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 const u8 *hash_location; 97 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 98 bool paws_reject = false; 99 100 tmp_opt.saw_tstamp = 0; 101 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) { 102 tcp_parse_options(skb, &tmp_opt, &hash_location, 0, NULL); 103 104 if (tmp_opt.saw_tstamp) { 105 tmp_opt.rcv_tsecr -= tcptw->tw_ts_offset; 106 tmp_opt.ts_recent = tcptw->tw_ts_recent; 107 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp; 108 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 109 } 110 } 111 112 if (tw->tw_substate == TCP_FIN_WAIT2) { 113 /* Just repeat all the checks of tcp_rcv_state_process() */ 114 115 /* Out of window, send ACK */ 116 if (paws_reject || 117 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 118 tcptw->tw_rcv_nxt, 119 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd)) 120 return TCP_TW_ACK; 121 122 if (th->rst) 123 goto kill; 124 125 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt)) 126 goto kill_with_rst; 127 128 /* Dup ACK? */ 129 if (!th->ack || 130 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) || 131 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) { 132 inet_twsk_put(tw); 133 return TCP_TW_SUCCESS; 134 } 135 136 /* New data or FIN. If new data arrive after half-duplex close, 137 * reset. 138 */ 139 if (!th->fin || 140 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) { 141 kill_with_rst: 142 inet_twsk_deschedule(tw, &tcp_death_row); 143 inet_twsk_put(tw); 144 return TCP_TW_RST; 145 } 146 147 /* FIN arrived, enter true time-wait state. */ 148 tw->tw_substate = TCP_TIME_WAIT; 149 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq; 150 if (tmp_opt.saw_tstamp) { 151 tcptw->tw_ts_recent_stamp = get_seconds(); 152 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 153 } 154 155 if (tcp_death_row.sysctl_tw_recycle && 156 tcptw->tw_ts_recent_stamp && 157 tcp_tw_remember_stamp(tw)) 158 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout, 159 TCP_TIMEWAIT_LEN); 160 else 161 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 162 TCP_TIMEWAIT_LEN); 163 return TCP_TW_ACK; 164 } 165 166 /* 167 * Now real TIME-WAIT state. 168 * 169 * RFC 1122: 170 * "When a connection is [...] on TIME-WAIT state [...] 171 * [a TCP] MAY accept a new SYN from the remote TCP to 172 * reopen the connection directly, if it: 173 * 174 * (1) assigns its initial sequence number for the new 175 * connection to be larger than the largest sequence 176 * number it used on the previous connection incarnation, 177 * and 178 * 179 * (2) returns to TIME-WAIT state if the SYN turns out 180 * to be an old duplicate". 181 */ 182 183 if (!paws_reject && 184 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 185 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 186 /* In window segment, it may be only reset or bare ack. */ 187 188 if (th->rst) { 189 /* This is TIME_WAIT assassination, in two flavors. 190 * Oh well... nobody has a sufficient solution to this 191 * protocol bug yet. 192 */ 193 if (sysctl_tcp_rfc1337 == 0) { 194 kill: 195 inet_twsk_deschedule(tw, &tcp_death_row); 196 inet_twsk_put(tw); 197 return TCP_TW_SUCCESS; 198 } 199 } 200 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 201 TCP_TIMEWAIT_LEN); 202 203 if (tmp_opt.saw_tstamp) { 204 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 205 tcptw->tw_ts_recent_stamp = get_seconds(); 206 } 207 208 inet_twsk_put(tw); 209 return TCP_TW_SUCCESS; 210 } 211 212 /* Out of window segment. 213 214 All the segments are ACKed immediately. 215 216 The only exception is new SYN. We accept it, if it is 217 not old duplicate and we are not in danger to be killed 218 by delayed old duplicates. RFC check is that it has 219 newer sequence number works at rates <40Mbit/sec. 220 However, if paws works, it is reliable AND even more, 221 we even may relax silly seq space cutoff. 