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 tmp_opt.rcv_tsecr -= tcptw->tw_ts_offset; 105 tmp_opt.ts_recent = tcptw->tw_ts_recent; 106 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp; 107 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 108 } 109 } 110 111 if (tw->tw_substate == TCP_FIN_WAIT2) { 112 /* Just repeat all the checks of tcp_rcv_state_process() */ 113 114 /* Out of window, send ACK */ 115 if (paws_reject || 116 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 117 tcptw->tw_rcv_nxt, 118 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd)) 119 return tcp_timewait_check_oow_rate_limit( 120 tw, skb, LINUX_MIB_TCPACKSKIPPEDFINWAIT2); 121 122 if (th->rst) 123 goto kill; 124 125 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt)) 126 return TCP_TW_RST; 127 128 /* Dup ACK? */ 129 if (!th->ack || 130 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) || 131 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) { 132 inet_twsk_put(tw); 133 return TCP_TW_SUCCESS; 134 } 135 136 /* New data or FIN. If new data arrive after half-duplex close, 137 * reset. 138 */ 139 if (!th->fin || 140 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) 141 return TCP_TW_RST; 142 143 /* FIN arrived, enter true time-wait state. */ 144 tw->tw_substate = TCP_TIME_WAIT; 145 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq; 146 if (tmp_opt.saw_tstamp) { 147 tcptw->tw_ts_recent_stamp = get_seconds(); 148 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 149 } 150 151 if (tcp_death_row->sysctl_tw_recycle && 152 tcptw->tw_ts_recent_stamp && 153 tcp_tw_remember_stamp(tw)) 154 inet_twsk_reschedule(tw, tw->tw_timeout); 155 else 156 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 157 return TCP_TW_ACK; 158 } 159 160 /* 161 * Now real TIME-WAIT state. 162 * 163 * RFC 1122: 164 * "When a connection is [...] on TIME-WAIT state [...] 165 * [a TCP] MAY accept a new SYN from the remote TCP to 166 * reopen the connection directly, if it: 167 * 168 * (1) assigns its initial sequence number for the new 169 * connection to be larger than the largest sequence 170 * number it used on the previous connection incarnation, 171 * and 172 * 173 * (2) returns to TIME-WAIT state if the SYN turns out 174 * to be an old duplicate". 175 */ 176 177 if (!paws_reject && 178 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 179 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 180 /* In window segment, it may be only reset or bare ack. */ 181 182 if (th->rst) { 183 /* This is TIME_WAIT assassination, in two flavors. 184 * Oh well... nobody has a sufficient solution to this 185 * protocol bug yet. 186 */ 187 if (sysctl_tcp_rfc1337 == 0) { 188 kill: 189 inet_twsk_deschedule_put(tw); 190 return TCP_TW_SUCCESS; 191 } 192 } 193 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 194 195 if (tmp_opt.saw_tstamp) { 196 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 197 tcptw->tw_ts_recent_stamp = get_seconds(); 198 } 199 200 inet_twsk_put(tw); 201 return TCP_TW_SUCCESS; 202 } 203 204 /* Out of window segment. 205 206 All the segments are ACKed immediately. 207 208 The only exception is new SYN. We accept it, if it is 209 not old duplicate and we are not in danger to be killed 210 by delayed old duplicates. RFC check is that it has 211 newer sequence number works at rates <40Mbit/sec. 212 However, if paws works, it is reliable AND even more, 213 we even may relax silly seq space cutoff. 214 215 RED-PEN: we violate main RFC requirement, if this SYN will appear 216 old duplicate (i.e. we receive RST in reply to SYN-ACK), 217 we must return socket to time-wait state. It is not good, 218 but not fatal yet. 219 */ 220 221 if (th->syn && !th->rst && !th->ack && !paws_reject && 222 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 223 (tmp_opt.saw_tstamp && 224 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 225 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 226 if (isn == 0) 227 isn++; 228 TCP_SKB_CB(skb)->tcp_tw_isn = isn; 229 return TCP_TW_SYN; 230 } 231 232 if (paws_reject) 233 __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 234 235 if (!th->rst) { 236 /* In this case we must reset the TIMEWAIT timer. 