tcp_minisocks.c revision 5d299f3d3c8a2fbc732b1bf03af36333ccec3130
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 */ 89enum tcp_tw_status 90tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb, 91 const struct tcphdr *th) 92{ 93 struct tcp_options_received tmp_opt; 94 const u8 *hash_location; 95 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 96 bool paws_reject = false; 97 98 tmp_opt.saw_tstamp = 0; 99 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) { 100 tcp_parse_options(skb, &tmp_opt, &hash_location, 0, NULL); 101 102 if (tmp_opt.saw_tstamp) { 103 tmp_opt.ts_recent = tcptw->tw_ts_recent; 104 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp; 105 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 106 } 107 } 108 109 if (tw->tw_substate == TCP_FIN_WAIT2) { 110 /* Just repeat all the checks of tcp_rcv_state_process() */ 111 112 /* Out of window, send ACK */ 113 if (paws_reject || 114 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 115 tcptw->tw_rcv_nxt, 116 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd)) 117 return TCP_TW_ACK; 118 119 if (th->rst) 120 goto kill; 121 122 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt)) 123 goto kill_with_rst; 124 125 /* Dup ACK? */ 126 if (!th->ack || 127 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) || 128 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) { 129 inet_twsk_put(tw); 130 return TCP_TW_SUCCESS; 131 } 132 133 /* New data or FIN. If new data arrive after half-duplex close, 134 * reset. 135 */ 136 if (!th->fin || 137 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) { 138kill_with_rst: 139 inet_twsk_deschedule(tw, &tcp_death_row); 140 inet_twsk_put(tw); 141 return TCP_TW_RST; 142 } 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 = get_seconds(); 149 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 150 } 151 152 if (tcp_death_row.sysctl_tw_recycle && 153 tcptw->tw_ts_recent_stamp && 154 tcp_tw_remember_stamp(tw)) 155 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout, 156 TCP_TIMEWAIT_LEN); 157 else 158 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 159 TCP_TIMEWAIT_LEN); 160 return TCP_TW_ACK; 161 } 162 163 /* 164 * Now real TIME-WAIT state. 165 * 166 * RFC 1122: 167 * "When a connection is [...] on TIME-WAIT state [...] 168 * [a TCP] MAY accept a new SYN from the remote TCP to 169 * reopen the connection directly, if it: 170 * 171 * (1) assigns its initial sequence number for the new 172 * connection to be larger than the largest sequence 173 * number it used on the previous connection incarnation, 174 * and 175 * 176 * (2) returns to TIME-WAIT state if the SYN turns out 177 * to be an old duplicate". 178 */ 179 180 if (!paws_reject && 181 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt && 182 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) { 183 /* In window segment, it may be only reset or bare ack. */ 184 185 if (th->rst) { 186 /* This is TIME_WAIT assassination, in two flavors. 187 * Oh well... nobody has a sufficient solution to this 188 * protocol bug yet. 189 */ 190 if (sysctl_tcp_rfc1337 == 0) { 191kill: 192 inet_twsk_deschedule(tw, &tcp_death_row); 193 inet_twsk_put(tw); 194 return TCP_TW_SUCCESS; 195 } 196 } 197 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 198 TCP_TIMEWAIT_LEN); 199 200 if (tmp_opt.saw_tstamp) { 201 tcptw->tw_ts_recent = tmp_opt.rcv_tsval; 202 tcptw->tw_ts_recent_stamp = get_seconds(); 203 } 204 205 inet_twsk_put(tw); 206 return TCP_TW_SUCCESS; 207 } 208 209 /* Out of window segment. 210 211 All the segments are ACKed immediately. 212 213 The only exception is new SYN. We accept it, if it is 214 not old duplicate and we are not in danger to be killed 215 by delayed old duplicates. RFC check is that it has 216 newer sequence number works at rates <40Mbit/sec. 217 However, if paws works, it is reliable AND even more, 218 we even may relax silly seq space cutoff. 