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(twsk_net(tw), 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 u32 rcv_wnd; 355 int mss; 356 357 mss = tcp_mss_clamp(tp, dst_metric_advmss(dst)); 358 window_clamp = READ_ONCE(tp->window_clamp); 359 /* Set this up on the first call only */ 360 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW); 361 362 /* limit the window selection if the user enforce a smaller rx buffer */ 363 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK && 364 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0)) 365 req->rsk_window_clamp = full_space; 366 367 rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req); 368 if (rcv_wnd == 0) 369 rcv_wnd = dst_metric(dst, RTAX_INITRWND); 370 else if (full_space < rcv_wnd * mss) 371 full_space = rcv_wnd * mss; 372 373 /* tcp_full_space because it is guaranteed to be the first packet */ 374 tcp_select_initial_window(full_space, 375 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0), 376 &req->rsk_rcv_wnd, 377 &req->rsk_window_clamp, 378 ireq->wscale_ok, 379 &rcv_wscale, 380 rcv_wnd); 381 ireq->rcv_wscale = rcv_wscale; 382 } 383 EXPORT_SYMBOL(tcp_openreq_init_rwin); 384 385 static void tcp_ecn_openreq_child(struct tcp_sock *tp, 386 const struct request_sock *req) 387 { 388 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 389 } 390 391 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst) 392 { 393 struct inet_connection_sock *icsk = inet_csk(sk); 394 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO); 395 bool ca_got_dst = false; 396 397 if (ca_key != TCP_CA_UNSPEC) { 398 const struct tcp_congestion_ops *ca; 399 400 rcu_read_lock(); 401 ca = tcp_ca_find_key(ca_key); 402 if (likely(ca && try_module_get(ca->owner))) { 403 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst); 404 icsk->icsk_ca_ops = ca; 405 ca_got_dst = true; 406 } 407 rcu_read_unlock(); 408 } 409 410 /* If no valid choice made yet, assign current system default ca. */ 411 if (!ca_got_dst && 412 (!icsk->icsk_ca_setsockopt || 413 !try_module_get(icsk->icsk_ca_ops->owner))) 414 tcp_assign_congestion_control(sk); 415 416 tcp_set_ca_state(sk, TCP_CA_Open); 417 } 418 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child); 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(const struct sock *sk, 427 struct request_sock *req, 428 struct sk_buff *skb) 429 { 430 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC); 431 432 if (newsk) { 433 const struct inet_request_sock *ireq = inet_rsk(req); 434 struct tcp_request_sock *treq = tcp_rsk(req); 435 struct inet_connection_sock *newicsk = inet_csk(newsk); 436 struct tcp_sock *newtp = tcp_sk(newsk); 437 438 /* Now setup tcp_sock */ 439 newtp->pred_flags = 0; 440 441 newtp->rcv_wup = newtp->copied_seq = 442 newtp->rcv_nxt = treq->rcv_isn + 1; 443 newtp->segs_in = 1; 444 445 newtp->snd_sml = newtp->snd_una = 446 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1; 447 448 INIT_LIST_HEAD(&newtp->tsq_node); 449 450 tcp_init_wl(newtp, treq->rcv_isn); 451 452 newtp->srtt_us = 0; 453 newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT); 454 minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U); 455 newicsk->icsk_rto = TCP_TIMEOUT_INIT; 456 newicsk->icsk_ack.lrcvtime = tcp_jiffies32; 457 458 newtp->packets_out = 0; 459 newtp->retrans_out = 0; 460 newtp->sacked_out = 0; 461 newtp->fackets_out = 0; 462 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH; 463 newtp->tlp_high_seq = 0; 464 newtp->lsndtime = tcp_jiffies32; 465 newsk->sk_txhash = treq->txhash; 466 newtp->last_oow_ack_time = 0; 467 newtp->total_retrans = req->num_retrans; 468 469 /* So many TCP implementations out there (incorrectly) count the 470 * initial SYN frame in their delayed-ACK and congestion control 471 * algorithms that we must have the following bandaid to talk 472 * efficiently to them. -DaveM 473 */ 474 newtp->snd_cwnd = TCP_INIT_CWND; 475 newtp->snd_cwnd_cnt = 0; 476 477 /* There's a bubble in the pipe until at least the first ACK. */ 