237 * 238 * If it is ACKless SYN it may be both old duplicate 239 * and new good SYN with random sequence number <rcv_nxt. 240 * Do not reschedule in the last case. 241 */ 242 if (paws_reject || th->ack) 243 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 244 245 return tcp_timewait_check_oow_rate_limit( 246 tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT); 247 } 248 inet_twsk_put(tw); 249 return TCP_TW_SUCCESS; 250 } 251 EXPORT_SYMBOL(tcp_timewait_state_process); 252 253 /* 254 * Move a socket to time-wait or dead fin-wait-2 state. 255 */ 256 void tcp_time_wait(struct sock *sk, int state, int timeo) 257 { 258 const struct inet_connection_sock *icsk = inet_csk(sk); 259 const struct tcp_sock *tp = tcp_sk(sk); 260 struct inet_timewait_sock *tw; 261 bool recycle_ok = false; 262 struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row; 263 264 if (tcp_death_row->sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp) 265 recycle_ok = tcp_remember_stamp(sk); 266 267 tw = inet_twsk_alloc(sk, tcp_death_row, state); 268 269 if (tw) { 270 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 271 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 272 struct inet_sock *inet = inet_sk(sk); 273 274 tw->tw_transparent = inet->transparent; 275 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 276 tcptw->tw_rcv_nxt = tp->rcv_nxt; 277 tcptw->tw_snd_nxt = tp->snd_nxt; 278 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 279 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 280 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 281 tcptw->tw_ts_offset = tp->tsoffset; 282 tcptw->tw_last_oow_ack_time = 0; 283 284 #if IS_ENABLED(CONFIG_IPV6) 285 if (tw->tw_family == PF_INET6) { 286 struct ipv6_pinfo *np = inet6_sk(sk); 287 288 tw->tw_v6_daddr = sk->sk_v6_daddr; 289 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr; 290 tw->tw_tclass = np->tclass; 291 tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK); 292 tw->tw_ipv6only = sk->sk_ipv6only; 293 } 294 #endif 295 296 #ifdef CONFIG_TCP_MD5SIG 297 /* 298 * The timewait bucket does not have the key DB from the 299 * sock structure. We just make a quick copy of the 300 * md5 key being used (if indeed we are using one) 301 * so the timewait ack generating code has the key. 302 */ 303 do { 304 struct tcp_md5sig_key *key; 305 tcptw->tw_md5_key = NULL; 306 key = tp->af_specific->md5_lookup(sk, sk); 307 if (key) { 308 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC); 309 if (tcptw->tw_md5_key && !tcp_alloc_md5sig_pool()) 310 BUG(); 311 } 312 } while (0); 313 #endif 314 315 /* Get the TIME_WAIT timeout firing. */ 316 if (timeo < rto) 317 timeo = rto; 318 319 if (recycle_ok) { 320 tw->tw_timeout = rto; 321 } else { 322 tw->tw_timeout = TCP_TIMEWAIT_LEN; 323 if (state == TCP_TIME_WAIT) 324 timeo = TCP_TIMEWAIT_LEN; 325 } 326 327 inet_twsk_schedule(tw, timeo); 328 /* Linkage updates. */ 329 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 330 inet_twsk_put(tw); 331 } else { 332 /* Sorry, if we're out of memory, just CLOSE this 333 * socket up. We've got bigger problems than 334 * non-graceful socket closings. 335 */ 336 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW); 337 } 338 339 tcp_update_metrics(sk); 340 tcp_done(sk); 341 } 342 343 void tcp_twsk_destructor(struct sock *sk) 344 { 345 #ifdef CONFIG_TCP_MD5SIG 346 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 347 348 if (twsk->tw_md5_key) 349 kfree_rcu(twsk->tw_md5_key, rcu); 350 #endif 351 } 352 EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 353 354 /* Warning : This function is called without sk_listener being locked. 