err.no Git - linux-2.6/blob - 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/sysctl.h>
  24 #include <linux/workqueue.h>
  25 #include <net/tcp.h>
  26 #include <net/inet_common.h>
  27 #include <net/xfrm.h>
  28
  29 #ifdef CONFIG_SYSCTL
  30 #define SYNC_INIT 0 /* let the user enable it */
  31 #else
  32 #define SYNC_INIT 1
  33 #endif
  34
  35 int sysctl_tcp_syncookies __read_mostly = SYNC_INIT;
  36 EXPORT_SYMBOL(sysctl_tcp_syncookies);
  37
  38 int sysctl_tcp_abort_on_overflow __read_mostly;
  39
  40 struct inet_timewait_death_row tcp_death_row = {
  41         .sysctl_max_tw_buckets = NR_FILE * 2,
  42         .period         = TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS,
  43         .death_lock     = __SPIN_LOCK_UNLOCKED(tcp_death_row.death_lock),
  44         .hashinfo       = &tcp_hashinfo,
  45         .tw_timer       = TIMER_INITIALIZER(inet_twdr_hangman, 0,
  46                                             (unsigned long)&tcp_death_row),
  47         .twkill_work    = __WORK_INITIALIZER(tcp_death_row.twkill_work,
  48                                              inet_twdr_twkill_work),
  49 /* Short-time timewait calendar */
  50
  51         .twcal_hand     = -1,
  52         .twcal_timer    = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0,
  53                                             (unsigned long)&tcp_death_row),
  54 };
  55
  56 EXPORT_SYMBOL_GPL(tcp_death_row);
  57
  58 static __inline__ int tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
  59 {
  60         if (seq == s_win)
  61                 return 1;
  62         if (after(end_seq, s_win) && before(seq, e_win))
  63                 return 1;
  64         return (seq == e_win && seq == end_seq);
  65 }
  66
  67 /*
  68  * * Main purpose of TIME-WAIT state is to close connection gracefully,
  69  *   when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
  70  *   (and, probably, tail of data) and one or more our ACKs are lost.
  71  * * What is TIME-WAIT timeout? It is associated with maximal packet
  72  *   lifetime in the internet, which results in wrong conclusion, that
  73  *   it is set to catch "old duplicate segments" wandering out of their path.
  74  *   It is not quite correct. This timeout is calculated so that it exceeds
  75  *   maximal retransmission timeout enough to allow to lose one (or more)
  76  *   segments sent by peer and our ACKs. This time may be calculated from RTO.
  77  * * When TIME-WAIT socket receives RST, it means that another end
  78  *   finally closed and we are allowed to kill TIME-WAIT too.
  79  * * Second purpose of TIME-WAIT is catching old duplicate segments.
  80  *   Well, certainly it is pure paranoia, but if we load TIME-WAIT
  81  *   with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
  82  * * If we invented some more clever way to catch duplicates
  83  *   (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
  84  *
  85  * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
  86  * When you compare it to RFCs, please, read section SEGMENT ARRIVES
  87  * from the very beginning.
