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_abort_on_overflow __read_mostly; 31 32 static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win) 33 { 34 if (seq == s_win) 35 return true; 36 if (after(end_seq, s_win) && before(seq, e_win)) 37 return true; 38 return seq == e_win && seq == end_seq; 39 } 40 41 static enum tcp_tw_status 42 tcp_timewait_check_oow_rate_limit(struct inet_timewait_sock *tw, 43 const struct sk_buff *skb, int mib_idx) 44 { 45 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 46 47 if (!tcp_oow_rate_limited(twsk_net(tw), skb, mib_idx, 48 &tcptw->tw_last_oow_ack_time)) { 49 /* Send ACK. Note, we do not put the bucket, 50 * it will be released by caller. 51 */ 52 return TCP_TW_ACK; 53 } 54 55 /* We are rate-limiting, so just release the tw sock and drop skb. */ 56 inet_twsk_put(tw); 57 return TCP_TW_SUCCESS; 58 } 59 60 /* 61 * * Main purpose of TIME-WAIT state is to close connection gracefully, 62 * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN 63 * (and, probably, tail of data) and one or more our ACKs are lost. 64 * * What is TIME-WAIT timeout? It is associated with maximal packet 65 * lifetime in the internet, which results in wrong conclusion, that 66 * it is set to catch "old duplicate segments" wandering out of their path. 67 * It is not quite correct. This timeout is calculated so that it exceeds 68 * maximal retransmission timeout enough to allow to lose one (or more) 69 * segments sent by peer and our ACKs. This time may be calculated from RTO. 70 * * When TIME-WAIT socket receives RST, it means that another end 71 * finally closed and we are allowed to kill TIME-WAIT too. 72 * * Second purpose of TIME-WAIT is catching old duplicate segments. 73 * Well, certainly it is pure paranoia, but if we load TIME-WAIT 74 * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs. 75 * * If we invented some more clever way to catch duplicates 76 * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs. 77 * 78 * The algorithm below is based on FORMAL INTERPRETATION of RFCs. 79 * When you compare it to RFCs, please, read section SEGMENT ARRIVES 80 * from the very beginning. 81 * 82 * NOTE. With recycling (and later with fin-wait-2) TW bucket 83 * is _not_ stateless. It means, that strictly speaking we must 84 * spinlock it. I do not want! Well, probability of misbehaviour 85 * is ridiculously low and, seems, we could use some mb() tricks 86 * to avoid misread sequence numbers, states etc. --ANK 87 * 88 * We don't need to initialize tmp_out.sack_ok as we don't use the results 89 */ 90 enum tcp_tw_status 91 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb, 92 const struct tcphdr *th) 93 { 94 struct tcp_options_received tmp_opt; 95 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 96 bool paws_reject = false; 97 struct inet_timewait_death_row *tcp_death_row = &sock_net((struct sock*)tw)->ipv4.tcp_death_row; 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 if (tcp_death_row->sysctl_tw_recycle && 153 tcptw->tw_ts_recent_stamp && 154 tcp_tw_remember_stamp(tw)) 155 inet_twsk_reschedule(tw, tw->tw_timeout); 156 else 157 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 158 return TCP_TW_ACK; 159 } 160 161 /* 162 * Now real TIME-WAIT state. 163 * 164 * RFC 1122: 165 * "When a connection is [...] on TIME-WAIT state [...] 166 * [a TCP] MAY accept a new SYN from the remote TCP to 167 * reopen the connection directly, if it: 168 * 169 * (1) assigns its initial sequence number for the new 170 * connection to be larger than the largest sequence 171 * number it used on the previous connection incarnation, 172 * and 173 * 174 * (2) returns to TIME-WAIT state if the SYN turns out 175 * to be an old duplicate". 176 */ 177 178 if (!paws_reject && 179 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 180 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 181 /* In window segment, it may be only reset or bare ack. */ 182 183 if (th->rst) { 184 /* This is TIME_WAIT assassination, in two flavors. 