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 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 const u8 *hash_location; 95 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 96 bool paws_reject = false; 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, NULL); 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 if (tcp_death_row.sysctl_tw_recycle && 153 tcptw->tw_ts_recent_stamp && 154 tcp_tw_remember_stamp(tw)) 155 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout, 156 TCP_TIMEWAIT_LEN); 157 else 158 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 159 TCP_TIMEWAIT_LEN); 160 return TCP_TW_ACK; 161 } 162 163 /* 164 * Now real TIME-WAIT state. 165 * 166 * RFC 1122: 167 * "When a connection is [...] on TIME-WAIT state [...] 168 * [a TCP] MAY accept a new SYN from the remote TCP to 169 * reopen the connection directly, if it: 170 * 171 * (1) assigns its initial sequence number for the new 172 * connection to be larger than the largest sequence 173 * number it used on the previous connection incarnation, 174 * and 175 * 176 * (2) returns to TIME-WAIT state if the SYN turns out 177 * to be an old duplicate". 178 */ 179 180 if (!paws_reject && 181 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 182 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 183 /* In window segment, it may be only reset or bare ack. */ 184 185 if (th->rst) { 186 /* This is TIME_WAIT assassination, in two flavors. 187 * Oh well... nobody has a sufficient solution to this 188 * protocol bug yet. 189 */ 190 if (sysctl_tcp_rfc1337 == 0) { 191 kill: 192 inet_twsk_deschedule(tw, &tcp_death_row); 193 inet_twsk_put(tw); 194 return TCP_TW_SUCCESS; 195 } 196 } 197 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 198 TCP_TIMEWAIT_LEN); 199 200 if (tmp_opt.saw_tstamp) { 201 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 202 tcptw->tw_ts_recent_stamp = get_seconds(); 203 } 204 205 inet_twsk_put(tw); 206 return TCP_TW_SUCCESS; 207 } 208 209 /* Out of window segment. 210 211 All the segments are ACKed immediately. 212 213 The only exception is new SYN. We accept it, if it is 214 not old duplicate and we are not in danger to be killed 215 by delayed old duplicates. RFC check is that it has 216 newer sequence number works at rates <40Mbit/sec. 217 However, if paws works, it is reliable AND even more, 218 we even may relax silly seq space cutoff. 219 220 RED-PEN: we violate main RFC requirement, if this SYN will appear 221 old duplicate (i.e. we receive RST in reply to SYN-ACK), 222 we must return socket to time-wait state. It is not good, 223 but not fatal yet. 224 */ 225 226 if (th->syn && !th->rst && !th->ack && !paws_reject && 227 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 228 (tmp_opt.saw_tstamp && 229 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 230 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 231 if (isn == 0) 232 isn++; 233 TCP_SKB_CB(skb)->when = isn; 234 return TCP_TW_SYN; 235 } 236 237 if (paws_reject) 238 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 239 240 if (!th->rst) { 241 /* In this case we must reset the TIMEWAIT timer. 242 * 243 * If it is ACKless SYN it may be both old duplicate 244 * and new good SYN with random sequence number <rcv_nxt. 245 * Do not reschedule in the last case. 246 */ 247 if (paws_reject || th->ack) 248 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 249 TCP_TIMEWAIT_LEN); 250 251 /* Send ACK. Note, we do not put the bucket, 252 * it will be released by caller. 253 */ 254 return TCP_TW_ACK; 255 } 256 inet_twsk_put(tw); 257 return TCP_TW_SUCCESS; 258 } 259 EXPORT_SYMBOL(tcp_timewait_state_process); 260 261 /* 262 * Move a socket to time-wait or dead fin-wait-2 state. 