222 223 RED-PEN: we violate main RFC requirement, if this SYN will appear 224 old duplicate (i.e. we receive RST in reply to SYN-ACK), 225 we must return socket to time-wait state. It is not good, 226 but not fatal yet. 227 */ 228 229 if (th->syn && !th->rst && !th->ack && !paws_reject && 230 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 231 (tmp_opt.saw_tstamp && 232 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 233 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 234 if (isn == 0) 235 isn++; 236 TCP_SKB_CB(skb)->when = isn; 237 return TCP_TW_SYN; 238 } 239 240 if (paws_reject) 241 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 242 243 if (!th->rst) { 244 /* In this case we must reset the TIMEWAIT timer. 245 * 246 * If it is ACKless SYN it may be both old duplicate 247 * and new good SYN with random sequence number <rcv_nxt. 248 * Do not reschedule in the last case. 249 */ 250 if (paws_reject || th->ack) 251 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 252 TCP_TIMEWAIT_LEN); 253 254 /* Send ACK. Note, we do not put the bucket, 255 * it will be released by caller. 256 */ 257 return TCP_TW_ACK; 258 } 259 inet_twsk_put(tw); 260 return TCP_TW_SUCCESS; 261 } 262 EXPORT_SYMBOL(tcp_timewait_state_process); 263 264 /* 265 * Move a socket to time-wait or dead fin-wait-2 state. 266 */ 267 void 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 bool recycle_ok = false; 273 274 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp) 275 recycle_ok = tcp_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 struct inet_sock *inet = inet_sk(sk); 284 285 tw->tw_transparent = inet->transparent; 286 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 287 tcptw->tw_rcv_nxt = tp->rcv_nxt; 288 tcptw->tw_snd_nxt = tp->snd_nxt; 289 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 290 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 291 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 292 tcptw->tw_ts_offset = tp->tsoffset; 293 294 #if IS_ENABLED(CONFIG_IPV6) 295 if (tw->tw_family == PF_INET6) { 296 struct ipv6_pinfo *np = inet6_sk(sk); 297 struct inet6_timewait_sock *tw6; 298 299 tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot); 300 tw6 = inet6_twsk((struct sock *)tw); 301 tw6->tw_v6_daddr = np->daddr; 302 tw6->tw_v6_rcv_saddr = np->rcv_saddr; 303 tw->tw_tclass = np->tclass; 304 tw->tw_ipv6only = np->ipv6only; 305 } 306 #endif 307 308 #ifdef CONFIG_TCP_MD5SIG 309 /* 310 * The timewait bucket does not have the key DB from the 311 * sock structure. We just make a quick copy of the 312 * md5 key being used (if indeed we are using one) 313 * so the timewait ack generating code has the key. 314 */ 315 do { 316 struct tcp_md5sig_key *key; 317 tcptw->tw_md5_key = NULL; 318 key = tp->af_specific->md5_lookup(sk, sk); 319 if (key != NULL) { 320 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC); 321 if (tcptw->tw_md5_key && tcp_alloc_md5sig_pool(sk) == NULL) 322 BUG(); 323 } 324 } while (0); 325 #endif 326 327 /* Linkage updates. */ 328 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 329 330 /* Get the TIME_WAIT timeout firing. */ 331 if (timeo < rto) 332 timeo = rto; 333 334 if (recycle_ok) { 335 tw->tw_timeout = rto; 336 } else { 337 tw->tw_timeout = TCP_TIMEWAIT_LEN; 338 if (state == TCP_TIME_WAIT) 339 timeo = TCP_TIMEWAIT_LEN; 340 } 341 342 inet_twsk_schedule(tw, &tcp_death_row, timeo, 343 TCP_TIMEWAIT_LEN); 344 inet_twsk_put(tw); 345 } else { 346 /* Sorry, if we're out of memory, just CLOSE this 347 * socket up. We've got bigger problems than 348 * non-graceful socket closings. 