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 /* VJ's idea. Save last timestamp seen from this destination 53 * and hold it at least for normal timewait interval to use for duplicate 54 * segment detection in subsequent connections, before they enter synchronized 55 * state. 56 */ 57 58 static bool tcp_remember_stamp(struct sock *sk) 59 { 60 const struct inet_connection_sock *icsk = inet_csk(sk); 61 struct tcp_sock *tp = tcp_sk(sk); 62 struct inet_peer *peer; 63 bool release_it; 64 65 peer = icsk->icsk_af_ops->get_peer(sk, &release_it); 66 if (peer) { 67 if ((s32)(peer->tcp_ts - tp->rx_opt.ts_recent) <= 0 || 68 ((u32)get_seconds() - peer->tcp_ts_stamp > TCP_PAWS_MSL && 69 peer->tcp_ts_stamp <= (u32)tp->rx_opt.ts_recent_stamp)) { 70 peer->tcp_ts_stamp = (u32)tp->rx_opt.ts_recent_stamp; 71 peer->tcp_ts = tp->rx_opt.ts_recent; 72 } 73 if (release_it) 74 inet_putpeer(peer); 75 return true; 76 } 77 78 return false; 79 } 80 81 static bool tcp_tw_remember_stamp(struct inet_timewait_sock *tw) 82 { 83 struct sock *sk = (struct sock *) tw; 84 struct inet_peer *peer; 85 86 peer = twsk_getpeer(sk); 87 if (peer) { 88 const struct tcp_timewait_sock *tcptw = tcp_twsk(sk); 89 90 if ((s32)(peer->tcp_ts - tcptw->tw_ts_recent) <= 0 || 91 ((u32)get_seconds() - peer->tcp_ts_stamp > TCP_PAWS_MSL && 92 peer->tcp_ts_stamp <= (u32)tcptw->tw_ts_recent_stamp)) { 93 peer->tcp_ts_stamp = (u32)tcptw->tw_ts_recent_stamp; 94 peer->tcp_ts = tcptw->tw_ts_recent; 95 } 96 inet_putpeer(peer); 97 return true; 98 } 99 return false; 100 } 101 102 static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win) 103 { 104 if (seq == s_win) 105 return true; 106 if (after(end_seq, s_win) && before(seq, e_win)) 107 return true; 108 return seq == e_win && seq == end_seq; 109 } 110 111 /* 112 * * Main purpose of TIME-WAIT state is to close connection gracefully, 113 * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN 114 * (and, probably, tail of data) and one or more our ACKs are lost. 115 * * What is TIME-WAIT timeout? It is associated with maximal packet 116 * lifetime in the internet, which results in wrong conclusion, that 117 * it is set to catch "old duplicate segments" wandering out of their path. 118 * It is not quite correct. This timeout is calculated so that it exceeds 119 * maximal retransmission timeout enough to allow to lose one (or more) 120 * segments sent by peer and our ACKs. This time may be calculated from RTO. 121 * * When TIME-WAIT socket receives RST, it means that another end 122 * finally closed and we are allowed to kill TIME-WAIT too. 123 * * Second purpose of TIME-WAIT is catching old duplicate segments. 124 * Well, certainly it is pure paranoia, but if we load TIME-WAIT 125 * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs. 126 * * If we invented some more clever way to catch duplicates 127 * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs. 128 * 129 * The algorithm below is based on FORMAL INTERPRETATION of RFCs. 130 * When you compare it to RFCs, please, read section SEGMENT ARRIVES 131 * from the very beginning. 132 * 133 * NOTE. With recycling (and later with fin-wait-2) TW bucket 134 * is _not_ stateless. It means, that strictly speaking we must 135 * spinlock it. I do not want! Well, probability of misbehaviour 136 * is ridiculously low and, seems, we could use some mb() tricks 137 * to avoid misread sequence numbers, states etc. --ANK 138 */ 139 enum tcp_tw_status 140 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb, 141 const struct tcphdr *th) 142 { 143 struct tcp_options_received tmp_opt; 144 const u8 *hash_location; 145 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 146 bool paws_reject = false; 147 148 tmp_opt.saw_tstamp = 0; 149 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) { 150 tcp_parse_options(skb, &tmp_opt, &hash_location, 0); 151 152 if (tmp_opt.saw_tstamp) { 153 tmp_opt.ts_recent = tcptw->tw_ts_recent; 154 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp; 155 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 156 } 157 } 158 159 if (tw->tw_substate == TCP_FIN_WAIT2) { 160 /* Just repeat all the checks of tcp_rcv_state_process() */ 161 162 /* Out of window, send ACK */ 163 if (paws_reject || 164 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 165 tcptw->tw_rcv_nxt, 166 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd)) 167 return TCP_TW_ACK; 168 169 if (th->rst) 170 goto kill; 171 172 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt)) 173 goto kill_with_rst; 174 175 /* Dup ACK? */ 176 if (!th->ack || 177 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) || 178 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) { 179 inet_twsk_put(tw); 180 return TCP_TW_SUCCESS; 181 } 182 183 /* New data or FIN. If new data arrive after half-duplex close, 184 * reset. 185 */ 186 if (!th->fin || 187 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) { 188 kill_with_rst: 189 inet_twsk_deschedule(tw, &tcp_death_row); 190 inet_twsk_put(tw); 191 return TCP_TW_RST; 192 } 193 194 /* FIN arrived, enter true time-wait state. */ 195 tw->tw_substate = TCP_TIME_WAIT; 196 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq; 197 if (tmp_opt.saw_tstamp) { 198 tcptw->tw_ts_recent_stamp = get_seconds(); 199 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 200 } 201 202 if (tcp_death_row.sysctl_tw_recycle && 203 tcptw->tw_ts_recent_stamp && 204 tcp_tw_remember_stamp(tw)) 205 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout, 206 TCP_TIMEWAIT_LEN); 207 else 208 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 209 TCP_TIMEWAIT_LEN); 210 return TCP_TW_ACK; 211 } 212 213 /* 214 * Now real TIME-WAIT state. 215 * 216 * RFC 1122: 217 * "When a connection is [...] on TIME-WAIT state [...] 218 * [a TCP] MAY accept a new SYN from the remote TCP to 219 * reopen the connection directly, if it: 220 * 221 * (1) assigns its initial sequence number for the new 222 * connection to be larger than the largest sequence 223 * number it used on the previous connection incarnation, 224 * and 225 * 226 * (2) returns to TIME-WAIT state if the SYN turns out 227 * to be an old duplicate". 228 */ 229 230 if (!paws_reject && 231 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 232 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 233 /* In window segment, it may be only reset or bare ack. */ 234 235 if (th->rst) { 236 /* This is TIME_WAIT assassination, in two flavors. 237 * Oh well... nobody has a sufficient solution to this 238 * protocol bug yet. 239 */ 240 if (sysctl_tcp_rfc1337 == 0) { 241 kill: 242 inet_twsk_deschedule(tw, &tcp_death_row); 243 inet_twsk_put(tw); 244 return TCP_TW_SUCCESS; 245 } 246 } 247 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 248 TCP_TIMEWAIT_LEN); 249 250 if (tmp_opt.saw_tstamp) { 251 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 252 tcptw->tw_ts_recent_stamp = get_seconds(); 253 } 254 255 inet_twsk_put(tw); 256 return TCP_TW_SUCCESS; 257 } 258 259 /* Out of window segment. 260 261 All the segments are ACKed immediately. 262 263 The only exception is new SYN. We accept it, if it is 264 not old duplicate and we are not in danger to be killed 265 by delayed old duplicates. RFC check is that it has 266 newer sequence number works at rates <40Mbit/sec. 267 However, if paws works, it is reliable AND even more, 268 we even may relax silly seq space cutoff. 269 270 RED-PEN: we violate main RFC requirement, if this SYN will appear 271 old duplicate (i.e. we receive RST in reply to SYN-ACK), 272 we must return socket to time-wait state. It is not good, 273 but not fatal yet. 274 */ 275 276 if (th->syn && !th->rst && !th->ack && !paws_reject && 277 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 278 (tmp_opt.saw_tstamp && 279 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 280 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 281 if (isn == 0) 282 isn++; 283 TCP_SKB_CB(skb)->when = isn; 284 return TCP_TW_SYN; 285 } 286 287 if (paws_reject) 288 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 289 290 if (!th->rst) { 291 /* In this case we must reset the TIMEWAIT timer. 