263 */ 264 void tcp_time_wait(struct sock *sk, int state, int timeo) 265 { 266 struct inet_timewait_sock *tw = NULL; 267 const struct inet_connection_sock *icsk = inet_csk(sk); 268 const struct tcp_sock *tp = tcp_sk(sk); 269 bool recycle_ok = false; 270 271 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp) 272 recycle_ok = tcp_remember_stamp(sk); 273 274 if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets) 275 tw = inet_twsk_alloc(sk, state); 276 277 if (tw != NULL) { 278 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 279 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 280 struct inet_sock *inet = inet_sk(sk); 281 282 tw->tw_transparent = inet->transparent; 283 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 284 tcptw->tw_rcv_nxt = tp->rcv_nxt; 285 tcptw->tw_snd_nxt = tp->snd_nxt; 286 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 287 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 288 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 289 290 #if IS_ENABLED(CONFIG_IPV6) 291 if (tw->tw_family == PF_INET6) { 292 struct ipv6_pinfo *np = inet6_sk(sk); 293 struct inet6_timewait_sock *tw6; 294 295 tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot); 296 tw6 = inet6_twsk((struct sock *)tw); 297 tw6->tw_v6_daddr = np->daddr; 298 tw6->tw_v6_rcv_saddr = np->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(sk) == NULL) 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 tcp_free_md5sig_pool(); 360 kfree_rcu(twsk->tw_md5_key, rcu); 361 } 362 #endif 363 } 364 EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 365 366 static 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 */ 378 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb) 379 { 380 struct sock *newsk = inet_csk_clone_lock(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 = tcp_sk(newsk); 387 struct tcp_sock *oldtp = tcp_sk(sk); 388 struct tcp_cookie_values *oldcvp = oldtp->cookie_values; 389 390 /* TCP Cookie Transactions require space for the cookie pair, 391 * as it differs for each connection. There is no need to 392 * copy any s_data_payload stored at the original socket. 393 * Failure will prevent resuming the connection. 394 * 395 * Presumed copied, in order of appearance: 396 * cookie_in_always, cookie_out_never 397 */ 398 if (oldcvp != NULL) { 399 struct tcp_cookie_values *newcvp = 400 kzalloc(sizeof(*newtp->cookie_values), 401 GFP_ATOMIC); 402 403 if (newcvp != NULL) { 404 kref_init(&newcvp->kref); 405 newcvp->cookie_desired = 406 oldcvp->cookie_desired; 407 newtp->cookie_values = newcvp; 408 } else { 409 /* Not Yet Implemented */ 410 newtp->cookie_values = NULL; 411 } 412 } 413 414 /* Now setup tcp_sock */ 415 newtp->pred_flags = 0; 416 417 newtp->rcv_wup = newtp->copied_seq = 418 newtp->rcv_nxt = treq->rcv_isn + 1; 419 420 newtp->snd_sml = newtp->snd_una = 421 newtp->snd_nxt = newtp->snd_up = 422 treq->snt_isn + 1 + tcp_s_data_size(oldtp); 423 424 tcp_prequeue_init(newtp); 425 INIT_LIST_HEAD(&newtp->tsq_node); 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 tcp_enable_early_retrans(newtp); 439 440 /* So many TCP implementations out there (incorrectly) count the 441 * initial SYN frame in their delayed-ACK and congestion control 442 * algorithms that we must have the following bandaid to talk 443 * efficiently to them. -DaveM 444 */ 445 newtp->snd_cwnd = TCP_INIT_CWND; 446 newtp->snd_cwnd_cnt = 0; 447 newtp->bytes_acked = 0; 448 449 newtp->frto_counter = 0; 450 newtp->frto_highmark = 0; 451 452 if (newicsk->icsk_ca_ops != &tcp_init_congestion_ops && 453 !try_module_get(newicsk->icsk_ca_ops->owner)) 454 newicsk->icsk_ca_ops = &tcp_init_congestion_ops; 455 456 tcp_set_ca_state(newsk, TCP_CA_Open); 457 tcp_init_xmit_timers(newsk); 458 skb_queue_head_init(&newtp->out_of_order_queue); 459 newtp->write_seq = newtp->pushed_seq = 460 treq->snt_isn + 1 + tcp_s_data_size(oldtp); 461 462 newtp->rx_opt.saw_tstamp = 0; 463 464 newtp->rx_opt.dsack = 0; 465 newtp->rx_opt.num_sacks = 0; 466 467 newtp->urg_data = 0; 468 469 if (sock_flag(newsk, SOCK_KEEPOPEN)) 470 inet_csk_reset_keepalive_timer(newsk, 471 keepalive_time_when(newtp)); 472 473 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 474 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 475 if (sysctl_tcp_fack) 476 tcp_enable_fack(newtp); 477 } 478 newtp->window_clamp = req->window_clamp; 479 newtp->rcv_ssthresh = req->rcv_wnd; 480 newtp->rcv_wnd = req->rcv_wnd; 481 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 482 if (newtp->rx_opt.wscale_ok) { 483 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 484 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 485 } else { 486 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 487 newtp->window_clamp = min(newtp->window_clamp, 65535U); 488 } 489 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 