linux/net/ipv4/tcp_minisocks.c
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   1/*
   2 * INET         An implementation of the TCP/IP protocol suite for the LINUX
   3 *              operating system.  INET is implemented using the  BSD Socket
   4 *              interface as the means of communication with the user level.
   5 *
   6 *              Implementation of the Transmission Control Protocol(TCP).
   7 *
   8 * Authors:     Ross Biro
   9 *              Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
  10 *              Mark Evans, <evansmp@uhura.aston.ac.uk>
  11 *              Corey Minyard <wf-rch!minyard@relay.EU.net>
  12 *              Florian La Roche, <flla@stud.uni-sb.de>
  13 *              Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
  14 *              Linus Torvalds, <torvalds@cs.helsinki.fi>
  15 *              Alan Cox, <gw4pts@gw4pts.ampr.org>
  16 *              Matthew Dillon, <dillon@apollo.west.oic.com>
  17 *              Arnt Gulbrandsen, <agulbra@nvg.unit.no>
  18 *              Jorge Cwik, <jorge@laser.satlink.net>
  19 */
  20
  21#include <linux/mm.h>
  22#include <linux/module.h>
  23#include <linux/slab.h>
  24#include <linux/sysctl.h>
  25#include <linux/workqueue.h>
  26#include <net/tcp.h>
  27#include <net/inet_common.h>
  28#include <net/xfrm.h>
  29
  30int sysctl_tcp_syncookies __read_mostly = 1;
  31EXPORT_SYMBOL(sysctl_tcp_syncookies);
  32
  33int sysctl_tcp_abort_on_overflow __read_mostly;
  34
  35struct inet_timewait_death_row tcp_death_row = {
  36        .sysctl_max_tw_buckets = NR_FILE * 2,
  37        .period         = TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS,
  38        .death_lock     = __SPIN_LOCK_UNLOCKED(tcp_death_row.death_lock),
  39        .hashinfo       = &tcp_hashinfo,
  40        .tw_timer       = TIMER_INITIALIZER(inet_twdr_hangman, 0,
  41                                            (unsigned long)&tcp_death_row),
  42        .twkill_work    = __WORK_INITIALIZER(tcp_death_row.twkill_work,
  43                                             inet_twdr_twkill_work),
  44/* Short-time timewait calendar */
  45
  46        .twcal_hand     = -1,
  47        .twcal_timer    = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0,
  48                                            (unsigned long)&tcp_death_row),
  49};
  50
  51EXPORT_SYMBOL_GPL(tcp_death_row);
  52
  53static __inline__ int tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
  54{
  55        if (seq == s_win)
  56                return 1;
  57        if (after(end_seq, s_win) && before(seq, e_win))
  58                return 1;
  59        return (seq == e_win && seq == end_seq);
  60}
  61
  62/*
  63 * * Main purpose of TIME-WAIT state is to close connection gracefully,
  64 *   when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
  65 *   (and, probably, tail of data) and one or more our ACKs are lost.
  66 * * What is TIME-WAIT timeout? It is associated with maximal packet
  67 *   lifetime in the internet, which results in wrong conclusion, that
  68 *   it is set to catch "old duplicate segments" wandering out of their path.
  69 *   It is not quite correct. This timeout is calculated so that it exceeds
  70 *   maximal retransmission timeout enough to allow to lose one (or more)
  71 *   segments sent by peer and our ACKs. This time may be calculated from RTO.
  72 * * When TIME-WAIT socket receives RST, it means that another end
  73 *   finally closed and we are allowed to kill TIME-WAIT too.
  74 * * Second purpose of TIME-WAIT is catching old duplicate segments.
  75 *   Well, certainly it is pure paranoia, but if we load TIME-WAIT
  76 *   with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
  77 * * If we invented some more clever way to catch duplicates
  78 *   (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
  79 *
  80 * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
  81 * When you compare it to RFCs, please, read section SEGMENT ARRIVES
  82 * from the very beginning.
