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 <net/tcp.h> 23 #include <net/xfrm.h> 24 #include <net/busy_poll.h> 25 26 static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win) 27 { 28 if (seq == s_win) 29 return true; 30 if (after(end_seq, s_win) && before(seq, e_win)) 31 return true; 32 return seq == e_win && seq == end_seq; 33 } 34 35 static enum tcp_tw_status 36 tcp_timewait_check_oow_rate_limit(struct inet_timewait_sock *tw, 37 const struct sk_buff *skb, int mib_idx) 38 { 39 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 40 41 if (!tcp_oow_rate_limited(twsk_net(tw), skb, mib_idx, 42 &tcptw->tw_last_oow_ack_time)) { 43 /* Send ACK. Note, we do not put the bucket, 44 * it will be released by caller. 45 */ 46 return TCP_TW_ACK; 47 } 48 49 /* We are rate-limiting, so just release the tw sock and drop skb. */ 50 inet_twsk_put(tw); 51 return TCP_TW_SUCCESS; 52 } 53 54 /* 55 * * Main purpose of TIME-WAIT state is to close connection gracefully, 56 * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN 57 * (and, probably, tail of data) and one or more our ACKs are lost. 58 * * What is TIME-WAIT timeout? It is associated with maximal packet 59 * lifetime in the internet, which results in wrong conclusion, that 60 * it is set to catch "old duplicate segments" wandering out of their path. 61 * It is not quite correct. This timeout is calculated so that it exceeds 62 * maximal retransmission timeout enough to allow to lose one (or more) 63 * segments sent by peer and our ACKs. This time may be calculated from RTO. 64 * * When TIME-WAIT socket receives RST, it means that another end 65 * finally closed and we are allowed to kill TIME-WAIT too. 66 * * Second purpose of TIME-WAIT is catching old duplicate segments. 67 * Well, certainly it is pure paranoia, but if we load TIME-WAIT 68 * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs. 69 * * If we invented some more clever way to catch duplicates 70 * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs. 71 * 72 * The algorithm below is based on FORMAL INTERPRETATION of RFCs. 73 * When you compare it to RFCs, please, read section SEGMENT ARRIVES 74 * from the very beginning. 75 * 76 * NOTE. With recycling (and later with fin-wait-2) TW bucket 77 * is _not_ stateless. It means, that strictly speaking we must 78 * spinlock it. I do not want! Well, probability of misbehaviour 79 * is ridiculously low and, seems, we could use some mb() tricks 80 * to avoid misread sequence numbers, states etc. --ANK 81 * 82 * We don't need to initialize tmp_out.sack_ok as we don't use the results 83 */ 84 enum tcp_tw_status 85 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb, 86 const struct tcphdr *th) 87 { 88 struct tcp_options_received tmp_opt; 89 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 90 bool paws_reject = false; 91 92 tmp_opt.saw_tstamp = 0; 93 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) { 94 tcp_parse_options(twsk_net(tw), skb, &tmp_opt, 0, NULL); 95 96 if (tmp_opt.saw_tstamp) { 97 if (tmp_opt.rcv_tsecr) 98 tmp_opt.rcv_tsecr -= tcptw->tw_ts_offset; 99 tmp_opt.ts_recent = tcptw->tw_ts_recent; 100 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp; 101 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 102 } 103 } 104 105 if (tw->tw_substate == TCP_FIN_WAIT2) { 106 /* Just repeat all the checks of tcp_rcv_state_process() */ 107 108 /* Out of window, send ACK */ 109 if (paws_reject || 110 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 111 tcptw->tw_rcv_nxt, 112 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd)) 113 return tcp_timewait_check_oow_rate_limit( 114 tw, skb, LINUX_MIB_TCPACKSKIPPEDFINWAIT2); 115 116 if (th->rst) 117 goto kill; 118 119 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt)) 120 return TCP_TW_RST; 121 122 /* Dup ACK? */ 123 if (!th->ack || 124 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) || 125 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) { 126 inet_twsk_put(tw); 127 return TCP_TW_SUCCESS; 128 } 129 130 /* New data or FIN. If new data arrive after half-duplex close, 131 * reset. 132 */ 133 if (!th->fin || 134 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) 135 return TCP_TW_RST; 136 137 /* FIN arrived, enter true time-wait state. */ 138 tw->tw_substate = TCP_TIME_WAIT; 139 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq; 140 if (tmp_opt.saw_tstamp) { 141 tcptw->tw_ts_recent_stamp = ktime_get_seconds(); 142 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 143 } 144 145 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 146 return TCP_TW_ACK; 147 } 148 149 /* 150 * Now real TIME-WAIT state. 