185 * Oh well... nobody has a sufficient solution to this 186 * protocol bug yet. 187 */ 188 if (sysctl_tcp_rfc1337 == 0) { 189 kill: 190 inet_twsk_deschedule_put(tw); 191 return TCP_TW_SUCCESS; 192 } 193 } 194 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 195 196 if (tmp_opt.saw_tstamp) { 197 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 198 tcptw->tw_ts_recent_stamp = get_seconds(); 199 } 200 201 inet_twsk_put(tw); 202 return TCP_TW_SUCCESS; 203 } 204 205 /* Out of window segment. 206 207 All the segments are ACKed immediately. 208 209 The only exception is new SYN. We accept it, if it is 210 not old duplicate and we are not in danger to be killed 211 by delayed old duplicates. RFC check is that it has 212 newer sequence number works at rates <40Mbit/sec. 213 However, if paws works, it is reliable AND even more, 214 we even may relax silly seq space cutoff. 215 216 RED-PEN: we violate main RFC requirement, if this SYN will appear 217 old duplicate (i.e. we receive RST in reply to SYN-ACK), 218 we must return socket to time-wait state. It is not good, 219 but not fatal yet. 220 */ 221 222 if (th->syn && !th->rst && !th->ack && !paws_reject && 223 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 224 (tmp_opt.saw_tstamp && 225 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 226 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 227 if (isn == 0) 228 isn++; 229 TCP_SKB_CB(skb)->tcp_tw_isn = isn; 230 return TCP_TW_SYN; 231 } 232 233 if (paws_reject) 234 __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 235 236 if (!th->rst) { 237 /* In this case we must reset the TIMEWAIT timer. 238 * 239 * If it is ACKless SYN it may be both old duplicate 240 * and new good SYN with random sequence number <rcv_nxt. 241 * Do not reschedule in the last case. 242 */ 243 if (paws_reject || th->ack) 244 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 245 246 return tcp_timewait_check_oow_rate_limit( 247 tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT); 248 } 249 inet_twsk_put(tw); 250 return TCP_TW_SUCCESS; 251 } 252 EXPORT_SYMBOL(tcp_timewait_state_process); 253 254 /* 255 * Move a socket to time-wait or dead fin-wait-2 state. 256 */ 257 void tcp_time_wait(struct sock *sk, int state, int timeo) 258 { 259 const struct inet_connection_sock *icsk = inet_csk(sk); 260 const struct tcp_sock *tp = tcp_sk(sk); 261 struct inet_timewait_sock *tw; 262 bool recycle_ok = false; 263 struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row; 264 265 if (tcp_death_row->sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp) 266 recycle_ok = tcp_remember_stamp(sk); 267 268 tw = inet_twsk_alloc(sk, tcp_death_row, state); 269 270 if (tw) { 271 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 272 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 273 struct inet_sock *inet = inet_sk(sk); 274 275 tw->tw_transparent = inet->transparent; 276 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 277 tcptw->tw_rcv_nxt = tp->rcv_nxt; 278 tcptw->tw_snd_nxt = tp->snd_nxt; 279 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 280 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 281 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 282 tcptw->tw_ts_offset = tp->tsoffset; 283 tcptw->tw_last_oow_ack_time = 0; 284 285 #if IS_ENABLED(CONFIG_IPV6) 286 if (tw->tw_family == PF_INET6) { 287 struct ipv6_pinfo *np = inet6_sk(sk); 288 289 tw->tw_v6_daddr = sk->sk_v6_daddr; 290 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr; 291 tw->tw_tclass = np->tclass; 292 tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK); 293 tw->tw_ipv6only = sk->sk_ipv6only; 294 } 295 #endif 296 297 #ifdef CONFIG_TCP_MD5SIG 298 /* 299 * The timewait bucket does not have the key DB from the 300 * sock structure. We just make a quick copy of the 301 * md5 key being used (if indeed we are using one) 302 * so the timewait ack generating code has the key. 303 */ 304 do { 305 struct tcp_md5sig_key *key; 306 tcptw->tw_md5_key = NULL; 307 key = tp->af_specific->md5_lookup(sk, sk); 308 if (key) { 309 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC); 310 if (tcptw->tw_md5_key && !tcp_alloc_md5sig_pool()) 311 BUG(); 312 } 313 } while (0); 314 #endif 315 316 /* Get the TIME_WAIT timeout firing. */ 317 if (timeo < rto) 318 timeo = rto; 319 320 if (recycle_ok) { 321 tw->tw_timeout = rto; 322 } else { 323 tw->tw_timeout = TCP_TIMEWAIT_LEN; 324 if (state == TCP_TIME_WAIT) 325 timeo = TCP_TIMEWAIT_LEN; 326 } 327 328 inet_twsk_schedule(tw, timeo); 329 /* Linkage updates. */ 330 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 331 inet_twsk_put(tw); 332 } else { 333 /* Sorry, if we're out of memory, just CLOSE this 334 * socket up. We've got bigger problems than 335 * non-graceful socket closings. 