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 tcptw->tw_tx_delay = tp->tcp_tx_delay; 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_txhash = sk->sk_txhash; 287 tw->tw_ipv6only = sk->sk_ipv6only; 288 } 289 #endif 290 291 #ifdef CONFIG_TCP_MD5SIG 292 /* 293 * The timewait bucket does not have the key DB from the 294 * sock structure. We just make a quick copy of the 295 * md5 key being used (if indeed we are using one) 296 * so the timewait ack generating code has the key. 297 */ 298 do { 299 tcptw->tw_md5_key = NULL; 300 if (static_branch_unlikely(&tcp_md5_needed)) { 301 struct tcp_md5sig_key *key; 302 303 key = tp->af_specific->md5_lookup(sk, sk); 304 if (key) { 305 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC); 306 BUG_ON(tcptw->tw_md5_key && !tcp_alloc_md5sig_pool()); 307 } 308 } 309 } while (0); 310 #endif 311 312 /* Get the TIME_WAIT timeout firing. */ 313 if (timeo < rto) 314 timeo = rto; 315 316 if (state == TCP_TIME_WAIT) 317 timeo = TCP_TIMEWAIT_LEN; 318 319 /* tw_timer is pinned, so we need to make sure BH are disabled 320 * in following section, otherwise timer handler could run before 321 * we complete the initialization. 322 */ 323 local_bh_disable(); 324 inet_twsk_schedule(tw, timeo); 325 /* Linkage updates. 326 * Note that access to tw after this point is illegal. 327 */ 328 inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 329 local_bh_enable(); 330 } else { 331 /* Sorry, if we're out of memory, just CLOSE this 332 * socket up. We've got bigger problems than 333 * non-graceful socket closings. 334 */ 335 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW); 336 } 337 338 tcp_update_metrics(sk); 339 tcp_done(sk); 340 } 341 EXPORT_SYMBOL(tcp_time_wait); 342 343 void tcp_twsk_destructor(struct sock *sk) 344 { 345 #ifdef CONFIG_TCP_MD5SIG 346 if (static_branch_unlikely(&tcp_md5_needed)) { 347 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 348 349 if (twsk->tw_md5_key) 350 kfree_rcu(twsk->tw_md5_key, rcu); 351 } 352 #endif 353 } 354 EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 355 356 /* Warning : This function is called without sk_listener being locked. 357 * Be sure to read socket fields once, as their value could change under us. 358 */ 359 void tcp_openreq_init_rwin(struct request_sock *req, 360 const struct sock *sk_listener, 361 const struct dst_entry *dst) 362 { 363 struct inet_request_sock *ireq = inet_rsk(req); 364 const struct tcp_sock *tp = tcp_sk(sk_listener); 365 int full_space = tcp_full_space(sk_listener); 366 u32 window_clamp; 367 __u8 rcv_wscale; 368 u32 rcv_wnd; 369 int mss; 370 371 mss = tcp_mss_clamp(tp, dst_metric_advmss(dst)); 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 rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req); 382 if (rcv_wnd == 0) 383 rcv_wnd = dst_metric(dst, RTAX_INITRWND); 384 else if (full_space < rcv_wnd * mss) 385 full_space = rcv_wnd * mss; 386 387 /* tcp_full_space because it is guaranteed to be the first packet */ 388 tcp_select_initial_window(sk_listener, full_space, 389 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0), 390 &req->rsk_rcv_wnd, 391 &req->rsk_window_clamp, 392 ireq->wscale_ok, 393 &rcv_wscale, 394 rcv_wnd); 395 ireq->rcv_wscale = rcv_wscale; 396 } 397 EXPORT_SYMBOL(tcp_openreq_init_rwin); 398 399 static void tcp_ecn_openreq_child(struct tcp_sock *tp, 400 const struct request_sock *req) 401 { 402 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 403 } 404 405 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst) 406 { 407 struct inet_connection_sock *icsk = inet_csk(sk); 408 u32 ca_key = dst_metric(dst, RTAX_CC_ALGO); 409 bool ca_got_dst = false; 410 411 if (ca_key != TCP_CA_UNSPEC) { 412 const struct tcp_congestion_ops *ca; 413 414 rcu_read_lock(); 415 ca = tcp_ca_find_key(ca_key); 416 if (likely(ca && try_module_get(ca->owner))) { 417 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst); 