1 // SPDX-License-Identifier: GPL-2.0-only 2 /* 3 * INET An implementation of the TCP/IP protocol suite for the LINUX 4 * operating system. INET is implemented using the BSD Socket 5 * interface as the means of communication with the user level. 6 * 7 * Implementation of the Transmission Control Protocol(TCP). 8 * 9 * Authors: Ross Biro 10 * Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG> 11 * Mark Evans, <evansmp@uhura.aston.ac.uk> 12 * Corey Minyard <wf-rch!minyard@relay.EU.net> 13 * Florian La Roche, <flla@stud.uni-sb.de> 14 * Charles Hedrick, <hedrick@klinzhai.rutgers.edu> 15 * Linus Torvalds, <torvalds@cs.helsinki.fi> 16 * Alan Cox, <gw4pts@gw4pts.ampr.org> 17 * Matthew Dillon, <dillon@apollo.west.oic.com> 18 * Arnt Gulbrandsen, <agulbra@nvg.unit.no> 19 * Jorge Cwik, <jorge@laser.satlink.net> 20 */ 21 22 #include <linux/mm.h> 23 #include <linux/module.h> 24 #include <linux/slab.h> 25 #include <linux/sysctl.h> 26 #include <linux/workqueue.h> 27 #include <linux/static_key.h> 28 #include <net/tcp.h> 29 #include <net/inet_common.h> 30 #include <net/xfrm.h> 31 #include <net/busy_poll.h> 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 = ktime_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 (twsk_net(tw)->ipv4.sysctl_tcp_rfc1337 == 0) { 184 kill: 185 inet_twsk_deschedule_put(tw); 186 return TCP_TW_SUCCESS; 187 } 188 } else { 189 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 190 } 191 192 if (tmp_opt.saw_tstamp) { 193 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 194 tcptw->tw_ts_recent_stamp = ktime_get_seconds(); 195 } 196 197 inet_twsk_put(tw); 198 return TCP_TW_SUCCESS; 199 } 200 201 /* Out of window segment. 202 203 All the segments are ACKed immediately. 204 205 The only exception is new SYN. We accept it, if it is 206 not old duplicate and we are not in danger to be killed 207 by delayed old duplicates. RFC check is that it has 208 newer sequence number works at rates <40Mbit/sec. 209 However, if paws works, it is reliable AND even more, 210 we even may relax silly seq space cutoff. 211 212 RED-PEN: we violate main RFC requirement, if this SYN will appear 213 old duplicate (i.e. we receive RST in reply to SYN-ACK), 214 we must return socket to time-wait state. It is not good, 215 but not fatal yet. 216 */ 217 218 if (th->syn && !th->rst && !th->ack && !paws_reject && 219 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 220 (tmp_opt.saw_tstamp && 221 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 222 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 223 if (isn == 0) 224 isn++; 225 TCP_SKB_CB(skb)->tcp_tw_isn = isn; 226 return TCP_TW_SYN; 227 } 228 229 if (paws_reject) 230 __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 231 232 if (!th->rst) { 233 /* In this case we must reset the TIMEWAIT timer. 234 * 235 * If it is ACKless SYN it may be both old duplicate 236 * and new good SYN with random sequence number <rcv_nxt. 237 * Do not reschedule in the last case. 238 */ 239 if (paws_reject || th->ack) 240 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 241 242 return tcp_timewait_check_oow_rate_limit( 243 tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT); 244 } 245 inet_twsk_put(tw); 246 return TCP_TW_SUCCESS; 247 } 248 EXPORT_SYMBOL(tcp_timewait_state_process); 249 250 /* 251 * Move a socket to time-wait or dead fin-wait-2 state. 252 */ 253 void tcp_time_wait(struct sock *sk, int state, int timeo) 254 { 255 const struct inet_connection_sock *icsk = inet_csk(sk); 256 const struct tcp_sock *tp = tcp_sk(sk); 257 struct inet_timewait_sock *tw; 258 struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row; 259 260 tw = inet_twsk_alloc(sk, tcp_death_row, state); 261 262 if (tw) { 263 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 264 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 265 struct inet_sock *inet = inet_sk(sk); 266 267 tw->tw_transparent = inet->transparent; 268 tw->tw_mark = sk->sk_mark; 269 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 270 tcptw->tw_rcv_nxt = tp->rcv_nxt; 271 tcptw->tw_snd_nxt = tp->snd_nxt; 272 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 273 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 274 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 275 tcptw->tw_ts_offset = tp->tsoffset; 276 tcptw->tw_last_oow_ack_time = 0; 277 278 #if IS_ENABLED(CONFIG_IPV6) 279 if (tw->tw_family == PF_INET6) { 280 struct ipv6_pinfo *np = inet6_sk(sk); 281 282 tw->tw_v6_daddr = sk->sk_v6_daddr; 283 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr; 284 tw->tw_tclass = np->tclass; 285 tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK); 286 tw->tw_ipv6only = sk->sk_ipv6only; 287 } 288 #endif 289 290 #ifdef CONFIG_TCP_MD5SIG 291 /* 292 * The timewait bucket does not have the key DB from the 293 * sock structure. We just make a quick copy of the 294 * md5 key being used (if indeed we are using one) 295 * so the timewait ack generating code has the key. 296 */ 297 do { 298 tcptw->tw_md5_key = NULL; 299 if (static_branch_unlikely(&tcp_md5_needed)) { 300 struct tcp_md5sig_key *key; 301 302 key = tp->af_specific->md5_lookup(sk, sk); 303 if (key) { 304 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC); 305 BUG_ON(tcptw->tw_md5_key && !tcp_alloc_md5sig_pool()); 306 } 307 } 308 } while (0); 309 #endif 310 311 /* Get the TIME_WAIT timeout firing. */ 312 if (timeo < rto) 313 timeo = rto; 314 315 if (state == TCP_TIME_WAIT) 316 timeo = TCP_TIMEWAIT_LEN; 317 318 /* tw_timer is pinned, so we need to make sure BH are disabled 319 * in following section, otherwise timer handler could run before 320 * we complete the initialization. 321 */ 322 local_bh_disable(); 323 inet_twsk_schedule(tw, timeo); 324 /* Linkage updates. 325 * Note that access to tw after this point is illegal. 326 */ 327 inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 328 local_bh_enable(); 329 } else { 330 /* Sorry, if we're out of memory, just CLOSE this 331 * socket up. We've got bigger problems than 332 * non-graceful socket closings. 333 */ 334 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW); 335 } 336 337 tcp_update_metrics(sk); 338 tcp_done(sk); 339 } 340 EXPORT_SYMBOL(tcp_time_wait); 341 342 void tcp_twsk_destructor(struct sock *sk) 343 { 344 #ifdef CONFIG_TCP_MD5SIG 345 if (static_branch_unlikely(&tcp_md5_needed)) { 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 } 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 u32 rcv_wnd; 368 int mss; 369 370 mss = tcp_mss_clamp(tp, dst_metric_advmss(dst)); 371 window_clamp = READ_ONCE(tp->window_clamp); 372 /* Set this up on the first call only */ 373 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW); 374 375 /* limit the window selection if the user enforce a smaller rx buffer */ 376 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK && 377 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0)) 378 req->rsk_window_clamp = full_space; 379 380 rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req); 381 if (rcv_wnd == 0) 382 rcv_wnd = dst_metric(dst, RTAX_INITRWND); 383 else if (full_space < rcv_wnd * mss) 384 full_space = rcv_wnd * mss; 385 386 /* tcp_full_space because it is guaranteed to be the first packet */ 387 tcp_select_initial_window(sk_listener, full_space, 388 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0), 389 &req->rsk_rcv_wnd, 390 &req->rsk_window_clamp, 391 ireq->wscale_ok, 392 &rcv_wscale, 393 rcv_wnd); 394 ireq->rcv_wscale = rcv_wscale; 395 } 396 EXPORT_SYMBOL(tcp_openreq_init_rwin); 397 398 static void tcp_ecn_openreq_child(struct tcp_sock *tp, 399 const struct request_sock *req) 400 { 401 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 402 } 403 404 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst) 405 { 406 struct inet_connection_sock *icsk = inet_csk(sk); 407 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO); 408 bool ca_got_dst = false; 409 410 if (ca_key != TCP_CA_UNSPEC) { 411 const struct tcp_congestion_ops *ca; 412 413 rcu_read_lock(); 414 ca = tcp_ca_find_key(ca_key); 415 if (likely(ca && try_module_get(ca->owner))) { 416 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst); 417 icsk->icsk_ca_ops = ca; 418 ca_got_dst = true; 419 } 420 rcu_read_unlock(); 421 } 422 423 /* If no valid choice made yet, assign current system default ca. */ 424 if (!ca_got_dst && 425 (!icsk->icsk_ca_setsockopt || 426 !try_module_get(icsk->icsk_ca_ops->owner))) 427 tcp_assign_congestion_control(sk); 428 429 tcp_set_ca_state(sk, TCP_CA_Open); 430 } 431 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child); 432 433 static void smc_check_reset_syn_req(struct tcp_sock *oldtp, 434 struct request_sock *req, 435 struct tcp_sock *newtp) 436 { 437 #if IS_ENABLED(CONFIG_SMC) 438 struct inet_request_sock *ireq; 439 440 if (static_branch_unlikely(&tcp_have_smc)) { 441 ireq = inet_rsk(req); 442 if (oldtp->syn_smc && !ireq->smc_ok) 443 newtp->syn_smc = 0; 444 } 445 #endif 446 } 447 448 /* This is not only more efficient than what we used to do, it eliminates 449 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 450 * 451 * Actually, we could lots of memory writes here. tp of listening 452 * socket contains all necessary default parameters. 