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  * Version:     $Id: tcp_minisocks.c,v 1.15 2002/02/01 22:01:04 davem Exp $
   9  *
  10  * Authors:     Ross Biro
  11  *              Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
  12  *              Mark Evans, <evansmp@uhura.aston.ac.uk>
  13  *              Corey Minyard <wf-rch!minyard@relay.EU.net>
  14  *              Florian La Roche, <flla@stud.uni-sb.de>
  15  *              Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
  16  *              Linus Torvalds, <torvalds@cs.helsinki.fi>
  17  *              Alan Cox, <gw4pts@gw4pts.ampr.org>
  18  *              Matthew Dillon, <dillon@apollo.west.oic.com>
  19  *              Arnt Gulbrandsen, <agulbra@nvg.unit.no>
  20  *              Jorge Cwik, <jorge@laser.satlink.net>
  21  */
  22
  23 #include <linux/mm.h>
  24 #include <linux/module.h>
  25 #include <linux/sysctl.h>
  26 #include <linux/workqueue.h>
  27 #include <net/tcp.h>
  28 #include <net/inet_common.h>
  29 #include <net/xfrm.h>
  30
  31 #ifdef CONFIG_SYSCTL
  32 #define SYNC_INIT 0 /* let the user enable it */
  33 #else
  34 #define SYNC_INIT 1
  35 #endif
  36
  37 int sysctl_tcp_syncookies __read_mostly = SYNC_INIT;
  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(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 /* This is not only more efficient than what we used to do, it eliminates
 372  * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
 373  *
 374  * Actually, we could lots of memory writes here. tp of listening
 375  * socket contains all necessary default parameters.
 376  */
 377 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
 378 {
 379         struct sock *newsk = inet_csk_clone(sk, req, GFP_ATOMIC);
 380
 381         if (newsk != NULL) {
 382                 const struct inet_request_sock *ireq = inet_rsk(req);
 383                 struct tcp_request_sock *treq = tcp_rsk(req);
 384                 struct inet_connection_sock *newicsk = inet_csk(newsk);
 385                 struct tcp_sock *newtp;
 386
 387                 /* Now setup tcp_sock */
 388                 newtp = tcp_sk(newsk);
 389                 newtp->pred_flags = 0;
 390                 newtp->rcv_wup = newtp->copied_seq = newtp->rcv_nxt = treq->rcv_isn + 1;
 391                 newtp->snd_sml = newtp->snd_una = newtp->snd_nxt = treq->snt_isn + 1;
 392
 393                 tcp_prequeue_init(newtp);
 394
 395                 tcp_init_wl(newtp, treq->snt_isn, treq->rcv_isn);
 396
 397                 newtp->srtt = 0;
 398                 newtp->mdev = TCP_TIMEOUT_INIT;
 399                 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
 400
 401                 newtp->packets_out = 0;
 402                 newtp->left_out = 0;
 403                 newtp->retrans_out = 0;
 404                 newtp->sacked_out = 0;
 405                 newtp->fackets_out = 0;
 406                 newtp->snd_ssthresh = 0x7fffffff;
 407
 408                 /* So many TCP implementations out there (incorrectly) count the
 409                  * initial SYN frame in their delayed-ACK and congestion control
 410                  * algorithms that we must have the following bandaid to talk
 411                  * efficiently to them.  -DaveM
 412                  */
 413                 newtp->snd_cwnd = 2;
 414                 newtp->snd_cwnd_cnt = 0;
 415                 newtp->bytes_acked = 0;
 416
 417                 newtp->frto_counter = 0;
 418                 newtp->frto_highmark = 0;
 419
 420                 newicsk->icsk_ca_ops = &tcp_init_congestion_ops;
 421
 422                 tcp_set_ca_state(newsk, TCP_CA_Open);
 423                 tcp_init_xmit_timers(newsk);
 424                 skb_queue_head_init(&newtp->out_of_order_queue);
 425                 newtp->write_seq = treq->snt_isn + 1;
 426                 newtp->pushed_seq = newtp->write_seq;
 427
 428                 newtp->rx_opt.saw_tstamp = 0;
 429
 430                 newtp->rx_opt.dsack = 0;
 431                 newtp->rx_opt.eff_sacks = 0;
 432
 433                 newtp->rx_opt.num_sacks = 0;
 434                 newtp->urg_data = 0;
 435
 436                 if (sock_flag(newsk, SOCK_KEEPOPEN))
 437                         inet_csk_reset_keepalive_timer(newsk,
 438                                                        keepalive_time_when(newtp));
 439
 440                 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