151 * 152 * RFC 1122: 153 * "When a connection is [...] on TIME-WAIT state [...] 154 * [a TCP] MAY accept a new SYN from the remote TCP to 155 * reopen the connection directly, if it: 156 * 157 * (1) assigns its initial sequence number for the new 158 * connection to be larger than the largest sequence 159 * number it used on the previous connection incarnation, 160 * and 161 * 162 * (2) returns to TIME-WAIT state if the SYN turns out 163 * to be an old duplicate". 164 */ 165 166 if (!paws_reject && 167 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 168 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 169 /* In window segment, it may be only reset or bare ack. */ 170 171 if (th->rst) { 172 /* This is TIME_WAIT assassination, in two flavors. 173 * Oh well... nobody has a sufficient solution to this 174 * protocol bug yet. 175 */ 176 if (!READ_ONCE(twsk_net(tw)->ipv4.sysctl_tcp_rfc1337)) { 177 kill: 178 inet_twsk_deschedule_put(tw); 179 return TCP_TW_SUCCESS; 180 } 181 } else { 182 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 183 } 184 185 if (tmp_opt.saw_tstamp) { 186 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 187 tcptw->tw_ts_recent_stamp = ktime_get_seconds(); 188 } 189 190 inet_twsk_put(tw); 191 return TCP_TW_SUCCESS; 192 } 193 194 /* Out of window segment. 195 196 All the segments are ACKed immediately. 197 198 The only exception is new SYN. We accept it, if it is 199 not old duplicate and we are not in danger to be killed 200 by delayed old duplicates. RFC check is that it has 201 newer sequence number works at rates <40Mbit/sec. 202 However, if paws works, it is reliable AND even more, 203 we even may relax silly seq space cutoff. 204 205 RED-PEN: we violate main RFC requirement, if this SYN will appear 206 old duplicate (i.e. we receive RST in reply to SYN-ACK), 207 we must return socket to time-wait state. It is not good, 208 but not fatal yet. 209 */ 210 211 if (th->syn && !th->rst && !th->ack && !paws_reject && 212 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 213 (tmp_opt.saw_tstamp && 214 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 215 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 216 if (isn == 0) 217 isn++; 218 TCP_SKB_CB(skb)->tcp_tw_isn = isn; 219 return TCP_TW_SYN; 220 } 221 222 if (paws_reject) 223 __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 224 225 if (!th->rst) { 226 /* In this case we must reset the TIMEWAIT timer. 227 * 228 * If it is ACKless SYN it may be both old duplicate 229 * and new good SYN with random sequence number <rcv_nxt. 230 * Do not reschedule in the last case. 231 */ 232 if (paws_reject || th->ack) 233 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN); 234 235 return tcp_timewait_check_oow_rate_limit( 236 tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT); 237 } 238 inet_twsk_put(tw); 239 return TCP_TW_SUCCESS; 240 } 241 EXPORT_SYMBOL(tcp_timewait_state_process); 242 243 /* 244 * Move a socket to time-wait or dead fin-wait-2 state. 245 */ 246 void tcp_time_wait(struct sock *sk, int state, int timeo) 247 { 248 const struct inet_connection_sock *icsk = inet_csk(sk); 249 const struct tcp_sock *tp = tcp_sk(sk); 250 struct inet_timewait_sock *tw; 251 struct inet_timewait_death_row *tcp_death_row = sock_net(sk)->ipv4.tcp_death_row; 252 253 tw = inet_twsk_alloc(sk, tcp_death_row, state); 254 255 if (tw) { 256 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 257 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 258 struct inet_sock *inet = inet_sk(sk); 259 260 tw->tw_transparent = inet->transparent; 261 tw->tw_mark = sk->sk_mark; 262 tw->tw_priority = sk->sk_priority; 263 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 264 tcptw->tw_rcv_nxt = tp->rcv_nxt; 265 tcptw->tw_snd_nxt = tp->snd_nxt; 266 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 267 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 268 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 269 tcptw->tw_ts_offset = tp->tsoffset; 270 tcptw->tw_last_oow_ack_time = 0; 271 tcptw->tw_tx_delay = tp->tcp_tx_delay; 272 #if IS_ENABLED(CONFIG_IPV6) 273 if (tw->tw_family == PF_INET6) { 274 struct ipv6_pinfo *np = inet6_sk(sk); 275 276 tw->tw_v6_daddr = sk->sk_v6_daddr; 277 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr; 278 tw->tw_tclass = np->tclass; 279 tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK); 