418 icsk->icsk_ca_ops = ca; 419 ca_got_dst = true; 420 } 421 rcu_read_unlock(); 422 } 423 424 /* If no valid choice made yet, assign current system default ca. */ 425 if (!ca_got_dst && 426 (!icsk->icsk_ca_setsockopt || 427 !try_module_get(icsk->icsk_ca_ops->owner))) 428 tcp_assign_congestion_control(sk); 429 430 tcp_set_ca_state(sk, TCP_CA_Open); 431 } 432 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child); 433 434 static void smc_check_reset_syn_req(struct tcp_sock *oldtp, 435 struct request_sock *req, 436 struct tcp_sock *newtp) 437 { 438 #if IS_ENABLED(CONFIG_SMC) 439 struct inet_request_sock *ireq; 440 441 if (static_branch_unlikely(&tcp_have_smc)) { 442 ireq = inet_rsk(req); 443 if (oldtp->syn_smc && !ireq->smc_ok) 444 newtp->syn_smc = 0; 445 } 446 #endif 447 } 448 449 /* This is not only more efficient than what we used to do, it eliminates 450 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 451 * 452 * Actually, we could lots of memory writes here. tp of listening 453 * socket contains all necessary default parameters. 454 */ 455 struct sock *tcp_create_openreq_child(const struct sock *sk, 456 struct request_sock *req, 457 struct sk_buff *skb) 458 { 459 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC); 460 const struct inet_request_sock *ireq = inet_rsk(req); 461 struct tcp_request_sock *treq = tcp_rsk(req); 462 struct inet_connection_sock *newicsk; 463 struct tcp_sock *oldtp, *newtp; 464 465 if (!newsk) 466 return NULL; 467 468 newicsk = inet_csk(newsk); 469 newtp = tcp_sk(newsk); 470 oldtp = tcp_sk(sk); 471 472 smc_check_reset_syn_req(oldtp, req, newtp); 473 474 /* Now setup tcp_sock */ 475 newtp->pred_flags = 0; 476 477 newtp->rcv_wup = newtp->copied_seq = 478 newtp->rcv_nxt = treq->rcv_isn + 1; 479 newtp->segs_in = 1; 480 481 newtp->snd_sml = newtp->snd_una = 482 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1; 483 484 INIT_LIST_HEAD(&newtp->tsq_node); 485 INIT_LIST_HEAD(&newtp->tsorted_sent_queue); 486 487 tcp_init_wl(newtp, treq->rcv_isn); 488 489 minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U); 490 newicsk->icsk_ack.lrcvtime = tcp_jiffies32; 491 492 newtp->lsndtime = tcp_jiffies32; 493 newsk->sk_txhash = treq->txhash; 494 newtp->total_retrans = req->num_retrans; 495 496 tcp_init_xmit_timers(newsk); 497 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1; 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 newtp->rx_opt.sack_ok = ireq->sack_ok; 505 newtp->window_clamp = req->rsk_window_clamp; 506 newtp->rcv_ssthresh = req->rsk_rcv_wnd; 507 newtp->rcv_wnd = req->rsk_rcv_wnd; 508 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 509 if (newtp->rx_opt.wscale_ok) { 510 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 511 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 512 } else { 513 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 514 newtp->window_clamp = min(newtp->window_clamp, 65535U); 515 } 516 newtp->snd_wnd = ntohs(tcp_hdr(skb)->window) << newtp->rx_opt.snd_wscale; 517 newtp->max_window = newtp->snd_wnd; 518 519 if (newtp->rx_opt.tstamp_ok) { 520 newtp->rx_opt.ts_recent = req->ts_recent; 521 newtp->rx_opt.ts_recent_stamp = ktime_get_seconds(); 522 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 523 } else { 524 newtp->rx_opt.ts_recent_stamp = 0; 525 newtp->tcp_header_len = sizeof(struct tcphdr); 526 } 527 if (req->num_timeout) { 528 newtp->undo_marker = treq->snt_isn; 529 newtp->retrans_stamp = div_u64(treq->snt_synack, 530 USEC_PER_SEC / TCP_TS_HZ); 531 } 532 newtp->tsoffset = treq->ts_off; 533 #ifdef CONFIG_TCP_MD5SIG 534 newtp->md5sig_info = NULL; /*XXX*/ 535 if (newtp->af_specific->md5_lookup(sk, newsk)) 536 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 537 #endif 538 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) 539 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 540 newtp->rx_opt.mss_clamp = req->mss; 541 tcp_ecn_openreq_child(newtp, req); 542 newtp->fastopen_req = NULL; 543 newtp->fastopen_rsk = NULL; 544 545 __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS); 546 547 return newsk; 548 } 549 EXPORT_SYMBOL(tcp_create_openreq_child); 550 551 /* 552 * Process an incoming packet for SYN_RECV sockets represented as a 553 * request_sock. Normally sk is the listener socket but for TFO it 554 * points to the child socket. 