453 */ 454 struct sock *tcp_create_openreq_child(const struct sock *sk, 455 struct request_sock *req, 456 struct sk_buff *skb) 457 { 458 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC); 459 const struct inet_request_sock *ireq = inet_rsk(req); 460 struct tcp_request_sock *treq = tcp_rsk(req); 461 struct inet_connection_sock *newicsk; 462 struct tcp_sock *oldtp, *newtp; 463 464 if (!newsk) 465 return NULL; 466 467 newicsk = inet_csk(newsk); 468 newtp = tcp_sk(newsk); 469 oldtp = tcp_sk(sk); 470 471 smc_check_reset_syn_req(oldtp, req, newtp); 472 473 /* Now setup tcp_sock */ 474 newtp->pred_flags = 0; 475 476 newtp->rcv_wup = newtp->copied_seq = 477 newtp->rcv_nxt = treq->rcv_isn + 1; 478 newtp->segs_in = 1; 479 480 newtp->snd_sml = newtp->snd_una = 481 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1; 482 483 INIT_LIST_HEAD(&newtp->tsq_node); 484 INIT_LIST_HEAD(&newtp->tsorted_sent_queue); 485 486 tcp_init_wl(newtp, treq->rcv_isn); 487 488 minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U); 489 newicsk->icsk_ack.lrcvtime = tcp_jiffies32; 490 491 newtp->lsndtime = tcp_jiffies32; 492 newsk->sk_txhash = treq->txhash; 493 newtp->total_retrans = req->num_retrans; 494 495 tcp_init_xmit_timers(newsk); 496 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1; 497 498 if (sock_flag(newsk, SOCK_KEEPOPEN)) 499 inet_csk_reset_keepalive_timer(newsk, 500 keepalive_time_when(newtp)); 501 502 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 503 newtp->rx_opt.sack_ok = ireq->sack_ok; 504 newtp->window_clamp = req->rsk_window_clamp; 505 newtp->rcv_ssthresh = req->rsk_rcv_wnd; 506 newtp->rcv_wnd = req->rsk_rcv_wnd; 507 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 508 if (newtp->rx_opt.wscale_ok) { 509 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 510 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 511 } else { 512 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 513 newtp->window_clamp = min(newtp->window_clamp, 65535U); 514 } 515 newtp->snd_wnd = ntohs(tcp_hdr(skb)->window) << newtp->rx_opt.snd_wscale; 516 newtp->max_window = newtp->snd_wnd; 517 518 if (newtp->rx_opt.tstamp_ok) { 519 newtp->rx_opt.ts_recent = req->ts_recent; 520 newtp->rx_opt.ts_recent_stamp = ktime_get_seconds(); 521 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 522 } else { 523 newtp->rx_opt.ts_recent_stamp = 0; 524 newtp->tcp_header_len = sizeof(struct tcphdr); 525 } 526 if (req->num_timeout) { 527 newtp->undo_marker = treq->snt_isn; 528 newtp->retrans_stamp = div_u64(treq->snt_synack, 529 USEC_PER_SEC / TCP_TS_HZ); 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_req = NULL; 542 newtp->fastopen_rsk = NULL; 543 544 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS); 545 546 return newsk; 547 } 548 EXPORT_SYMBOL(tcp_create_openreq_child); 549 550 /* 551 * Process an incoming packet for SYN_RECV sockets represented as a 552 * request_sock. Normally sk is the listener socket but for TFO it 553 * points to the child socket. 554 * 555 * XXX (TFO) - The current impl contains a special check for ack 556 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better? 557 * 558 * We don't need to initialize tmp_opt.sack_ok as we don't use the results 559 */ 560 561 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 562 struct request_sock *req, 563 bool fastopen, bool *req_stolen) 564 { 565 struct tcp_options_received tmp_opt; 566 struct sock *child; 567 const struct tcphdr *th = tcp_hdr(skb); 568 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 569 bool paws_reject = false; 570 bool own_req; 571 572 tmp_opt.saw_tstamp = 0; 573 if (th->doff > (sizeof(struct tcphdr)>>2)) { 574 tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL); 575 576 if (tmp_opt.saw_tstamp) { 577 tmp_opt.ts_recent = req->ts_recent; 578 if (tmp_opt.rcv_tsecr) 579 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off; 580 /* We do not store true stamp, but it is not required, 581 * it can be estimated (approximately) 582 * from another data. 583 */ 584 tmp_opt.ts_recent_stamp = ktime_get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout); 585 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 586 } 587 } 588 589 /* Check for pure retransmitted SYN. */ 590 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 591 flg == TCP_FLAG_SYN && 592 !paws_reject) { 593 /* 594 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 595 * this case on figure 6 and figure 8, but formal 596 * protocol description says NOTHING. 