280 tw->tw_txhash = sk->sk_txhash; 281 tw->tw_ipv6only = sk->sk_ipv6only; 282 } 283 #endif 284 285 #ifdef CONFIG_TCP_MD5SIG 286 /* 287 * The timewait bucket does not have the key DB from the 288 * sock structure. We just make a quick copy of the 289 * md5 key being used (if indeed we are using one) 290 * so the timewait ack generating code has the key. 291 */ 292 do { 293 tcptw->tw_md5_key = NULL; 294 if (static_branch_unlikely(&tcp_md5_needed)) { 295 struct tcp_md5sig_key *key; 296 297 key = tp->af_specific->md5_lookup(sk, sk); 298 if (key) { 299 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC); 300 BUG_ON(tcptw->tw_md5_key && !tcp_alloc_md5sig_pool()); 301 } 302 } 303 } while (0); 304 #endif 305 306 /* Get the TIME_WAIT timeout firing. */ 307 if (timeo < rto) 308 timeo = rto; 309 310 if (state == TCP_TIME_WAIT) 311 timeo = TCP_TIMEWAIT_LEN; 312 313 /* tw_timer is pinned, so we need to make sure BH are disabled 314 * in following section, otherwise timer handler could run before 315 * we complete the initialization. 316 */ 317 local_bh_disable(); 318 inet_twsk_schedule(tw, timeo); 319 /* Linkage updates. 320 * Note that access to tw after this point is illegal. 321 */ 322 inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 323 local_bh_enable(); 324 } else { 325 /* Sorry, if we're out of memory, just CLOSE this 326 * socket up. We've got bigger problems than 327 * non-graceful socket closings. 328 */ 329 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW); 330 } 331 332 tcp_update_metrics(sk); 333 tcp_done(sk); 334 } 335 EXPORT_SYMBOL(tcp_time_wait); 336 337 void tcp_twsk_destructor(struct sock *sk) 338 { 339 #ifdef CONFIG_TCP_MD5SIG 340 if (static_branch_unlikely(&tcp_md5_needed)) { 341 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 342 343 if (twsk->tw_md5_key) 344 kfree_rcu(twsk->tw_md5_key, rcu); 345 } 346 #endif 347 } 348 EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 349 350 /* Warning : This function is called without sk_listener being locked. 351 * Be sure to read socket fields once, as their value could change under us. 352 */ 353 void tcp_openreq_init_rwin(struct request_sock *req, 354 const struct sock *sk_listener, 355 const struct dst_entry *dst) 356 { 357 struct inet_request_sock *ireq = inet_rsk(req); 358 const struct tcp_sock *tp = tcp_sk(sk_listener); 359 int full_space = tcp_full_space(sk_listener); 360 u32 window_clamp; 361 __u8 rcv_wscale; 362 u32 rcv_wnd; 363 int mss; 364 365 mss = tcp_mss_clamp(tp, dst_metric_advmss(dst)); 366 window_clamp = READ_ONCE(tp->window_clamp); 367 /* Set this up on the first call only */ 368 req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW); 369 370 /* limit the window selection if the user enforce a smaller rx buffer */ 371 if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK && 372 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0)) 373 req->rsk_window_clamp = full_space; 374 375 rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req); 376 if (rcv_wnd == 0) 377 rcv_wnd = dst_metric(dst, RTAX_INITRWND); 378 else if (full_space < rcv_wnd * mss) 379 full_space = rcv_wnd * mss; 380 381 /* tcp_full_space because it is guaranteed to be the first packet */ 382 tcp_select_initial_window(sk_listener, 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 rcv_wnd); 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 && bpf_try_module_get(ca, 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 !bpf_try_module_get(icsk->icsk_ca_ops, 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 static void smc_check_reset_syn_req(struct tcp_sock *oldtp, 429 struct request_sock *req, 430 struct tcp_sock *newtp) 431 { 432 #if IS_ENABLED(CONFIG_SMC) 433 struct inet_request_sock *ireq; 434 435 if (static_branch_unlikely(&tcp_have_smc)) { 436 ireq = inet_rsk(req); 437 if (oldtp->syn_smc && !ireq->smc_ok) 438 newtp->syn_smc = 0; 439 } 440 #endif 441 } 442 443 /* This is not only more efficient than what we used to do, it eliminates 444 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 445 * 446 * Actually, we could lots of memory writes here. tp of listening 447 * socket contains all necessary default parameters. 