  88  *
  89  * NOTE. With recycling (and later with fin-wait-2) TW bucket
  90  * is _not_ stateless. It means, that strictly speaking we must
  91  * spinlock it. I do not want! Well, probability of misbehaviour
  92  * is ridiculously low and, seems, we could use some mb() tricks
  93  * to avoid misread sequence numbers, states etc.  --ANK
  94  */
  95 enum tcp_tw_status
  96 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
  97                            const struct tcphdr *th)
  98 {
  99         struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
 100         struct tcp_options_received tmp_opt;
 101         int paws_reject = 0;
 102
 103         tmp_opt.saw_tstamp = 0;
 104         if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
 105                 tcp_parse_options(skb, &tmp_opt, 0);
 106
 107                 if (tmp_opt.saw_tstamp) {
 108                         tmp_opt.ts_recent       = tcptw->tw_ts_recent;
 109                         tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
 110                         paws_reject = tcp_paws_check(&tmp_opt, th->rst);
 111                 }
 112         }
 113
 114         if (tw->tw_substate == TCP_FIN_WAIT2) {
 115                 /* Just repeat all the checks of tcp_rcv_state_process() */
 116
 117                 /* Out of window, send ACK */
 118                 if (paws_reject ||
 119                     !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
 120                                    tcptw->tw_rcv_nxt,
 121                                    tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
 122                         return TCP_TW_ACK;
 123
 124                 if (th->rst)
 125                         goto kill;
 126
 127                 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
 128                         goto kill_with_rst;
 129
 130                 /* Dup ACK? */
 131                 if (!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 kill_with_rst:
 143                         inet_twsk_deschedule(tw, &tcp_death_row);
 144                         inet_twsk_put(tw);
 145                         return TCP_TW_RST;
 146                 }
 147
 148                 /* FIN arrived, enter true time-wait state. */
 149                 tw->tw_substate   = TCP_TIME_WAIT;
 150                 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
 151                 if (tmp_opt.saw_tstamp) {
 152                         tcptw->tw_ts_recent_stamp = get_seconds();
 153                         tcptw->tw_ts_recent       = tmp_opt.rcv_tsval;
 154                 }
 155
 156                 /* I am shamed, but failed to make it more elegant.
 157                  * Yes, it is direct reference to IP, which is impossible
 158                  * to generalize to IPv6. Taking into account that IPv6
 159                  * do not understand recycling in any case, it not
 160                  * a big problem in practice. --ANK */
 161                 if (tw->tw_family == AF_INET &&
 162                     tcp_death_row.sysctl_tw_recycle && tcptw->tw_ts_recent_stamp &&
 163                     tcp_v4_tw_remember_stamp(tw))
 164                         inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout,
 165                                            TCP_TIMEWAIT_LEN);
 166                 else
 167                         inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
 168                                            TCP_TIMEWAIT_LEN);
 169                 return TCP_TW_ACK;
 170         }
 171
 172         /*
 173          *      Now real TIME-WAIT state.
 174          *
 175          *      RFC 1122:
 176          *      "When a connection is [...] on TIME-WAIT state [...]
 177          *      [a TCP] MAY accept a new SYN from the remote TCP to
 178          *      reopen the connection directly, if it:
 179          *
 180          *      (1)  assigns its initial sequence number for the new
 181          *      connection to be larger than the largest sequence
 182          *      number it used on the previous connection incarnation,
 183          *      and
 184          *
 185          *      (2)  returns to TIME-WAIT state if the SYN turns out
 186          *      to be an old duplicate".
 187          */
 188
 189         if (!paws_reject &&
 190             (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
 191              (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
 192                 /* In window segment, it may be only reset or bare ack. */
 193
 194                 if (th->rst) {
 195                         /* This is TIME_WAIT assassination, in two flavors.
 196                          * Oh well... nobody has a sufficient solution to this
 197                          * protocol bug yet.
 198                          */
 199                         if (sysctl_tcp_rfc1337 == 0) {
 200 kill:
 201                                 inet_twsk_deschedule(tw, &tcp_death_row);
 202                                 inet_twsk_put(tw);
 203                                 return TCP_TW_SUCCESS;
 204                         }
 205                 }
 206                 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
 207                                    TCP_TIMEWAIT_LEN);
 208
 209                 if (tmp_opt.saw_tstamp) {
 210                         tcptw->tw_ts_recent       = tmp_opt.rcv_tsval;
 211                         tcptw->tw_ts_recent_stamp = get_seconds();
 212                 }
 213
 214                 inet_twsk_put(tw);
 215                 return TCP_TW_SUCCESS;
 216         }
 217
 218         /* Out of window segment.
 219
 220            All the segments are ACKed immediately.
 221
 222            The only exception is new SYN. We accept it, if it is
 223            not old duplicate and we are not in danger to be killed
 224            by delayed old duplicates. RFC check is that it has
 225            newer sequence number works at rates <40Mbit/sec.
 226            However, if paws works, it is reliable AND even more,
 227            we even may relax silly seq space cutoff.
