net/ipv4/tcp_minisocks.c

   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
  30 int sysctl_tcp_syncookies __read_mostly = 1;
  31 EXPORT_SYMBOL(sysctl_tcp_syncookies);
  32
  33 int sysctl_tcp_abort_on_overflow __read_mostly;
  34
  35 struct 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 };
  50 EXPORT_SYMBOL_GPL(tcp_death_row);
  51
  52 static 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 enum tcp_tw_status
  90 tcp_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) {
 138 kill_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) {
 191 kill:
 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 }
 259 EXPORT_SYMBOL(tcp_timewait_state_process);
 260
 261 /*
 262  * Move a socket to time-wait or dead fin-wait-2 state.
 263  */
 264 void 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
 353 void 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 }
 364 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
 365
 366 static 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  */
 378 struct 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                 /* TCP Cookie Transactions require space for the cookie pair,
 391                  * as it differs for each connection.  There is no need to
 392                  * copy any s_data_payload stored at the original socket.
 393                  * Failure will prevent resuming the connection.
 394                  *
 395                  * Presumed copied, in order of appearance:
 396                  *      cookie_in_always, cookie_out_never
 397                  */
 398                 if (oldcvp != NULL) {
 399                         struct tcp_cookie_values *newcvp =
 400                                 kzalloc(sizeof(*newtp->cookie_values),
 401                                         GFP_ATOMIC);
 402
 403                         if (newcvp != NULL) {
 404                                 kref_init(&newcvp->kref);
 405                                 newcvp->cookie_desired =
 406                                                 oldcvp->cookie_desired;
 407                                 newtp->cookie_values = newcvp;
 408                         } else {
 409                                 /* Not Yet Implemented */
 410                                 newtp->cookie_values = NULL;
 411                         }
 412                 }
 413
 414                 /* Now setup tcp_sock */
 415                 newtp->pred_flags = 0;
 416
 417                 newtp->rcv_wup = newtp->copied_seq =
 418                 newtp->rcv_nxt = treq->rcv_isn + 1;
 419
 420                 newtp->snd_sml = newtp->snd_una =
 421                 newtp->snd_nxt = newtp->snd_up =
 422                         treq->snt_isn + 1 + tcp_s_data_size(oldtp);
 423
 424                 tcp_prequeue_init(newtp);
 425                 INIT_LIST_HEAD(&newtp->tsq_node);
 426
 427                 tcp_init_wl(newtp, treq->rcv_isn);
 428
 429                 newtp->srtt = 0;
 430                 newtp->mdev = TCP_TIMEOUT_INIT;
 431                 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
 432
 433                 newtp->packets_out = 0;
 434                 newtp->retrans_out = 0;
 435                 newtp->sacked_out = 0;
 436                 newtp->fackets_out = 0;
 437                 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
 438                 tcp_enable_early_retrans(newtp);
 439
 440                 /* So many TCP implementations out there (incorrectly) count the
 441                  * initial SYN frame in their delayed-ACK and congestion control
 442                  * algorithms that we must have the following bandaid to talk
 443                  * efficiently to them.  -DaveM
 444                  */
 445                 newtp->snd_cwnd = TCP_INIT_CWND;
 446                 newtp->snd_cwnd_cnt = 0;
 447                 newtp->bytes_acked = 0;
 448
 449                 newtp->frto_counter = 0;
 450                 newtp->frto_highmark = 0;
 451
 452                 if (newicsk->icsk_ca_ops != &tcp_init_congestion_ops &&
 453                     !try_module_get(newicsk->icsk_ca_ops->owner))
 454                         newicsk->icsk_ca_ops = &tcp_init_congestion_ops;
 455
 456                 tcp_set_ca_state(newsk, TCP_CA_Open);
 457                 tcp_init_xmit_timers(newsk);
 458                 skb_queue_head_init(&newtp->out_of_order_queue);
 459                 newtp->write_seq = newtp->pushed_seq =
 460                         treq->snt_isn + 1 + tcp_s_data_size(oldtp);
 461
 462                 newtp->rx_opt.saw_tstamp = 0;
 463
 464                 newtp->rx_opt.dsack = 0;
 465                 newtp->rx_opt.num_sacks = 0;
 466
 467                 newtp->urg_data = 0;
 468
 469                 if (sock_flag(newsk, SOCK_KEEPOPEN))
 470                         inet_csk_reset_keepalive_timer(newsk,
 471                                                        keepalive_time_when(newtp));
 472
 473                 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
 474                 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
 475                         if (sysctl_tcp_fack)
 476                                 tcp_enable_fack(newtp);
 477                 }
 478                 newtp->window_clamp = req->window_clamp;
 479                 newtp->rcv_ssthresh = req->rcv_wnd;
 480                 newtp->rcv_wnd = req->rcv_wnd;
 481                 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
 482                 if (newtp->rx_opt.wscale_ok) {
 483                         newtp->rx_opt.snd_wscale = ireq->snd_wscale;
 484                         newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
 485                 } else {
 486                         newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
 487                         newtp->window_clamp = min(newtp->window_clamp, 65535U);
 488                 }
 489                 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
 490                                   newtp->rx_opt.snd_wscale);
 491                 newtp->max_window = newtp->snd_wnd;
 492
 493                 if (newtp->rx_opt.tstamp_ok) {
 494                         newtp->rx_opt.ts_recent = req->ts_recent;
 495                         newtp->rx_opt.ts_recent_stamp = get_seconds();
 496                         newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
 497                 } else {
 498                         newtp->rx_opt.ts_recent_stamp = 0;
 499                         newtp->tcp_header_len = sizeof(struct tcphdr);
 500                 }
 501 #ifdef CONFIG_TCP_MD5SIG
 502                 newtp->md5sig_info = NULL;      /*XXX*/
 503                 if (newtp->af_specific->md5_lookup(sk, newsk))
 504                         newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
 505 #endif
 506                 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
 507                         newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
 508                 newtp->rx_opt.mss_clamp = req->mss;
 509                 TCP_ECN_openreq_child(newtp, req);
 510
 511                 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS);
 512         }
 513         return newsk;
 514 }
 515 EXPORT_SYMBOL(tcp_create_openreq_child);
 516
 517 /*
 518  *      Process an incoming packet for SYN_RECV sockets represented
 519  *      as a request_sock.
