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_req = NULL; 527 newtp->fastopen_rsk = NULL; 528 newtp->syn_data_acked = 0; 529 newtp->rack.mstamp.v64 = 0; 530 newtp->rack.advanced = 0; 531 532 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS); 533 } 534 return newsk; 535 } 536 EXPORT_SYMBOL(tcp_create_openreq_child); 537 538 /* 539 * Process an incoming packet for SYN_RECV sockets represented as a 540 * request_sock. Normally sk is the listener socket but for TFO it 541 * points to the child socket. 542 * 543 * XXX (TFO) - The current impl contains a special check for ack 544 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better? 545 * 546 * We don't need to initialize tmp_opt.sack_ok as we don't use the results 547 */ 548 549 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 550 struct request_sock *req, 551 bool fastopen) 552 { 553 struct tcp_options_received tmp_opt; 554 struct sock *child; 555 const struct tcphdr *th = tcp_hdr(skb); 556 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 557 bool paws_reject = false; 558 bool own_req; 559 560 tmp_opt.saw_tstamp = 0; 561 if (th->doff > (sizeof(struct tcphdr)>>2)) { 562 tcp_parse_options(skb, &tmp_opt, 0, NULL); 563 564 if (tmp_opt.saw_tstamp) { 565 tmp_opt.ts_recent = req->ts_recent; 566 if (tmp_opt.rcv_tsecr) 567 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off; 568 /* We do not store true stamp, but it is not required, 569 * it can be estimated (approximately) 570 * from another data. 571 */ 572 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout); 573 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 574 } 575 } 576 577 /* Check for pure retransmitted SYN. */ 578 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 579 flg == TCP_FLAG_SYN && 580 !paws_reject) { 581 /* 582 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 583 * this case on figure 6 and figure 8, but formal 584 * protocol description says NOTHING. 585 * To be more exact, it says that we should send ACK, 586 * because this segment (at least, if it has no data) 587 * is out of window. 588 * 589 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 590 * describe SYN-RECV state. All the description 591 * is wrong, we cannot believe to it and should 592 * rely only on common sense and implementation 593 * experience. 594 * 595 * Enforce "SYN-ACK" according to figure 8, figure 6 596 * of RFC793, fixed by RFC1122. 597 * 598 * Note that even if there is new data in the SYN packet 599 * they will be thrown away too. 600 * 601 * Reset timer after retransmitting SYNACK, similar to 602 * the idea of fast retransmit in recovery. 603 */ 604 if (!tcp_oow_rate_limited(sock_net(sk), skb, 605 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 606 &tcp_rsk(req)->last_oow_ack_time) && 607 608 !inet_rtx_syn_ack(sk, req)) { 609 unsigned long expires = jiffies; 610 611 expires += min(TCP_TIMEOUT_INIT << req->num_timeout, 612 TCP_RTO_MAX); 613 if (!fastopen) 614 mod_timer_pending(&req->rsk_timer, expires); 615 else 616 req->rsk_timer.expires = expires; 617 } 618 return NULL; 619 } 620 621 /* Further reproduces section "SEGMENT ARRIVES" 622 for state SYN-RECEIVED of RFC793. 623 It is broken, however, it does not work only 624 when SYNs are crossed. 625 626 You would think that SYN crossing is impossible here, since 627 we should have a SYN_SENT socket (from connect()) on our end, 628 but this is not true if the crossed SYNs were sent to both 629 ends by a malicious third party. We must defend against this, 630 and to do that we first verify the ACK (as per RFC793, page 631 36) and reset if it is invalid. Is this a true full defense? 632 To convince ourselves, let us consider a way in which the ACK 633 test can still pass in this 'malicious crossed SYNs' case. 634 Malicious sender sends identical SYNs (and thus identical sequence 635 numbers) to both A and B: 636 637 A: gets SYN, seq=7 638 B: gets SYN, seq=7 639 640 By our good fortune, both A and B select the same initial 641 send sequence number of seven :-) 642 643 A: sends SYN|ACK, seq=7, ack_seq=8 644 B: sends SYN|ACK, seq=7, ack_seq=8 645 646 So we are now A eating this SYN|ACK, ACK test passes. So 647 does sequence test, SYN is truncated, and thus we consider 648 it a bare ACK. 649 650 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 651 bare ACK. Otherwise, we create an established connection. Both 652 ends (listening sockets) accept the new incoming connection and try 653 to talk to each other. 8-) 654 655 Note: This case is both harmless, and rare. Possibility is about the 656 same as us discovering intelligent life on another plant tomorrow. 657 658 But generally, we should (RFC lies!) to accept ACK 659 from SYNACK both here and in tcp_rcv_state_process(). 660 tcp_rcv_state_process() does not, hence, we do not too. 661 662 Note that the case is absolutely generic: 663 we cannot optimize anything here without 664 violating protocol. All the checks must be made 665 before attempt to create socket. 