349 */ 350 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW); 351 } 352 353 tcp_update_metrics(sk); 354 tcp_done(sk); 355 } 356 357 void tcp_twsk_destructor(struct sock *sk) 358 { 359 #ifdef CONFIG_TCP_MD5SIG 360 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 361 362 if (twsk->tw_md5_key) { 363 tcp_free_md5sig_pool(); 364 kfree_rcu(twsk->tw_md5_key, rcu); 365 } 366 #endif 367 } 368 EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 369 370 static inline void TCP_ECN_openreq_child(struct tcp_sock *tp, 371 struct request_sock *req) 372 { 373 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 374 } 375 376 /* This is not only more efficient than what we used to do, it eliminates 377 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 378 * 379 * Actually, we could lots of memory writes here. tp of listening 380 * socket contains all necessary default parameters. 381 */ 382 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb) 383 { 384 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC); 385 386 if (newsk != NULL) { 387 const struct inet_request_sock *ireq = inet_rsk(req); 388 struct tcp_request_sock *treq = tcp_rsk(req); 389 struct inet_connection_sock *newicsk = inet_csk(newsk); 390 struct tcp_sock *newtp = tcp_sk(newsk); 391 struct tcp_sock *oldtp = tcp_sk(sk); 392 struct tcp_cookie_values *oldcvp = oldtp->cookie_values; 393 394 /* TCP Cookie Transactions require space for the cookie pair, 395 * as it differs for each connection. There is no need to 396 * copy any s_data_payload stored at the original socket. 397 * Failure will prevent resuming the connection. 398 * 399 * Presumed copied, in order of appearance: 400 * cookie_in_always, cookie_out_never 401 */ 402 if (oldcvp != NULL) { 403 struct tcp_cookie_values *newcvp = 404 kzalloc(sizeof(*newtp->cookie_values), 405 GFP_ATOMIC); 406 407 if (newcvp != NULL) { 408 kref_init(&newcvp->kref); 409 newcvp->cookie_desired = 410 oldcvp->cookie_desired; 411 newtp->cookie_values = newcvp; 412 } else { 413 /* Not Yet Implemented */ 414 newtp->cookie_values = NULL; 415 } 416 } 417 418 /* Now setup tcp_sock */ 419 newtp->pred_flags = 0; 420 421 newtp->rcv_wup = newtp->copied_seq = 422 newtp->rcv_nxt = treq->rcv_isn + 1; 423 424 newtp->snd_sml = newtp->snd_una = 425 newtp->snd_nxt = newtp->snd_up = 426 treq->snt_isn + 1 + tcp_s_data_size(oldtp); 427 428 tcp_prequeue_init(newtp); 429 INIT_LIST_HEAD(&newtp->tsq_node); 430 431 tcp_init_wl(newtp, treq->rcv_isn); 432 433 newtp->srtt = 0; 434 newtp->mdev = TCP_TIMEOUT_INIT; 435 newicsk->icsk_rto = TCP_TIMEOUT_INIT; 436 437 newtp->packets_out = 0; 438 newtp->retrans_out = 0; 439 newtp->sacked_out = 0; 440 newtp->fackets_out = 0; 441 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH; 442 tcp_enable_early_retrans(newtp); 443 444 /* So many TCP implementations out there (incorrectly) count the 445 * initial SYN frame in their delayed-ACK and congestion control 446 * algorithms that we must have the following bandaid to talk 447 * efficiently to them. -DaveM 448 */ 449 newtp->snd_cwnd = TCP_INIT_CWND; 450 newtp->snd_cwnd_cnt = 0; 451 452 newtp->frto_counter = 0; 453 newtp->frto_highmark = 0; 454 455 if (newicsk->icsk_ca_ops != &tcp_init_congestion_ops && 456 !try_module_get(newicsk->icsk_ca_ops->owner)) 457 newicsk->icsk_ca_ops = &tcp_init_congestion_ops; 458 459 tcp_set_ca_state(newsk, TCP_CA_Open); 460 tcp_init_xmit_timers(newsk); 461 skb_queue_head_init(&newtp->out_of_order_queue); 462 newtp->write_seq = newtp->pushed_seq = 463 treq->snt_isn + 1 + tcp_s_data_size(oldtp); 464 465 newtp->rx_opt.saw_tstamp = 0; 466 467 newtp->rx_opt.dsack = 0; 468 newtp->rx_opt.num_sacks = 0; 469 470 newtp->urg_data = 0; 471 472 if (sock_flag(newsk, SOCK_KEEPOPEN)) 473 