 228
 229            RED-PEN: we violate main RFC requirement, if this SYN will appear
 230            old duplicate (i.e. we receive RST in reply to SYN-ACK),
 231            we must return socket to time-wait state. It is not good,
 232            but not fatal yet.
 233          */
 234
 235         if (th->syn && !th->rst && !th->ack && !paws_reject &&
 236             (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
 237              (tmp_opt.saw_tstamp &&
 238               (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
 239                 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
 240                 if (isn == 0)
 241                         isn++;
 242                 TCP_SKB_CB(skb)->when = isn;
 243                 return TCP_TW_SYN;
 244         }
 245
 246         if (paws_reject)
 247                 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
 248
 249         if (!th->rst) {
 250                 /* In this case we must reset the TIMEWAIT timer.
 251                  *
 252                  * If it is ACKless SYN it may be both old duplicate
 253                  * and new good SYN with random sequence number <rcv_nxt.
 254                  * Do not reschedule in the last case.
 255                  */
 256                 if (paws_reject || th->ack)
 257                         inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
 258                                            TCP_TIMEWAIT_LEN);
 259
 260                 /* Send ACK. Note, we do not put the bucket,
 261                  * it will be released by caller.
 262                  */
 263                 return TCP_TW_ACK;
 264         }
 265         inet_twsk_put(tw);
 266         return TCP_TW_SUCCESS;
 267 }
 268
 269 /*
 270  * Move a socket to time-wait or dead fin-wait-2 state.
 271  */
 272 void tcp_time_wait(struct sock *sk, int state, int timeo)
 273 {
 274         struct inet_timewait_sock *tw = NULL;
 275         const struct inet_connection_sock *icsk = inet_csk(sk);
 276         const struct tcp_sock *tp = tcp_sk(sk);
 277         int recycle_ok = 0;
 278
 279         if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp)
 280                 recycle_ok = icsk->icsk_af_ops->remember_stamp(sk);
 281
 282         if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets)
 283                 tw = inet_twsk_alloc(sk, state);
 284
 285         if (tw != NULL) {
 286                 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
 287                 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
 288
 289                 tw->tw_rcv_wscale       = tp->rx_opt.rcv_wscale;
 290                 tcptw->tw_rcv_nxt       = tp->rcv_nxt;
 291                 tcptw->tw_snd_nxt       = tp->snd_nxt;
 292                 tcptw->tw_rcv_wnd       = tcp_receive_window(tp);
 293                 tcptw->tw_ts_recent     = tp->rx_opt.ts_recent;
 294                 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
 295
 296 #if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
 297                 if (tw->tw_family == PF_INET6) {
 298                         struct ipv6_pinfo *np = inet6_sk(sk);
 299                         struct inet6_timewait_sock *tw6;
 300
 301                         tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot);
 302                         tw6 = inet6_twsk((struct sock *)tw);
 303                         ipv6_addr_copy(&tw6->tw_v6_daddr, &np->daddr);
 304                         ipv6_addr_copy(&tw6->tw_v6_rcv_saddr, &np->rcv_saddr);
 305                         tw->tw_ipv6only = np->ipv6only;
 306                 }
 307 #endif
 308
 309 #ifdef CONFIG_TCP_MD5SIG
 310                 /*
 311                  * The timewait bucket does not have the key DB from the
 312                  * sock structure. We just make a quick copy of the
 313                  * md5 key being used (if indeed we are using one)
 314                  * so the timewait ack generating code has the key.
