tcp_minisocks.c revision 9b717a8d245075ffb8e95a2dfb4ee97ce4747457
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 30int sysctl_tcp_syncookies __read_mostly = 1; 31EXPORT_SYMBOL(sysctl_tcp_syncookies); 32 33int sysctl_tcp_abort_on_overflow __read_mostly; 34 35struct inet_timewait_death_row tcp_death_row = { 36 .sysctl_max_tw_buckets = NR_FILE * 2, 37 .period = TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS, 38 .death_lock = __SPIN_LOCK_UNLOCKED(tcp_death_row.death_lock), 39 .hashinfo = &tcp_hashinfo, 40 .tw_timer = TIMER_INITIALIZER(inet_twdr_hangman, 0, 41 (unsigned long)&tcp_death_row), 42 .twkill_work = __WORK_INITIALIZER(tcp_death_row.twkill_work, 43 inet_twdr_twkill_work), 44/* Short-time timewait calendar */ 45 46 .twcal_hand = -1, 47 .twcal_timer = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0, 48 (unsigned long)&tcp_death_row), 49}; 50EXPORT_SYMBOL_GPL(tcp_death_row); 51 52static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win) 53{ 54 if (seq == s_win) 55 return true; 56 if (after(end_seq, s_win) && before(seq, e_win)) 57 return true; 58 return seq == e_win && seq == end_seq; 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 */ 91enum tcp_tw_status 92tcp_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 const u8 *hash_location; 97 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 98 bool paws_reject = false; 99 100 tmp_opt.saw_tstamp = 0; 101 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) { 102 tcp_parse_options(skb, &tmp_opt, &hash_location, 0, NULL); 103 104 if (tmp_opt.saw_tstamp) { 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_TW_ACK; 121 122 if (th->rst) 123 goto kill; 124 125 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt)) 126 goto kill_with_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) { 141kill_with_rst: 142 inet_twsk_deschedule(tw, &tcp_death_row); 143 inet_twsk_put(tw); 144 return TCP_TW_RST; 145 } 146 147 /* FIN arrived, enter true time-wait state. */ 148 tw->tw_substate = TCP_TIME_WAIT; 149 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq; 150 if (tmp_opt.saw_tstamp) { 151 tcptw->tw_ts_recent_stamp = get_seconds(); 152 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 153 } 154 155 if (tcp_death_row.sysctl_tw_recycle && 156 tcptw->tw_ts_recent_stamp && 157 tcp_tw_remember_stamp(tw)) 158 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout, 159 TCP_TIMEWAIT_LEN); 160 else 161 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 162 TCP_TIMEWAIT_LEN); 163 return TCP_TW_ACK; 164 } 165 166 /* 167 * Now real TIME-WAIT state. 168 * 169 * RFC 1122: 170 * "When a connection is [...] on TIME-WAIT state [...] 171 * [a TCP] MAY accept a new SYN from the remote TCP to 172 * reopen the connection directly, if it: 173 * 174 * (1) assigns its initial sequence number for the new 175 * connection to be larger than the largest sequence 176 * number it used on the previous connection incarnation, 177 * and 178 * 179 * (2) returns to TIME-WAIT state if the SYN turns out 180 * to be an old duplicate". 181 */ 182 183 if (!paws_reject && 184 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 185 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 186 /* In window segment, it may be only reset or bare ack. */ 187 188 if (th->rst) { 189 /* This is TIME_WAIT assassination, in two flavors. 190 * Oh well... nobody has a sufficient solution to this 191 * protocol bug yet. 192 */ 193 if (sysctl_tcp_rfc1337 == 0) { 194kill: 195 inet_twsk_deschedule(tw, &tcp_death_row); 196 inet_twsk_put(tw); 197 return TCP_TW_SUCCESS; 198 } 199 } 200 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 201 TCP_TIMEWAIT_LEN); 202 203 if (tmp_opt.saw_tstamp) { 204 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 205 tcptw->tw_ts_recent_stamp = get_seconds(); 206 } 207 208 inet_twsk_put(tw); 209 return TCP_TW_SUCCESS; 210 } 211 212 /* Out of window segment. 