  83 *
  84 * NOTE. With recycling (and later with fin-wait-2) TW bucket
  85 * is _not_ stateless. It means, that strictly speaking we must
  86 * spinlock it. I do not want! Well, probability of misbehaviour
  87 * is ridiculously low and, seems, we could use some mb() tricks
  88 * to avoid misread sequence numbers, states etc.  --ANK
  89 */
  90enum tcp_tw_status
  91tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
  92                           const struct tcphdr *th)
  93{
  94        struct tcp_options_received tmp_opt;
  95        u8 *hash_location;
  96        struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
  97        int paws_reject = 0;
  98
  99        tmp_opt.saw_tstamp = 0;
 100        if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
 101                tcp_parse_options(skb, &tmp_opt, &hash_location, 0);
 102
 103                if (tmp_opt.saw_tstamp) {
 104                        tmp_opt.ts_recent       = tcptw->tw_ts_recent;
 105                        tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
 106                        paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
 107                }
 108        }
 109
 110        if (tw->tw_substate == TCP_FIN_WAIT2) {
 111                /* Just repeat all the checks of tcp_rcv_state_process() */
 112
 113                /* Out of window, send ACK */
 114                if (paws_reject ||
 115                    !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
 116                                   tcptw->tw_rcv_nxt,
 117                                   tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
 118                        return TCP_TW_ACK;
 119
 120                if (th->rst)
 121                        goto kill;
 122
 123                if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
 124                        goto kill_with_rst;
 125
 126                /* Dup ACK? */
 127                if (!th->ack ||
 128                    !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
 129                    TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
 130                        inet_twsk_put(tw);
 131                        return TCP_TW_SUCCESS;
 132                }
 133
 134                /* New data or FIN. If new data arrive after half-duplex close,
 135                 * reset.
 136                 */
 137                if (!th->fin ||
 138                    TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) {
 139kill_with_rst:
 140                        inet_twsk_deschedule(tw, &tcp_death_row);
 141                        inet_twsk_put(tw);
 142                        return TCP_TW_RST;
 143                }
 144
 145                /* FIN arrived, enter true time-wait state. */
 146                tw->tw_substate   = TCP_TIME_WAIT;
 147                tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
 148                if (tmp_opt.saw_tstamp) {
 149                        tcptw->tw_ts_recent_stamp = get_seconds();
 150                        tcptw->tw_ts_recent       = tmp_opt.rcv_tsval;
 151                }
 152
 153                /* I am shamed, but failed to make it more elegant.
 154                 * Yes, it is direct reference to IP, which is impossible
 155                 * to generalize to IPv6. Taking into account that IPv6
 156                 * do not understand recycling in any case, it not
 157                 * a big problem in practice. --ANK */
 158                if (tw->tw_family == AF_INET &&
 159                    tcp_death_row.sysctl_tw_recycle && tcptw->tw_ts_recent_stamp &&
 160                    tcp_v4_tw_remember_stamp(tw))
 161                        inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout,
 162                                           TCP_TIMEWAIT_LEN);
 163                else
 164                        inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
 165                                           TCP_TIMEWAIT_LEN);
 166                return TCP_TW_ACK;
 167        }
 168
 169        /*
 170         *      Now real TIME-WAIT state.
 171         *
 172         *      RFC 1122:
 173         *      "When a connection is [...] on TIME-WAIT state [...]
 174         *      [a TCP] MAY accept a new SYN from the remote TCP to
 175         *      reopen the connection directly, if it:
 176         *
 177         *      (1)  assigns its initial sequence number for the new
 178         *      connection to be larger than the largest sequence
 179         *      number it used on the previous connection incarnation,
 180         *      and
 181         *
 182         *      (2)  returns to TIME-WAIT state if the SYN turns out
 183         *      to be an old duplicate".
 184         */
 185
 186        if (!paws_reject &&
 187            (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
 188             (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
 189                /* In window segment, it may be only reset or bare ack. */
 190
 191                if (th->rst) {
 192                        /* This is TIME_WAIT assassination, in two flavors.
 193                         * Oh well... nobody has a sufficient solution to this
 194                         * protocol bug yet.