 315                  */
 316                 do {
 317                         struct tcp_md5sig_key *key;
 318                         memset(tcptw->tw_md5_key, 0, sizeof(tcptw->tw_md5_key));
 319                         tcptw->tw_md5_keylen = 0;
 320                         key = tp->af_specific->md5_lookup(sk, sk);
 321                         if (key != NULL) {
 322                                 memcpy(&tcptw->tw_md5_key, key->key, key->keylen);
 323                                 tcptw->tw_md5_keylen = key->keylen;
 324                                 if (tcp_alloc_md5sig_pool() == NULL)
 325                                         BUG();
 326                         }
 327                 } while (0);
 328 #endif
 329
 330                 /* Linkage updates. */
 331                 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
 332
 333                 /* Get the TIME_WAIT timeout firing. */
 334                 if (timeo < rto)
 335                         timeo = rto;
 336
 337                 if (recycle_ok) {
 338                         tw->tw_timeout = rto;
 339                 } else {
 340                         tw->tw_timeout = TCP_TIMEWAIT_LEN;
 341                         if (state == TCP_TIME_WAIT)
 342                                 timeo = TCP_TIMEWAIT_LEN;
 343                 }
 344
 345                 inet_twsk_schedule(tw, &tcp_death_row, timeo,
 346                                    TCP_TIMEWAIT_LEN);
 347                 inet_twsk_put(tw);
 348         } else {
 349                 /* Sorry, if we're out of memory, just CLOSE this
 350                  * socket up.  We've got bigger problems than
 351                  * non-graceful socket closings.
 352                  */
 353                 LIMIT_NETDEBUG(KERN_INFO "TCP: time wait bucket table overflow\n");
 354         }
 355
 356         tcp_update_metrics(sk);
 357         tcp_done(sk);
 358 }
 359
 360 void tcp_twsk_destructor(struct sock *sk)
 361 {
 362 #ifdef CONFIG_TCP_MD5SIG
 363         struct tcp_timewait_sock *twsk = tcp_twsk(sk);
 364         if (twsk->tw_md5_keylen)
 365                 tcp_put_md5sig_pool();
 366 #endif
 367 }
 368
 369 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
 370
 371 static inline void TCP_ECN_openreq_child(struct tcp_sock *tp,
 372                                          struct request_sock *req)
 373 {
 374         tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
 375 }
 376
 377 /* This is not only more efficient than what we used to do, it eliminates
 378  * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
 379  *
 380  * Actually, we could lots of memory writes here. tp of listening
 381  * socket contains all necessary default parameters.
 382  */
 383 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
 384 {
 385         struct sock *newsk = inet_csk_clone(sk, req, GFP_ATOMIC);
 386
 387         if (newsk != NULL) {
 388                 const struct inet_request_sock *ireq = inet_rsk(req);
 389                 struct tcp_request_sock *treq = tcp_rsk(req);
 390                 struct inet_connection_sock *newicsk = inet_csk(newsk);
 391                 struct tcp_sock *newtp;
 392
 393                 /* Now setup tcp_sock */
 394                 newtp = tcp_sk(newsk);
 395                 newtp->pred_flags = 0;
 396                 newtp->rcv_wup = newtp->copied_seq = newtp->rcv_nxt = treq->rcv_isn + 1;
 397                 newtp->snd_sml = newtp->snd_una = newtp->snd_nxt = treq->snt_isn + 1;
 398
 399                 tcp_prequeue_init(newtp);
 400
 401                 tcp_init_wl(newtp, treq->snt_isn, treq->rcv_isn);
 402
 403                 newtp->srtt = 0;
 404                 newtp->mdev = TCP_TIMEOUT_INIT;
 405                 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
 406
 407                 newtp->packets_out = 0;
 408                 newtp->retrans_out = 0;
 409                 newtp->sacked_out = 0;
 410                 newtp->fackets_out = 0;
 411                 newtp->snd_ssthresh = 0x7fffffff;
 412
 413                 /* So many TCP implementations out there (incorrectly) count the
 414                  * initial SYN frame in their delayed-ACK and congestion control
 415                  * algorithms that we must have the following bandaid to talk
 416                  * efficiently to them.  -DaveM
 417                  */
 418                 newtp->snd_cwnd = 2;
 419                 newtp->snd_cwnd_cnt = 0;
 420                 newtp->bytes_acked = 0;
 421
 422                 newtp->frto_counter = 0;
 423                 newtp->frto_highmark = 0;
 424
 425                 newicsk->icsk_ca_ops = &tcp_init_congestion_ops;
 426
 427                 tcp_set_ca_state(newsk, TCP_CA_Open);
 428                 tcp_init_xmit_timers(newsk);