336 */ 337 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW); 338 } 339 340 tcp_update_metrics(sk); 341 tcp_done(sk); 342 } 343 344 void tcp_twsk_destructor(struct sock *sk) 345 { 346 #ifdef CONFIG_TCP_MD5SIG 347 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 348 349 if (twsk->tw_md5_key) 350 kfree_rcu(twsk->tw_md5_key, rcu); 351 #endif 352 } 353 EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 354 355 /* Warning : This function is called without sk_listener being locked. 356 * Be sure to read socket fields once, as their value could change under us. 357 */ 358 void tcp_openreq_init_rwin(struct request_sock *req, 359 const struct sock *sk_listener, 360 const struct dst_entry *dst) 361 { 362 struct inet_request_sock *ireq = inet_rsk(req); 363 const struct tcp_sock *tp = tcp_sk(sk_listener); 364 int full_space = tcp_full_space(sk_listener); 365 u32 window_clamp; 366 __u8 rcv_wscale; 367 int mss; 368 369 mss = tcp_mss_clamp(tp, dst_metric_advmss(dst)); 370 window_clamp = READ_ONCE(tp->window_clamp); 371 /* Set this up on the first call only */ 372 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW); 373 374 /* limit the window selection if the user enforce a smaller rx buffer */ 375 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK && 376 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0)) 377 req->rsk_window_clamp = full_space; 378 379 /* tcp_full_space because it is guaranteed to be the first packet */ 380 tcp_select_initial_window(full_space, 381 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0), 382 &req->rsk_rcv_wnd, 383 &req->rsk_window_clamp, 384 ireq->wscale_ok, 385 &rcv_wscale, 386 dst_metric(dst, RTAX_INITRWND)); 387 ireq->rcv_wscale = rcv_wscale; 388 } 389 EXPORT_SYMBOL(tcp_openreq_init_rwin); 390 391 static void tcp_ecn_openreq_child(struct tcp_sock *tp, 392 const struct request_sock *req) 393 { 394 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 395 } 396 397 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst) 398 { 399 struct inet_connection_sock *icsk = inet_csk(sk); 400 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO); 401 bool ca_got_dst = false; 402 403 if (ca_key != TCP_CA_UNSPEC) { 404 const struct tcp_congestion_ops *ca; 405 406 rcu_read_lock(); 407 ca = tcp_ca_find_key(ca_key); 408 if (likely(ca && try_module_get(ca->owner))) { 409 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst); 410 icsk->icsk_ca_ops = ca; 411 ca_got_dst = true; 412 } 413 rcu_read_unlock(); 414 } 415 416 /* If no valid choice made yet, assign current system default ca. */ 417 if (!ca_got_dst && 418 (!icsk->icsk_ca_setsockopt || 419 !try_module_get(icsk->icsk_ca_ops->owner))) 420 tcp_assign_congestion_control(sk); 421 422 tcp_set_ca_state(sk, TCP_CA_Open); 423 } 424 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child); 425 426 /* This is not only more efficient than what we used to do, it eliminates 427 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 428 * 429 * Actually, we could lots of memory writes here. tp of listening 430 * socket contains all necessary default parameters. 431 */ 432 struct sock *tcp_create_openreq_child(const struct sock *sk, 433 struct request_sock *req, 434 struct sk_buff *skb) 435 { 436 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC); 437 438 if (newsk) { 439 const struct inet_request_sock *ireq = inet_rsk(req); 440 struct tcp_request_sock *treq = tcp_rsk(req); 441 struct inet_connection_sock *newicsk = inet_csk(newsk); 442 struct tcp_sock *newtp = tcp_sk(newsk); 443 444 /* Now setup tcp_sock */ 445 newtp->pred_flags = 0; 446 447 newtp->rcv_wup = newtp->copied_seq = 448 newtp->rcv_nxt = treq->rcv_isn + 1; 449 newtp->segs_in = 1; 450 451 newtp->snd_sml = newtp->snd_una = 452 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1; 453 454 tcp_prequeue_init(newtp); 455 INIT_LIST_HEAD(&newtp->tsq_node); 456 457 tcp_init_wl(newtp, treq->rcv_isn); 458 459 newtp->srtt_us = 0; 460 newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT); 