and rare. Possibility is about the 596 same as us discovering intelligent life on another plant tomorrow. 597 598 But generally, we should (RFC lies!) to accept ACK 599 from SYNACK both here and in tcp_rcv_state_process(). 600 tcp_rcv_state_process() does not, hence, we do not too. 601 602 Note that the case is absolutely generic: 603 we cannot optimize anything here without 604 violating protocol. All the checks must be made 605 before attempt to create socket. 606 */ 607 608 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 609 * and the incoming segment acknowledges something not yet 610 * sent (the segment carries an unacceptable ACK) ... 611 * a reset is sent." 612 * 613 * Invalid ACK: reset will be sent by listening socket. 614 * Note that the ACK validity check for a Fast Open socket is done 615 * elsewhere and is checked directly against the child socket rather 616 * than req because user data may have been sent out. 617 */ 618 if ((flg & TCP_FLAG_ACK) && !fastopen && 619 (TCP_SKB_CB(skb)->ack_seq != 620 tcp_rsk(req)->snt_isn + 1)) 621 return sk; 622 623 /* Also, it would be not so bad idea to check rcv_tsecr, which 624 * is essentially ACK extension and too early or too late values 625 * should cause reset in unsynchronized states. 626 */ 627 628 /* RFC793: "first check sequence number". */ 629 630 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 631 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rcv_wnd)) { 632 /* Out of window: send ACK and drop. */ 633 if (!(flg & TCP_FLAG_RST)) 634 req->rsk_ops->send_ack(sk, skb, req); 635 if (paws_reject) 636 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 637 return NULL; 638 } 639 640 /* In sequence, PAWS is OK. */ 641 642 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt)) 643 req->ts_recent = tmp_opt.rcv_tsval; 644 645 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 646 /* Truncate SYN, it is out of window starting 647 at tcp_rsk(req)->rcv_isn + 1. */ 648 flg &= ~TCP_FLAG_SYN; 649 } 650 651 /* RFC793: "second check the RST bit" and 652 * "fourth, check the SYN bit" 653 */ 654 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 655 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 656 goto embryonic_reset; 657 } 658 659 /* ACK sequence verified above, just make sure ACK is 660 * set. If ACK not set, just silently drop the packet. 661 * 662 * XXX (TFO) - if we ever allow "data after SYN", the 663 * following check needs to be removed. 664 */ 665 if (!(flg & TCP_FLAG_ACK)) 666 return NULL; 667 668 /* For Fast Open no more processing is needed (sk is the 669 * child socket). 670 */ 671 if (fastopen) 672 return sk; 673 674 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 675 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 676 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 677 inet_rsk(req)->acked = 1; 678 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP); 679 return NULL; 680 } 681 682 /* OK, ACK is valid, create big socket and 683 * feed this segment to it. It will repeat all 684 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 685 * ESTABLISHED STATE. If it will be dropped after 686 * socket is created, wait for troubles. 687 */ 688 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL); 689 if (child == NULL) 690 goto listen_overflow; 691 692 inet_csk_reqsk_queue_unlink(sk, req, prev); 693 inet_csk_reqsk_queue_removed(sk, req); 694 695 inet_csk_reqsk_queue_add(sk, req, child); 696 return child; 697 698 listen_overflow: 699 if (!sysctl_tcp_abort_on_overflow) { 700 inet_rsk(req)->acked = 1; 701 return NULL; 702 } 703 704 embryonic_reset: 705 if (!(flg & TCP_FLAG_RST)) { 706 /* Received a bad SYN pkt - for TFO We try not to reset 707 * the local connection unless it's really necessary to 708 * avoid becoming vulnerable to outside attack aiming at 709 * resetting legit local connections. 710 */ 711 req->rsk_ops->send_reset(sk, skb); 712 } else if (fastopen) { /* received a valid RST pkt */ 713 reqsk_fastopen_remove(sk, req, true); 714 tcp_reset(sk); 715 } 716 if (!fastopen) { 717 inet_csk_reqsk_queue_drop(sk, req, prev); 718 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 719 } 720 return NULL; 721 } 722 EXPORT_SYMBOL(tcp_check_req); 723 724 /* 725 * Queue segment on the new socket if the new socket is active, 726 * otherwise we just shortcircuit this and continue with 727 * the new socket. 728 * 729 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV 730 * when entering. But other states are possible due to a race condition 731 * where after __inet_lookup_established() fails but before the listener 732 * locked is obtained, other packets cause the same connection to 733 * be created. 734 */ 735 736 int tcp_child_process(struct sock *parent, struct sock *child, 737 struct sk_buff *skb) 738 { 739 int ret = 0; 740 int state = child->sk_state; 741 742 if (!sock_owned_by_user(child)) { 743 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb), 744 skb->len); 745 /* Wakeup parent, send SIGIO */ 746 if (state == TCP_SYN_RECV && child->sk_state != state) 747 parent->sk_data_ready(parent, 0); 748 } else { 749 /* Alas, it is possible again, because we do lookup 750 * in main socket hash table and lock on listening 751 * socket does not protect us more. 752 */ 753 __sk_add_backlog(child, skb); 754 } 755 756 bh_unlock_sock(child); 757 sock_put(child); 758 return ret; 759 } 760 EXPORT_SYMBOL(tcp_child_process); 761