inet_csk_reset_keepalive_timer(newsk, 474 keepalive_time_when(newtp)); 475 476 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 477 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 478 if (sysctl_tcp_fack) 479 tcp_enable_fack(newtp); 480 } 481 newtp->window_clamp = req->window_clamp; 482 newtp->rcv_ssthresh = req->rcv_wnd; 483 newtp->rcv_wnd = req->rcv_wnd; 484 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 485 if (newtp->rx_opt.wscale_ok) { 486 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 487 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 488 } else { 489 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 490 newtp->window_clamp = min(newtp->window_clamp, 65535U); 491 } 492 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 493 newtp->rx_opt.snd_wscale); 494 newtp->max_window = newtp->snd_wnd; 495 496 if (newtp->rx_opt.tstamp_ok) { 497 newtp->rx_opt.ts_recent = req->ts_recent; 498 newtp->rx_opt.ts_recent_stamp = get_seconds(); 499 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 500 } else { 501 newtp->rx_opt.ts_recent_stamp = 0; 502 newtp->tcp_header_len = sizeof(struct tcphdr); 503 } 504 newtp->tsoffset = 0; 505 #ifdef CONFIG_TCP_MD5SIG 506 newtp->md5sig_info = NULL; /*XXX*/ 507 if (newtp->af_specific->md5_lookup(sk, newsk)) 508 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 509 #endif 510 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) 511 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 512 newtp->rx_opt.mss_clamp = req->mss; 513 TCP_ECN_openreq_child(newtp, req); 514 newtp->fastopen_rsk = NULL; 515 newtp->syn_data_acked = 0; 516 517 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS); 518 } 519 return newsk; 520 } 521 EXPORT_SYMBOL(tcp_create_openreq_child); 522 523 /* 524 * Process an incoming packet for SYN_RECV sockets represented as a 525 * request_sock. Normally sk is the listener socket but for TFO it 526 * points to the child socket. 527 * 528 * XXX (TFO) - The current impl contains a special check for ack 529 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better? 530 * 531 * We don't need to initialize tmp_opt.sack_ok as we don't use the results 532 */ 533 534 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 535 struct request_sock *req, 536 struct request_sock **prev, 537 bool fastopen) 538 { 539 struct tcp_options_received tmp_opt; 540 const u8 *hash_location; 541 struct sock *child; 542 const struct tcphdr *th = tcp_hdr(skb); 543 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 544 bool paws_reject = false; 545 546 BUG_ON(fastopen == (sk->sk_state == TCP_LISTEN)); 547 548 tmp_opt.saw_tstamp = 0; 549 if (th->doff > (sizeof(struct tcphdr)>>2)) { 550 tcp_parse_options(skb, &tmp_opt, &hash_location, 0, NULL); 551 552 if (tmp_opt.saw_tstamp) { 553 tmp_opt.ts_recent = req->ts_recent; 554 /* We do not store true stamp, but it is not required, 555 * it can be estimated (approximately) 556 * from another data. 557 */ 558 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout); 559 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 560 } 561 } 562 563 /* Check for pure retransmitted SYN. */ 564 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 565 flg == TCP_FLAG_SYN && 566 !paws_reject) { 567 /* 568 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 569 * this case on figure 6 and figure 8, but formal 570 * protocol description says NOTHING. 571 * To be more exact, it says that we should send ACK, 572 * because this segment (at least, if it has no data) 573 * is out of window. 574 * 575 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 576 * describe SYN-RECV state. All the description 577 * is wrong, we cannot believe to it and should 578 * rely only on common sense and implementation 579 * experience. 