461 minmax_reset(&newtp->rtt_min, tcp_time_stamp, ~0U); 462 newicsk->icsk_rto = TCP_TIMEOUT_INIT; 463 464 newtp->packets_out = 0; 465 newtp->retrans_out = 0; 466 newtp->sacked_out = 0; 467 newtp->fackets_out = 0; 468 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH; 469 newtp->tlp_high_seq = 0; 470 newtp->lsndtime = treq->snt_synack.stamp_jiffies; 471 newsk->sk_txhash = treq->txhash; 472 newtp->last_oow_ack_time = 0; 473 newtp->total_retrans = req->num_retrans; 474 475 /* So many TCP implementations out there (incorrectly) count the 476 * initial SYN frame in their delayed-ACK and congestion control 477 * algorithms that we must have the following bandaid to talk 478 * efficiently to them. -DaveM 479 */ 480 newtp->snd_cwnd = TCP_INIT_CWND; 481 newtp->snd_cwnd_cnt = 0; 482 483 /* There's a bubble in the pipe until at least the first ACK. */ 484 newtp->app_limited = ~0U; 485 486 tcp_init_xmit_timers(newsk); 487 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1; 488 489 newtp->rx_opt.saw_tstamp = 0; 490 491 newtp->rx_opt.dsack = 0; 492 newtp->rx_opt.num_sacks = 0; 493 494 newtp->urg_data = 0; 495 496 if (sock_flag(newsk, SOCK_KEEPOPEN)) 497 inet_csk_reset_keepalive_timer(newsk, 498 keepalive_time_when(newtp)); 499 500 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 501 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 502 if (sysctl_tcp_fack) 503 tcp_enable_fack(newtp); 504 } 505 newtp->window_clamp = req->rsk_window_clamp; 506 newtp->rcv_ssthresh = req->rsk_rcv_wnd; 507 newtp->rcv_wnd = req->rsk_rcv_wnd; 508 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 509 if (newtp->rx_opt.wscale_ok) { 510 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 511 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 512 } else { 513 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 514 newtp->window_clamp = min(newtp->window_clamp, 65535U); 515 } 516 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 517 newtp->rx_opt.snd_wscale); 518 newtp->max_window = newtp->snd_wnd; 519 520 if (newtp->rx_opt.tstamp_ok) { 521 newtp->rx_opt.ts_recent = req->ts_recent; 522 newtp->rx_opt.ts_recent_stamp = get_seconds(); 523 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 524 } else { 525 newtp->rx_opt.ts_recent_stamp = 0; 526 newtp->tcp_header_len = sizeof(struct tcphdr); 527 } 528 newtp->tsoffset = treq->ts_off; 529 #ifdef CONFIG_TCP_MD5SIG 530 newtp->md5sig_info = NULL; /*XXX*/ 531 if (newtp->af_specific->md5_lookup(sk, newsk)) 532 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 533 #endif 534 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) 535 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 536 newtp->rx_opt.mss_clamp = req->mss; 537 tcp_ecn_openreq_child(newtp, req); 538 newtp->fastopen_rsk = NULL; 539 newtp->syn_data_acked = 0; 540 newtp->rack.mstamp.v64 = 0; 541 newtp->rack.advanced = 0; 542 543 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS); 544 } 545 return newsk; 546 } 547 EXPORT_SYMBOL(tcp_create_openreq_child); 548 549 /* 550 * Process an incoming packet for SYN_RECV sockets represented as a 551 * request_sock. Normally sk is the listener socket but for TFO it 552 * points to the child socket. 553 * 554 * XXX (TFO) - The current impl contains a special check for ack 555 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better? 556 * 557 * We don't need to initialize tmp_opt.sack_ok as we don't use the results 558 */ 559 560 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 561 struct request_sock *req, 562 bool fastopen) 563 { 564 struct tcp_options_received tmp_opt; 565 struct sock *child; 566 const struct tcphdr *th = tcp_hdr(skb); 567 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 568 bool paws_reject = false; 569 bool own_req; 570 571 tmp_opt.saw_tstamp = 0; 572 if (th->doff > (sizeof(struct tcphdr)>>2)) { 573 tcp_parse_options(skb, &tmp_opt, 0, NULL); 574 575 if (tmp_opt.saw_tstamp) { 576 tmp_opt.ts_recent = req->ts_recent; 577 if (tmp_opt.rcv_tsecr) 578 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off; 579 /* We do not store true stamp, but it is not required, 580 * it can be estimated (approximately) 581 * from another data. 582 */ 583 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout); 584 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 585 } 586 } 587 588 /* Check for pure retransmitted SYN. */ 589 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 590 flg == TCP_FLAG_SYN && 591 !paws_reject) { 592 /* 593 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 594 * this case on figure 6 and figure 8, but formal 595 * protocol description says NOTHING. 