 195                         */
 196                        if (sysctl_tcp_rfc1337 == 0) {
 197kill:
 198                                inet_twsk_deschedule(tw, &tcp_death_row);
 199                                inet_twsk_put(tw);
 200                                return TCP_TW_SUCCESS;
 201                        }
 202                }
 203                inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
 204                                   TCP_TIMEWAIT_LEN);
 205
 206                if (tmp_opt.saw_tstamp) {
 207                        tcptw->tw_ts_recent       = tmp_opt.rcv_tsval;
 208                        tcptw->tw_ts_recent_stamp = get_seconds();
 209                }
 210
 211                inet_twsk_put(tw);
 212                return TCP_TW_SUCCESS;
 213        }
 214
 215        /* Out of window segment.
 216
 217           All the segments are ACKed immediately.
 218
 219           The only exception is new SYN. We accept it, if it is
 220           not old duplicate and we are not in danger to be killed
 221           by delayed old duplicates. RFC check is that it has
 222           newer sequence number works at rates <40Mbit/sec.
 223           However, if paws works, it is reliable AND even more,
 224           we even may relax silly seq space cutoff.
 225
 226           RED-PEN: we violate main RFC requirement, if this SYN will appear
 227           old duplicate (i.e. we receive RST in reply to SYN-ACK),
 228           we must return socket to time-wait state. It is not good,
 229           but not fatal yet.
 230         */
 231
 232        if (th->syn && !th->rst && !th->ack && !paws_reject &&
 233            (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
 234             (tmp_opt.saw_tstamp &&
 235              (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
 236                u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
 237                if (isn == 0)
 238                        isn++;
 239                TCP_SKB_CB(skb)->when = isn;
 240                return TCP_TW_SYN;
 241        }
 242
 243        if (paws_reject)
 244                NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
 245
 246        if (!th->rst) {
 247                /* In this case we must reset the TIMEWAIT timer.
 248                 *
 249                 * If it is ACKless SYN it may be both old duplicate
 250                 * and new good SYN with random sequence number <rcv_nxt.
 251                 * Do not reschedule in the last case.
 252                 */
 253                if (paws_reject || th->ack)
 254                        inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
 255                                           TCP_TIMEWAIT_LEN);
 256
 257                /* Send ACK. Note, we do not put the bucket,
 258                 * it will be released by caller.
 259                 */
 260                return TCP_TW_ACK;
 261        }
 262        inet_twsk_put(tw);
 263        return TCP_TW_SUCCESS;
 264}
 265
 266/*
 267 * Move a socket to time-wait or dead fin-wait-2 state.
 268 */
 269void tcp_time_wait(struct sock *sk, int state, int timeo)
 270{
 271        struct inet_timewait_sock *tw = NULL;
 272        const struct inet_connection_sock *icsk = inet_csk(sk);
 273        const struct tcp_sock *tp = tcp_sk(sk);
 274        int recycle_ok = 0;
 275
 276        if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp)
 277                recycle_ok = icsk->icsk_af_ops->remember_stamp(sk);
 278
 279        if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets)
 280                tw = inet_twsk_alloc(sk, state);
 281
 282        if (tw != NULL) {
 283                struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
 284                const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
 285
 286                tw->tw_rcv_wscale       = tp->rx_opt.rcv_wscale;
 287                tcptw->tw_rcv_nxt       = tp->rcv_nxt;
 288                tcptw->tw_snd_nxt       = tp->snd_nxt;
 289                tcptw->tw_rcv_wnd       = tcp_receive_window(tp);
 290                tcptw->tw_ts_recent     = tp->rx_opt.ts_recent;
 291                tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
 292
 293#if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
 294                if (tw->tw_family == PF_INET6) {
 295                        struct ipv6_pinfo *np = inet6_sk(sk);
 296                        struct inet6_timewait_sock *tw6;
 297
 298                        tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot);
 299                        tw6 = inet6_twsk((struct sock *)tw);
 300                        ipv6_addr_copy(&tw6->tw_v6_daddr, &np->daddr);
 301                        ipv6_addr_copy(&tw6->tw_v6_rcv_saddr, &np->rcv_saddr);
 302                        tw->tw_ipv6only = np->ipv6only;
 303                }
 304#endif
 305
 306#ifdef CONFIG_TCP_MD5SIG
 307                /*
 308                 * The timewait bucket does not have the key DB from the
 309                 * sock structure. We just make a quick copy of the
 310                 * md5 key being used (if indeed we are using one)
 311                 * so the timewait ack generating code has the key.