580 * 581 * Enforce "SYN-ACK" according to figure 8, figure 6 582 * of RFC793, fixed by RFC1122. 583 * 584 * Note that even if there is new data in the SYN packet 585 * they will be thrown away too. 586 */ 587 inet_rtx_syn_ack(sk, req); 588 return NULL; 589 } 590 591 /* Further reproduces section "SEGMENT ARRIVES" 592 for state SYN-RECEIVED of RFC793. 593 It is broken, however, it does not work only 594 when SYNs are crossed. 595 596 You would think that SYN crossing is impossible here, since 597 we should have a SYN_SENT socket (from connect()) on our end, 598 but this is not true if the crossed SYNs were sent to both 599 ends by a malicious third party. We must defend against this, 600 and to do that we first verify the ACK (as per RFC793, page 601 36) and reset if it is invalid. Is this a true full defense? 602 To convince ourselves, let us consider a way in which the ACK 603 test can still pass in this 'malicious crossed SYNs' case. 604 Malicious sender sends identical SYNs (and thus identical sequence 605 numbers) to both A and B: 606 607 A: gets SYN, seq=7 608 B: gets SYN, seq=7 609 610 By our good fortune, both A and B select the same initial 611 send sequence number of seven :-) 612 613 A: sends SYN|ACK, seq=7, ack_seq=8 614 B: sends SYN|ACK, seq=7, ack_seq=8 615 616 So we are now A eating this SYN|ACK, ACK test passes. So 617 does sequence test, SYN is truncated, and thus we consider 618 it a bare ACK. 619 620 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 621 bare ACK. Otherwise, we create an established connection. Both 622 ends (listening sockets) accept the new incoming connection and try 623 to talk to each other. 8-) 624 625 Note: This case is both harmless, and rare. Possibility is about the 626 same as us discovering intelligent life on another plant tomorrow. 627 628 But generally, we should (RFC lies!) to accept ACK 629 from SYNACK both here and in tcp_rcv_state_process(). 630 tcp_rcv_state_process() does not, hence, we do not too. 631 632 Note that the case is absolutely generic: 633 we cannot optimize anything here without 634 violating protocol. All the checks must be made 635 before attempt to create socket. 636 */ 637 638 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 639 * and the incoming segment acknowledges something not yet 640 * sent (the segment carries an unacceptable ACK) ... 641 * a reset is sent." 642 * 643 * Invalid ACK: reset will be sent by listening socket. 644 * Note that the ACK validity check for a Fast Open socket is done 645 * elsewhere and is checked directly against the child socket rather 646 * than req because user data may have been sent out. 647 */ 648 if ((flg & TCP_FLAG_ACK) && !fastopen && 649 (TCP_SKB_CB(skb)->ack_seq != 650 tcp_rsk(req)->snt_isn + 1 + tcp_s_data_size(tcp_sk(sk)))) 651 return sk; 652 653 /* Also, it would be not so bad idea to check rcv_tsecr, which 654 * is essentially ACK extension and too early or too late values 655 * should cause reset in unsynchronized states. 656 */ 657 658 /* RFC793: "first check sequence number". */ 659 660 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 661 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rcv_wnd)) { 662 /* Out of window: send ACK and drop. */ 663 if (!(flg & TCP_FLAG_RST)) 664 req->rsk_ops->send_ack(sk, skb, req); 665 if (paws_reject) 666 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 667 return NULL; 668 } 669 670 /* In sequence, PAWS is OK. */ 671 672 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt)) 673 req->ts_recent = tmp_opt.rcv_tsval; 674 675 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 676 /* Truncate SYN, it is out of window starting 677 at tcp_rsk(req)->rcv_isn + 1. */ 678 flg &= ~TCP_FLAG_SYN; 679 } 680 681 /* RFC793: "second check the RST bit" and 682 * "fourth, check the SYN bit" 683 */ 684 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 685 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 686 goto embryonic_reset; 687 } 688 689 /* ACK sequence verified above, just make sure ACK is 690 * set. If ACK not set, just silently drop the packet. 