 429                 skb_queue_head_init(&newtp->out_of_order_queue);
 430                 newtp->write_seq = treq->snt_isn + 1;
 431                 newtp->pushed_seq = newtp->write_seq;
 432
 433                 newtp->rx_opt.saw_tstamp = 0;
 434
 435                 newtp->rx_opt.dsack = 0;
 436                 newtp->rx_opt.eff_sacks = 0;
 437
 438                 newtp->rx_opt.num_sacks = 0;
 439                 newtp->urg_data = 0;
 440
 441                 if (sock_flag(newsk, SOCK_KEEPOPEN))
 442                         inet_csk_reset_keepalive_timer(newsk,
 443                                                        keepalive_time_when(newtp));
 444
 445                 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
 446                 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
 447                         if (sysctl_tcp_fack)
 448                                 tcp_enable_fack(newtp);
 449                 }
 450                 newtp->window_clamp = req->window_clamp;
 451                 newtp->rcv_ssthresh = req->rcv_wnd;
 452                 newtp->rcv_wnd = req->rcv_wnd;
 453                 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
 454                 if (newtp->rx_opt.wscale_ok) {
 455                         newtp->rx_opt.snd_wscale = ireq->snd_wscale;
 456                         newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
 457                 } else {
 458                         newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
 459                         newtp->window_clamp = min(newtp->window_clamp, 65535U);
 460                 }
 461                 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
 462                                   newtp->rx_opt.snd_wscale);
 463                 newtp->max_window = newtp->snd_wnd;
 464
 465                 if (newtp->rx_opt.tstamp_ok) {
 466                         newtp->rx_opt.ts_recent = req->ts_recent;
 467                         newtp->rx_opt.ts_recent_stamp = get_seconds();
 468                         newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
 469                 } else {
 470                         newtp->rx_opt.ts_recent_stamp = 0;
 471                         newtp->tcp_header_len = sizeof(struct tcphdr);
 472                 }
 473 #ifdef CONFIG_TCP_MD5SIG
 474                 newtp->md5sig_info = NULL;      /*XXX*/
 475                 if (newtp->af_specific->md5_lookup(sk, newsk))
 476                         newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
 477 #endif
 478                 if (skb->len >= TCP_MIN_RCVMSS+newtp->tcp_header_len)
 479                         newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
 480                 newtp->rx_opt.mss_clamp = req->mss;
 481                 TCP_ECN_openreq_child(newtp, req);
 482
 483                 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS);
 484         }
 485         return newsk;
 486 }
 487
 488 /*
 489  *      Process an incoming packet for SYN_RECV sockets represented
 490  *      as a request_sock.
 491  */
 492
 493 struct sock *tcp_check_req(struct sock *sk,struct sk_buff *skb,
 494                            struct request_sock *req,
 495                            struct request_sock **prev)
 496 {
 497         const struct tcphdr *th = tcp_hdr(skb);
 498         __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
 499         int paws_reject = 0;
 500         struct tcp_options_received tmp_opt;
 501         struct sock *child;
 502
 503         tmp_opt.saw_tstamp = 0;
 504         if (th->doff > (sizeof(struct tcphdr)>>2)) {
 505                 tcp_parse_options(skb, &tmp_opt, 0);
 506
 507                 if (tmp_opt.saw_tstamp) {
 508                         tmp_opt.ts_recent = req->ts_recent;
 509                         /* We do not store true stamp, but it is not required,
 510                          * it can be estimated (approximately)
 511                          * from another data.
 512                          */
 513                         tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans);
 514                         paws_reject = tcp_paws_check(&tmp_opt, th->rst);
 515                 }
 516         }
 517
 518         /* Check for pure retransmitted SYN. */
 519         if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
 520             flg == TCP_FLAG_SYN &&
 521             !paws_reject) {
 522                 /*
 523                  * RFC793 draws (Incorrectly! It was fixed in RFC1122)
 524                  * this case on figure 6 and figure 8, but formal
 525                  * protocol description says NOTHING.
 526                  * To be more exact, it says that we should send ACK,
 527                  * because this segment (at least, if it has no data)
 528                  * is out of window.