 441                 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
 442                         if (sysctl_tcp_fack)
 443                                 newtp->rx_opt.sack_ok |= 2;
 444                 }
 445                 newtp->window_clamp = req->window_clamp;
 446                 newtp->rcv_ssthresh = req->rcv_wnd;
 447                 newtp->rcv_wnd = req->rcv_wnd;
 448                 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
 449                 if (newtp->rx_opt.wscale_ok) {
 450                         newtp->rx_opt.snd_wscale = ireq->snd_wscale;
 451                         newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
 452                 } else {
 453                         newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
 454                         newtp->window_clamp = min(newtp->window_clamp, 65535U);
 455                 }
 456                 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
 457                                   newtp->rx_opt.snd_wscale);
 458                 newtp->max_window = newtp->snd_wnd;
 459
 460                 if (newtp->rx_opt.tstamp_ok) {
 461                         newtp->rx_opt.ts_recent = req->ts_recent;
 462                         newtp->rx_opt.ts_recent_stamp = get_seconds();
 463                         newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
 464                 } else {
 465                         newtp->rx_opt.ts_recent_stamp = 0;
 466                         newtp->tcp_header_len = sizeof(struct tcphdr);
 467                 }
 468 #ifdef CONFIG_TCP_MD5SIG
 469                 newtp->md5sig_info = NULL;      /*XXX*/
 470                 if (newtp->af_specific->md5_lookup(sk, newsk))
 471                         newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
 472 #endif
 473                 if (skb->len >= TCP_MIN_RCVMSS+newtp->tcp_header_len)
 474                         newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
 475                 newtp->rx_opt.mss_clamp = req->mss;
 476                 TCP_ECN_openreq_child(newtp, req);
 477
 478                 TCP_INC_STATS_BH(TCP_MIB_PASSIVEOPENS);
 479         }
 480         return newsk;
 481 }
 482
 483 /*
 484  *      Process an incoming packet for SYN_RECV sockets represented
 485  *      as a request_sock.
 486  */
 487
 488 struct sock *tcp_check_req(struct sock *sk,struct sk_buff *skb,
 489                            struct request_sock *req,
 490                            struct request_sock **prev)
 491 {
 492         const struct tcphdr *th = tcp_hdr(skb);
 493         __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
 494         int paws_reject = 0;
 495         struct tcp_options_received tmp_opt;
 496         struct sock *child;
 497
 498         tmp_opt.saw_tstamp = 0;
 499         if (th->doff > (sizeof(struct tcphdr)>>2)) {
 500                 tcp_parse_options(skb, &tmp_opt, 0);
 501
 502                 if (tmp_opt.saw_tstamp) {
 503                         tmp_opt.ts_recent = req->ts_recent;
 504                         /* We do not store true stamp, but it is not required,
 505                          * it can be estimated (approximately)
 506                          * from another data.
 507                          */
 508                         tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans);
 509                         paws_reject = tcp_paws_check(&tmp_opt, th->rst);
 510                 }
 511         }
 512
 513         /* Check for pure retransmitted SYN. */
 514         if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
 515             flg == TCP_FLAG_SYN &&
 516             !paws_reject) {
 517                 /*
 518                  * RFC793 draws (Incorrectly! It was fixed in RFC1122)
 519                  * this case on figure 6 and figure 8, but formal
 520                  * protocol description says NOTHING.
 521                  * To be more exact, it says that we should send ACK,
 522                  * because this segment (at least, if it has no data)
 523                  * is out of window.
 524                  *
 525                  *  CONCLUSION: RFC793 (even with RFC1122) DOES NOT
 526                  *  describe SYN-RECV state. All the description
 527                  *  is wrong, we cannot believe to it and should
 528                  *  rely only on common sense and implementation
 529                  *  experience.
