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 #include <net/busy_poll.h> 30 31 int sysctl_tcp_abort_on_overflow __read_mostly; 32 33 static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win) 34 { 35 if (seq == s_win) 36 return true; 37 if (after(end_seq, s_win) && before(seq, e_win)) 38 return true; 39 return seq == e_win && seq == end_seq; 40 } 41 42 static enum tcp_tw_status 43 tcp_timewait_check_oow_rate_limit(struct inet_timewait_sock *tw, 44 const struct sk_buff *skb, int mib_idx) 45 { 46 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 47 48 if (!tcp_oow_rate_limited(twsk_net(tw), skb, mib_idx, 49 &tcptw->tw_last_oow_ack_time)) { 50 /* Send ACK. Note, we do not put the bucket, 51 * it will be released by caller. 52 */ 53 return TCP_TW_ACK; 54 } 55 56 /* We are rate-limiting, so just release the tw sock and drop skb. */ 57 inet_twsk_put(tw); 58 return TCP_TW_SUCCESS; 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 if (tmp_opt.rcv_tsecr) 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_timewait_check_oow_rate_limit( 121 tw, skb, LINUX_MIB_TCPACKSKIPPEDFINWAIT2); 122 123 if (th->rst) 124 goto kill; 125 126 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt)) 127 return TCP_TW_RST; 128 129 /* Dup ACK? */ 130 if (!th->ack || 131 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) || 132 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) { 133 inet_twsk_put(tw); 134 return TCP_TW_SUCCESS; 135 } 136 137 /* New data or FIN. If new data arrive after half-duplex close, 138 * reset. 139 */ 140 if (!th->fin || 141 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) 142 return TCP_TW_RST; 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 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 153 return TCP_TW_ACK; 154 } 155 156 /* 157 * Now real TIME-WAIT state. 158 * 159 * RFC 1122: 160 * "When a connection is [...] on TIME-WAIT state [...] 161 * [a TCP] MAY accept a new SYN from the remote TCP to 162 * reopen the connection directly, if it: 163 * 164 * (1) assigns its initial sequence number for the new 165 * connection to be larger than the largest sequence 166 * number it used on the previous connection incarnation, 167 * and 168 * 169 * (2) returns to TIME-WAIT state if the SYN turns out 170 * to be an old duplicate". 171 */ 172 173 if (!paws_reject && 174 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 175 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 176 /* In window segment, it may be only reset or bare ack. */ 177 178 if (th->rst) { 179 /* This is TIME_WAIT assassination, in two flavors. 180 * Oh well... nobody has a sufficient solution to this 181 * protocol bug yet. 182 */ 183 if (sysctl_tcp_rfc1337 == 0) { 184 kill: 185 inet_twsk_deschedule_put(tw); 186 return TCP_TW_SUCCESS; 187 } 188 } 189 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 190 191 if (tmp_opt.saw_tstamp) { 192 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 193 tcptw->tw_ts_recent_stamp = get_seconds(); 194 } 195 196 inet_twsk_put(tw); 197 return TCP_TW_SUCCESS; 198 } 199 200 /* Out of window segment. 201 202 All the segments are ACKed immediately. 203 204 The only exception is new SYN. We accept it, if it is 205 not old duplicate and we are not in danger to be killed 206 by delayed old duplicates. RFC check is that it has 207 newer sequence number works at rates <40Mbit/sec. 208 However, if paws works, it is reliable AND even more, 209 we even may relax silly seq space cutoff. 210 211 RED-PEN: we violate main RFC requirement, if this SYN will appear 212 old duplicate (i.e. we receive RST in reply to SYN-ACK), 213 we must return socket to time-wait state. It is not good, 214 but not fatal yet. 215 */ 216 217 if (th->syn && !th->rst && !th->ack && !paws_reject && 218 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 219 (tmp_opt.saw_tstamp && 220 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 221 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 222 if (isn == 0) 223 isn++; 224 TCP_SKB_CB(skb)->tcp_tw_isn = isn; 225 return TCP_TW_SYN; 226 } 227 228 if (paws_reject) 229 __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 230 231 if (!th->rst) { 232 /* In this case we must reset the TIMEWAIT timer. 