213 214 All the segments are ACKed immediately. 215 216 The only exception is new SYN. We accept it, if it is 217 not old duplicate and we are not in danger to be killed 218 by delayed old duplicates. RFC check is that it has 219 newer sequence number works at rates <40Mbit/sec. 220 However, if paws works, it is reliable AND even more, 221 we even may relax silly seq space cutoff. 222 223 RED-PEN: we violate main RFC requirement, if this SYN will appear 224 old duplicate (i.e. we receive RST in reply to SYN-ACK), 225 we must return socket to time-wait state. It is not good, 226 but not fatal yet. 227 */ 228 229 if (th->syn && !th->rst && !th->ack && !paws_reject && 230 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 231 (tmp_opt.saw_tstamp && 232 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 233 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 234 if (isn == 0) 235 isn++; 236 TCP_SKB_CB(skb)->when = isn; 237 return TCP_TW_SYN; 238 } 239 240 if (paws_reject) 241 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 242 243 if (!th->rst) { 244 /* In this case we must reset the TIMEWAIT timer. 245 * 246 * If it is ACKless SYN it may be both old duplicate 247 * and new good SYN with random sequence number <rcv_nxt. 248 * Do not reschedule in the last case. 249 */ 250 if (paws_reject || th->ack) 251 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 252 TCP_TIMEWAIT_LEN); 253 254 /* Send ACK. Note, we do not put the bucket, 255 * it will be released by caller. 256 */ 257 return TCP_TW_ACK; 258 } 259 inet_twsk_put(tw); 260 return TCP_TW_SUCCESS; 261} 262EXPORT_SYMBOL(tcp_timewait_state_process); 263 264/* 265 * Move a socket to time-wait or dead fin-wait-2 state. 266 */ 267void tcp_time_wait(struct sock *sk, int state, int timeo) 268{ 269 struct inet_timewait_sock *tw = NULL; 270 const struct inet_connection_sock *icsk = inet_csk(sk); 271 const struct tcp_sock *tp = tcp_sk(sk); 272 bool recycle_ok = false; 273 274 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp) 275 recycle_ok = tcp_remember_stamp(sk); 276 277 if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets) 278 tw = inet_twsk_alloc(sk, state); 279 280 if (tw != NULL) { 281 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 282 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 283 struct inet_sock *inet = inet_sk(sk); 284 285 tw->tw_transparent = inet->transparent; 286 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 287 tcptw->tw_rcv_nxt = tp->rcv_nxt; 288 tcptw->tw_snd_nxt = tp->snd_nxt; 289 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 290 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 291 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 292 tcptw->tw_ts_offset = tp->tsoffset; 293 294#if IS_ENABLED(CONFIG_IPV6) 295 if (tw->tw_family == PF_INET6) { 296 struct ipv6_pinfo *np = inet6_sk(sk); 297 struct inet6_timewait_sock *tw6; 298 299 tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot); 300 tw6 = inet6_twsk((struct sock *)tw); 301 tw6->tw_v6_daddr = np->daddr; 302 tw6->tw_v6_rcv_saddr = np->rcv_saddr; 303 tw->tw_tclass = np->tclass; 304 tw->tw_ipv6only = np->ipv6only; 305 } 306#endif 307 308#ifdef CONFIG_TCP_MD5SIG 309 /* 310 * The timewait bucket does not have the key DB from the 311 * sock structure. We just make a quick copy of the 312 * md5 key being used (if indeed we are using one) 313 * so the timewait ack generating code has the key. 314 */ 315 do { 316 struct tcp_md5sig_key *key; 317 tcptw->tw_md5_key = NULL; 318 key = tp->af_specific->md5_lookup(sk, sk); 319 if (key != NULL) { 320 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC); 321 if (tcptw->tw_md5_key && tcp_alloc_md5sig_pool(sk) == NULL) 322 BUG(); 323 } 324 } while (0); 325#endif 326 327 /* Linkage updates. */ 328 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 329 330 /* Get the TIME_WAIT timeout firing. */ 331 if (timeo < rto) 332 timeo = rto; 333 334 if (recycle_ok) { 335 tw->tw_timeout = rto; 336 } else { 337 tw->tw_timeout = TCP_TIMEWAIT_LEN; 338 if (state == TCP_TIME_WAIT) 339 timeo = TCP_TIMEWAIT_LEN; 340 } 341 342 inet_twsk_schedule(tw, &tcp_death_row, timeo, 343 TCP_TIMEWAIT_LEN); 344 inet_twsk_put(tw); 345 } else { 346 /* Sorry, if we're out of memory, just CLOSE this 347 * socket up. We've got bigger problems than 348 * non-graceful socket closings. 