linux/net/ipv4/tcp_minisocks.c
<<
>>
Prefs
   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
  35int sysctl_tcp_syncookies __read_mostly = SYNC_INIT;
  36EXPORT_SYMBOL(sysctl_tcp_syncookies);
  37
  38int sysctl_tcp_abort_on_overflow __read_mostly;
  39
  40struct 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
  56EXPORT_SYMBOL_GPL(tcp_death_row);
  57
  58static __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 */
  95enum tcp_tw_status
  96tcp_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_reject(&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) {
 142kill_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) {
 200kill:
 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 */
 272void 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
 360void 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
 369EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
 370
 371static 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 */
 383struct 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                newtp->snd_up = treq->snt_isn + 1;
 399
 400                tcp_prequeue_init(newtp);
 401
 402                tcp_init_wl(newtp, treq->rcv_isn);
 403
 404                newtp->srtt = 0;
 405                newtp->mdev = TCP_TIMEOUT_INIT;
 406                newicsk->icsk_rto = TCP_TIMEOUT_INIT;
 407
 408                newtp->packets_out = 0;
 409                newtp->retrans_out = 0;
 410                newtp->sacked_out = 0;
 411                newtp->fackets_out = 0;
 412                newtp->snd_ssthresh = 0x7fffffff;
 413
 414                /* So many TCP implementations out there (incorrectly) count the
 415                 * initial SYN frame in their delayed-ACK and congestion control
 416                 * algorithms that we must have the following bandaid to talk
 417                 * efficiently to them.  -DaveM
 418                 */
 419                newtp->snd_cwnd = 2;
 420                newtp->snd_cwnd_cnt = 0;
 421                newtp->bytes_acked = 0;
 422
 423                newtp->frto_counter = 0;
 424                newtp->frto_highmark = 0;
 425
 426                newicsk->icsk_ca_ops = &tcp_init_congestion_ops;
 427
 428                tcp_set_ca_state(newsk, TCP_CA_Open);
 429                tcp_init_xmit_timers(newsk);
 430                skb_queue_head_init(&newtp->out_of_order_queue);
 431                newtp->write_seq = treq->snt_isn + 1;
 432                newtp->pushed_seq = newtp->write_seq;
 433
 434                newtp->rx_opt.saw_tstamp = 0;
 435
 436                newtp->rx_opt.dsack = 0;
 437                newtp->rx_opt.num_sacks = 0;
 438
 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
 493struct 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_reject(&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(sk, 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, req, NULL);
 657        if (child == NULL)
 658                goto listen_overflow;
 659#ifdef CONFIG_TCP_MD5SIG
 660        else {
 661                /* Copy over the MD5 key from the original socket */
 662                struct tcp_md5sig_key *key;
 663                struct tcp_sock *tp = tcp_sk(sk);
 664                key = tp->af_specific->md5_lookup(sk, child);
 665                if (key != NULL) {
 666                        /*
 667                         * We're using one, so create a matching key on the
 668                         * newsk structure. If we fail to get memory then we
 669                         * end up not copying the key across. Shucks.
 670                         */
 671                        char *newkey = kmemdup(key->key, key->keylen,
 672                                               GFP_ATOMIC);
 673                        if (newkey) {
 674                                if (!tcp_alloc_md5sig_pool())
 675                                        BUG();
 676                                tp->af_specific->md5_add(child, child, newkey,
 677                                                         key->keylen);
 678                        }
 679                }
 680        }
 681#endif
 682
 683        inet_csk_reqsk_queue_unlink(sk, req, prev);
 684        inet_csk_reqsk_queue_removed(sk, req);
 685
 686        inet_csk_reqsk_queue_add(sk, req, child);
 687        return child;
 688
 689listen_overflow:
 690        if (!sysctl_tcp_abort_on_overflow) {
 691                inet_rsk(req)->acked = 1;
 692                return NULL;
 693        }
 694
 695embryonic_reset:
 696        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
 697        if (!(flg & TCP_FLAG_RST))
 698                req->rsk_ops->send_reset(sk, skb);
 699
 700        inet_csk_reqsk_queue_drop(sk, req, prev);
 701        return NULL;
 702}
 703
 704/*
 705 * Queue segment on the new socket if the new socket is active,
 706 * otherwise we just shortcircuit this and continue with
 707 * the new socket.
 708 */
 709
 710int tcp_child_process(struct sock *parent, struct sock *child,
 711                      struct sk_buff *skb)
 712{
 713        int ret = 0;
 714        int state = child->sk_state;
 715
 716        if (!sock_owned_by_user(child)) {
 717                ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
 718                                            skb->len);
 719                /* Wakeup parent, send SIGIO */
 720                if (state == TCP_SYN_RECV && child->sk_state != state)
 721                        parent->sk_data_ready(parent, 0);
 722        } else {
 723                /* Alas, it is possible again, because we do lookup
 724                 * in main socket hash table and lock on listening
 725                 * socket does not protect us more.
 726                 */
 727                sk_add_backlog(child, skb);
 728        }
 729
 730        bh_unlock_sock(child);
 731        sock_put(child);
 732        return ret;
 733}
 734
 735EXPORT_SYMBOL(tcp_check_req);
 736EXPORT_SYMBOL(tcp_child_process);
 737EXPORT_SYMBOL(tcp_create_openreq_child);
 738EXPORT_SYMBOL(tcp_timewait_state_process);
 739