233 * 234 * If it is ACKless SYN it may be both old duplicate 235 * and new good SYN with random sequence number <rcv_nxt. 236 * Do not reschedule in the last case. 237 */ 238 if (paws_reject || th->ack) 239 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 240 241 return tcp_timewait_check_oow_rate_limit( 242 tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT); 243 } 244 inet_twsk_put(tw); 245 return TCP_TW_SUCCESS; 246 } 247 EXPORT_SYMBOL(tcp_timewait_state_process); 248 249 /* 250 * Move a socket to time-wait or dead fin-wait-2 state. 251 */ 252 void tcp_time_wait(struct sock *sk, int state, int timeo) 253 { 254 const struct inet_connection_sock *icsk = inet_csk(sk); 255 const struct tcp_sock *tp = tcp_sk(sk); 256 struct inet_timewait_sock *tw; 257 struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row; 258 259 tw = inet_twsk_alloc(sk, tcp_death_row, state); 260 261 if (tw) { 262 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 263 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 264 struct inet_sock *inet = inet_sk(sk); 265 266 tw->tw_transparent = inet->transparent; 267 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 268 tcptw->tw_rcv_nxt = tp->rcv_nxt; 269 tcptw->tw_snd_nxt = tp->snd_nxt; 270 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 271 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 272 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 273 tcptw->tw_ts_offset = tp->tsoffset; 274 tcptw->tw_last_oow_ack_time = 0; 275 276 #if IS_ENABLED(CONFIG_IPV6) 277 if (tw->tw_family == PF_INET6) { 278 struct ipv6_pinfo *np = inet6_sk(sk); 279 280 tw->tw_v6_daddr = sk->sk_v6_daddr; 281 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr; 282 tw->tw_tclass = np->tclass; 283 tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK); 284 tw->tw_ipv6only = sk->sk_ipv6only; 285 } 286 #endif 287 288 #ifdef CONFIG_TCP_MD5SIG 289 /* 290 * The timewait bucket does not have the key DB from the 291 * sock structure. We just make a quick copy of the 292 * md5 key being used (if indeed we are using one) 293 * so the timewait ack generating code has the key. 294 */ 295 do { 296 struct tcp_md5sig_key *key; 297 tcptw->tw_md5_key = NULL; 298 key = tp->af_specific->md5_lookup(sk, sk); 299 if (key) { 300 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC); 301 if (tcptw->tw_md5_key && !tcp_alloc_md5sig_pool()) 302 BUG(); 303 } 304 } while (0); 305 #endif 306 307 /* Get the TIME_WAIT timeout firing. */ 308 if (timeo < rto) 309 timeo = rto; 310 311 tw->tw_timeout = TCP_TIMEWAIT_LEN; 312 if (state == TCP_TIME_WAIT) 313 timeo = TCP_TIMEWAIT_LEN; 314 315 inet_twsk_schedule(tw, timeo); 316 /* Linkage updates. */ 317 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 318 inet_twsk_put(tw); 319 } else { 320 /* Sorry, if we're out of memory, just CLOSE this 321 * socket up. We've got bigger problems than 322 * non-graceful socket closings. 323 */ 324 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW); 325 } 326 327 tcp_update_metrics(sk); 328 tcp_done(sk); 329 } 330 331 void tcp_twsk_destructor(struct sock *sk) 332 { 333 #ifdef CONFIG_TCP_MD5SIG 334 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 335 336 if (twsk->tw_md5_key) 337 kfree_rcu(twsk->tw_md5_key, rcu); 338 #endif 339 } 340 EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 341 342 /* Warning : This function is called without sk_listener being locked. 343 * Be sure to read socket fields once, as their value could change under us. 344 */ 345 void tcp_openreq_init_rwin(struct request_sock *req, 346 const struct sock *sk_listener, 347 const struct dst_entry *dst) 348 { 349 struct inet_request_sock *ireq = inet_rsk(req); 350 const struct tcp_sock *tp = tcp_sk(sk_listener); 351 int full_space = tcp_full_space(sk_listener); 352 u32 window_clamp; 353 __u8 rcv_wscale; 354 int mss; 355 356 mss = tcp_mss_clamp(tp, dst_metric_advmss(dst)); 357 window_clamp = READ_ONCE(tp->window_clamp); 358 /* Set this up on the first call only */ 359 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW); 360 361 /* limit the window selection if the user enforce a smaller rx buffer */ 362 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK && 363 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0)) 364 req->rsk_window_clamp = full_space; 365 366 /* tcp_full_space because it is guaranteed to be the first packet */ 367 tcp_select_initial_window(full_space, 368 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0), 369 &req->rsk_rcv_wnd, 370 &req->rsk_window_clamp, 371 ireq->wscale_ok, 372 &rcv_wscale, 373 dst_metric(dst, RTAX_INITRWND)); 374 ireq->rcv_wscale = rcv_wscale; 375 } 376 EXPORT_SYMBOL(tcp_openreq_init_rwin); 377 378 static void tcp_ecn_openreq_child(struct tcp_sock *tp, 379 const struct request_sock *req) 380 { 381 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 382 } 383 384 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst) 385 { 386 struct inet_connection_sock *icsk = inet_csk(sk); 387 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO); 388 bool ca_got_dst = false; 389 390 if (ca_key != TCP_CA_UNSPEC) { 391 const struct tcp_congestion_ops *ca; 392 393 rcu_read_lock(); 394 ca = tcp_ca_find_key(ca_key); 395 if (likely(ca && try_module_get(ca->owner))) { 396 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst); 397 icsk->icsk_ca_ops = ca; 398 ca_got_dst = true; 399 } 400 rcu_read_unlock(); 401 } 402 403 /* If no valid choice made yet, assign current system default ca. */ 404 if (!ca_got_dst && 405 (!icsk->icsk_ca_setsockopt || 406 !try_module_get(icsk->icsk_ca_ops->owner))) 407 tcp_assign_congestion_control(sk); 408 409 tcp_set_ca_state(sk, TCP_CA_Open); 410 } 411 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child); 412 413 /* This is not only more efficient than what we used to do, it eliminates 414 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 415 * 416 * Actually, we could lots of memory writes here. tp of listening 417 * socket contains all necessary default parameters. 418 */ 419 struct sock *tcp_create_openreq_child(const struct sock *sk, 420 struct request_sock *req, 421 struct sk_buff *skb) 422 { 423 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC); 424 425 if (newsk) { 426 const struct inet_request_sock *ireq = inet_rsk(req); 427 struct tcp_request_sock *treq = tcp_rsk(req); 428 struct inet_connection_sock *newicsk = inet_csk(newsk); 429 struct tcp_sock *newtp = tcp_sk(newsk); 430 431 /* Now setup tcp_sock */ 432 newtp->pred_flags = 0; 433 434 newtp->rcv_wup = newtp->copied_seq = 435 newtp->rcv_nxt = treq->rcv_isn + 1; 436 newtp->segs_in = 1; 437 438 newtp->snd_sml = newtp->snd_una = 439 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1; 440 441 tcp_prequeue_init(newtp); 442 INIT_LIST_HEAD(&newtp->tsq_node); 443 444 tcp_init_wl(newtp, treq->rcv_isn); 445 446 newtp->srtt_us = 0; 447 newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT); 448 minmax_reset(&newtp->rtt_min, tcp_time_stamp, ~0U); 449 newicsk->icsk_rto = TCP_TIMEOUT_INIT; 450 newicsk->icsk_ack.lrcvtime = tcp_time_stamp; 451 452 newtp->packets_out = 0; 453 newtp->retrans_out = 0; 454 newtp->sacked_out = 0; 455 newtp->fackets_out = 0; 456 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH; 457 newtp->tlp_high_seq = 0; 458 newtp->lsndtime = treq->snt_synack.stamp_jiffies; 459 newsk->sk_txhash = treq->txhash; 460 newtp->last_oow_ack_time = 0; 461 newtp->total_retrans = req->num_retrans; 462 463 /* So many TCP implementations out there (incorrectly) count the 464 * initial SYN frame in their delayed-ACK and congestion control 465 * algorithms that we must have the following bandaid to talk 466 * efficiently to them. -DaveM 467 */ 468 newtp->snd_cwnd = TCP_INIT_CWND; 469 newtp->snd_cwnd_cnt = 0; 470 471 /* There's a bubble in the pipe until at least the first ACK. */ 472 newtp->app_limited = ~0U; 473 474 tcp_init_xmit_timers(newsk); 475 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1; 476 477 newtp->rx_opt.saw_tstamp = 0; 478 479 newtp->rx_opt.dsack = 0; 480 newtp->rx_opt.num_sacks = 0; 481 482 newtp->urg_data = 0; 483 484 if (sock_flag(newsk, SOCK_KEEPOPEN)) 485 inet_csk_reset_keepalive_timer(newsk, 486 keepalive_time_when(newtp)); 487 488 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 489 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 490 if (sysctl_tcp_fack) 491 tcp_enable_fack(newtp); 492 } 493 newtp->window_clamp = req->rsk_window_clamp; 494 newtp->rcv_ssthresh = req->rsk_rcv_wnd; 495 newtp->rcv_wnd = req->rsk_rcv_wnd; 496 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 497 if (newtp->rx_opt.wscale_ok) { 498 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 499 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 500 } else { 501 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 502 newtp->window_clamp = min(newtp->window_clamp, 65535U); 503 } 504 