355 * Be sure to read socket fields once, as their value could change under us. 356 */ 357 void tcp_openreq_init_rwin(struct request_sock *req, 358 const struct sock *sk_listener, 359 const struct dst_entry *dst) 360 { 361 struct inet_request_sock *ireq = inet_rsk(req); 362 const struct tcp_sock *tp = tcp_sk(sk_listener); 363 u16 user_mss = READ_ONCE(tp->rx_opt.user_mss); 364 int full_space = tcp_full_space(sk_listener); 365 int mss = dst_metric_advmss(dst); 366 u32 window_clamp; 367 __u8 rcv_wscale; 368 369 if (user_mss && user_mss < mss) 370 mss = user_mss; 371 372 window_clamp = READ_ONCE(tp->window_clamp); 373 /* Set this up on the first call only */ 374 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW); 375 376 /* limit the window selection if the user enforce a smaller rx buffer */ 377 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK && 378 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0)) 379 req->rsk_window_clamp = full_space; 380 381 /* tcp_full_space because it is guaranteed to be the first packet */ 382 tcp_select_initial_window(full_space, 383 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0), 384 &req->rsk_rcv_wnd, 385 &req->rsk_window_clamp, 386 ireq->wscale_ok, 387 &rcv_wscale, 388 dst_metric(dst, RTAX_INITRWND)); 389 ireq->rcv_wscale = rcv_wscale; 390 } 391 EXPORT_SYMBOL(tcp_openreq_init_rwin); 392 393 static void tcp_ecn_openreq_child(struct tcp_sock *tp, 394 const struct request_sock *req) 395 { 396 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 397 } 398 399 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst) 400 { 401 struct inet_connection_sock *icsk = inet_csk(sk); 402 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO); 403 bool ca_got_dst = false; 404 405 if (ca_key != TCP_CA_UNSPEC) { 406 const struct tcp_congestion_ops *ca; 407 408 rcu_read_lock(); 409 ca = tcp_ca_find_key(ca_key); 410 if (likely(ca && try_module_get(ca->owner))) { 411 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst); 412 icsk->icsk_ca_ops = ca; 413 ca_got_dst = true; 414 } 415 rcu_read_unlock(); 416 } 417 418 /* If no valid choice made yet, assign current system default ca. */ 419 if (!ca_got_dst && 420 (!icsk->icsk_ca_setsockopt || 421 !try_module_get(icsk->icsk_ca_ops->owner))) 422 tcp_assign_congestion_control(sk); 423 424 tcp_set_ca_state(sk, TCP_CA_Open); 425 } 426 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child); 427 428 /* This is not only more efficient than what we used to do, it eliminates 429 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 430 * 431 * Actually, we could lots of memory writes here. tp of listening 432 * socket contains all necessary default parameters. 433 */ 434 struct sock *tcp_create_openreq_child(const struct sock *sk, 435 struct request_sock *req, 436 struct sk_buff *skb) 437 { 438 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC); 439 440 if (newsk) { 441 const struct inet_request_sock *ireq = inet_rsk(req); 442 struct tcp_request_sock *treq = tcp_rsk(req); 443 struct inet_connection_sock *newicsk = inet_csk(newsk); 444 struct tcp_sock *newtp = tcp_sk(newsk); 445 446 /* Now setup tcp_sock */ 447 newtp->pred_flags = 0; 448 449 newtp->rcv_wup = newtp->copied_seq = 450 newtp->rcv_nxt = treq->rcv_isn + 1; 451 newtp->segs_in = 1; 452 453 newtp->snd_sml = newtp->snd_una = 454 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1; 455 456 tcp_prequeue_init(newtp); 457 INIT_LIST_HEAD(&newtp->tsq_node); 458 459 tcp_init_wl(newtp, treq->rcv_isn); 460 461 newtp->srtt_us = 0; 462 newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT); 463 minmax_reset(&newtp->rtt_min, tcp_time_stamp, ~0U); 464 newicsk->icsk_rto = TCP_TIMEOUT_INIT; 465 466 newtp->packets_out = 0; 467 newtp->retrans_out = 0; 468 newtp->sacked_out = 0; 469 newtp->fackets_out = 0; 470 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH; 471 tcp_enable_early_retrans(newtp); 472 newtp->tlp_high_seq = 0; 473 newtp->lsndtime = treq->snt_synack.stamp_jiffies; 474 newsk->sk_txhash = treq->txhash; 475 newtp->last_oow_ack_time = 0; 476 newtp->total_retrans = req->num_retrans; 477 478 /* So many TCP implementations out there (incorrectly) count the 479 * initial SYN frame in their delayed-ACK and congestion control 480 * algorithms that we must have