448 */ 449 struct sock *tcp_create_openreq_child(const struct sock *sk, 450 struct request_sock *req, 451 struct sk_buff *skb) 452 { 453 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC); 454 const struct inet_request_sock *ireq = inet_rsk(req); 455 struct tcp_request_sock *treq = tcp_rsk(req); 456 struct inet_connection_sock *newicsk; 457 struct tcp_sock *oldtp, *newtp; 458 u32 seq; 459 460 if (!newsk) 461 return NULL; 462 463 newicsk = inet_csk(newsk); 464 newtp = tcp_sk(newsk); 465 oldtp = tcp_sk(sk); 466 467 smc_check_reset_syn_req(oldtp, req, newtp); 468 469 /* Now setup tcp_sock */ 470 newtp->pred_flags = 0; 471 472 seq = treq->rcv_isn + 1; 473 newtp->rcv_wup = seq; 474 WRITE_ONCE(newtp->copied_seq, seq); 475 WRITE_ONCE(newtp->rcv_nxt, seq); 476 newtp->segs_in = 1; 477 478 seq = treq->snt_isn + 1; 479 newtp->snd_sml = newtp->snd_una = seq; 480 WRITE_ONCE(newtp->snd_nxt, seq); 481 newtp->snd_up = seq; 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 WRITE_ONCE(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 (treq->af_specific->req_md5_lookup(sk, req_to_sk(req))) 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 RCU_INIT_POINTER(newtp->fastopen_rsk, NULL); 543 544 tcp_bpf_clone(sk, newsk); 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, bool *req_stolen) 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(sock_net(sk), 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 = ktime_get_seconds() - reqsk_timeout(req, TCP_RTO_MAX) / HZ; 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 += reqsk_timeout(req, TCP_RTO_MAX); 626 if (!fastopen) 627 mod_timer_pending(&req->rsk_timer, expires); 628 else 629 req->rsk_timer.expires = expires; 630 } 631 return NULL; 632 } 633 634 /* Further reproduces section "SEGMENT ARRIVES" 635 for state SYN-RECEIVED of RFC793. 636 It is broken, however, it does not work only 637 when SYNs are crossed. 638 639 You would think that SYN crossing is impossible here, since 640 we should have a SYN_SENT socket (from connect()) on our end, 641 but this is not true if the crossed SYNs were sent to both 642 ends by a malicious third party. We must defend against this, 643 and to do that we first verify the ACK (as per RFC793, page 644 36) and reset if it is invalid. Is this a true full defense? 645 To convince ourselves, let us consider a way in which the ACK 646 test can still pass in this 'malicious crossed SYNs' case. 647 Malicious sender sends identical SYNs (and thus identical sequence 648 numbers) to both A and B: 649 650 A: gets SYN, seq=7 651 B: gets SYN, seq=7 652 653 By our good fortune, both A and B select the same initial 654 send sequence number of seven :-) 655 656 A: sends SYN|ACK, seq=7, ack_seq=8 657 B: sends SYN|ACK, seq=7, ack_seq=8 658 659 So we are now A eating this SYN|ACK, ACK test passes. So 660 does sequence test, SYN is truncated, and thus we consider 661 it a bare ACK. 662 663 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 664 bare ACK. Otherwise, we create an established connection. Both 665 ends (listening sockets) accept the new incoming connection and try 666 to talk to each other. 8-) 667 668 Note: This case is both harmless, and rare. Possibility is about the 669 same as us discovering intelligent life on another plant tomorrow. 670 671 But generally, we should (RFC lies!) to accept ACK 672 from SYNACK both here and in tcp_rcv_state_process(). 673 tcp_rcv_state_process() does not, hence, we do not too. 674 675 Note that the case is absolutely generic: 676 we cannot optimize anything here without 677 violating protocol. All the checks must be made 678 before attempt to create socket. 679 */ 680 681 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 682 * and the incoming segment acknowledges something not yet 683 * sent (the segment carries an unacceptable ACK) ... 684 * a reset is sent." 685 * 686 * Invalid ACK: reset will be sent by listening socket. 687 * Note that the ACK validity check for a Fast Open socket is done 688 * elsewhere and is checked directly against the child socket rather 689 * than req because user data may have been sent out. 690 */ 691 if ((flg & TCP_FLAG_ACK) && !fastopen && 692 (TCP_SKB_CB(skb)->ack_seq != 693 tcp_rsk(req)->snt_isn + 1)) 694 return sk; 695 696 /* Also, it would be not so bad idea to check rcv_tsecr, which 697 * is essentially ACK extension and too early or too late values 698 * should cause reset in unsynchronized states. 699 */ 700 701 /* RFC793: "first check sequence number". */ 702 703 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 704 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) { 705 /* Out of window: send ACK and drop. */ 706 if (!