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 505 newtp->rx_opt.snd_wscale); 506 newtp->max_window = newtp->snd_wnd; 507 508 if (newtp->rx_opt.tstamp_ok) { 509 newtp->rx_opt.ts_recent = req->ts_recent; 510 newtp->rx_opt.ts_recent_stamp = get_seconds(); 511 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 512 } else { 513 newtp->rx_opt.ts_recent_stamp = 0; 514 newtp->tcp_header_len = sizeof(struct tcphdr); 515 } 516 newtp->tsoffset = treq->ts_off; 517 #ifdef CONFIG_TCP_MD5SIG 518 newtp->md5sig_info = NULL; /*XXX*/ 519 if (newtp->af_specific->md5_lookup(sk, newsk)) 520 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 521 #endif 522 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) 523 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 524 newtp->rx_opt.mss_clamp = req->mss; 525 tcp_ecn_openreq_child(newtp, req); 526 newtp->fastopen_rsk = NULL; 527 newtp->syn_data_acked = 0; 528 newtp->rack.mstamp.v64 = 0; 529 newtp->rack.advanced = 0; 530 531 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS); 532 } 533 return newsk; 534 } 535 EXPORT_SYMBOL(tcp_create_openreq_child); 536 537 /* 538 * Process an incoming packet for SYN_RECV sockets represented as a 539 * request_sock. Normally sk is the listener socket but for TFO it 540 * points to the child socket. 541 * 542 * XXX (TFO) - The current impl contains a special check for ack 543 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better? 544 * 545 * We don't need to initialize tmp_opt.sack_ok as we don't use the results 546 */ 547 548 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 549 struct request_sock *req, 550 bool fastopen) 551 { 552 struct tcp_options_received tmp_opt; 553 struct sock *child; 554 const struct tcphdr *th = tcp_hdr(skb); 555 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 556 bool paws_reject = false; 557 bool own_req; 558 559 tmp_opt.saw_tstamp = 0; 560 if (th->doff > (sizeof(struct tcphdr)>>2)) { 561 tcp_parse_options(skb, &tmp_opt, 0, NULL); 562 563 if (tmp_opt.saw_tstamp) { 564 tmp_opt.ts_recent = req->ts_recent; 565 if (tmp_opt.rcv_tsecr) 566 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off; 567 /* We do not store true stamp, but it is not required, 568 * it can be estimated (approximately) 569 * from another data. 570 */ 571 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout); 572 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 573 } 574 } 575 576 /* Check for pure retransmitted SYN. */ 577 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 578 flg == TCP_FLAG_SYN && 579 !paws_reject) { 580 /* 581 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 582 * this case on figure 6 and figure 8, but formal 583 * protocol description says NOTHING. 584 * To be more exact, it says that we should send ACK, 585 * because this segment (at least, if it has no data) 586 * is out of window. 587 * 588 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 589 * describe SYN-RECV state. All the description 590 * is wrong, we cannot believe to it and should 591 * rely only on common sense and implementation 592 * experience. 593 * 594 * Enforce "SYN-ACK" according to figure 8, figure 6 595 * of RFC793, fixed by RFC1122. 596 * 597 * Note that even if there is new data in the SYN packet 598 * they will be thrown away too. 599 * 600 * Reset timer after retransmitting SYNACK, similar to 601 * the idea of fast retransmit in recovery. 602 */ 603 if (!tcp_oow_rate_limited(sock_net(sk), skb, 604 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 605 &tcp_rsk(req)->last_oow_ack_time) && 606 607 !inet_rtx_syn_ack(sk, req)) { 608 unsigned long expires = jiffies; 609 610 expires += min(TCP_TIMEOUT_INIT << req->num_timeout, 611 TCP_RTO_MAX); 612 if (!fastopen) 613 mod_timer_pending(&req->rsk_timer, expires); 614 else 615 req->rsk_timer.expires = expires; 616 } 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 * Note that the ACK validity check for a Fast Open socket is done 674 * elsewhere and is checked directly against the child socket rather 675 * than req because user data may have been sent out. 676 */ 677 if ((flg & TCP_FLAG_ACK) && !fastopen && 678 (TCP_SKB_CB(skb)->ack_seq != 679 tcp_rsk(req)->snt_isn + 1)) 680 return sk; 681 682 /* Also, it would be not so bad idea to check rcv_tsecr, which 683 * is essentially ACK extension and too early or too late values 684 * should cause reset in unsynchronized states. 