478 newtp->app_limited = ~0U; 479 480 tcp_init_xmit_timers(newsk); 481 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1; 482 483 newtp->rx_opt.saw_tstamp = 0; 484 485 newtp->rx_opt.dsack = 0; 486 newtp->rx_opt.num_sacks = 0; 487 488 newtp->urg_data = 0; 489 490 if (sock_flag(newsk, SOCK_KEEPOPEN)) 491 inet_csk_reset_keepalive_timer(newsk, 492 keepalive_time_when(newtp)); 493 494 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 495 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 496 if (sysctl_tcp_fack) 497 tcp_enable_fack(newtp); 498 } 499 newtp->window_clamp = req->rsk_window_clamp; 500 newtp->rcv_ssthresh = req->rsk_rcv_wnd; 501 newtp->rcv_wnd = req->rsk_rcv_wnd; 502 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 503 if (newtp->rx_opt.wscale_ok) { 504 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 505 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 506 } else { 507 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 508 newtp->window_clamp = min(newtp->window_clamp, 65535U); 509 } 510 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 511 newtp->rx_opt.snd_wscale); 512 newtp->max_window = newtp->snd_wnd; 513 514 if (newtp->rx_opt.tstamp_ok) { 515 newtp->rx_opt.ts_recent = req->ts_recent; 516 newtp->rx_opt.ts_recent_stamp = get_seconds(); 517 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 518 } else { 519 newtp->rx_opt.ts_recent_stamp = 0; 520 newtp->tcp_header_len = sizeof(struct tcphdr); 521 } 522 newtp->tsoffset = treq->ts_off; 523 #ifdef CONFIG_TCP_MD5SIG 524 newtp->md5sig_info = NULL; /*XXX*/ 525 if (newtp->af_specific->md5_lookup(sk, newsk)) 526 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 527 #endif 528 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) 529 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 530 newtp->rx_opt.mss_clamp = req->mss; 531 tcp_ecn_openreq_child(newtp, req); 532 newtp->fastopen_req = NULL; 533 newtp->fastopen_rsk = NULL; 534 newtp->syn_data_acked = 0; 535 newtp->rack.mstamp = 0; 536 newtp->rack.advanced = 0; 537 538 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS); 539 } 540 return newsk; 541 } 542 EXPORT_SYMBOL(tcp_create_openreq_child); 543 544 /* 545 * Process an incoming packet for SYN_RECV sockets represented as a 546 * request_sock. Normally sk is the listener socket but for TFO it 547 * points to the child socket. 548 * 549 * XXX (TFO) - The current impl contains a special check for ack 550 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better? 551 * 552 * We don't need to initialize tmp_opt.sack_ok as we don't use the results 553 */ 554 555 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 556 struct request_sock *req, 557 bool fastopen) 558 { 559 struct tcp_options_received tmp_opt; 560 struct sock *child; 561 const struct tcphdr *th = tcp_hdr(skb); 562 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 563 bool paws_reject = false; 564 bool own_req; 565 566 tmp_opt.saw_tstamp = 0; 567 if (th->doff > (sizeof(struct tcphdr)>>2)) { 568 tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL); 569 570 if (tmp_opt.saw_tstamp) { 571 tmp_opt.ts_recent = req->ts_recent; 572 if (tmp_opt.rcv_tsecr) 573 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off; 574 /* We do not store true stamp, but it is not required, 575 * it can be estimated (approximately) 576 * from another data. 577 */ 578 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout); 579 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 580 } 581 } 582 583 /* Check for pure retransmitted SYN. */ 584 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 585 flg == TCP_FLAG_SYN && 586 !paws_reject) { 587 /* 588 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 589 * this case on figure 6 and figure 8, but formal 590 * protocol description says NOTHING. 591 * To be more exact, it says that we should send ACK, 592 * because this segment (at least, if it has no data) 593 * is out of window. 