 520  */
 521
 522 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 523                            struct request_sock *req,
 524                            struct request_sock **prev)
 525 {
 526         struct tcp_options_received tmp_opt;
 527         const u8 *hash_location;
 528         struct sock *child;
 529         const struct tcphdr *th = tcp_hdr(skb);
 530         __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
 531         bool paws_reject = false;
 532
 533         tmp_opt.saw_tstamp = 0;
 534         if (th->doff > (sizeof(struct tcphdr)>>2)) {
 535                 tcp_parse_options(skb, &tmp_opt, &hash_location, 0, NULL);
 536
 537                 if (tmp_opt.saw_tstamp) {
 538                         tmp_opt.ts_recent = req->ts_recent;
 539                         /* We do not store true stamp, but it is not required,
 540                          * it can be estimated (approximately)
 541                          * from another data.
 542                          */
 543                         tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans);
 544                         paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
 545                 }
 546         }
 547
 548         /* Check for pure retransmitted SYN. */
 549         if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
 550             flg == TCP_FLAG_SYN &&
 551             !paws_reject) {
 552                 /*
 553                  * RFC793 draws (Incorrectly! It was fixed in RFC1122)
 554                  * this case on figure 6 and figure 8, but formal
 555                  * protocol description says NOTHING.
 556                  * To be more exact, it says that we should send ACK,
 557                  * because this segment (at least, if it has no data)
 558                  * is out of window.
 559                  *
 560                  *  CONCLUSION: RFC793 (even with RFC1122) DOES NOT
 561                  *  describe SYN-RECV state. All the description
 562                  *  is wrong, we cannot believe to it and should
 563                  *  rely only on common sense and implementation
 564                  *  experience.
 565                  *
 566                  * Enforce "SYN-ACK" according to figure 8, figure 6
 567                  * of RFC793, fixed by RFC1122.
 568                  */
 569                 req->rsk_ops->rtx_syn_ack(sk, req, NULL);
 570                 return NULL;
 571         }
 572
 573         /* Further reproduces section "SEGMENT ARRIVES"
 574            for state SYN-RECEIVED of RFC793.
 575            It is broken, however, it does not work only
 576            when SYNs are crossed.
 577
 578            You would think that SYN crossing is impossible here, since
 579            we should have a SYN_SENT socket (from connect()) on our end,
 580            but this is not true if the crossed SYNs were sent to both
 581            ends by a malicious third party.  We must defend against this,
 582            and to do that we first verify the ACK (as per RFC793, page
 583            36) and reset if it is invalid.  Is this a true full defense?
 584            To convince ourselves, let us consider a way in which the ACK
 585            test can still pass in this 'malicious crossed SYNs' case.
 586            Malicious sender sends identical SYNs (and thus identical sequence
 587            numbers) to both A and B:
 588
 589                 A: gets SYN, seq=7
 590                 B: gets SYN, seq=7
 591
 592            By our good fortune, both A and B select the same initial
 593            send sequence number of seven :-)
 594
 595                 A: sends SYN|ACK, seq=7, ack_seq=8
 596                 B: sends SYN|ACK, seq=7, ack_seq=8
 597
 598            So we are now A eating this SYN|ACK, ACK test passes.  So
 599            does sequence test, SYN is truncated, and thus we consider
 600            it a bare ACK.
 601
 602            If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
 603            bare ACK.  Otherwise, we create an established connection.  Both
 604            ends (listening sockets) accept the new incoming connection and try
 605            to talk to each other. 8-)
 606
 607            Note: This case is both harmless, and rare.  Possibility is about the
 608            same as us discovering intelligent life on another plant tomorrow.
 609
 610            But generally, we should (RFC lies!) to accept ACK
 611            from SYNACK both here and in tcp_rcv_state_process().