the following bandaid to talk 481 * efficiently to them. -DaveM 482 */ 483 newtp->snd_cwnd = TCP_INIT_CWND; 484 newtp->snd_cwnd_cnt = 0; 485 486 /* There's a bubble in the pipe until at least the first ACK. */ 487 newtp->app_limited = ~0U; 488 489 tcp_init_xmit_timers(newsk); 490 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1; 491 492 newtp->rx_opt.saw_tstamp = 0; 493 494 newtp->rx_opt.dsack = 0; 495 newtp->rx_opt.num_sacks = 0; 496 497 newtp->urg_data = 0; 498 499 if (sock_flag(newsk, SOCK_KEEPOPEN)) 500 inet_csk_reset_keepalive_timer(newsk, 501 keepalive_time_when(newtp)); 502 503 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 504 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 505 if (sysctl_tcp_fack) 506 tcp_enable_fack(newtp); 507 } 508 newtp->window_clamp = req->rsk_window_clamp; 509 newtp->rcv_ssthresh = req->rsk_rcv_wnd; 510 newtp->rcv_wnd = req->rsk_rcv_wnd; 511 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 512 if (newtp->rx_opt.wscale_ok) { 513 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 514 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 515 } else { 516 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 517 newtp->window_clamp = min(newtp->window_clamp, 65535U); 518 } 519 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 520 newtp->rx_opt.snd_wscale); 521 newtp->max_window = newtp->snd_wnd; 522 523 if (newtp->rx_opt.tstamp_ok) { 524 newtp->rx_opt.ts_recent = req->ts_recent; 525 newtp->rx_opt.ts_recent_stamp = get_seconds(); 526 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 527 } else { 528 newtp->rx_opt.ts_recent_stamp = 0; 529 newtp->tcp_header_len = sizeof(struct tcphdr); 530 } 531 newtp->tsoffset = treq->ts_off; 532 #ifdef CONFIG_TCP_MD5SIG 533 newtp->md5sig_info = NULL; /*XXX*/ 534 if (newtp->af_specific->md5_lookup(sk, newsk)) 535 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 536 #endif 537 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) 538 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 539 newtp->rx_opt.mss_clamp = req->mss; 540 tcp_ecn_openreq_child(newtp, req); 541 newtp->fastopen_rsk = NULL; 542 newtp->syn_data_acked = 0; 543 newtp->rack.mstamp.v64 = 0; 544 newtp->rack.advanced = 0; 545 546 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS); 547 } 548 return newsk; 549 } 550 EXPORT_SYMBOL(tcp_create_openreq_child); 551 552 /* 553 * Process an incoming packet for SYN_RECV sockets represented as a 554 * request_sock. Normally sk is the listener socket but for TFO it 555 * points to the child socket. 556 * 557 * XXX (TFO) - The current impl contains a special check for ack 558 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better? 559 * 560 * We don't need to initialize tmp_opt.sack_ok as we don't use the results 561 */ 562 563 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 564 struct request_sock *req, 565 bool fastopen) 566 { 567 struct tcp_options_received tmp_opt; 568 struct sock *child; 569 const struct tcphdr *th = tcp_hdr(skb); 570 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 571 bool paws_reject = false; 572 bool own_req; 573 574 tmp_opt.saw_tstamp = 0; 575 if (th->doff > (sizeof(struct tcphdr)>>2)) { 576 tcp_parse_options(skb, &tmp_opt, 0, NULL); 577 578 if (tmp_opt.saw_tstamp) { 579 tmp_opt.ts_recent = req->ts_recent; 580 if (tmp_opt.rcv_tsecr) 581 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off; 582 /* We do not store true stamp, but it is not required, 583 * it can be estimated (approximately) 584 * from another data. 585 */ 586 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout); 587 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 588 } 589 } 590 591 /* Check for pure retransmitted SYN. */ 592 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 593 flg == TCP_FLAG_SYN && 594 !paws_reject) { 595 /* 596 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 597 * this case on figure 6 and figure 8, but formal 598 * protocol description says NOTHING. 