 530                  *
 531                  * Enforce "SYN-ACK" according to figure 8, figure 6
 532                  * of RFC793, fixed by RFC1122.
 533                  */
 534                 req->rsk_ops->rtx_syn_ack(sk, req, NULL);
 535                 return NULL;
 536         }
 537
 538         /* Further reproduces section "SEGMENT ARRIVES"
 539            for state SYN-RECEIVED of RFC793.
 540            It is broken, however, it does not work only
 541            when SYNs are crossed.
 542
 543            You would think that SYN crossing is impossible here, since
 544            we should have a SYN_SENT socket (from connect()) on our end,
 545            but this is not true if the crossed SYNs were sent to both
 546            ends by a malicious third party.  We must defend against this,
 547            and to do that we first verify the ACK (as per RFC793, page
 548            36) and reset if it is invalid.  Is this a true full defense?
 549            To convince ourselves, let us consider a way in which the ACK
 550            test can still pass in this 'malicious crossed SYNs' case.
 551            Malicious sender sends identical SYNs (and thus identical sequence
 552            numbers) to both A and B:
 553
 554                 A: gets SYN, seq=7
 555                 B: gets SYN, seq=7
 556
 557            By our good fortune, both A and B select the same initial
 558            send sequence number of seven :-)
 559
 560                 A: sends SYN|ACK, seq=7, ack_seq=8
 561                 B: sends SYN|ACK, seq=7, ack_seq=8
 562
 563            So we are now A eating this SYN|ACK, ACK test passes.  So
 564            does sequence test, SYN is truncated, and thus we consider
 565            it a bare ACK.
 566
 567            If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
 568            bare ACK.  Otherwise, we create an established connection.  Both
 569            ends (listening sockets) accept the new incoming connection and try
 570            to talk to each other. 8-)
 571
 572            Note: This case is both harmless, and rare.  Possibility is about the
 573            same as us discovering intelligent life on another plant tomorrow.
 574
 575            But generally, we should (RFC lies!) to accept ACK
 576            from SYNACK both here and in tcp_rcv_state_process().
 577            tcp_rcv_state_process() does not, hence, we do not too.
 578
 579            Note that the case is absolutely generic:
 580            we cannot optimize anything here without
 581            violating protocol. All the checks must be made
 582            before attempt to create socket.
 583          */
 584
 585         /* RFC793 page 36: "If the connection is in any non-synchronized state ...
 586          *                  and the incoming segment acknowledges something not yet
 587          *                  sent (the segment carries an unacceptable ACK) ...
 588          *                  a reset is sent."
 589          *
 590          * Invalid ACK: reset will be sent by listening socket
 591          */
 592         if ((flg & TCP_FLAG_ACK) &&
 593             (TCP_SKB_CB(skb)->ack_seq != tcp_rsk(req)->snt_isn + 1))
 594                 return sk;
 595
 596         /* Also, it would be not so bad idea to check rcv_tsecr, which
 597          * is essentially ACK extension and too early or too late values
 598          * should cause reset in unsynchronized states.
 599          */
 600
 601         /* RFC793: "first check sequence number". */
 602
 603         if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
 604                                           tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) {
 605                 /* Out of window: send ACK and drop. */
 606                 if (!(flg & TCP_FLAG_RST))
 607                         req->rsk_ops->send_ack(skb, req);
 608                 if (paws_reject)
 609                         NET_INC_STATS_BH(LINUX_MIB_PAWSESTABREJECTED);
 610                 return NULL;
 611         }
 612
 613         /* In sequence, PAWS is OK. */
 614
 615         if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1))
 616                         req->ts_recent = tmp_opt.rcv_tsval;
 617
 618                 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
 619                         /* Truncate SYN, it is out of window starting
 620                            at tcp_rsk(req)->rcv_isn + 1. */
 621                         flg &= ~TCP_FLAG_SYN;
 622                 }
 623
 624                 /* RFC793: "second check the RST bit" and
 625                  *         "fourth, check the SYN bit"
 626                  */
 627                 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
 628                         TCP_INC_STATS_BH(TCP_MIB_ATTEMPTFAILS);
 629                         goto embryonic_reset;
 630                 }
 631
 632                 /* ACK sequence verified above, just make sure ACK is
 633                  * set.  If ACK not set, just silently drop the packet.