 312                 */
 313                do {
 314                        struct tcp_md5sig_key *key;
 315                        memset(tcptw->tw_md5_key, 0, sizeof(tcptw->tw_md5_key));
 316                        tcptw->tw_md5_keylen = 0;
 317                        key = tp->af_specific->md5_lookup(sk, sk);
 318                        if (key != NULL) {
 319                                memcpy(&tcptw->tw_md5_key, key->key, key->keylen);
 320                                tcptw->tw_md5_keylen = key->keylen;
 321                                if (tcp_alloc_md5sig_pool(sk) == NULL)
 322                                        BUG();
 323                        }
 324                } while (0);
 325#endif
 326
 327                /* Linkage updates. */
 328                __inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
 329
 330                /* Get the TIME_WAIT timeout firing. */
 331                if (timeo < rto)
 332                        timeo = rto;
 333
 334                if (recycle_ok) {
 335                        tw->tw_timeout = rto;
 336                } else {
 337                        tw->tw_timeout = TCP_TIMEWAIT_LEN;
 338                        if (state == TCP_TIME_WAIT)
 339                                timeo = TCP_TIMEWAIT_LEN;
 340                }
 341
 342                inet_twsk_schedule(tw, &tcp_death_row, timeo,
 343                                   TCP_TIMEWAIT_LEN);
 344                inet_twsk_put(tw);
 345        } else {
 346                /* Sorry, if we're out of memory, just CLOSE this
 347                 * socket up.  We've got bigger problems than
 348                 * non-graceful socket closings.
 349                 */
 350                LIMIT_NETDEBUG(KERN_INFO "TCP: time wait bucket table overflow\n");
 351        }
 352
 353        tcp_update_metrics(sk);
 354        tcp_done(sk);
 355}
 356
 357void tcp_twsk_destructor(struct sock *sk)
 358{
 359#ifdef CONFIG_TCP_MD5SIG
 360        struct tcp_timewait_sock *twsk = tcp_twsk(sk);
 361        if (twsk->tw_md5_keylen)
 362                tcp_free_md5sig_pool();
 363#endif
 364}
 365
 366EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
 367
 368static inline void TCP_ECN_openreq_child(struct tcp_sock *tp,
 369                                         struct request_sock *req)
 370{
 371        tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
 372}
 373
 374/* This is not only more efficient than what we used to do, it eliminates
 375 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
 376 *
 377 * Actually, we could lots of memory writes here. tp of listening
 378 * socket contains all necessary default parameters.
 379 */
 380struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
 381{
 382        struct sock *newsk = inet_csk_clone(sk, req, GFP_ATOMIC);
 383
 384        if (newsk != NULL) {
 385                const struct inet_request_sock *ireq = inet_rsk(req);
 386                struct tcp_request_sock *treq = tcp_rsk(req);
 387                struct inet_connection_sock *newicsk = inet_csk(newsk);
 388                struct tcp_sock *newtp = tcp_sk(newsk);
 389                struct tcp_sock *oldtp = tcp_sk(sk);
 390                struct tcp_cookie_values *oldcvp = oldtp->cookie_values;
 391
 392                /* TCP Cookie Transactions require space for the cookie pair,
 393                 * as it differs for each connection.  There is no need to
 394                 * copy any s_data_payload stored at the original socket.
 395                 * Failure will prevent resuming the connection.