691 * 692 * XXX (TFO) - if we ever allow "data after SYN", the 693 * following check needs to be removed. 694 */ 695 if (!(flg & TCP_FLAG_ACK)) 696 return NULL; 697 698 /* Got ACK for our SYNACK, so update baseline for SYNACK RTT sample. */ 699 if (tmp_opt.saw_tstamp && tmp_opt.rcv_tsecr) 700 tcp_rsk(req)->snt_synack = tmp_opt.rcv_tsecr; 701 else if (req->num_retrans) /* don't take RTT sample if retrans && ~TS */ 702 tcp_rsk(req)->snt_synack = 0; 703 704 /* For Fast Open no more processing is needed (sk is the 705 * child socket). 706 */ 707 if (fastopen) 708 return sk; 709 710 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 711 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 712 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 713 inet_rsk(req)->acked = 1; 714 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP); 715 return NULL; 716 } 717 718 /* OK, ACK is valid, create big socket and 719 * feed this segment to it. It will repeat all 720 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 721 * ESTABLISHED STATE. If it will be dropped after 722 * socket is created, wait for troubles. 723 */ 724 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL); 725 if (child == NULL) 726 goto listen_overflow; 727 728 inet_csk_reqsk_queue_unlink(sk, req, prev); 729 inet_csk_reqsk_queue_removed(sk, req); 730 731 inet_csk_reqsk_queue_add(sk, req, child); 732 return child; 733 734 listen_overflow: 735 if (!sysctl_tcp_abort_on_overflow) { 736 inet_rsk(req)->acked = 1; 737 return NULL; 738 } 739 740 embryonic_reset: 741 if (!(flg & TCP_FLAG_RST)) { 742 /* Received a bad SYN pkt - for TFO We try not to reset 743 * the local connection unless it's really necessary to 744 * avoid becoming vulnerable to outside attack aiming at 745 * resetting legit local connections. 746 */ 747 req->rsk_ops->send_reset(sk, skb); 748 } else if (fastopen) { /* received a valid RST pkt */ 749 reqsk_fastopen_remove(sk, req, true); 750 tcp_reset(sk); 751 } 752 if (!fastopen) { 753 inet_csk_reqsk_queue_drop(sk, req, prev); 754 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 755 } 756 return NULL; 757 } 758 EXPORT_SYMBOL(tcp_check_req); 759 760 /* 761 * Queue segment on the new socket if the new socket is active, 762 * otherwise we just shortcircuit this and continue with 763 * the new socket. 764 * 765 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV 766 * when entering. But other states are possible due to a race condition 767 * where after __inet_lookup_established() fails but before the listener 768 * locked is obtained, other packets cause the same connection to 769 * be created. 770 */ 771 772 int tcp_child_process(struct sock *parent, struct sock *child, 773 struct sk_buff *skb) 774 { 775 int ret = 0; 776 int state = child->sk_state; 777 778 if (!sock_owned_by_user(child)) { 779 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb), 780 skb->len); 781 /* Wakeup parent, send SIGIO */ 782 if (state == TCP_SYN_RECV && child->sk_state != state) 783 parent->sk_data_ready(parent, 0); 784 } else { 785 /* Alas, it is possible again, because we do lookup 786 * in main socket hash table and lock on listening 787 * socket does not protect us more. 788 */ 789 __sk_add_backlog(child, skb); 790 } 791 792 bh_unlock_sock(child); 793 sock_put(child); 794 return ret; 795 } 796 EXPORT_SYMBOL(tcp_child_process); 797