555 * 556 * XXX (TFO) - The current impl contains a special check for ack 557 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better? 558 * 559 * We don't need to initialize tmp_opt.sack_ok as we don't use the results 560 */ 561 562 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 563 struct request_sock *req, 564 bool fastopen, bool *req_stolen) 565 { 566 struct tcp_options_received tmp_opt; 567 struct sock *child; 568 const struct tcphdr *th = tcp_hdr(skb); 569 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 570 bool paws_reject = false; 571 bool own_req; 572 573 tmp_opt.saw_tstamp = 0; 574 if (th->doff > (sizeof(struct tcphdr)>>2)) { 575 tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL); 576 577 if (tmp_opt.saw_tstamp) { 578 tmp_opt.ts_recent = req->ts_recent; 579 if (tmp_opt.rcv_tsecr) 580 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off; 581 /* We do not store true stamp, but it is not required, 582 * it can be estimated (approximately) 583 * from another data. 584 */ 585 tmp_opt.ts_recent_stamp = ktime_get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout); 586 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 587 } 588 } 589 590 /* Check for pure retransmitted SYN. */ 591 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 592 flg == TCP_FLAG_SYN && 593 !paws_reject) { 594 /* 595 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 596 * this case on figure 6 and figure 8, but formal 597 * protocol description says NOTHING. 598 * To be more exact, it says that we should send ACK, 599 * because this segment (at least, if it has no data) 600 * is out of window. 601 * 602 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 603 * describe SYN-RECV state. All the description 604 * is wrong, we cannot believe to it and should 605 * rely only on common sense and implementation 606 * experience. 607 * 608 * Enforce "SYN-ACK" according to figure 8, figure 6 609 * of RFC793, fixed by RFC1122. 610 * 611 * Note that even if there is new data in the SYN packet 612 * they will be thrown away too. 613 * 614 * Reset timer after retransmitting SYNACK, similar to 615 * the idea of fast retransmit in recovery. 616 */ 617 if (!tcp_oow_rate_limited(sock_net(sk), skb, 618 LINUX_MIB_TCPACKSKIPPEDSYNRECV, 619 &tcp_rsk(req)->last_oow_ack_time) && 620 621 !inet_rtx_syn_ack(sk, req)) { 622 unsigned long expires = jiffies; 623 624 expires += min(TCP_TIMEOUT_INIT << req->num_timeout, 625 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 sock_rps_save_rxhash(child, skb); 770 tcp_synack_rtt_meas(child, req); 771 *req_stolen = !own_req; 772 return inet_csk_complete_hashdance(sk, child, req, own_req); 773 774 listen_overflow: 775 if (!sock_net(sk)->ipv4.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 /* record NAPI ID of child */ 819 sk_mark_napi_id(child, skb); 820 821 tcp_segs_in(tcp_sk(child), skb); 822 if (!sock_owned_by_user(child)) { 823 ret = tcp_rcv_state_process(child, skb); 824 /* Wakeup parent, send SIGIO */ 825 if (state == TCP_SYN_RECV && child->sk_state != state) 826 parent->sk_data_ready(parent); 827 } else { 828 /* Alas, it is possible again, because we do lookup 829 * in main socket hash table and lock on listening 830 * socket does not protect us more. 831 */ 832 __sk_add_backlog(child, skb); 833 } 834 835 bh_unlock_sock(child); 836 sock_put(child); 837 return ret; 838 } 839 EXPORT_SYMBOL(tcp_child_process); 840