 612            tcp_rcv_state_process() does not, hence, we do not too.
 613
 614            Note that the case is absolutely generic:
 615            we cannot optimize anything here without
 616            violating protocol. All the checks must be made
 617            before attempt to create socket.
 618          */
 619
 620         /* RFC793 page 36: "If the connection is in any non-synchronized state ...
 621          *                  and the incoming segment acknowledges something not yet
 622          *                  sent (the segment carries an unacceptable ACK) ...
 623          *                  a reset is sent."
 624          *
 625          * Invalid ACK: reset will be sent by listening socket
 626          */
 627         if ((flg & TCP_FLAG_ACK) &&
 628             (TCP_SKB_CB(skb)->ack_seq !=
 629              tcp_rsk(req)->snt_isn + 1 + tcp_s_data_size(tcp_sk(sk))))
 630                 return sk;
 631
 632         /* Also, it would be not so bad idea to check rcv_tsecr, which
 633          * is essentially ACK extension and too early or too late values
 634          * should cause reset in unsynchronized states.
 635          */
 636
 637         /* RFC793: "first check sequence number". */
 638
 639         if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
 640                                           tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) {
 641                 /* Out of window: send ACK and drop. */
 642                 if (!(flg & TCP_FLAG_RST))
 643                         req->rsk_ops->send_ack(sk, skb, req);
 644                 if (paws_reject)
 645                         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
 646                 return NULL;
 647         }
 648
 649         /* In sequence, PAWS is OK. */
 650
 651         if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1))
 652                 req->ts_recent = tmp_opt.rcv_tsval;
 653
 654         if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
 655                 /* Truncate SYN, it is out of window starting
 656                    at tcp_rsk(req)->rcv_isn + 1. */
 657                 flg &= ~TCP_FLAG_SYN;
 658         }
 659
 660         /* RFC793: "second check the RST bit" and
 661          *         "fourth, check the SYN bit"
 662          */
 663         if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
 664                 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
 665                 goto embryonic_reset;
 666         }
 667
 668         /* ACK sequence verified above, just make sure ACK is
 669          * set.  If ACK not set, just silently drop the packet.
 670          */
 671         if (!(flg & TCP_FLAG_ACK))
 672                 return NULL;
 673
 674         /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
 675         if (req->retrans < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
 676             TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
 677                 inet_rsk(req)->acked = 1;
 678                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
 679                 return NULL;
 680         }
 681         if (tmp_opt.saw_tstamp && tmp_opt.rcv_tsecr)
 682                 tcp_rsk(req)->snt_synack = tmp_opt.rcv_tsecr;
 683         else if (req->retrans) /* don't take RTT sample if retrans && ~TS */
 684                 tcp_rsk(req)->snt_synack = 0;
 685
 686         /* OK, ACK is valid, create big socket and
 687          * feed this segment to it. It will repeat all
 688          * the tests. THIS SEGMENT MUST MOVE SOCKET TO
 689          * ESTABLISHED STATE. If it will be dropped after
 690          * socket is created, wait for troubles.
 691          */
 692         child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL);
 693         if (child == NULL)
 694                 goto listen_overflow;
 695
 696         inet_csk_reqsk_queue_unlink(sk, req, prev);
 697         inet_csk_reqsk_queue_removed(sk, req);
 698
 699         inet_csk_reqsk_queue_add(sk, req, child);
 700         return child;
 701
 702 listen_overflow:
 703         if (!sysctl_tcp_abort_on_overflow) {
 704                 inet_rsk(req)->acked = 1;
 705                 return NULL;
 706         }
 707
 708 embryonic_reset:
 709         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
 710         if (!(flg & TCP_FLAG_RST))
 711                 req->rsk_ops->send_reset(sk, skb);
 712
 713         inet_csk_reqsk_queue_drop(sk, req, prev);
 714         return NULL;
 715 }
 716 EXPORT_SYMBOL(tcp_check_req);
 717
 718 /*
 719  * Queue segment on the new socket if the new socket is active,
 720  * otherwise we just shortcircuit this and continue with
 721  * the new socket.
 722  */
 723
 724 int tcp_child_process(struct sock *parent, struct sock *child,
 725                       struct sk_buff *skb)
 726 {
 727         int ret = 0;
 728         int state = child->sk_state;
 729
 730         if (!sock_owned_by_user(child)) {
 731                 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
 732                                             skb->len);
 733                 /* Wakeup parent, send SIGIO */
 734                 if (state == TCP_SYN_RECV && child->sk_state != state)
 735                         parent->sk_data_ready(parent, 0);
 736         } else {
 737                 /* Alas, it is possible again, because we do lookup
 738                  * in main socket hash table and lock on listening
 739                  * socket does not protect us more.
 740                  */
 741                 __sk_add_backlog(child, skb);
 742         }
 743
 744         bh_unlock_sock(child);
 745         sock_put(child);
 746         return ret;
 747 }
 748 EXPORT_SYMBOL(tcp_child_process);