596 * To be more exact, it says that we should send ACK, 597 * because this segment (at least, if it has no data) 598 * is out of window. 599 * 600 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 601 * describe SYN-RECV state. All the description 602 * is wrong, we cannot believe to it and should 603 * rely only on common sense and implementation 604 * experience. 605 * 606 * Enforce "SYN-ACK" according to figure 8, figure 6 607 * of RFC793, fixed by RFC1122. 608 * 609 * Note that even if there is new data in the SYN packet 610 * they will be thrown away too. 611 * 612 * Reset timer after retransmitting SYNACK, similar to 613 * the idea of fast retransmit in recovery. 614 */ 615 if (!tcp_oow_rate_limited(sock_net(sk), skb, 616 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 617 &tcp_rsk(req)->last_oow_ack_time) && 618 619 !inet_rtx_syn_ack(sk, req)) { 620 unsigned long expires = jiffies; 621 622 expires += min(TCP_TIMEOUT_INIT << req->num_timeout, 623 TCP_RTO_MAX); 624 if (!fastopen) 625 mod_timer_pending(&req->rsk_timer, expires); 626 else 627 req->rsk_timer.expires = expires; 628 } 629 return NULL; 630 } 631 632 /* Further reproduces section "SEGMENT ARRIVES" 633 for state SYN-RECEIVED of RFC793. 634 It is broken, however, it does not work only 635 when SYNs are crossed. 636 637 You would think that SYN crossing is impossible here, since 638 we should have a SYN_SENT socket (from connect()) on our end, 639 but this is not true if the crossed SYNs were sent to both 640 ends by a malicious third party. We must defend against this, 641 and to do that we first verify the ACK (as per RFC793, page 642 36) and reset if it is invalid. Is this a true full defense? 643 To convince ourselves, let us consider a way in which the ACK 644 test can still pass in this 'malicious crossed SYNs' case. 645 Malicious sender sends identical SYNs (and thus identical sequence 646 numbers) to both A and B: 647 648 A: gets SYN, seq=7 649 B: gets SYN, seq=7 650 651 By our good fortune, both A and B select the same initial 652 send sequence number of seven :-) 653 654 A: sends SYN|ACK, seq=7, ack_seq=8 655 B: sends SYN|ACK, seq=7, ack_seq=8 656 657 So we are now A eating this SYN|ACK, ACK test passes. So 658 does sequence test, SYN is truncated, and thus we consider 659 it a bare ACK. 660 661 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 662 bare ACK. Otherwise, we create an established connection. Both 663 ends (listening sockets) accept the new incoming connection and try 664 to talk to each other. 8-) 665 666 Note: This case is both harmless, and rare. Possibility is about the 667 same as us discovering intelligent life on another plant tomorrow. 668 669 But generally, we should (RFC lies!) to accept ACK 670 from SYNACK both here and in tcp_rcv_state_process(). 671 tcp_rcv_state_process() does not, hence, we do not too. 672 673 Note that the case is absolutely generic: 674 we cannot optimize anything here without 675 violating protocol. All the checks must be made 676 before attempt to create socket. 677 */ 678 679 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 680 * and the incoming segment acknowledges something not yet 681 * sent (the segment carries an unacceptable ACK) ... 682 * a reset is sent." 683 * 684 * Invalid ACK: reset will be sent by listening socket. 685 * Note that the ACK validity check for a Fast Open socket is done 686 * elsewhere and is checked directly against the child socket rather 687 * than req because user data may have been sent out. 688 */ 689 if ((flg & TCP_FLAG_ACK) && !fastopen && 690 (TCP_SKB_CB(skb)->ack_seq != 691 tcp_rsk(req)->snt_isn + 1)) 692 return sk; 693 694 /* Also, it would be not so bad idea to check rcv_tsecr, which 695 * is essentially ACK extension and too early or too late values 696 * should cause reset in unsynchronized states. 