666 */ 667 668 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 669 * and the incoming segment acknowledges something not yet 670 * sent (the segment carries an unacceptable ACK) ... 671 * a reset is sent." 672 * 673 * Invalid ACK: reset will be sent by listening socket. 674 * Note that the ACK validity check for a Fast Open socket is done 675 * elsewhere and is checked directly against the child socket rather 676 * than req because user data may have been sent out. 677 */ 678 if ((flg & TCP_FLAG_ACK) && !fastopen && 679 (TCP_SKB_CB(skb)->ack_seq != 680 tcp_rsk(req)->snt_isn + 1)) 681 return sk; 682 683 /* Also, it would be not so bad idea to check rcv_tsecr, which 684 * is essentially ACK extension and too early or too late values 685 * should cause reset in unsynchronized states. 686 */ 687 688 /* RFC793: "first check sequence number". */ 689 690 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 691 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) { 692 /* Out of window: send ACK and drop. */ 693 if (!(flg & TCP_FLAG_RST) && 694 !tcp_oow_rate_limited(sock_net(sk), skb, 695 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 696 &tcp_rsk(req)->last_oow_ack_time)) 697 req->rsk_ops->send_ack(sk, skb, req); 698 if (paws_reject) 699 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 700 return NULL; 701 } 702 703 /* In sequence, PAWS is OK. */ 704 705 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt)) 706 req->ts_recent = tmp_opt.rcv_tsval; 707 708 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 709 /* Truncate SYN, it is out of window starting 710 at tcp_rsk(req)->rcv_isn + 1. */ 711 flg &= ~TCP_FLAG_SYN; 712 } 713 714 /* RFC793: "second check the RST bit" and 715 * "fourth, check the SYN bit" 716 */ 717 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 718 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 719 goto embryonic_reset; 720 } 721 722 /* ACK sequence verified above, just make sure ACK is 723 * set. If ACK not set, just silently drop the packet. 724 * 725 * XXX (TFO) - if we ever allow "data after SYN", the 726 * following check needs to be removed. 727 */ 728 if (!(flg & TCP_FLAG_ACK)) 729 return NULL; 730 731 /* For Fast Open no more processing is needed (sk is the 732 * child socket). 733 */ 734 if (fastopen) 735 return sk; 736 737 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 738 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 739 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 740 inet_rsk(req)->acked = 1; 741 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP); 742 return NULL; 743 } 744 745 /* OK, ACK is valid, create big socket and 746 * feed this segment to it. It will repeat all 747 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 748 * ESTABLISHED STATE. If it will be dropped after 749 * socket is created, wait for troubles. 750 */ 751 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL, 752 req, &own_req); 753 if (!child) 754 goto listen_overflow; 755 756 sock_rps_save_rxhash(child, skb); 757 tcp_synack_rtt_meas(child, req); 758 return inet_csk_complete_hashdance(sk, child, req, own_req); 759 760 listen_overflow: 761 if (!sysctl_tcp_abort_on_overflow) { 762 inet_rsk(req)->acked = 1; 763 return NULL; 764 } 765 766 embryonic_reset: 767 if (!(flg & TCP_FLAG_RST)) { 768 /* Received a bad SYN pkt - for TFO We try not to reset 769 * the local connection unless it's really necessary to 770 * avoid becoming vulnerable to outside attack aiming at 771 * resetting legit local connections. 772 */ 773 req->rsk_ops->send_reset(sk, skb); 774 } else if (fastopen) { /* received a valid RST pkt */ 775 reqsk_fastopen_remove(sk, req, true); 776 tcp_reset(sk); 777 } 778 if (!fastopen) { 779 inet_csk_reqsk_queue_drop(sk, req); 780 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 781 } 782 return NULL; 783 } 784 EXPORT_SYMBOL(tcp_check_req); 785 786 /* 787 * Queue segment on the new socket if the new socket is active, 788 * otherwise we just shortcircuit this and continue with 789 * the new socket. 790 * 791 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV 792 * when entering. But other states are possible due to a race condition 793 * where after __inet_lookup_established() fails but before the listener 794 * locked is obtained, other packets cause the same connection to 795 * be created. 796 */ 797 798 int tcp_child_process(struct sock *parent, struct sock *child, 799 struct sk_buff *skb) 800 { 801 int ret = 0; 802 int state = child->sk_state; 803 804 /* record NAPI ID of child */ 805 sk_mark_napi_id(child, skb); 806 807 tcp_segs_in(tcp_sk(child), skb); 808 if (!sock_owned_by_user(child)) { 809 ret = tcp_rcv_state_process(child, skb); 810 /* Wakeup parent, send SIGIO */ 811 if (state == TCP_SYN_RECV && child->sk_state != state) 812 parent->sk_data_ready(parent); 813 } else { 814 /* Alas, it is possible again, because we do lookup 815 * in main socket hash table and lock on listening 816 * socket does not protect us more. 817 */ 818 __sk_add_backlog(child, skb); 819 } 820 821 bh_unlock_sock(child); 822 sock_put(child); 823 return ret; 824 } 825 EXPORT_SYMBOL(tcp_child_process); 826