292 * 293 * If it is ACKless SYN it may be both old duplicate 294 * and new good SYN with random sequence number <rcv_nxt. 295 * Do not reschedule in the last case. 296 */ 297 if (paws_reject || th->ack) 298 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 299 TCP_TIMEWAIT_LEN); 300 301 /* Send ACK. Note, we do not put the bucket, 302 * it will be released by caller. 303 */ 304 return TCP_TW_ACK; 305 } 306 inet_twsk_put(tw); 307 return TCP_TW_SUCCESS; 308 } 309 EXPORT_SYMBOL(tcp_timewait_state_process); 310 311 /* 312 * Move a socket to time-wait or dead fin-wait-2 state. 313 */ 314 void tcp_time_wait(struct sock *sk, int state, int timeo) 315 { 316 struct inet_timewait_sock *tw = NULL; 317 const struct inet_connection_sock *icsk = inet_csk(sk); 318 const struct tcp_sock *tp = tcp_sk(sk); 319 bool recycle_ok = false; 320 321 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp) 322 recycle_ok = tcp_remember_stamp(sk); 323 324 if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets) 325 tw = inet_twsk_alloc(sk, state); 326 327 if (tw != NULL) { 328 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 329 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 330 331 tw->tw_transparent = inet_sk(sk)->transparent; 332 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 333 tcptw->tw_rcv_nxt = tp->rcv_nxt; 334 tcptw->tw_snd_nxt = tp->snd_nxt; 335 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 336 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 337 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 338 339 #if IS_ENABLED(CONFIG_IPV6) 340 if (tw->tw_family == PF_INET6) { 341 struct ipv6_pinfo *np = inet6_sk(sk); 342 struct inet6_timewait_sock *tw6; 343 344 tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot); 345 tw6 = inet6_twsk((struct sock *)tw); 346 tw6->tw_v6_daddr = np->daddr; 347 tw6->tw_v6_rcv_saddr = np->rcv_saddr; 348 tw->tw_tclass = np->tclass; 349 tw->tw_ipv6only = np->ipv6only; 350 } 351 #endif 352 353 #ifdef CONFIG_TCP_MD5SIG 354 /* 355 * The timewait bucket does not have the key DB from the 356 * sock structure. We just make a quick copy of the 357 * md5 key being used (if indeed we are using one) 358 * so the timewait ack generating code has the key. 359 */ 360 do { 361 struct tcp_md5sig_key *key; 362 tcptw->tw_md5_key = NULL; 363 key = tp->af_specific->md5_lookup(sk, sk); 364 if (key != NULL) { 365 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC); 366 if (tcptw->tw_md5_key && tcp_alloc_md5sig_pool(sk) == NULL) 367 BUG(); 368 } 369 } while (0); 370 #endif 371 372 /* Linkage updates. */ 373 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 374 375 /* Get the TIME_WAIT timeout firing. */ 376 if (timeo < rto) 377 timeo = rto; 378 379 if (recycle_ok) { 380 tw->tw_timeout = rto; 381 } else { 382 tw->tw_timeout = TCP_TIMEWAIT_LEN; 383 if (state == TCP_TIME_WAIT) 384 timeo = TCP_TIMEWAIT_LEN; 385 } 386 387 inet_twsk_schedule(tw, &tcp_death_row, timeo, 388 TCP_TIMEWAIT_LEN); 389 inet_twsk_put(tw); 390 } else { 391 /* Sorry, if we're out of memory, just CLOSE this 392 * socket up. We've got bigger problems than 393 * non-graceful socket closings. 394 */ 395 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW); 396 } 397 398 tcp_update_metrics(sk); 399 tcp_done(sk); 400 } 401 402 void tcp_twsk_destructor(struct sock *sk) 403 { 404 #ifdef CONFIG_TCP_MD5SIG 405 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 406 if (twsk->tw_md5_key) { 407 tcp_free_md5sig_pool(); 408 kfree_rcu(twsk->tw_md5_key, rcu); 409 } 410 #endif 411 } 412 EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 413 414 static inline void TCP_ECN_openreq_child(struct tcp_sock *tp, 415 struct request_sock *req) 416 { 417 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 418 } 419 420 /* This is not only more efficient than what we used to do, it eliminates 421 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 422 * 423 * Actually, we could lots of memory writes here. tp of listening 424 * socket contains all necessary default parameters. 