219 220 RED-PEN: we violate main RFC requirement, if this SYN will appear 221 old duplicate (i.e. we receive RST in reply to SYN-ACK), 222 we must return socket to time-wait state. It is not good, 223 but not fatal yet. 224 */ 225 226 if (th->syn && !th->rst && !th->ack && !paws_reject && 227 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) || 228 (tmp_opt.saw_tstamp && 229 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) { 230 u32 isn = tcptw->tw_snd_nxt + 65535 + 2; 231 if (isn == 0) 232 isn++; 233 TCP_SKB_CB(skb)->when = isn; 234 return TCP_TW_SYN; 235 } 236 237 if (paws_reject) 238 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED); 239 240 if (!th->rst) { 241 /* In this case we must reset the TIMEWAIT timer. 242 * 243 * If it is ACKless SYN it may be both old duplicate 244 * and new good SYN with random sequence number <rcv_nxt. 245 * Do not reschedule in the last case. 246 */ 247 if (paws_reject || th->ack) 248 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN, 249 TCP_TIMEWAIT_LEN); 250 251 /* Send ACK. Note, we do not put the bucket, 252 * it will be released by caller. 253 */ 254 return TCP_TW_ACK; 255 } 256 inet_twsk_put(tw); 257 return TCP_TW_SUCCESS; 258} 259EXPORT_SYMBOL(tcp_timewait_state_process); 260 261/* 262 * Move a socket to time-wait or dead fin-wait-2 state. 263 */ 264void tcp_time_wait(struct sock *sk, int state, int timeo) 265{ 266 struct inet_timewait_sock *tw = NULL; 267 const struct inet_connection_sock *icsk = inet_csk(sk); 268 const struct tcp_sock *tp = tcp_sk(sk); 269 bool recycle_ok = false; 270 271 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp) 272 recycle_ok = tcp_remember_stamp(sk); 273 274 if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets) 275 tw = inet_twsk_alloc(sk, state); 276 277 if (tw != NULL) { 278 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw); 279 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1); 280 struct inet_sock *inet = inet_sk(sk); 281 282 tw->tw_transparent = inet->transparent; 283 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale; 284 tcptw->tw_rcv_nxt = tp->rcv_nxt; 285 tcptw->tw_snd_nxt = tp->snd_nxt; 286 tcptw->tw_rcv_wnd = tcp_receive_window(tp); 287 tcptw->tw_ts_recent = tp->rx_opt.ts_recent; 288 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; 289 290#if IS_ENABLED(CONFIG_IPV6) 291 if (tw->tw_family == PF_INET6) { 292 struct ipv6_pinfo *np = inet6_sk(sk); 293 struct inet6_timewait_sock *tw6; 294 295 tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot); 296 tw6 = inet6_twsk((struct sock *)tw); 297 tw6->tw_v6_daddr = np->daddr; 298 tw6->tw_v6_rcv_saddr = np->rcv_saddr; 299 tw->tw_tclass = np->tclass; 300 tw->tw_ipv6only = np->ipv6only; 301 } 302#endif 303 304#ifdef CONFIG_TCP_MD5SIG 305 /* 306 * The timewait bucket does not have the key DB from the 307 * sock structure. We just make a quick copy of the 308 * md5 key being used (if indeed we are using one) 309 * so the timewait ack generating code has the key. 310 */ 311 do { 312 struct tcp_md5sig_key *key; 313 tcptw->tw_md5_key = NULL; 314 key = tp->af_specific->md5_lookup(sk, sk); 315 if (key != NULL) { 316 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC); 317 if (tcptw->tw_md5_key && tcp_alloc_md5sig_pool(sk) == NULL) 318 BUG(); 319 } 320 } while (0); 321#endif 322 323 /* Linkage updates. */ 324 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo); 325 326 /* Get the TIME_WAIT timeout firing. */ 327 if (timeo < rto) 328 timeo = rto; 329 330 if (recycle_ok) { 331 tw->tw_timeout = rto; 332 } else { 333 tw->tw_timeout = TCP_TIMEWAIT_LEN; 334 if (state == TCP_TIME_WAIT) 335 timeo = TCP_TIMEWAIT_LEN; 336 } 337 338 inet_twsk_schedule(tw, &tcp_death_row, timeo, 339 TCP_TIMEWAIT_LEN); 340 inet_twsk_put(tw); 341 } else { 342 /* Sorry, if we're out of memory, just CLOSE this 343 * socket up. We've got bigger problems than 344 * non-graceful socket closings. 345 */ 346 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW); 347 } 348 349 tcp_update_metrics(sk); 350 tcp_done(sk); 351} 352 353void tcp_twsk_destructor(struct sock *sk) 354{ 355#ifdef CONFIG_TCP_MD5SIG 356 struct tcp_timewait_sock *twsk = tcp_twsk(sk); 357 358 if (twsk->tw_md5_key) { 359 tcp_free_md5sig_pool(); 360 kfree_rcu(twsk->tw_md5_key, rcu); 361 } 362#endif 363} 364EXPORT_SYMBOL_GPL(tcp_twsk_destructor); 365 366static inline void TCP_ECN_openreq_child(struct tcp_sock *tp, 367 struct request_sock *req) 368{ 369 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0; 370} 371 372/* This is not only more efficient than what we used to do, it eliminates 373 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM 374 * 375 * Actually, we could lots of memory writes here. tp of listening 376 * socket contains all necessary default parameters. 