349 */ 350 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW); 351 } 352 353 tcp_update_metrics(sk); 354 tcp_done(sk); 355} 356 357void tcp_twsk_destructor(struct sock *sk) 358{ 359#ifdef CONFIG_TCP_MD5SIG 360 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 361 362 if (twsk->tw_md5_key) { 363 tcp_free_md5sig_pool(); 364 kfree_rcu(twsk->tw_md5_key, rcu); 365 } 366#endif 367} 368EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 369 370static inline void TCP_ECN_openreq_child(struct tcp_sock *tp, 371 struct request_sock *req) 372{ 373 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 374} 375 376/* This is not only more efficient than what we used to do, it eliminates 377 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 378 * 379 * Actually, we could lots of memory writes here. tp of listening 380 * socket contains all necessary default parameters. 381 */ 382struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb) 383{ 384 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC); 385 386 if (newsk != NULL) { 387 const struct inet_request_sock *ireq = inet_rsk(req); 388 struct tcp_request_sock *treq = tcp_rsk(req); 389 struct inet_connection_sock *newicsk = inet_csk(newsk); 390 struct tcp_sock *newtp = tcp_sk(newsk); 391 struct tcp_sock *oldtp = tcp_sk(sk); 392 struct tcp_cookie_values *oldcvp = oldtp->cookie_values; 393 394 /* TCP Cookie Transactions require space for the cookie pair, 395 * as it differs for each connection. There is no need to 396 * copy any s_data_payload stored at the original socket. 397 * Failure will prevent resuming the connection. 398 * 399 * Presumed copied, in order of appearance: 400 * cookie_in_always, cookie_out_never 401 */ 402 if (oldcvp != NULL) { 403 struct tcp_cookie_values *newcvp = 404 kzalloc(sizeof(*newtp->cookie_values), 405 GFP_ATOMIC); 406 407 if (newcvp != NULL) { 408 kref_init(&newcvp->kref); 409 newcvp->cookie_desired = 410 oldcvp->cookie_desired; 411 newtp->cookie_values = newcvp; 412 } else { 413 /* Not Yet Implemented */ 414 newtp->cookie_values = NULL; 415 } 416 } 417 418 /* Now setup tcp_sock */ 419 newtp->pred_flags = 0; 420 421 newtp->rcv_wup = newtp->copied_seq = 422 newtp->rcv_nxt = treq->rcv_isn + 1; 423 424 newtp->snd_sml = newtp->snd_una = 425 newtp->snd_nxt = newtp->snd_up = 426 treq->snt_isn + 1 + tcp_s_data_size(oldtp); 427 428 tcp_prequeue_init(newtp); 429 INIT_LIST_HEAD(&newtp->tsq_node); 430 431 tcp_init_wl(newtp, treq->rcv_isn); 432 433 newtp->srtt = 0; 434 newtp->mdev = TCP_TIMEOUT_INIT; 435 newicsk->icsk_rto = TCP_TIMEOUT_INIT; 436 437 newtp->packets_out = 0; 438 newtp->retrans_out = 0; 439 newtp->sacked_out = 0; 440 newtp->fackets_out = 0; 441 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH; 442 tcp_enable_early_retrans(newtp); 443 newtp->tlp_high_seq = 0; 444 445 /* So many TCP implementations out there (incorrectly) count the 446 * initial SYN frame in their delayed-ACK and congestion control 447 * algorithms that we must have the following bandaid to talk 448 * efficiently to them. -DaveM 449 */ 450 newtp->snd_cwnd = TCP_INIT_CWND; 451 newtp->snd_cwnd_cnt = 0; 452 453 newtp->frto_counter = 0; 454 newtp->frto_highmark = 0; 455 456 if (newicsk->icsk_ca_ops != &tcp_init_congestion_ops && 457 !try_module_get(newicsk->icsk_ca_ops->owner)) 458 newicsk->icsk_ca_ops = &tcp_init_congestion_ops; 459 460 