 529                  *
 530                  *  CONCLUSION: RFC793 (even with RFC1122) DOES NOT
 531                  *  describe SYN-RECV state. All the description
 532                  *  is wrong, we cannot believe to it and should
 533                  *  rely only on common sense and implementation
 534                  *  experience.
 535                  *
 536                  * Enforce "SYN-ACK" according to figure 8, figure 6
 537                  * of RFC793, fixed by RFC1122.
 538                  */
 539                 req->rsk_ops->rtx_syn_ack(sk, req);
 540                 return NULL;
 541         }
 542
 543         /* Further reproduces section "SEGMENT ARRIVES"
 544            for state SYN-RECEIVED of RFC793.
 545            It is broken, however, it does not work only
 546            when SYNs are crossed.
 547
 548            You would think that SYN crossing is impossible here, since
 549            we should have a SYN_SENT socket (from connect()) on our end,
 550            but this is not true if the crossed SYNs were sent to both
 551            ends by a malicious third party.  We must defend against this,
 552            and to do that we first verify the ACK (as per RFC793, page
 553            36) and reset if it is invalid.  Is this a true full defense?
 554            To convince ourselves, let us consider a way in which the ACK
 555            test can still pass in this 'malicious crossed SYNs' case.
 556            Malicious sender sends identical SYNs (and thus identical sequence
 557            numbers) to both A and B:
 558
 559                 A: gets SYN, seq=7
 560                 B: gets SYN, seq=7
 561
 562            By our good fortune, both A and B select the same initial
 563            send sequence number of seven :-)
 564
 565                 A: sends SYN|ACK, seq=7, ack_seq=8
 566                 B: sends SYN|ACK, seq=7, ack_seq=8
 567
 568            So we are now A eating this SYN|ACK, ACK test passes.  So
 569            does sequence test, SYN is truncated, and thus we consider
 570            it a bare ACK.
 571
 572            If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
 573            bare ACK.  Otherwise, we create an established connection.  Both
 574            ends (listening sockets) accept the new incoming connection and try
 575            to talk to each other. 8-)
 576
 577            Note: This case is both harmless, and rare.  Possibility is about the
 578            same as us discovering intelligent life on another plant tomorrow.
 579
 580            But generally, we should (RFC lies!) to accept ACK
 581            from SYNACK both here and in tcp_rcv_state_process().
 582            tcp_rcv_state_process() does not, hence, we do not too.
 583
 584            Note that the case is absolutely generic:
 585            we cannot optimize anything here without
 586            violating protocol. All the checks must be made
 587            before attempt to create socket.
 588          */
 589
 590         /* RFC793 page 36: "If the connection is in any non-synchronized state ...
 591          *                  and the incoming segment acknowledges something not yet
 592          *                  sent (the segment carries an unacceptable ACK) ...
 593          *                  a reset is sent."
 594          *
 595          * Invalid ACK: reset will be sent by listening socket
 596          */
 597         if ((flg & TCP_FLAG_ACK) &&
 598             (TCP_SKB_CB(skb)->ack_seq != tcp_rsk(req)->snt_isn + 1))
 599                 return sk;
 600
 601         /* Also, it would be not so bad idea to check rcv_tsecr, which
 602          * is essentially ACK extension and too early or too late values
 603          * should cause reset in unsynchronized states.
 604          */
 605
 606         /* RFC793: "first check sequence number". */
 607
 608         if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
 609                                           tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) {
 610                 /* Out of window: send ACK and drop. */
 611                 if (!(flg & TCP_FLAG_RST))
 612                         req->rsk_ops->send_ack(skb, req);
 613                 if (paws_reject)
 614                         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
 615                 return NULL;
 616         }
 617
 618         /* In sequence, PAWS is OK. */
 619
 620         if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1))
 621                         req->ts_recent = tmp_opt.rcv_tsval;
 622
 623                 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
 624                         /* Truncate SYN, it is out of window starting
 625                            at tcp_rsk(req)->rcv_isn + 1. */
 626                         flg &= ~TCP_FLAG_SYN;
 627                 }
 628
 629                 /* RFC793: "second check the RST bit" and
 630                  *         "fourth, check the SYN bit"
 631                  */
 632                 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
 633                         TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
 634                         goto embryonic_reset;
 635                 }
 636
 637                 /* ACK sequence verified above, just make sure ACK is
 638                  * set.  If ACK not set, just silently drop the packet.