 396                 *
 397                 * Presumed copied, in order of appearance:
 398                 *      cookie_in_always, cookie_out_never
 399                 */
 400                if (oldcvp != NULL) {
 401                        struct tcp_cookie_values *newcvp =
 402                                kzalloc(sizeof(*newtp->cookie_values),
 403                                        GFP_ATOMIC);
 404
 405                        if (newcvp != NULL) {
 406                                kref_init(&newcvp->kref);
 407                                newcvp->cookie_desired =
 408                                                oldcvp->cookie_desired;
 409                                newtp->cookie_values = newcvp;
 410                        } else {
 411                                /* Not Yet Implemented */
 412                                newtp->cookie_values = NULL;
 413                        }
 414                }
 415
 416                /* Now setup tcp_sock */
 417                newtp->pred_flags = 0;
 418
 419                newtp->rcv_wup = newtp->copied_seq =
 420                newtp->rcv_nxt = treq->rcv_isn + 1;
 421
 422                newtp->snd_sml = newtp->snd_una =
 423                newtp->snd_nxt = newtp->snd_up =
 424                        treq->snt_isn + 1 + tcp_s_data_size(oldtp);
 425
 426                tcp_prequeue_init(newtp);
 427
 428                tcp_init_wl(newtp, treq->rcv_isn);
 429
 430                newtp->srtt = 0;
 431                newtp->mdev = TCP_TIMEOUT_INIT;
 432                newicsk->icsk_rto = TCP_TIMEOUT_INIT;
 433
 434                newtp->packets_out = 0;
 435                newtp->retrans_out = 0;
 436                newtp->sacked_out = 0;
 437                newtp->fackets_out = 0;
 438                newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
 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 = 2;
 446                newtp->snd_cwnd_cnt = 0;
 447                newtp->bytes_acked = 0;
 448
 449                newtp->frto_counter = 0;
 450                newtp->frto_highmark = 0;
 451
 452                newicsk->icsk_ca_ops = &tcp_init_congestion_ops;
 453
 454                tcp_set_ca_state(newsk, TCP_CA_Open);
 455                tcp_init_xmit_timers(newsk);
 456                skb_queue_head_init(&newtp->out_of_order_queue);
 457                newtp->write_seq = newtp->pushed_seq =
 458                        treq->snt_isn + 1 + tcp_s_data_size(oldtp);
 459
 460                newtp->rx_opt.saw_tstamp = 0;
 461
 462                newtp->rx_opt.dsack = 0;
 463                newtp->rx_opt.num_sacks = 0;
 464
 465                newtp->urg_data = 0;
 466
 467                if (sock_flag(newsk, SOCK_KEEPOPEN))
 468                        inet_csk_reset_keepalive_timer(newsk,
 469                                                       keepalive_time_when(newtp));
 470
 471                newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
 472                if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
 473                        if (sysctl_tcp_fack)
 474                                tcp_enable_fack(newtp);
 475                }
 476                newtp->window_clamp = req->window_clamp;
 477                newtp->rcv_ssthresh = req->rcv_wnd;
 478                newtp->rcv_wnd = req->rcv_wnd;
 479                newtp->rx_opt.wscale_ok = ireq->wscale_ok;
 480                if (newtp->rx_opt.wscale_ok) {
 481                        newtp->rx_opt.snd_wscale = ireq->snd_wscale;
 482                        newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
 483                } else {
 484                        newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
 485                        newtp->window_clamp = min(newtp->window_clamp, 65535U);
 486                }
 487                newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
 488                                  newtp->rx_opt.snd_wscale);
 489                newtp->max_window = newtp->snd_wnd;
 490
 491                if (newtp->rx_opt.tstamp_ok) {
 492                        newtp->rx_opt.ts_recent = req->ts_recent;
 493                        newtp->rx_opt.ts_recent_stamp = get_seconds();
 494                        newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
 495                } else {
 496                        newtp->rx_opt.ts_recent_stamp = 0;
 497                        newtp->tcp_header_len = sizeof(struct tcphdr);
 498                }
 499#ifdef CONFIG_TCP_MD5SIG
 500                newtp->md5sig_info = NULL;      /*XXX*/
 501                if (newtp->af_specific->md5_lookup(sk, newsk))
 502                        newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
 503#endif
 504                if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
 505                        newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
 506                newtp->rx_opt.mss_clamp = req->mss;
 507                TCP_ECN_openreq_child(newtp, req);
 508
 509                TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS);
 510        }
 511        return newsk;
 512}
 513
 514/*
 515 *      Process an incoming packet for SYN_RECV sockets represented
 516 *      as a request_sock.