tcp_set_ca_state(newsk, TCP_CA_Open); 461 tcp_init_xmit_timers(newsk); 462 skb_queue_head_init(&newtp->out_of_order_queue); 463 newtp->write_seq = newtp->pushed_seq = 464 treq->snt_isn + 1 + tcp_s_data_size(oldtp); 465 466 newtp->rx_opt.saw_tstamp = 0; 467 468 newtp->rx_opt.dsack = 0; 469 newtp->rx_opt.num_sacks = 0; 470 471 newtp->urg_data = 0; 472 473 if (sock_flag(newsk, SOCK_KEEPOPEN)) 474 inet_csk_reset_keepalive_timer(newsk, 475 keepalive_time_when(newtp)); 476 477 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 478 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 479 if (sysctl_tcp_fack) 480 tcp_enable_fack(newtp); 481 } 482 newtp->window_clamp = req->window_clamp; 483 newtp->rcv_ssthresh = req->rcv_wnd; 484 newtp->rcv_wnd = req->rcv_wnd; 485 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 486 if (newtp->rx_opt.wscale_ok) { 487 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 488 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 489 } else { 490 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 491 newtp->window_clamp = min(newtp->window_clamp, 65535U); 492 } 493 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 494 newtp->rx_opt.snd_wscale); 495 newtp->max_window = newtp->snd_wnd; 496 497 if (newtp->rx_opt.tstamp_ok) { 498 newtp->rx_opt.ts_recent = req->ts_recent; 499 newtp->rx_opt.ts_recent_stamp = get_seconds(); 500 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 501 } else { 502 newtp->rx_opt.ts_recent_stamp = 0; 503 newtp->tcp_header_len = sizeof(struct tcphdr); 504 } 505 newtp->tsoffset = 0; 506#ifdef CONFIG_TCP_MD5SIG 507 newtp->md5sig_info = NULL; /*XXX*/ 508 if (newtp->af_specific->md5_lookup(sk, newsk)) 509 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 510#endif 511 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) 512 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 513 newtp->rx_opt.mss_clamp = req->mss; 514 TCP_ECN_openreq_child(newtp, req); 515 newtp->fastopen_rsk = NULL; 516 newtp->syn_data_acked = 0; 517 518 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS); 519 } 520 return newsk; 521} 522EXPORT_SYMBOL(tcp_create_openreq_child); 523 524/* 525 * Process an incoming packet for SYN_RECV sockets represented as a 526 * request_sock. Normally sk is the listener socket but for TFO it 527 * points to the child socket. 528 * 529 * XXX (TFO) - The current impl contains a special check for ack 530 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better? 531 * 532 * We don't need to initialize tmp_opt.sack_ok as we don't use the results 533 */ 534 535struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 536 struct request_sock *req, 537 struct request_sock **prev, 538 bool fastopen) 539{ 540 struct tcp_options_received tmp_opt; 541 const u8 *hash_location; 542 struct sock *child; 543 const struct tcphdr *th = tcp_hdr(skb); 544 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 545 bool paws_reject = false; 546 547 BUG_ON(fastopen == (sk->sk_state == TCP_LISTEN)); 548 549 tmp_opt.saw_tstamp = 0; 550 if (th->doff > (sizeof(struct tcphdr)>>2)) { 551 tcp_parse_options(skb, &tmp_opt, &hash_location, 0, NULL); 552 553 if (tmp_opt.saw_tstamp) { 554 tmp_opt.ts_recent = req->ts_recent; 555 /* We do not store true stamp, but it is not required, 556 * it can be estimated (approximately) 557 * from another data. 558 */ 559 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout); 560 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 561 } 562 } 563 564 /* Check for pure retransmitted SYN. */ 565 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 566 flg == TCP_FLAG_SYN && 567 !paws_reject) { 568 /* 569 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 570 * this case on figure 6 and figure 8, but formal 571 * protocol description says NOTHING. 572 * To be more exact, it says that we should send ACK, 573 * because this segment (at least, if it has no data) 574 * is out of window. 