490 newtp->rx_opt.snd_wscale); 491 newtp->max_window = newtp->snd_wnd; 492 493 if (newtp->rx_opt.tstamp_ok) { 494 newtp->rx_opt.ts_recent = req->ts_recent; 495 newtp->rx_opt.ts_recent_stamp = get_seconds(); 496 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 497 } else { 498 newtp->rx_opt.ts_recent_stamp = 0; 499 newtp->tcp_header_len = sizeof(struct tcphdr); 500 } 501 #ifdef CONFIG_TCP_MD5SIG 502 newtp->md5sig_info = NULL; /*XXX*/ 503 if (newtp->af_specific->md5_lookup(sk, newsk)) 504 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 505 #endif 506 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) 507 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 508 newtp->rx_opt.mss_clamp = req->mss; 509 TCP_ECN_openreq_child(newtp, req); 510 511 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS); 512 } 513 return newsk; 514 } 515 EXPORT_SYMBOL(tcp_create_openreq_child); 516 517 /* 518 * Process an incoming packet for SYN_RECV sockets represented 519 * as a request_sock. 520 */ 521 522 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 523 struct request_sock *req, 524 struct request_sock **prev) 525 { 526 struct tcp_options_received tmp_opt; 527 const u8 *hash_location; 528 struct sock *child; 529 const struct tcphdr *th = tcp_hdr(skb); 530 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 531 bool paws_reject = false; 532 533 tmp_opt.saw_tstamp = 0; 534 if (th->doff > (sizeof(struct tcphdr)>>2)) { 535 tcp_parse_options(skb, &tmp_opt, &hash_location, 0, NULL); 536 537 if (tmp_opt.saw_tstamp) { 538 tmp_opt.ts_recent = req->ts_recent; 539 /* We do not store true stamp, but it is not required, 540 * it can be estimated (approximately) 541 * from another data. 542 */ 543 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans); 544 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 545 } 546 } 547 548 /* Check for pure retransmitted SYN. */ 549 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 550 flg == TCP_FLAG_SYN && 551 !paws_reject) { 552 /* 553 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 554 * this case on figure 6 and figure 8, but formal 555 * protocol description says NOTHING. 556 * To be more exact, it says that we should send ACK, 557 * because this segment (at least, if it has no data) 558 * is out of window. 559 * 560 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 561 * describe SYN-RECV state. All the description 562 * is wrong, we cannot believe to it and should 563 * rely only on common sense and implementation 564 * experience. 565 * 566 * Enforce "SYN-ACK" according to figure 8, figure 6 567 * of RFC793, fixed by RFC1122. 568 */ 569 req->rsk_ops->rtx_syn_ack(sk, req, NULL); 570 return NULL; 571 } 572 573 /* Further reproduces section "SEGMENT ARRIVES" 574 for state SYN-RECEIVED of RFC793. 575 It is broken, however, it does not work only 576 when SYNs are crossed. 577 578 You would think that SYN crossing is impossible here, since 579 we should have a SYN_SENT socket (from connect()) on our end, 580 but this is not true if the crossed SYNs were sent to both 581 ends by a malicious third party. We must defend against this, 582 and to do that we first verify the ACK (as per RFC793, page 583 36) and reset if it is invalid. Is this a true full defense? 584 To convince ourselves, let us consider a way in which the ACK 585 test can still pass in this 'malicious crossed SYNs' case. 586 Malicious sender sends identical SYNs (and thus identical sequence 587 numbers) to both A and B: 588 589 A: gets SYN, seq=7 590 B: gets SYN, seq=7 591 592 By our good fortune, both A and B select the same initial 593 send sequence number of seven :-) 594 595 A: sends SYN|ACK, seq=7, ack_seq=8 596 B: sends SYN|ACK, seq=7, ack_seq=8 597 598 So we are now A eating this SYN|ACK, ACK test passes. So 599 does sequence test, SYN is truncated, and thus we consider 600 it a bare ACK. 601 602 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 603 bare ACK. Otherwise, we create an established connection. Both 604 ends (listening sockets) accept the new incoming connection and try 605 to talk to each other. 8-) 606 607 Note: This case is both harmless, and rare. Possibility is about the 608 same as us discovering intelligent life on another plant tomorrow. 