425 */ 426 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb) 427 { 428 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC); 429 430 if (newsk != NULL) { 431 const struct inet_request_sock *ireq = inet_rsk(req); 432 struct tcp_request_sock *treq = tcp_rsk(req); 433 struct inet_connection_sock *newicsk = inet_csk(newsk); 434 struct tcp_sock *newtp = tcp_sk(newsk); 435 struct tcp_sock *oldtp = tcp_sk(sk); 436 struct tcp_cookie_values *oldcvp = oldtp->cookie_values; 437 438 /* TCP Cookie Transactions require space for the cookie pair, 439 * as it differs for each connection. There is no need to 440 * copy any s_data_payload stored at the original socket. 441 * Failure will prevent resuming the connection. 442 * 443 * Presumed copied, in order of appearance: 444 * cookie_in_always, cookie_out_never 445 */ 446 if (oldcvp != NULL) { 447 struct tcp_cookie_values *newcvp = 448 kzalloc(sizeof(*newtp->cookie_values), 449 GFP_ATOMIC); 450 451 if (newcvp != NULL) { 452 kref_init(&newcvp->kref); 453 newcvp->cookie_desired = 454 oldcvp->cookie_desired; 455 newtp->cookie_values = newcvp; 456 } else { 457 /* Not Yet Implemented */ 458 newtp->cookie_values = NULL; 459 } 460 } 461 462 /* Now setup tcp_sock */ 463 newtp->pred_flags = 0; 464 465 newtp->rcv_wup = newtp->copied_seq = 466 newtp->rcv_nxt = treq->rcv_isn + 1; 467 468 newtp->snd_sml = newtp->snd_una = 469 newtp->snd_nxt = newtp->snd_up = 470 treq->snt_isn + 1 + tcp_s_data_size(oldtp); 471 472 tcp_prequeue_init(newtp); 473 474 tcp_init_wl(newtp, treq->rcv_isn); 475 476 newtp->srtt = 0; 477 newtp->mdev = TCP_TIMEOUT_INIT; 478 newicsk->icsk_rto = TCP_TIMEOUT_INIT; 479 480 newtp->packets_out = 0; 481 newtp->retrans_out = 0; 482 newtp->sacked_out = 0; 483 newtp->fackets_out = 0; 484 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH; 485 tcp_enable_early_retrans(newtp); 486 487 /* So many TCP implementations out there (incorrectly) count the 488 * initial SYN frame in their delayed-ACK and congestion control 489 * algorithms that we must have the following bandaid to talk 490 * efficiently to them. -DaveM 491 */ 492 newtp->snd_cwnd = TCP_INIT_CWND; 493 newtp->snd_cwnd_cnt = 0; 494 newtp->bytes_acked = 0; 495 496 newtp->frto_counter = 0; 497 newtp->frto_highmark = 0; 498 499 if (newicsk->icsk_ca_ops != &tcp_init_congestion_ops && 500 !try_module_get(newicsk->icsk_ca_ops->owner)) 501 newicsk->icsk_ca_ops = &tcp_init_congestion_ops; 502 503 tcp_set_ca_state(newsk, TCP_CA_Open); 504 tcp_init_xmit_timers(newsk); 505 skb_queue_head_init(&newtp->out_of_order_queue); 506 newtp->write_seq = newtp->pushed_seq = 507 treq->snt_isn + 1 + tcp_s_data_size(oldtp); 508 509 newtp->rx_opt.saw_tstamp = 0; 510 511 newtp->rx_opt.dsack = 0; 512 newtp->rx_opt.num_sacks = 0; 513 514 newtp->urg_data = 0; 515 516 if (sock_flag(newsk, SOCK_KEEPOPEN)) 517 inet_csk_reset_keepalive_timer(newsk, 518 keepalive_time_when(newtp)); 519 520 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 521 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 522 if (sysctl_tcp_fack) 523 tcp_enable_fack(newtp); 524 } 525 newtp->window_clamp = req->window_clamp; 526 newtp->rcv_ssthresh = req->rcv_wnd; 527 newtp->rcv_wnd = req->rcv_wnd; 528 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 529 if (newtp->rx_opt.wscale_ok) { 530 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 531 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 532 } else { 533 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 534 newtp->window_clamp = min(newtp->window_clamp, 65535U); 535 } 536 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 537 newtp->rx_opt.snd_wscale); 538 newtp->max_window = newtp->snd_wnd; 539 540 if (newtp->rx_opt.tstamp_ok) { 541 newtp->rx_opt.ts_recent = req->ts_recent; 542 newtp->rx_opt.ts_recent_stamp = get_seconds(); 543 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 544 } else { 545 newtp->rx_opt.ts_recent_stamp = 0; 546 newtp->tcp_header_len = sizeof(struct tcphdr); 547 } 548 #ifdef CONFIG_TCP_MD5SIG 549 newtp->md5sig_info = NULL; /*XXX*/ 550 if (newtp->af_specific->md5_lookup(sk, newsk)) 551 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 