575 * 576 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 577 * describe SYN-RECV state. All the description 578 * is wrong, we cannot believe to it and should 579 * rely only on common sense and implementation 580 * experience. 581 * 582 * Enforce "SYN-ACK" according to figure 8, figure 6 583 * of RFC793, fixed by RFC1122. 584 * 585 * Note that even if there is new data in the SYN packet 586 * they will be thrown away too. 587 */ 588 inet_rtx_syn_ack(sk, req); 589 return NULL; 590 } 591 592 /* Further reproduces section "SEGMENT ARRIVES" 593 for state SYN-RECEIVED of RFC793. 594 It is broken, however, it does not work only 595 when SYNs are crossed. 596 597 You would think that SYN crossing is impossible here, since 598 we should have a SYN_SENT socket (from connect()) on our end, 599 but this is not true if the crossed SYNs were sent to both 600 ends by a malicious third party. We must defend against this, 601 and to do that we first verify the ACK (as per RFC793, page 602 36) and reset if it is invalid. Is this a true full defense? 603 To convince ourselves, let us consider a way in which the ACK 604 test can still pass in this 'malicious crossed SYNs' case. 605 Malicious sender sends identical SYNs (and thus identical sequence 606 numbers) to both A and B: 607 608 A: gets SYN, seq=7 609 B: gets SYN, seq=7 610 611 By our good fortune, both A and B select the same initial 612 send sequence number of seven :-) 613 614 A: sends SYN|ACK, seq=7, ack_seq=8 615 B: sends SYN|ACK, seq=7, ack_seq=8 616 617 So we are now A eating this SYN|ACK, ACK test passes. So 618 does sequence test, SYN is truncated, and thus we consider 619 it a bare ACK. 620 621 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 622 bare ACK. Otherwise, we create an established connection. Both 623 ends (listening sockets) accept the new incoming connection and try 624 to talk to each other. 8-) 625 626 Note: This case is both harmless, and rare. Possibility is about the 627 same as us discovering intelligent life on another plant tomorrow. 628 629 But generally, we should (RFC lies!) to accept ACK 630 from SYNACK both here and in tcp_rcv_state_process(). 631 tcp_rcv_state_process() does not, hence, we do not too. 632 633 Note that the case is absolutely generic: 634 we cannot optimize anything here without 635 violating protocol. All the checks must be made 636 before attempt to create socket. 637 */ 638 639 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 640 * and the incoming segment acknowledges something not yet 641 * sent (the segment carries an unacceptable ACK) ... 642 * a reset is sent." 643 * 644 * Invalid ACK: reset will be sent by listening socket. 645 * Note that the ACK validity check for a Fast Open socket is done 646 * elsewhere and is checked directly against the child socket rather 647 * than req because user data may have been sent out. 648 */ 649 if ((flg & TCP_FLAG_ACK) && !fastopen && 650 (TCP_SKB_CB(skb)->ack_seq != 651 tcp_rsk(req)->snt_isn + 1 + tcp_s_data_size(tcp_sk(sk)))) 652 return sk; 653 654 /* Also, it would be not so bad idea to check rcv_tsecr, which 655 * is essentially ACK extension and too early or too late values 656 * should cause reset in unsynchronized states. 657 */ 658 659 /* RFC793: "first check sequence number". */ 660 661 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 662 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rcv_wnd)) { 663 /* Out of window: send ACK and drop. */ 664 if (!