599 * To be more exact, it says that we should send ACK, 600 * because this segment (at least, if it has no data) 601 * is out of window. 602 * 603 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 604 * describe SYN-RECV state. All the description 605 * is wrong, we cannot believe to it and should 606 * rely only on common sense and implementation 607 * experience. 608 * 609 * Enforce "SYN-ACK" according to figure 8, figure 6 610 * of RFC793, fixed by RFC1122. 611 * 612 * Note that even if there is new data in the SYN packet 613 * they will be thrown away too. 614 * 615 * Reset timer after retransmitting SYNACK, similar to 616 * the idea of fast retransmit in recovery. 617 */ 618 if (!tcp_oow_rate_limited(sock_net(sk), skb, 619 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 620 &tcp_rsk(req)->last_oow_ack_time) && 621 622 !inet_rtx_syn_ack(sk, req)) { 623 unsigned long expires = jiffies; 624 625 expires += min(TCP_TIMEOUT_INIT << req->num_timeout, 626 TCP_RTO_MAX); 627 if (!fastopen) 628 mod_timer_pending(&req->rsk_timer, expires); 629 else 630 req->rsk_timer.expires = expires; 631 } 632 return NULL; 633 } 634 635 /* Further reproduces section "SEGMENT ARRIVES" 636 for state SYN-RECEIVED of RFC793. 637 It is broken, however, it does not work only 638 when SYNs are crossed. 639 640 You would think that SYN crossing is impossible here, since 641 we should have a SYN_SENT socket (from connect()) on our end, 642 but this is not true if the crossed SYNs were sent to both 643 ends by a malicious third party. We must defend against this, 644 and to do that we first verify the ACK (as per RFC793, page 645 36) and reset if it is invalid. Is this a true full defense? 646 To convince ourselves, let us consider a way in which the ACK 647 test can still pass in this 'malicious crossed SYNs' case. 648 Malicious sender sends identical SYNs (and thus identical sequence 649 numbers) to both A and B: 650 651 A: gets SYN, seq=7 652 B: gets SYN, seq=7 653 654 By our good fortune, both A and B select the same initial 655 send sequence number of seven :-) 656 657 A: sends SYN|ACK, seq=7, ack_seq=8 658 B: sends SYN|ACK, seq=7, ack_seq=8 659 660 So we are now A eating this SYN|ACK, ACK test passes. So 661 does sequence test, SYN is truncated, and thus we consider 662 it a bare ACK. 663 664 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 665 bare ACK. Otherwise, we create an established connection. Both 666 ends (listening sockets) accept the new incoming connection and try 667 to talk to each other. 8-) 668 669 Note: This case is both harmless, and rare. Possibility is about the 670 same as us discovering intelligent life on another plant tomorrow. 671 672 But generally, we should (RFC lies!) to accept ACK 673 from SYNACK both here and in tcp_rcv_state_process(). 674 tcp_rcv_state_process() does not, hence, we do not too. 675 676 Note that the case is absolutely generic: 677 we cannot optimize anything here without 678 violating protocol. All the checks must be made 679 before attempt to create socket. 680 */ 681 682 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 683 * and the incoming segment acknowledges something not yet 684 * sent (the segment carries an unacceptable ACK) ... 685 * a reset is sent." 686 * 687 * Invalid ACK: reset will be sent by listening socket. 688 * Note that the ACK validity check for a Fast Open socket is done 689 * elsewhere and is checked directly against the child socket rather 690 * than req because user data may have been sent out. 691 */ 692 if ((flg & TCP_FLAG_ACK) && !fastopen && 693 (TCP_SKB_CB(skb)->ack_seq != 694 tcp_rsk(req)->snt_isn + 1)) 695 return sk; 696 697 /* Also, it would be not so bad idea to check rcv_tsecr, which 698 * is essentially ACK extension and too early or too late values 699 * should cause reset in unsynchronized states. 700 */ 701 702 /* RFC793: "first check sequence number". */ 703 704 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 705 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) { 706 /* Out of window: send ACK and drop. */ 707 if (!