594 * 595 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 596 * describe SYN-RECV state. All the description 597 * is wrong, we cannot believe to it and should 598 * rely only on common sense and implementation 599 * experience. 600 * 601 * Enforce "SYN-ACK" according to figure 8, figure 6 602 * of RFC793, fixed by RFC1122. 603 * 604 * Note that even if there is new data in the SYN packet 605 * they will be thrown away too. 606 * 607 * Reset timer after retransmitting SYNACK, similar to 608 * the idea of fast retransmit in recovery. 609 */ 610 if (!tcp_oow_rate_limited(sock_net(sk), skb, 611 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 612 &tcp_rsk(req)->last_oow_ack_time) && 613 614 !inet_rtx_syn_ack(sk, req)) { 615 unsigned long expires = jiffies; 616 617 expires += min(TCP_TIMEOUT_INIT << req->num_timeout, 618 TCP_RTO_MAX); 619 if (!fastopen) 620 mod_timer_pending(&req->rsk_timer, expires); 621 else 622 req->rsk_timer.expires = expires; 623 } 624 return NULL; 625 } 626 627 /* Further reproduces section "SEGMENT ARRIVES" 628 for state SYN-RECEIVED of RFC793. 629 It is broken, however, it does not work only 630 when SYNs are crossed. 631 632 You would think that SYN crossing is impossible here, since 633 we should have a SYN_SENT socket (from connect()) on our end, 634 but this is not true if the crossed SYNs were sent to both 635 ends by a malicious third party. We must defend against this, 636 and to do that we first verify the ACK (as per RFC793, page 637 36) and reset if it is invalid. Is this a true full defense? 638 To convince ourselves, let us consider a way in which the ACK 639 test can still pass in this 'malicious crossed SYNs' case. 640 Malicious sender sends identical SYNs (and thus identical sequence 641 numbers) to both A and B: 642 643 A: gets SYN, seq=7 644 B: gets SYN, seq=7 645 646 By our good fortune, both A and B select the same initial 647 send sequence number of seven :-) 648 649 A: sends SYN|ACK, seq=7, ack_seq=8 650 B: sends SYN|ACK, seq=7, ack_seq=8 651 652 So we are now A eating this SYN|ACK, ACK test passes. So 653 does sequence test, SYN is truncated, and thus we consider 654 it a bare ACK. 655 656 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 657 bare ACK. Otherwise, we create an established connection. Both 658 ends (listening sockets) accept the new incoming connection and try 659 to talk to each other. 8-) 660 661 Note: This case is both harmless, and rare. Possibility is about the 662 same as us discovering intelligent life on another plant tomorrow. 663 664 But generally, we should (RFC lies!) to accept ACK 665 from SYNACK both here and in tcp_rcv_state_process(). 666 tcp_rcv_state_process() does not, hence, we do not too. 667 668 Note that the case is absolutely generic: 669 we cannot optimize anything here without 670 violating protocol. All the checks must be made 671 before attempt to create socket. 672 */ 673 674 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 675 * and the incoming segment acknowledges something not yet 676 * sent (the segment carries an unacceptable ACK) ... 677 * a reset is sent." 678 * 679 * Invalid ACK: reset will be sent by listening socket. 680 * Note that the ACK validity check for a Fast Open socket is done 681 * elsewhere and is checked directly against the child socket rather 682 * than req because user data may have been sent out. 683 */ 684 if ((flg & TCP_FLAG_ACK) && !fastopen && 685 (TCP_SKB_CB(skb)->ack_seq != 686 tcp_rsk(req)->snt_isn + 1)) 687 return sk; 688 689 /* Also, it would be not so bad idea to check rcv_tsecr, which 690 * is essentially ACK extension and too early or too late values 691 * should cause reset in unsynchronized states. 692 */ 693 694 /* RFC793: "first check sequence number". */ 695 696 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 697 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) { 698 /* Out of window: send ACK and drop. */ 699 if (!