(flg & TCP_FLAG_RST)) 665 req->rsk_ops->send_ack(sk, skb, req); 666 if (paws_reject) 667 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 668 return NULL; 669 } 670 671 /* In sequence, PAWS is OK. */ 672 673 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt)) 674 req->ts_recent = tmp_opt.rcv_tsval; 675 676 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 677 /* Truncate SYN, it is out of window starting 678 at tcp_rsk(req)->rcv_isn + 1. */ 679 flg &= ~TCP_FLAG_SYN; 680 } 681 682 /* RFC793: "second check the RST bit" and 683 * "fourth, check the SYN bit" 684 */ 685 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 686 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 687 goto embryonic_reset; 688 } 689 690 /* ACK sequence verified above, just make sure ACK is 691 * set. If ACK not set, just silently drop the packet. 692 * 693 * XXX (TFO) - if we ever allow "data after SYN", the 694 * following check needs to be removed. 695 */ 696 if (!(flg & TCP_FLAG_ACK)) 697 return NULL; 698 699 /* Got ACK for our SYNACK, so update baseline for SYNACK RTT sample. */ 700 if (tmp_opt.saw_tstamp && tmp_opt.rcv_tsecr) 701 tcp_rsk(req)->snt_synack = tmp_opt.rcv_tsecr; 702 else if (req->num_retrans) /* don't take RTT sample if retrans && ~TS */ 703 tcp_rsk(req)->snt_synack = 0; 704 705 /* For Fast Open no more processing is needed (sk is the 706 * child socket). 707 */ 708 if (fastopen) 709 return sk; 710 711 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 712 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 713 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 714 inet_rsk(req)->acked = 1; 715 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP); 716 return NULL; 717 } 718 719 /* OK, ACK is valid, create big socket and 720 * feed this segment to it. It will repeat all 721 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 722 * ESTABLISHED STATE. If it will be dropped after 723 * socket is created, wait for troubles. 724 */ 725 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL); 726 if (child == NULL) 727 goto listen_overflow; 728 729 inet_csk_reqsk_queue_unlink(sk, req, prev); 730 inet_csk_reqsk_queue_removed(sk, req); 731 732 inet_csk_reqsk_queue_add(sk, req, child); 733 return child; 734 735listen_overflow: 736 if (!sysctl_tcp_abort_on_overflow) { 737 inet_rsk(req)->acked = 1; 738 return NULL; 739 } 740 741embryonic_reset: 742 if (!(flg & TCP_FLAG_RST)) { 743 /* Received a bad SYN pkt - for TFO We try not to reset 744 * the local connection unless it's really necessary to 745 * avoid becoming vulnerable to outside attack aiming at 746 * resetting legit local connections. 747 */ 748 req->rsk_ops->send_reset(sk, skb); 749 } else if (fastopen) { /* received a valid RST pkt */ 750 reqsk_fastopen_remove(sk, req, true); 751 tcp_reset(sk); 752 } 753 if (!fastopen) { 754 inet_csk_reqsk_queue_drop(sk, req, prev); 755 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 756 } 757 return NULL; 758} 759EXPORT_SYMBOL(tcp_check_req); 760 761/* 762 * Queue segment on the new socket if the new socket is active, 763 * otherwise we just shortcircuit this and continue with 764 * the new socket. 765 * 766 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV 767 * when entering. But other states are possible due to a race condition 768 * where after __inet_lookup_established() fails but before the listener 769 * locked is obtained, other packets cause the same connection to 770 * be created. 771 */ 772 773int tcp_child_process(struct sock *parent, struct sock *child, 774 struct sk_buff *skb) 775{ 776 int ret = 0; 777 int state = child->sk_state; 778 779 if (!sock_owned_by_user(child)) { 780 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb), 781 skb->len); 782 /* Wakeup parent, send SIGIO */ 783 if (state == TCP_SYN_RECV && child->sk_state != state) 784 parent->sk_data_ready(parent, 0); 785 } else { 786 /* Alas, it is possible again, because we do lookup 787 * in main socket hash table and lock on listening 788 * socket does not protect us more. 789 */ 790 __sk_add_backlog(child, skb); 791 } 792 793 bh_unlock_sock(child); 794 sock_put(child); 795 return ret; 796} 797EXPORT_SYMBOL(tcp_child_process); 798