697 */ 698 699 /* RFC793: "first check sequence number". */ 700 701 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 702 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) { 703 /* Out of window: send ACK and drop. */ 704 if (!(flg & TCP_FLAG_RST) && 705 !tcp_oow_rate_limited(sock_net(sk), skb, 706 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 707 &tcp_rsk(req)->last_oow_ack_time)) 708 req->rsk_ops->send_ack(sk, skb, req); 709 if (paws_reject) 710 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 711 return NULL; 712 } 713 714 /* In sequence, PAWS is OK. */ 715 716 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt)) 717 req->ts_recent = tmp_opt.rcv_tsval; 718 719 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 720 /* Truncate SYN, it is out of window starting 721 at tcp_rsk(req)->rcv_isn + 1. */ 722 flg &= ~TCP_FLAG_SYN; 723 } 724 725 /* RFC793: "second check the RST bit" and 726 * "fourth, check the SYN bit" 727 */ 728 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 729 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 730 goto embryonic_reset; 731 } 732 733 /* ACK sequence verified above, just make sure ACK is 734 * set. If ACK not set, just silently drop the packet. 735 * 736 * XXX (TFO) - if we ever allow "data after SYN", the 737 * following check needs to be removed. 738 */ 739 if (!(flg & TCP_FLAG_ACK)) 740 return NULL; 741 742 /* For Fast Open no more processing is needed (sk is the 743 * child socket). 744 */ 745 if (fastopen) 746 return sk; 747 748 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 749 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 750 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 751 inet_rsk(req)->acked = 1; 752 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP); 753 return NULL; 754 } 755 756 /* OK, ACK is valid, create big socket and 757 * feed this segment to it. It will repeat all 758 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 759 * ESTABLISHED STATE. If it will be dropped after 760 * socket is created, wait for troubles. 761 */ 762 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL, 763 req, &own_req); 764 if (!child) 765 goto listen_overflow; 766 767 sock_rps_save_rxhash(child, skb); 768 tcp_synack_rtt_meas(child, req); 769 return inet_csk_complete_hashdance(sk, child, req, own_req); 770 771 listen_overflow: 772 if (!sysctl_tcp_abort_on_overflow) { 773 inet_rsk(req)->acked = 1; 774 return NULL; 775 } 776 777 embryonic_reset: 778 if (!(flg & TCP_FLAG_RST)) { 779 /* Received a bad SYN pkt - for TFO We try not to reset 780 * the local connection unless it's really necessary to 781 * avoid becoming vulnerable to outside attack aiming at 782 * resetting legit local connections. 783 */ 784 req->rsk_ops->send_reset(sk, skb); 785 } else if (fastopen) { /* received a valid RST pkt */ 786 reqsk_fastopen_remove(sk, req, true); 787 tcp_reset(sk); 788 } 789 if (!fastopen) { 790 inet_csk_reqsk_queue_drop(sk, req); 791 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 792 } 793 return NULL; 794 } 795 EXPORT_SYMBOL(tcp_check_req); 796 797 /* 798 * Queue segment on the new socket if the new socket is active, 799 * otherwise we just shortcircuit this and continue with 800 * the new socket. 801 * 802 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV 803 * when entering. But other states are possible due to a race condition 804 * where after __inet_lookup_established() fails but before the listener 805 * locked is obtained, other packets cause the same connection to 806 * be created. 807 */ 808 809 int tcp_child_process(struct sock *parent, struct sock *child, 810 struct sk_buff *skb) 811 { 812 int ret = 0; 813 int state = child->sk_state; 814 815 tcp_segs_in(tcp_sk(child), skb); 816 if (!sock_owned_by_user(child)) { 817 ret = tcp_rcv_state_process(child, skb); 818 /* Wakeup parent, send SIGIO */ 819 if (state == TCP_SYN_RECV && child->sk_state != state) 820 parent->sk_data_ready(parent); 821 } else { 822 /* Alas, it is possible again, because we do lookup 823 * in main socket hash table and lock on listening 824 * socket does not protect us more. 825 */ 826 __sk_add_backlog(child, skb); 827 } 828 829 bh_unlock_sock(child); 830 sock_put(child); 831 return ret; 832 } 833 EXPORT_SYMBOL(tcp_child_process); 834