(flg & TCP_FLAG_RST) && 700 !tcp_oow_rate_limited(sock_net(sk), skb, 701 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 702 &tcp_rsk(req)->last_oow_ack_time)) 703 req->rsk_ops->send_ack(sk, skb, req); 704 if (paws_reject) 705 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 706 return NULL; 707 } 708 709 /* In sequence, PAWS is OK. */ 710 711 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt)) 712 req->ts_recent = tmp_opt.rcv_tsval; 713 714 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 715 /* Truncate SYN, it is out of window starting 716 at tcp_rsk(req)->rcv_isn + 1. */ 717 flg &= ~TCP_FLAG_SYN; 718 } 719 720 /* RFC793: "second check the RST bit" and 721 * "fourth, check the SYN bit" 722 */ 723 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 724 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 725 goto embryonic_reset; 726 } 727 728 /* ACK sequence verified above, just make sure ACK is 729 * set. If ACK not set, just silently drop the packet. 730 * 731 * XXX (TFO) - if we ever allow "data after SYN", the 732 * following check needs to be removed. 733 */ 734 if (!(flg & TCP_FLAG_ACK)) 735 return NULL; 736 737 /* For Fast Open no more processing is needed (sk is the 738 * child socket). 739 */ 740 if (fastopen) 741 return sk; 742 743 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 744 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 745 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 746 inet_rsk(req)->acked = 1; 747 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP); 748 return NULL; 749 } 750 751 /* OK, ACK is valid, create big socket and 752 * feed this segment to it. It will repeat all 753 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 754 * ESTABLISHED STATE. If it will be dropped after 755 * socket is created, wait for troubles. 756 */ 757 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL, 758 req, &own_req); 759 if (!child) 760 goto listen_overflow; 761 762 sock_rps_save_rxhash(child, skb); 763 tcp_synack_rtt_meas(child, req); 764 return inet_csk_complete_hashdance(sk, child, req, own_req); 765 766 listen_overflow: 767 if (!sysctl_tcp_abort_on_overflow) { 768 inet_rsk(req)->acked = 1; 769 return NULL; 770 } 771 772 embryonic_reset: 773 if (!(flg & TCP_FLAG_RST)) { 774 /* Received a bad SYN pkt - for TFO We try not to reset 775 * the local connection unless it's really necessary to 776 * avoid becoming vulnerable to outside attack aiming at 777 * resetting legit local connections. 778 */ 779 req->rsk_ops->send_reset(sk, skb); 780 } else if (fastopen) { /* received a valid RST pkt */ 781 reqsk_fastopen_remove(sk, req, true); 782 tcp_reset(sk); 783 } 784 if (!fastopen) { 785 inet_csk_reqsk_queue_drop(sk, req); 786 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 787 } 788 return NULL; 789 } 790 EXPORT_SYMBOL(tcp_check_req); 791 792 /* 793 * Queue segment on the new socket if the new socket is active, 794 * otherwise we just shortcircuit this and continue with 795 * the new socket. 796 * 797 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV 798 * when entering. But other states are possible due to a race condition 799 * where after __inet_lookup_established() fails but before the listener 800 * locked is obtained, other packets cause the same connection to 801 * be created. 802 */ 803 804 int tcp_child_process(struct sock *parent, struct sock *child, 805 struct sk_buff *skb) 806 { 807 int ret = 0; 808 int state = child->sk_state; 809 810 /* record NAPI ID of child */ 811 sk_mark_napi_id(child, skb); 812 813 tcp_segs_in(tcp_sk(child), skb); 814 if (!sock_owned_by_user(child)) { 815 ret = tcp_rcv_state_process(child, skb); 816 /* Wakeup parent, send SIGIO */ 817 if (state == TCP_SYN_RECV && child->sk_state != state) 818 parent->sk_data_ready(parent); 819 } else { 820 /* Alas, it is possible again, because we do lookup 821 * in main socket hash table and lock on listening 822 * socket does not protect us more. 823 */ 824 __sk_add_backlog(child, skb); 825 } 826 827 bh_unlock_sock(child); 828 sock_put(child); 829 return ret; 830 } 831 EXPORT_SYMBOL(tcp_child_process); 832