 517 */
 518
 519struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 520                           struct request_sock *req,
 521                           struct request_sock **prev)
 522{
 523        struct tcp_options_received tmp_opt;
 524        u8 *hash_location;
 525        struct sock *child;
 526        const struct tcphdr *th = tcp_hdr(skb);
 527        __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
 528        int paws_reject = 0;
 529
 530        tmp_opt.saw_tstamp = 0;
 531        if (th->doff > (sizeof(struct tcphdr)>>2)) {
 532                tcp_parse_options(skb, &tmp_opt, &hash_location, 0);
 533
 534                if (tmp_opt.saw_tstamp) {
 535                        tmp_opt.ts_recent = req->ts_recent;
 536                        /* We do not store true stamp, but it is not required,
 537                         * it can be estimated (approximately)
 538                         * from another data.
 539                         */
 540                        tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans);
 541                        paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
 542                }
 543        }
 544
 545        /* Check for pure retransmitted SYN. */
 546        if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
 547            flg == TCP_FLAG_SYN &&
 548            !paws_reject) {
 549                /*
 550                 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
 551                 * this case on figure 6 and figure 8, but formal
 552                 * protocol description says NOTHING.
 553                 * To be more exact, it says that we should send ACK,
 554                 * because this segment (at least, if it has no data)
 555                 * is out of window.
 556                 *
 557                 *  CONCLUSION: RFC793 (even with RFC1122) DOES NOT
 558                 *  describe SYN-RECV state. All the description
 559                 *  is wrong, we cannot believe to it and should
 560                 *  rely only on common sense and implementation
 561                 *  experience.
 562                 *
 563                 * Enforce "SYN-ACK" according to figure 8, figure 6
 564                 * of RFC793, fixed by RFC1122.
 565                 */
 566                req->rsk_ops->rtx_syn_ack(sk, req, NULL);
 567                return NULL;
 568        }
 569
 570        /* Further reproduces section "SEGMENT ARRIVES"
 571           for state SYN-RECEIVED of RFC793.
 572           It is broken, however, it does not work only
 573           when SYNs are crossed.
 574
 575           You would think that SYN crossing is impossible here, since
 576           we should have a SYN_SENT socket (from connect()) on our end,
 577           but this is not true if the crossed SYNs were sent to both
 578           ends by a malicious third party.  We must defend against this,
 579           and to do that we first verify the ACK (as per RFC793, page
 580           36) and reset if it is invalid.  Is this a true full defense?
 581           To convince ourselves, let us consider a way in which the ACK
 582           test can still pass in this 'malicious crossed SYNs' case.
 583           Malicious sender sends identical SYNs (and thus identical sequence
 584           numbers) to both A and B:
 585
 586                A: gets SYN, seq=7
 587                B: gets SYN, seq=7
 588
 589           By our good fortune, both A and B select the same initial
 590           send sequence number of seven :-)
 591
 592                A: sends SYN|ACK, seq=7, ack_seq=8
 593                B: sends SYN|ACK, seq=7, ack_seq=8
 594
 595           So we are now A eating this SYN|ACK, ACK test passes.  So
 596           does sequence test, SYN is truncated, and thus we consider
 597           it a bare ACK.
 598
 599           If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
 600           bare ACK.  Otherwise, we create an established connection.  Both
 601           ends (listening sockets) accept the new incoming connection and try
 602           to talk to each other. 8-)
 603
 604           Note: This case is both harmless, and rare.  Possibility is about the
 605           same as us discovering intelligent life on another plant tomorrow.
 606
 607           But generally, we should (RFC lies!) to accept ACK
 608           from SYNACK both here and in tcp_rcv_state_process().
 609           tcp_rcv_state_process() does not, hence, we do not too.
 610
 611           Note that the case is absolutely generic:
 612           we cannot optimize anything here without
 613           violating protocol. All the checks must be made
 614           before attempt to create socket.
 615         */
 616
 617        /* RFC793 page 36: "If the connection is in any non-synchronized state ...
 618         *                  and the incoming segment acknowledges something not yet
 619         *                  sent (the segment carries an unacceptable ACK) ...
 620         *                  a reset is sent."