377 */ 378struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb) 379{ 380 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC); 381 382 if (newsk != NULL) { 383 const struct inet_request_sock *ireq = inet_rsk(req); 384 struct tcp_request_sock *treq = tcp_rsk(req); 385 struct inet_connection_sock *newicsk = inet_csk(newsk); 386 struct tcp_sock *newtp = tcp_sk(newsk); 387 struct tcp_sock *oldtp = tcp_sk(sk); 388 struct tcp_cookie_values *oldcvp = oldtp->cookie_values; 389 390 newicsk->icsk_af_ops->sk_rx_dst_set(newsk, skb); 391 392 /* TCP Cookie Transactions require space for the cookie pair, 393 * as it differs for each connection. There is no need to 394 * copy any s_data_payload stored at the original socket. 395 * Failure will prevent resuming the connection. 396 * 397 * Presumed copied, in order of appearance: 398 * cookie_in_always, cookie_out_never 399 */ 400 if (oldcvp != NULL) { 401 struct tcp_cookie_values *newcvp = 402 kzalloc(sizeof(*newtp->cookie_values), 403 GFP_ATOMIC); 404 405 if (newcvp != NULL) { 406 kref_init(&newcvp->kref); 407 newcvp->cookie_desired = 408 oldcvp->cookie_desired; 409 newtp->cookie_values = newcvp; 410 } else { 411 /* Not Yet Implemented */ 412 newtp->cookie_values = NULL; 413 } 414 } 415 416 /* Now setup tcp_sock */ 417 newtp->pred_flags = 0; 418 419 newtp->rcv_wup = newtp->copied_seq = 420 newtp->rcv_nxt = treq->rcv_isn + 1; 421 422 newtp->snd_sml = newtp->snd_una = 423 newtp->snd_nxt = newtp->snd_up = 424 treq->snt_isn + 1 + tcp_s_data_size(oldtp); 425 426 tcp_prequeue_init(newtp); 427 INIT_LIST_HEAD(&newtp->tsq_node); 428 429 tcp_init_wl(newtp, treq->rcv_isn); 430 431 newtp->srtt = 0; 432 newtp->mdev = TCP_TIMEOUT_INIT; 433 newicsk->icsk_rto = TCP_TIMEOUT_INIT; 434 435 newtp->packets_out = 0; 436 newtp->retrans_out = 0; 437 newtp->sacked_out = 0; 438 newtp->fackets_out = 0; 439 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH; 440 tcp_enable_early_retrans(newtp); 441 442 /* So many TCP implementations out there (incorrectly) count the 443 * initial SYN frame in their delayed-ACK and congestion control 444 * algorithms that we must have the following bandaid to talk 445 * efficiently to them. -DaveM 446 */ 447 newtp->snd_cwnd = TCP_INIT_CWND; 448 newtp->snd_cwnd_cnt = 0; 449 newtp->bytes_acked = 0; 450 451 newtp->frto_counter = 0; 452 newtp->frto_highmark = 0; 453 454 if (newicsk->icsk_ca_ops != &tcp_init_congestion_ops && 455 !try_module_get(newicsk->icsk_ca_ops->owner)) 456 newicsk->icsk_ca_ops = &tcp_init_congestion_ops; 457 458 tcp_set_ca_state(newsk, TCP_CA_Open); 459 tcp_init_xmit_timers(newsk); 460 skb_queue_head_init(&newtp->out_of_order_queue); 461 newtp->write_seq = newtp->pushed_seq = 462 treq->snt_isn + 1 + tcp_s_data_size(oldtp); 463 464 newtp->rx_opt.saw_tstamp = 0; 465 466 newtp->rx_opt.dsack = 0; 467 newtp->rx_opt.num_sacks = 0; 468 469 newtp->urg_data = 0; 470 471 if (sock_flag(newsk, SOCK_KEEPOPEN)) 472 inet_csk_reset_keepalive_timer(newsk, 473 keepalive_time_when(newtp)); 474 475 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok; 476 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) { 477 if (sysctl_tcp_fack) 478 tcp_enable_fack(newtp); 479 } 480 newtp->window_clamp = req->window_clamp; 481 newtp->rcv_ssthresh = req->rcv_wnd; 482 newtp->rcv_wnd = req->rcv_wnd; 483 newtp->rx_opt.wscale_ok = ireq->wscale_ok; 484 if (newtp->rx_opt.wscale_ok) { 485 newtp->rx_opt.snd_wscale = ireq->snd_wscale; 486 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale; 487 } else { 488 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0; 489 newtp->window_clamp = min(newtp->window_clamp, 65535U); 490 } 491 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << 492 newtp->rx_opt.snd_wscale); 493 newtp->max_window = newtp->snd_wnd; 494 495 if (newtp->rx_opt.tstamp_ok) { 496 newtp->rx_opt.ts_recent = req->ts_recent; 497 newtp->rx_opt.ts_recent_stamp = get_seconds(); 498 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED; 499 } else { 500 