 634                  */
 635                 if (!(flg & TCP_FLAG_ACK))
 636                         return NULL;
 637
 638                 /* If TCP_DEFER_ACCEPT is set, drop bare ACK. */
 639                 if (inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
 640                     TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
 641                         inet_rsk(req)->acked = 1;
 642                         return NULL;
 643                 }
 644
 645                 /* OK, ACK is valid, create big socket and
 646                  * feed this segment to it. It will repeat all
 647                  * the tests. THIS SEGMENT MUST MOVE SOCKET TO
 648                  * ESTABLISHED STATE. If it will be dropped after
 649                  * socket is created, wait for troubles.
 650                  */
 651                 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb,
 652                                                                  req, NULL);
 653                 if (child == NULL)
 654                         goto listen_overflow;
 655 #ifdef CONFIG_TCP_MD5SIG
 656                 else {
 657                         /* Copy over the MD5 key from the original socket */
 658                         struct tcp_md5sig_key *key;
 659                         struct tcp_sock *tp = tcp_sk(sk);
 660                         key = tp->af_specific->md5_lookup(sk, child);
 661                         if (key != NULL) {
 662                                 /*
 663                                  * We're using one, so create a matching key on the
 664                                  * newsk structure. If we fail to get memory then we
 665                                  * end up not copying the key across. Shucks.
 666                                  */
 667                                 char *newkey = kmemdup(key->key, key->keylen,
 668                                                        GFP_ATOMIC);
 669                                 if (newkey) {
 670                                         if (!tcp_alloc_md5sig_pool())
 671                                                 BUG();
 672                                         tp->af_specific->md5_add(child, child,
 673                                                                  newkey,
 674                                                                  key->keylen);
 675                                 }
 676                         }
 677                 }
 678 #endif
 679
 680                 inet_csk_reqsk_queue_unlink(sk, req, prev);
 681                 inet_csk_reqsk_queue_removed(sk, req);
 682
 683                 inet_csk_reqsk_queue_add(sk, req, child);
 684                 return child;
 685
 686         listen_overflow:
 687                 if (!sysctl_tcp_abort_on_overflow) {
 688                         inet_rsk(req)->acked = 1;
 689                         return NULL;
 690                 }
 691
 692         embryonic_reset:
 693                 NET_INC_STATS_BH(LINUX_MIB_EMBRYONICRSTS);
 694                 if (!(flg & TCP_FLAG_RST))
 695                         req->rsk_ops->send_reset(sk, skb);
 696
 697                 inet_csk_reqsk_queue_drop(sk, req, prev);
 698                 return NULL;
 699 }
 700
 701 /*
 702  * Queue segment on the new socket if the new socket is active,
 703  * otherwise we just shortcircuit this and continue with
 704  * the new socket.
 705  */
 706
 707 int tcp_child_process(struct sock *parent, struct sock *child,
 708                       struct sk_buff *skb)
 709 {
 710         int ret = 0;
 711         int state = child->sk_state;
 712
 713         if (!sock_owned_by_user(child)) {
 714                 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
 715                                             skb->len);
 716                 /* Wakeup parent, send SIGIO */
 717                 if (state == TCP_SYN_RECV && child->sk_state != state)
 718                         parent->sk_data_ready(parent, 0);
 719         } else {
 720                 /* Alas, it is possible again, because we do lookup
 721                  * in main socket hash table and lock on listening
 722                  * socket does not protect us more.
 723                  */
 724                 sk_add_backlog(child, skb);
 725         }
 726
 727         bh_unlock_sock(child);
 728         sock_put(child);
 729         return ret;
 730 }
 731
 732 EXPORT_SYMBOL(tcp_check_req);
 733 EXPORT_SYMBOL(tcp_child_process);
 734 EXPORT_SYMBOL(tcp_create_openreq_child);
 735 EXPORT_SYMBOL(tcp_timewait_state_process);