 621         *
 622         * Invalid ACK: reset will be sent by listening socket
 623         */
 624        if ((flg & TCP_FLAG_ACK) &&
 625            (TCP_SKB_CB(skb)->ack_seq !=
 626             tcp_rsk(req)->snt_isn + 1 + tcp_s_data_size(tcp_sk(sk))))
 627                return sk;
 628
 629        /* Also, it would be not so bad idea to check rcv_tsecr, which
 630         * is essentially ACK extension and too early or too late values
 631         * should cause reset in unsynchronized states.
 632         */
 633
 634        /* RFC793: "first check sequence number". */
 635
 636        if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
 637                                          tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) {
 638                /* Out of window: send ACK and drop. */
 639                if (!(flg & TCP_FLAG_RST))
 640                        req->rsk_ops->send_ack(sk, skb, req);
 641                if (paws_reject)
 642                        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
 643                return NULL;
 644        }
 645
 646        /* In sequence, PAWS is OK. */
 647
 648        if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1))
 649                req->ts_recent = tmp_opt.rcv_tsval;
 650
 651        if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
 652                /* Truncate SYN, it is out of window starting
 653                   at tcp_rsk(req)->rcv_isn + 1. */
 654                flg &= ~TCP_FLAG_SYN;
 655        }
 656
 657        /* RFC793: "second check the RST bit" and
 658         *         "fourth, check the SYN bit"
 659         */
 660        if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
 661                TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
 662                goto embryonic_reset;
 663        }
 664
 665        /* ACK sequence verified above, just make sure ACK is
 666         * set.  If ACK not set, just silently drop the packet.
 667         */
 668        if (!(flg & TCP_FLAG_ACK))
 669                return NULL;
 670
 671        /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
 672        if (req->retrans < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
 673            TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
 674                inet_rsk(req)->acked = 1;
 675                NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
 676                return NULL;
 677        }
 678
 679        /* OK, ACK is valid, create big socket and
 680         * feed this segment to it. It will repeat all
 681         * the tests. THIS SEGMENT MUST MOVE SOCKET TO
 682         * ESTABLISHED STATE. If it will be dropped after
 683         * socket is created, wait for troubles.
 684         */
 685        child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL);
 686        if (child == NULL)
 687                goto listen_overflow;
 688
 689        inet_csk_reqsk_queue_unlink(sk, req, prev);
 690        inet_csk_reqsk_queue_removed(sk, req);
 691
 692        inet_csk_reqsk_queue_add(sk, req, child);
 693        return child;
 694
 695listen_overflow:
 696        if (!sysctl_tcp_abort_on_overflow) {
 697                inet_rsk(req)->acked = 1;
 698                return NULL;
 699        }
 700
 701embryonic_reset:
 702        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
 703        if (!(flg & TCP_FLAG_RST))
 704                req->rsk_ops->send_reset(sk, skb);
 705
 706        inet_csk_reqsk_queue_drop(sk, req, prev);
 707        return NULL;
 708}
 709
 710/*
 711 * Queue segment on the new socket if the new socket is active,
 712 * otherwise we just shortcircuit this and continue with
 713 * the new socket.
 714 */
 715
 716int tcp_child_process(struct sock *parent, struct sock *child,
 717                      struct sk_buff *skb)
 718{
 719        int ret = 0;
 720        int state = child->sk_state;
 721
 722        if (!sock_owned_by_user(child)) {
 723                ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
 724                                            skb->len);
 725                /* Wakeup parent, send SIGIO */
 726                if (state == TCP_SYN_RECV && child->sk_state != state)
 727                        parent->sk_data_ready(parent, 0);
 728        } else {
 729                /* Alas, it is possible again, because we do lookup
 730                 * in main socket hash table and lock on listening
 731                 * socket does not protect us more.
 732                 */
 733                __sk_add_backlog(child, skb);
 734        }
 735
 736        bh_unlock_sock(child);
 737        sock_put(child);
 738        return ret;
 739}
 740
 741EXPORT_SYMBOL(tcp_check_req);
 742EXPORT_SYMBOL(tcp_child_process);
 743EXPORT_SYMBOL(tcp_create_openreq_child);
 744EXPORT_SYMBOL(tcp_timewait_state_process);
 745