597 * To be more exact, it says that we should send ACK, 598 * because this segment (at least, if it has no data) 599 * is out of window. 600 * 601 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 602 * describe SYN-RECV state. All the description 603 * is wrong, we cannot believe to it and should 604 * rely only on common sense and implementation 605 * experience. 606 * 607 * Enforce "SYN-ACK" according to figure 8, figure 6 608 * of RFC793, fixed by RFC1122. 609 * 610 * Note that even if there is new data in the SYN packet 611 * they will be thrown away too. 612 * 613 * Reset timer after retransmitting SYNACK, similar to 614 * the idea of fast retransmit in recovery. 615 */ 616 if (!tcp_oow_rate_limited(sock_net(sk), skb, 617 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 618 &tcp_rsk(req)->last_oow_ack_time) && 619 620 !inet_rtx_syn_ack(sk, req)) { 621 unsigned long expires = jiffies; 622 623 expires += min(TCP_TIMEOUT_INIT << req->num_timeout, 624 TCP_RTO_MAX); 625 if (!fastopen) 626 mod_timer_pending(&req->rsk_timer, expires); 627 else 628 req->rsk_timer.expires = expires; 629 } 630 return NULL; 631 } 632 633 /* Further reproduces section "SEGMENT ARRIVES" 634 for state SYN-RECEIVED of RFC793. 635 It is broken, however, it does not work only 636 when SYNs are crossed. 637 638 You would think that SYN crossing is impossible here, since 639 we should have a SYN_SENT socket (from connect()) on our end, 640 but this is not true if the crossed SYNs were sent to both 641 ends by a malicious third party. We must defend against this, 642 and to do that we first verify the ACK (as per RFC793, page 643 36) and reset if it is invalid. Is this a true full defense? 644 To convince ourselves, let us consider a way in which the ACK 645 test can still pass in this 'malicious crossed SYNs' case. 646 Malicious sender sends identical SYNs (and thus identical sequence 647 numbers) to both A and B: 648 649 A: gets SYN, seq=7 650 B: gets SYN, seq=7 651 652 By our good fortune, both A and B select the same initial 653 send sequence number of seven :-) 654 655 A: sends SYN|ACK, seq=7, ack_seq=8 656 B: sends SYN|ACK, seq=7, ack_seq=8 657 658 So we are now A eating this SYN|ACK, ACK test passes. So 659 does sequence test, SYN is truncated, and thus we consider 660 it a bare ACK. 661 662 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 663 bare ACK. Otherwise, we create an established connection. Both 664 ends (listening sockets) accept the new incoming connection and try 665 to talk to each other. 8-) 666 667 Note: This case is both harmless, and rare. Possibility is about the 668 same as us discovering intelligent life on another plant tomorrow. 669 670 But generally, we should (RFC lies!) to accept ACK 671 from SYNACK both here and in tcp_rcv_state_process(). 672 tcp_rcv_state_process() does not, hence, we do not too. 673 674 Note that the case is absolutely generic: 675 we cannot optimize anything here without 676 violating protocol. All the checks must be made 677 before attempt to create socket. 678 */ 679 680 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 681 * and the incoming segment acknowledges something not yet 682 * sent (the segment carries an unacceptable ACK) ... 683 * a reset is sent." 684 * 685 * Invalid ACK: reset will be sent by listening socket. 686 * Note that the ACK validity check for a Fast Open socket is done 687 * elsewhere and is checked directly against the child socket rather 688 * than req because user data may have been sent out. 689 */ 690 if ((flg & TCP_FLAG_ACK) && !fastopen && 691 (TCP_SKB_CB(skb)->ack_seq != 692 tcp_rsk(req)->snt_isn + 1)) 693 return sk; 694 695 /* Also, it would be not so bad idea to check rcv_tsecr, which 696 * is essentially ACK extension and too early or too late values 697 * should cause reset in unsynchronized states. 