 639                  */
 640                 if (!(flg & TCP_FLAG_ACK))
 641                         return NULL;
 642
 643                 /* If TCP_DEFER_ACCEPT is set, drop bare ACK. */
 644                 if (inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
 645                     TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
 646                         inet_rsk(req)->acked = 1;
 647                         return NULL;
 648                 }
 649
 650                 /* OK, ACK is valid, create big socket and
 651                  * feed this segment to it. It will repeat all
 652                  * the tests. THIS SEGMENT MUST MOVE SOCKET TO
 653                  * ESTABLISHED STATE. If it will be dropped after
 654                  * socket is created, wait for troubles.
 655                  */
 656                 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb,
 657                                                                  req, NULL);
 658                 if (child == NULL)
 659                         goto listen_overflow;
 660 #ifdef CONFIG_TCP_MD5SIG
 661                 else {
 662                         /* Copy over the MD5 key from the original socket */
 663                         struct tcp_md5sig_key *key;
 664                         struct tcp_sock *tp = tcp_sk(sk);
 665                         key = tp->af_specific->md5_lookup(sk, child);
 666                         if (key != NULL) {
 667                                 /*
 668                                  * We're using one, so create a matching key on the
 669                                  * newsk structure. If we fail to get memory then we
 670                                  * end up not copying the key across. Shucks.
 671                                  */
 672                                 char *newkey = kmemdup(key->key, key->keylen,
 673                                                        GFP_ATOMIC);
 674                                 if (newkey) {
 675                                         if (!tcp_alloc_md5sig_pool())
 676                                                 BUG();
 677                                         tp->af_specific->md5_add(child, child,
 678                                                                  newkey,
 679                                                                  key->keylen);
 680                                 }
 681                         }
 682                 }
 683 #endif
 684
 685                 inet_csk_reqsk_queue_unlink(sk, req, prev);
 686                 inet_csk_reqsk_queue_removed(sk, req);
 687
 688                 inet_csk_reqsk_queue_add(sk, req, child);
 689                 return child;
 690
 691         listen_overflow:
 692                 if (!sysctl_tcp_abort_on_overflow) {
 693                         inet_rsk(req)->acked = 1;
 694                         return NULL;
 695                 }
 696
 697         embryonic_reset:
 698                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
 699                 if (!(flg & TCP_FLAG_RST))
 700                         req->rsk_ops->send_reset(sk, skb);
 701
 702                 inet_csk_reqsk_queue_drop(sk, req, prev);
 703                 return NULL;
 704 }
 705
 706 /*
 707  * Queue segment on the new socket if the new socket is active,
 708  * otherwise we just shortcircuit this and continue with
 709  * the new socket.
 710  */
 711
 712 int tcp_child_process(struct sock *parent, struct sock *child,
 713                       struct sk_buff *skb)
 714 {
 715         int ret = 0;
 716         int state = child->sk_state;
 717
 718         if (!sock_owned_by_user(child)) {
 719                 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
 720                                             skb->len);
 721                 /* Wakeup parent, send SIGIO */
 722                 if (state == TCP_SYN_RECV && child->sk_state != state)
 723                         parent->sk_data_ready(parent, 0);
 724         } else {
 725                 /* Alas, it is possible again, because we do lookup
 726                  * in main socket hash table and lock on listening
 727                  * socket does not protect us more.
 728                  */
 729                 sk_add_backlog(child, skb);
 730         }
 731
 732         bh_unlock_sock(child);
 733         sock_put(child);
 734         return ret;
 735 }
 736
 737 EXPORT_SYMBOL(tcp_check_req);
 738 EXPORT_SYMBOL(tcp_child_process);
 739 EXPORT_SYMBOL(tcp_create_openreq_child);
 740 EXPORT_SYMBOL(tcp_timewait_state_process);