(flg & TCP_FLAG_RST) && 707 !tcp_oow_rate_limited(sock_net(sk), skb, 708 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 709 &tcp_rsk(req)->last_oow_ack_time)) 710 req->rsk_ops->send_ack(sk, skb, req); 711 if (paws_reject) 712 __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 713 return NULL; 714 } 715 716 /* In sequence, PAWS is OK. */ 717 718 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt)) 719 req->ts_recent = tmp_opt.rcv_tsval; 720 721 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 722 /* Truncate SYN, it is out of window starting 723 at tcp_rsk(req)->rcv_isn + 1. */ 724 flg &= ~TCP_FLAG_SYN; 725 } 726 727 /* RFC793: "second check the RST bit" and 728 * "fourth, check the SYN bit" 729 */ 730 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 731 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 732 goto embryonic_reset; 733 } 734 735 /* ACK sequence verified above, just make sure ACK is 736 * set. If ACK not set, just silently drop the packet. 737 * 738 * XXX (TFO) - if we ever allow "data after SYN", the 739 * following check needs to be removed. 740 */ 741 if (!(flg & TCP_FLAG_ACK)) 742 return NULL; 743 744 /* For Fast Open no more processing is needed (sk is the 745 * child socket). 746 */ 747 if (fastopen) 748 return sk; 749 750 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 751 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 752 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 753 inet_rsk(req)->acked = 1; 754 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP); 755 return NULL; 756 } 757 758 /* OK, ACK is valid, create big socket and 759 * feed this segment to it. It will repeat all 760 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 761 * ESTABLISHED STATE. If it will be dropped after 762 * socket is created, wait for troubles. 763 */ 764 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL, 765 req, &own_req); 766 if (!child) 767 goto listen_overflow; 768 769 if (own_req && rsk_drop_req(req)) { 770 reqsk_queue_removed(&inet_csk(req->rsk_listener)->icsk_accept_queue, req); 771 inet_csk_reqsk_queue_drop_and_put(req->rsk_listener, req); 772 return child; 773 } 774 775 sock_rps_save_rxhash(child, skb); 776 tcp_synack_rtt_meas(child, req); 777 *req_stolen = !own_req; 778 return inet_csk_complete_hashdance(sk, child, req, own_req); 779 780 listen_overflow: 781 if (sk != req->rsk_listener) 782 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPMIGRATEREQFAILURE); 783 784 if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_abort_on_overflow)) { 785 inet_rsk(req)->acked = 1; 786 return NULL; 787 } 788 789 embryonic_reset: 790 if (!(flg & TCP_FLAG_RST)) { 791 /* Received a bad SYN pkt - for TFO We try not to reset 792 * the local connection unless it's really necessary to 793 * avoid becoming vulnerable to outside attack aiming at 794 * resetting legit local connections. 795 */ 796 req->rsk_ops->send_reset(sk, skb); 797 } else if (fastopen) { /* received a valid RST pkt */ 798 reqsk_fastopen_remove(sk, req, true); 799 tcp_reset(sk, skb); 800 } 801 if (!fastopen) { 802 bool unlinked = inet_csk_reqsk_queue_drop(sk, req); 803 804 if (unlinked) 805 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 806 *req_stolen = !unlinked; 807 } 808 return NULL; 809 } 810 EXPORT_SYMBOL(tcp_check_req); 811 812 /* 813 * Queue segment on the new socket if the new socket is active, 814 * otherwise we just shortcircuit this and continue with 815 * the new socket. 816 * 817 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV 818 * when entering. But other states are possible due to a race condition 819 * where after __inet_lookup_established() fails but before the listener 820 * locked is obtained, other packets cause the same connection to 821 * be created. 822 */ 823 824 int tcp_child_process(struct sock *parent, struct sock *child, 825 struct sk_buff *skb) 826 __releases(&((child)->sk_lock.slock)) 827 { 828 int ret = 0; 829 int state = child->sk_state; 830 831 /* record sk_napi_id and sk_rx_queue_mapping of child. */ 832 sk_mark_napi_id_set(child, skb); 833 834 tcp_segs_in(tcp_sk(child), skb); 835 if (!sock_owned_by_user(child)) { 836 ret = tcp_rcv_state_process(child, skb); 837 /* Wakeup parent, send SIGIO */ 838 if (state == TCP_SYN_RECV && child->sk_state != state) 839 parent->sk_data_ready(parent); 840 } else { 841 /* Alas, it is possible again, because we do lookup 842 * in main socket hash table and lock on listening 843 * socket does not protect us more. 844 */ 845 __sk_add_backlog(child, skb); 846 } 847 848 bh_unlock_sock(child); 849 sock_put(child); 850 return ret; 851 } 852 EXPORT_SYMBOL(tcp_child_process); 853