609 610 But generally, we should (RFC lies!) to accept ACK 611 from SYNACK both here and in tcp_rcv_state_process(). 612 tcp_rcv_state_process() does not, hence, we do not too. 613 614 Note that the case is absolutely generic: 615 we cannot optimize anything here without 616 violating protocol. All the checks must be made 617 before attempt to create socket. 618 */ 619 620 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 621 * and the incoming segment acknowledges something not yet 622 * sent (the segment carries an unacceptable ACK) ... 623 * a reset is sent." 624 * 625 * Invalid ACK: reset will be sent by listening socket 626 */ 627 if ((flg & TCP_FLAG_ACK) && 628 (TCP_SKB_CB(skb)->ack_seq != 629 tcp_rsk(req)->snt_isn + 1 + tcp_s_data_size(tcp_sk(sk)))) 630 return sk; 631 632 /* Also, it would be not so bad idea to check rcv_tsecr, which 633 * is essentially ACK extension and too early or too late values 634 * should cause reset in unsynchronized states. 635 */ 636 637 /* RFC793: "first check sequence number". */ 638 639 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 640 tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) { 641 /* Out of window: send ACK and drop. */ 642 if (!(flg & TCP_FLAG_RST)) 643 req->rsk_ops->send_ack(sk, skb, req); 644 if (paws_reject) 645 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 646 return NULL; 647 } 648 649 /* In sequence, PAWS is OK. */ 650 651 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1)) 652 req->ts_recent = tmp_opt.rcv_tsval; 653 654 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 655 /* Truncate SYN, it is out of window starting 656 at tcp_rsk(req)->rcv_isn + 1. */ 657 flg &= ~TCP_FLAG_SYN; 658 } 659 660 /* RFC793: "second check the RST bit" and 661 * "fourth, check the SYN bit" 662 */ 663 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 664 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 665 goto embryonic_reset; 666 } 667 668 /* ACK sequence verified above, just make sure ACK is 669 * set. If ACK not set, just silently drop the packet. 670 */ 671 if (!(flg & TCP_FLAG_ACK)) 672 return NULL; 673 674 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 675 if (req->retrans < 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 if (tmp_opt.saw_tstamp && tmp_opt.rcv_tsecr) 682 tcp_rsk(req)->snt_synack = tmp_opt.rcv_tsecr; 683 else if (req->retrans) /* don't take RTT sample if retrans && ~TS */ 684 tcp_rsk(req)->snt_synack = 0; 685 686 /* OK, ACK is valid, create big socket and 687 * feed this segment to it. It will repeat all 688 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 689 * ESTABLISHED STATE. If it will be dropped after 690 * socket is created, wait for troubles. 691 */ 692 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL); 693 if (child == NULL) 694 goto listen_overflow; 695 696 inet_csk_reqsk_queue_unlink(sk, req, prev); 697 inet_csk_reqsk_queue_removed(sk, req); 698 699 inet_csk_reqsk_queue_add(sk, req, child); 700 return child; 701 702 listen_overflow: 703 if (!sysctl_tcp_abort_on_overflow) { 704 inet_rsk(req)->acked = 1; 705 return NULL; 706 } 707 708 embryonic_reset: 709 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 710 if (!(flg & TCP_FLAG_RST)) 711 req->rsk_ops->send_reset(sk, skb); 712 713 inet_csk_reqsk_queue_drop(sk, req, prev); 714 return NULL; 715 } 716 EXPORT_SYMBOL(tcp_check_req); 717 718 /* 719 * Queue segment on the new socket if the new socket is active, 720 * otherwise we just shortcircuit this and continue with 721 * the new socket. 722 */ 723 724 int tcp_child_process(struct sock *parent, struct sock *child, 725 struct sk_buff *skb) 726 { 727 int ret = 0; 728 int state = child->sk_state; 729 730 if (!sock_owned_by_user(child)) { 731 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb), 732 skb->len); 733 /* Wakeup parent, send SIGIO */ 734 if (state == TCP_SYN_RECV && child->sk_state != state) 735 parent->sk_data_ready(parent, 0); 736 } else { 737 /* Alas, it is possible again, because we do lookup 738 * in main socket hash table and lock on listening 739 * socket does not protect us more. 740 */ 741 __sk_add_backlog(child, skb); 742 } 743 744 bh_unlock_sock(child); 745 sock_put(child); 746 return ret; 747 } 748 EXPORT_SYMBOL(tcp_child_process); 749