newtp->rx_opt.ts_recent_stamp = 0; 501 newtp->tcp_header_len = sizeof(struct tcphdr); 502 } 503#ifdef CONFIG_TCP_MD5SIG 504 newtp->md5sig_info = NULL; /*XXX*/ 505 if (newtp->af_specific->md5_lookup(sk, newsk)) 506 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED; 507#endif 508 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len) 509 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len; 510 newtp->rx_opt.mss_clamp = req->mss; 511 TCP_ECN_openreq_child(newtp, req); 512 513 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS); 514 } 515 return newsk; 516} 517EXPORT_SYMBOL(tcp_create_openreq_child); 518 519/* 520 * Process an incoming packet for SYN_RECV sockets represented 521 * as a request_sock. 522 */ 523 524struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, 525 struct request_sock *req, 526 struct request_sock **prev) 527{ 528 struct tcp_options_received tmp_opt; 529 const u8 *hash_location; 530 struct sock *child; 531 const struct tcphdr *th = tcp_hdr(skb); 532 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); 533 bool paws_reject = false; 534 535 tmp_opt.saw_tstamp = 0; 536 if (th->doff > (sizeof(struct tcphdr)>>2)) { 537 tcp_parse_options(skb, &tmp_opt, &hash_location, 0, NULL); 538 539 if (tmp_opt.saw_tstamp) { 540 tmp_opt.ts_recent = req->ts_recent; 541 /* We do not store true stamp, but it is not required, 542 * it can be estimated (approximately) 543 * from another data. 544 */ 545 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans); 546 paws_reject = tcp_paws_reject(&tmp_opt, th->rst); 547 } 548 } 549 550 /* Check for pure retransmitted SYN. */ 551 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn && 552 flg == TCP_FLAG_SYN && 553 !paws_reject) { 554 /* 555 * RFC793 draws (Incorrectly! It was fixed in RFC1122) 556 * this case on figure 6 and figure 8, but formal 557 * protocol description says NOTHING. 558 * To be more exact, it says that we should send ACK, 559 * because this segment (at least, if it has no data) 560 * is out of window. 561 * 562 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT 563 * describe SYN-RECV state. All the description 564 * is wrong, we cannot believe to it and should 565 * rely only on common sense and implementation 566 * experience. 567 * 568 * Enforce "SYN-ACK" according to figure 8, figure 6 569 * of RFC793, fixed by RFC1122. 570 */ 571 req->rsk_ops->rtx_syn_ack(sk, req, NULL); 572 return NULL; 573 } 574 575 /* Further reproduces section "SEGMENT ARRIVES" 576 for state SYN-RECEIVED of RFC793. 577 It is broken, however, it does not work only 578 when SYNs are crossed. 579 580 You would think that SYN crossing is impossible here, since 581 we should have a SYN_SENT socket (from connect()) on our end, 582 but this is not true if the crossed SYNs were sent to both 583 ends by a malicious third party. We must defend against this, 584 and to do that we first verify the ACK (as per RFC793, page 585 36) and reset if it is invalid. Is this a true full defense? 586 To convince ourselves, let us consider a way in which the ACK 587 test can still pass in this 'malicious crossed SYNs' case. 588 Malicious sender sends identical SYNs (and thus identical sequence 589 numbers) to both A and B: 590 591 A: gets SYN, seq=7 592 B: gets SYN, seq=7 593 594 By our good fortune, both A and B select the same initial 595 send sequence number of seven :-) 596 597 A: sends SYN|ACK, seq=7, ack_seq=8 598 B: sends SYN|ACK, seq=7, ack_seq=8 599 600 So we are now A eating this SYN|ACK, ACK test passes. So 601 does sequence test, SYN is truncated, and thus we consider 602 it a bare ACK. 603 604 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this 605 bare ACK. Otherwise, we create an established connection. Both 606 ends (listening sockets) accept the new incoming connection and try 607 to talk to each other. 8-) 608 609 Note: This case is both harmless, and rare. Possibility is about the 610 same as us discovering intelligent life on another plant tomorrow. 611 612 But generally, we should (RFC lies!) to accept ACK 613 from SYNACK both here and in tcp_rcv_state_process(). 614 tcp_rcv_state_process() does not, hence, we do not too. 615 616 Note that the case is absolutely generic: 617 we cannot optimize anything here without 618 violating protocol. All the checks must be made 619 before attempt to create socket. 