552 #endif 553 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) 554 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 555 newtp->rx_opt.mss_clamp = req->mss; 556 TCP_ECN_openreq_child(newtp, req); 557 558 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS); 559 } 560 return newsk; 561 } 562 EXPORT_SYMBOL(tcp_create_openreq_child); 563 564 /* 565 * Process an incoming packet for SYN_RECV sockets represented 566 * as a request_sock. 567 */ 568 569 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 570 struct request_sock *req, 571 struct request_sock **prev) 572 { 573 struct tcp_options_received tmp_opt; 574 const u8 *hash_location; 575 struct sock *child; 576 const struct tcphdr *th = tcp_hdr(skb); 577 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 578 bool paws_reject = false; 579 580 tmp_opt.saw_tstamp = 0; 581 if (th->doff > (sizeof(struct tcphdr)>>2)) { 582 tcp_parse_options(skb, &tmp_opt, &hash_location, 0); 583 584 if (tmp_opt.saw_tstamp) { 585 tmp_opt.ts_recent = req->ts_recent; 586 /* We do not store true stamp, but it is not required, 587 * it can be estimated (approximately) 588 * from another data. 589 */ 590 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans); 591 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 592 } 593 } 594 595 /* Check for pure retransmitted SYN. */ 596 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 597 flg == TCP_FLAG_SYN && 598 !paws_reject) { 599 /* 600 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 601 * this case on figure 6 and figure 8, but formal 602 * protocol description says NOTHING. 603 * To be more exact, it says that we should send ACK, 604 * because this segment (at least, if it has no data) 605 * is out of window. 606 * 607 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 608 * describe SYN-RECV state. All the description 609 * is wrong, we cannot believe to it and should 610 * rely only on common sense and implementation 611 * experience. 612 * 613 * Enforce "SYN-ACK" according to figure 8, figure 6 614 * of RFC793, fixed by RFC1122. 615 */ 616 req->rsk_ops->rtx_syn_ack(sk, req, NULL); 617 return NULL; 618 } 619 620 /* Further reproduces section "SEGMENT ARRIVES" 621 for state SYN-RECEIVED of RFC793. 622 It is broken, however, it does not work only 623 when SYNs are crossed. 624 625 You would think that SYN crossing is impossible here, since 626 we should have a SYN_SENT socket (from connect()) on our end, 627 but this is not true if the crossed SYNs were sent to both 628 ends by a malicious third party. We must defend against this, 629 and to do that we first verify the ACK (as per RFC793, page 630 36) and reset if it is invalid. Is this a true full defense? 631 To convince ourselves, let us consider a way in which the ACK 632 test can still pass in this 'malicious crossed SYNs' case. 633 Malicious sender sends identical SYNs (and thus identical sequence 634 numbers) to both A and B: 635 636 A: gets SYN, seq=7 637 B: gets SYN, seq=7 638 639 By our good fortune, both A and B select the same initial 640 send sequence number of seven :-) 641 642 A: sends SYN|ACK, seq=7, ack_seq=8 643 B: sends SYN|ACK, seq=7, ack_seq=8 644 645 So we are now A eating this SYN|ACK, ACK test passes. So 646 does sequence test, SYN is truncated, and thus we consider 647 it a bare ACK. 648 649 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 650 bare ACK. Otherwise, we create an established connection. Both 651 ends (listening sockets) accept the new incoming connection and try 652 to talk to each other. 8-) 653 654 Note: This case is both harmless, and rare. Possibility is about the 655 same as us discovering intelligent life on another plant tomorrow. 656 657 But generally, we should (RFC lies!) to accept ACK 658 from SYNACK both here and in tcp_rcv_state_process(). 659 tcp_rcv_state_process() does not, hence, we do not too. 660 661 Note that the case is absolutely generic: 662 we cannot optimize anything here without 663 violating protocol. All the checks must be made 664 before attempt to create socket. 