(flg & TCP_FLAG_RST) && 708 !tcp_oow_rate_limited(sock_net(sk), skb, 709 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 710 &tcp_rsk(req)->last_oow_ack_time)) 711 req->rsk_ops->send_ack(sk, skb, req); 712 if (paws_reject) 713 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 714 return NULL; 715 } 716 717 /* In sequence, PAWS is OK. */ 718 719 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt)) 720 req->ts_recent = tmp_opt.rcv_tsval; 721 722 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 723 /* Truncate SYN, it is out of window starting 724 at tcp_rsk(req)->rcv_isn + 1. */ 725 flg &= ~TCP_FLAG_SYN; 726 } 727 728 /* RFC793: "second check the RST bit" and 729 * "fourth, check the SYN bit" 730 */ 731 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 732 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 733 goto embryonic_reset; 734 } 735 736 /* ACK sequence verified above, just make sure ACK is 737 * set. If ACK not set, just silently drop the packet. 738 * 739 * XXX (TFO) - if we ever allow "data after SYN", the 740 * following check needs to be removed. 741 */ 742 if (!(flg & TCP_FLAG_ACK)) 743 return NULL; 744 745 /* For Fast Open no more processing is needed (sk is the 746 * child socket). 747 */ 748 if (fastopen) 749 return sk; 750 751 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 752 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 753 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 754 inet_rsk(req)->acked = 1; 755 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP); 756 return NULL; 757 } 758 759 /* OK, ACK is valid, create big socket and 760 * feed this segment to it. It will repeat all 761 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 762 * ESTABLISHED STATE. If it will be dropped after 763 * socket is created, wait for troubles. 764 */ 765 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL, 766 req, &own_req); 767 if (!child) 768 goto listen_overflow; 769 770 sock_rps_save_rxhash(child, skb); 771 tcp_synack_rtt_meas(child, req); 772 return inet_csk_complete_hashdance(sk, child, req, own_req); 773 774 listen_overflow: 775 if (!sysctl_tcp_abort_on_overflow) { 776 inet_rsk(req)->acked = 1; 777 return NULL; 778 } 779 780 embryonic_reset: 781 if (!(flg & TCP_FLAG_RST)) { 782 /* Received a bad SYN pkt - for TFO We try not to reset 783 * the local connection unless it's really necessary to 784 * avoid becoming vulnerable to outside attack aiming at 785 * resetting legit local connections. 786 */ 787 req->rsk_ops->send_reset(sk, skb); 788 } else if (fastopen) { /* received a valid RST pkt */ 789 reqsk_fastopen_remove(sk, req, true); 790 tcp_reset(sk); 791 } 792 if (!fastopen) { 793 inet_csk_reqsk_queue_drop(sk, req); 794 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 795 } 796 return NULL; 797 } 798 EXPORT_SYMBOL(tcp_check_req); 799 800 /* 801 * Queue segment on the new socket if the new socket is active, 802 * otherwise we just shortcircuit this and continue with 803 * the new socket. 804 * 805 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV 806 * when entering. But other states are possible due to a race condition 807 * where after __inet_lookup_established() fails but before the listener 808 * locked is obtained, other packets cause the same connection to 809 * be created. 810 */ 811 812 int tcp_child_process(struct sock *parent, struct sock *child, 813 struct sk_buff *skb) 814 { 815 int ret = 0; 816 int state = child->sk_state; 817 818 tcp_segs_in(tcp_sk(child), skb); 819 if (!sock_owned_by_user(child)) { 820 ret = tcp_rcv_state_process(child, skb); 821 /* Wakeup parent, send SIGIO */ 822 if (state == TCP_SYN_RECV && child->sk_state != state) 823 parent->sk_data_ready(parent); 824 } else { 825 /* Alas, it is possible again, because we do lookup 826 * in main socket hash table and lock on listening 827 * socket does not protect us more. 828 */ 829 __sk_add_backlog(child, skb); 830 } 831 832 bh_unlock_sock(child); 833 sock_put(child); 834 return ret; 835 } 836 EXPORT_SYMBOL(tcp_child_process); 837