620 */ 621 622 /* RFC793 page 36: "If the connection is in any non-synchronized state ... 623 * and the incoming segment acknowledges something not yet 624 * sent (the segment carries an unacceptable ACK) ... 625 * a reset is sent." 626 * 627 * Invalid ACK: reset will be sent by listening socket 628 */ 629 if ((flg & TCP_FLAG_ACK) && 630 (TCP_SKB_CB(skb)->ack_seq != 631 tcp_rsk(req)->snt_isn + 1 + tcp_s_data_size(tcp_sk(sk)))) 632 return sk; 633 634 /* Also, it would be not so bad idea to check rcv_tsecr, which 635 * is essentially ACK extension and too early or too late values 636 * should cause reset in unsynchronized states. 637 */ 638 639 /* RFC793: "first check sequence number". */ 640 641 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq, 642 tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) { 643 /* Out of window: send ACK and drop. */ 644 if (!(flg & TCP_FLAG_RST)) 645 req->rsk_ops->send_ack(sk, skb, req); 646 if (paws_reject) 647 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); 648 return NULL; 649 } 650 651 /* In sequence, PAWS is OK. */ 652 653 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1)) 654 req->ts_recent = tmp_opt.rcv_tsval; 655 656 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) { 657 /* Truncate SYN, it is out of window starting 658 at tcp_rsk(req)->rcv_isn + 1. */ 659 flg &= ~TCP_FLAG_SYN; 660 } 661 662 /* RFC793: "second check the RST bit" and 663 * "fourth, check the SYN bit" 664 */ 665 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) { 666 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS); 667 goto embryonic_reset; 668 } 669 670 /* ACK sequence verified above, just make sure ACK is 671 * set. If ACK not set, just silently drop the packet. 672 */ 673 if (!(flg & TCP_FLAG_ACK)) 674 return NULL; 675 676 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */ 677 if (req->retrans < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept && 678 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) { 679 inet_rsk(req)->acked = 1; 680 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP); 681 return NULL; 682 } 683 if (tmp_opt.saw_tstamp && tmp_opt.rcv_tsecr) 684 tcp_rsk(req)->snt_synack = tmp_opt.rcv_tsecr; 685 else if (req->retrans) /* don't take RTT sample if retrans && ~TS */ 686 tcp_rsk(req)->snt_synack = 0; 687 688 /* OK, ACK is valid, create big socket and 689 * feed this segment to it. It will repeat all 690 * the tests. THIS SEGMENT MUST MOVE SOCKET TO 691 * ESTABLISHED STATE. If it will be dropped after 692 * socket is created, wait for troubles. 693 */ 694 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL); 695 if (child == NULL) 696 goto listen_overflow; 697 698 inet_csk_reqsk_queue_unlink(sk, req, prev); 699 inet_csk_reqsk_queue_removed(sk, req); 700 701 inet_csk_reqsk_queue_add(sk, req, child); 702 return child; 703 704listen_overflow: 705 if (!sysctl_tcp_abort_on_overflow) { 706 inet_rsk(req)->acked = 1; 707 return NULL; 708 } 709 710embryonic_reset: 711 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS); 712 if (!(flg & TCP_FLAG_RST)) 713 req->rsk_ops->send_reset(sk, skb); 714 715 inet_csk_reqsk_queue_drop(sk, req, prev); 716 return NULL; 717} 718EXPORT_SYMBOL(tcp_check_req); 719 720/* 721 * Queue segment on the new socket if the new socket is active, 722 * otherwise we just shortcircuit this and continue with 723 * the new socket. 724 */ 725 726int tcp_child_process(struct sock *parent, struct sock *child, 727 struct sk_buff *skb) 728{ 729 int ret = 0; 730 int state = child->sk_state; 731 732 if (!sock_owned_by_user(child)) { 733 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb), 734 skb->len); 735 /* Wakeup parent, send SIGIO */ 736 if (state == TCP_SYN_RECV && child->sk_state != state) 737 parent->sk_data_ready(parent, 0); 738 } else { 739 /* Alas, it is possible again, because we do lookup 740 * in main socket hash table and lock on listening 741 * socket does not protect us more. 742 */ 743 __sk_add_backlog(child, skb); 744 } 745 746 bh_unlock_sock(child); 747 sock_put(child); 748 return ret; 749} 750EXPORT_SYMBOL(tcp_child_process); 751