665 */ 666 667 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 668 * and the incoming segment acknowledges something not yet 669 * sent (the segment carries an unacceptable ACK) ... 670 * a reset is sent." 671 * 672 * Invalid ACK: reset will be sent by listening socket 673 */ 674 if ((flg & TCP_FLAG_ACK) && 675 (TCP_SKB_CB(skb)->ack_seq != 676 tcp_rsk(req)->snt_isn + 1 + tcp_s_data_size(tcp_sk(sk)))) 677 return sk; 678 679 /* Also, it would be not so bad idea to check rcv_tsecr, which 680 * is essentially ACK extension and too early or too late values 681 * should cause reset in unsynchronized states. 682 */ 683 684 /* RFC793: "first check sequence number". */ 685 686 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 687 tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) { 688 /* Out of window: send ACK and drop. */ 689 if (!(flg & TCP_FLAG_RST)) 690 req->rsk_ops->send_ack(sk, skb, req); 691 if (paws_reject) 692 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 693 return NULL; 694 } 695 696 /* In sequence, PAWS is OK. */ 697 698 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1)) 699 req->ts_recent = tmp_opt.rcv_tsval; 700 701 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 702 /* Truncate SYN, it is out of window starting 703 at tcp_rsk(req)->rcv_isn + 1. */ 704 flg &= ~TCP_FLAG_SYN; 705 } 706 707 /* RFC793: "second check the RST bit" and 708 * "fourth, check the SYN bit" 709 */ 710 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 711 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 712 goto embryonic_reset; 713 } 714 715 /* ACK sequence verified above, just make sure ACK is 716 * set. If ACK not set, just silently drop the packet. 717 */ 718 if (!(flg & TCP_FLAG_ACK)) 719 return NULL; 720 721 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 722 if (req->retrans < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 723 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 724 inet_rsk(req)->acked = 1; 725 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP); 726 return NULL; 727 } 728 if (tmp_opt.saw_tstamp && tmp_opt.rcv_tsecr) 729 tcp_rsk(req)->snt_synack = tmp_opt.rcv_tsecr; 730 else if (req->retrans) /* don't take RTT sample if retrans && ~TS */ 731 tcp_rsk(req)->snt_synack = 0; 732 733 /* OK, ACK is valid, create big socket and 734 * feed this segment to it. It will repeat all 735 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 736 * ESTABLISHED STATE. If it will be dropped after 737 * socket is created, wait for troubles. 738 */ 739 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL); 740 if (child == NULL) 741 goto listen_overflow; 742 743 inet_csk_reqsk_queue_unlink(sk, req, prev); 744 inet_csk_reqsk_queue_removed(sk, req); 745 746 inet_csk_reqsk_queue_add(sk, req, child); 747 return child; 748 749 listen_overflow: 750 if (!sysctl_tcp_abort_on_overflow) { 751 inet_rsk(req)->acked = 1; 752 return NULL; 753 } 754 755 embryonic_reset: 756 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 757 if (!(flg & TCP_FLAG_RST)) 758 req->rsk_ops->send_reset(sk, skb); 759 760 inet_csk_reqsk_queue_drop(sk, req, prev); 761 return NULL; 762 } 763 EXPORT_SYMBOL(tcp_check_req); 764 765 /* 766 * Queue segment on the new socket if the new socket is active, 767 * otherwise we just shortcircuit this and continue with 768 * the new socket. 769 */ 770 771 int tcp_child_process(struct sock *parent, struct sock *child, 772 struct sk_buff *skb) 773 { 774 int ret = 0; 775 int state = child->sk_state; 776 777 if (!sock_owned_by_user(child)) { 778 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb), 779 skb->len); 780 /* Wakeup parent, send SIGIO */ 781 if (state == TCP_SYN_RECV && child->sk_state != state) 782 parent->sk_data_ready(parent, 0); 783 } else { 784 /* Alas, it is possible again, because we do lookup 785 * in main socket hash table and lock on listening 786 * socket does not protect us more. 787 */ 788 __sk_add_backlog(child, skb); 789 } 790 791 bh_unlock_sock(child); 792 sock_put(child); 793 return ret; 794 } 795 EXPORT_SYMBOL(tcp_child_process); 796