685 */ 686 687 /* RFC793: "first check sequence number". */ 688 689 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 690 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) { 691 /* Out of window: send ACK and drop. */ 692 if (!(flg & TCP_FLAG_RST) && 693 !tcp_oow_rate_limited(sock_net(sk), skb, 694 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 695 &tcp_rsk(req)->last_oow_ack_time)) 696 req->rsk_ops->send_ack(sk, skb, req); 697 if (paws_reject) 698 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 699 return NULL; 700 } 701 702 /* In sequence, PAWS is OK. */ 703 704 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt)) 705 req->ts_recent = tmp_opt.rcv_tsval; 706 707 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 708 /* Truncate SYN, it is out of window starting 709 at tcp_rsk(req)->rcv_isn + 1. */ 710 flg &= ~TCP_FLAG_SYN; 711 } 712 713 /* RFC793: "second check the RST bit" and 714 * "fourth, check the SYN bit" 715 */ 716 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 717 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 718 goto embryonic_reset; 719 } 720 721 /* ACK sequence verified above, just make sure ACK is 722 * set. If ACK not set, just silently drop the packet. 723 * 724 * XXX (TFO) - if we ever allow "data after SYN", the 725 * following check needs to be removed. 726 */ 727 if (!(flg & TCP_FLAG_ACK)) 728 return NULL; 729 730 /* For Fast Open no more processing is needed (sk is the 731 * child socket). 732 */ 733 if (fastopen) 734 return sk; 735 736 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 737 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 738 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 739 inet_rsk(req)->acked = 1; 740 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP); 741 return NULL; 742 } 743 744 /* OK, ACK is valid, create big socket and 745 * feed this segment to it. It will repeat all 746 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 747 * ESTABLISHED STATE. If it will be dropped after 748 * socket is created, wait for troubles. 749 */ 750 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL, 751 req, &own_req); 752 if (!child) 753 goto listen_overflow; 754 755 sock_rps_save_rxhash(child, skb); 756 tcp_synack_rtt_meas(child, req); 757 return inet_csk_complete_hashdance(sk, child, req, own_req); 758 759 listen_overflow: 760 if (!sysctl_tcp_abort_on_overflow) { 761 inet_rsk(req)->acked = 1; 762 return NULL; 763 } 764 765 embryonic_reset: 766 if (!(flg & TCP_FLAG_RST)) { 767 /* Received a bad SYN pkt - for TFO We try not to reset 768 * the local connection unless it's really necessary to 769 * avoid becoming vulnerable to outside attack aiming at 770 * resetting legit local connections. 771 */ 772 req->rsk_ops->send_reset(sk, skb); 773 } else if (fastopen) { /* received a valid RST pkt */ 774 reqsk_fastopen_remove(sk, req, true); 775 tcp_reset(sk); 776 } 777 if (!fastopen) { 778 inet_csk_reqsk_queue_drop(sk, req); 779 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 780 } 781 return NULL; 782 } 783 EXPORT_SYMBOL(tcp_check_req); 784 785 /* 786 * Queue segment on the new socket if the new socket is active, 787 * otherwise we just shortcircuit this and continue with 788 * the new socket. 789 * 790 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV 791 * when entering. But other states are possible due to a race condition 792 * where after __inet_lookup_established() fails but before the listener 793 * locked is obtained, other packets cause the same connection to 794 * be created. 795 */ 796 797 int tcp_child_process(struct sock *parent, struct sock *child, 798 struct sk_buff *skb) 799 { 800 int ret = 0; 801 int state = child->sk_state; 802 803 /* record NAPI ID of child */ 804 sk_mark_napi_id(child, skb); 805 806 tcp_segs_in(tcp_sk(child), skb); 807 if (!sock_owned_by_user(child)) { 808 ret = tcp_rcv_state_process(child, skb); 809 /* Wakeup parent, send SIGIO */ 810 if (state == TCP_SYN_RECV && child->sk_state != state) 811 parent->sk_data_ready(parent); 812 } else { 813 /* Alas, it is possible again, because we do lookup 814 * in main socket hash table and lock on listening 815 * socket does not protect us more. 816 */ 817 __sk_add_backlog(child, skb); 818 } 819 820 bh_unlock_sock(child); 821 sock_put(child); 822 return ret; 823 } 824 EXPORT_SYMBOL(tcp_child_process); 825