698 */ 699 700 /* RFC793: "first check sequence number". */ 701 702 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 703 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) { 704 /* Out of window: send ACK and drop. */ 705 if (!(flg & TCP_FLAG_RST) && 706 !tcp_oow_rate_limited(sock_net(sk), skb, 707 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 708 &tcp_rsk(req)->last_oow_ack_time)) 709 req->rsk_ops->send_ack(sk, skb, req); 710 if (paws_reject) 711 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 712 return NULL; 713 } 714 715 /* In sequence, PAWS is OK. */ 716 717 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt)) 718 req->ts_recent = tmp_opt.rcv_tsval; 719 720 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 721 /* Truncate SYN, it is out of window starting 722 at tcp_rsk(req)->rcv_isn + 1. */ 723 flg &= ~TCP_FLAG_SYN; 724 } 725 726 /* RFC793: "second check the RST bit" and 727 * "fourth, check the SYN bit" 728 */ 729 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 730 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 731 goto embryonic_reset; 732 } 733 734 /* ACK sequence verified above, just make sure ACK is 735 * set. If ACK not set, just silently drop the packet. 736 * 737 * XXX (TFO) - if we ever allow "data after SYN", the 738 * following check needs to be removed. 739 */ 740 if (!(flg & TCP_FLAG_ACK)) 741 return NULL; 742 743 /* For Fast Open no more processing is needed (sk is the 744 * child socket). 745 */ 746 if (fastopen) 747 return sk; 748 749 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 750 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 751 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 752 inet_rsk(req)->acked = 1; 753 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP); 754 return NULL; 755 } 756 757 /* OK, ACK is valid, create big socket and 758 * feed this segment to it. It will repeat all 759 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 760 * ESTABLISHED STATE. If it will be dropped after 761 * socket is created, wait for troubles. 762 */ 763 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL, 764 req, &own_req); 765 if (!child) 766 goto listen_overflow; 767 768 sock_rps_save_rxhash(child, skb); 769 tcp_synack_rtt_meas(child, req); 770 *req_stolen = !own_req; 771 return inet_csk_complete_hashdance(sk, child, req, own_req); 772 773 listen_overflow: 774 if (!sock_net(sk)->ipv4.sysctl_tcp_abort_on_overflow) { 775 inet_rsk(req)->acked = 1; 776 return NULL; 777 } 778 779 embryonic_reset: 780 if (!(flg & TCP_FLAG_RST)) { 781 /* Received a bad SYN pkt - for TFO We try not to reset 782 * the local connection unless it's really necessary to 783 * avoid becoming vulnerable to outside attack aiming at 784 * resetting legit local connections. 785 */ 786 req->rsk_ops->send_reset(sk, skb); 787 } else if (fastopen) { /* received a valid RST pkt */ 788 reqsk_fastopen_remove(sk, req, true); 789 tcp_reset(sk); 790 } 791 if (!fastopen) { 792 inet_csk_reqsk_queue_drop(sk, req); 793 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 794 } 795 return NULL; 796 } 797 EXPORT_SYMBOL(tcp_check_req); 798 799 /* 800 * Queue segment on the new socket if the new socket is active, 801 * otherwise we just shortcircuit this and continue with 802 * the new socket. 803 * 804 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV 805 * when entering. But other states are possible due to a race condition 806 * where after __inet_lookup_established() fails but before the listener 807 * locked is obtained, other packets cause the same connection to 808 * be created. 809 */ 810 811 int tcp_child_process(struct sock *parent, struct sock *child, 812 struct sk_buff *skb) 813 { 814 int ret = 0; 815 int state = child->sk_state; 816 817 /* record NAPI ID of child */ 818 sk_mark_napi_id(child, skb); 819 820 tcp_segs_in(tcp_sk(child), skb); 821 if (!sock_owned_by_user(child)) { 822 ret = tcp_rcv_state_process(child, skb); 823 /* Wakeup parent, send SIGIO */ 824 if (state == TCP_SYN_RECV && child->sk_state != state) 825 parent->sk_data_ready(parent); 826 } else { 827 /* Alas, it is possible again, because we do lookup 828 * in main socket hash table and lock on listening 829 * socket does not protect us more. 830 */ 831 __sk_add_backlog(child, skb); 832 } 833 834 bh_unlock_sock(child); 835 sock_put(child); 836 return ret; 837 } 838 EXPORT_SYMBOL(tcp_child_process); 839