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Linux/net/ipv4/tcp_minisocks.c

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  1 // SPDX-License-Identifier: GPL-2.0-only
  2 /*
  3  * INET         An implementation of the TCP/IP protocol suite for the LINUX
  4  *              operating system.  INET is implemented using the  BSD Socket
  5  *              interface as the means of communication with the user level.
  6  *
  7  *              Implementation of the Transmission Control Protocol(TCP).
  8  *
  9  * Authors:     Ross Biro
 10  *              Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
 11  *              Mark Evans, <evansmp@uhura.aston.ac.uk>
 12  *              Corey Minyard <wf-rch!minyard@relay.EU.net>
 13  *              Florian La Roche, <flla@stud.uni-sb.de>
 14  *              Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
 15  *              Linus Torvalds, <torvalds@cs.helsinki.fi>
 16  *              Alan Cox, <gw4pts@gw4pts.ampr.org>
 17  *              Matthew Dillon, <dillon@apollo.west.oic.com>
 18  *              Arnt Gulbrandsen, <agulbra@nvg.unit.no>
 19  *              Jorge Cwik, <jorge@laser.satlink.net>
 20  */
 21 
 22 #include <linux/mm.h>
 23 #include <linux/module.h>
 24 #include <linux/slab.h>
 25 #include <linux/sysctl.h>
 26 #include <linux/workqueue.h>
 27 #include <linux/static_key.h>
 28 #include <net/tcp.h>
 29 #include <net/inet_common.h>
 30 #include <net/xfrm.h>
 31 #include <net/busy_poll.h>
 32 
 33 static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
 34 {
 35         if (seq == s_win)
 36                 return true;
 37         if (after(end_seq, s_win) && before(seq, e_win))
 38                 return true;
 39         return seq == e_win && seq == end_seq;
 40 }
 41 
 42 static enum tcp_tw_status
 43 tcp_timewait_check_oow_rate_limit(struct inet_timewait_sock *tw,
 44                                   const struct sk_buff *skb, int mib_idx)
 45 {
 46         struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
 47 
 48         if (!tcp_oow_rate_limited(twsk_net(tw), skb, mib_idx,
 49                                   &tcptw->tw_last_oow_ack_time)) {
 50                 /* Send ACK. Note, we do not put the bucket,
 51                  * it will be released by caller.
 52                  */
 53                 return TCP_TW_ACK;
 54         }
 55 
 56         /* We are rate-limiting, so just release the tw sock and drop skb. */
 57         inet_twsk_put(tw);
 58         return TCP_TW_SUCCESS;
 59 }
 60 
 61 /*
 62  * * Main purpose of TIME-WAIT state is to close connection gracefully,
 63  *   when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
 64  *   (and, probably, tail of data) and one or more our ACKs are lost.
 65  * * What is TIME-WAIT timeout? It is associated with maximal packet
 66  *   lifetime in the internet, which results in wrong conclusion, that
 67  *   it is set to catch "old duplicate segments" wandering out of their path.
 68  *   It is not quite correct. This timeout is calculated so that it exceeds
 69  *   maximal retransmission timeout enough to allow to lose one (or more)
 70  *   segments sent by peer and our ACKs. This time may be calculated from RTO.
 71  * * When TIME-WAIT socket receives RST, it means that another end
 72  *   finally closed and we are allowed to kill TIME-WAIT too.
 73  * * Second purpose of TIME-WAIT is catching old duplicate segments.
 74  *   Well, certainly it is pure paranoia, but if we load TIME-WAIT
 75  *   with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
 76  * * If we invented some more clever way to catch duplicates
 77  *   (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
 78  *
 79  * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
 80  * When you compare it to RFCs, please, read section SEGMENT ARRIVES
 81  * from the very beginning.
 82  *
 83  * NOTE. With recycling (and later with fin-wait-2) TW bucket
 84  * is _not_ stateless. It means, that strictly speaking we must
 85  * spinlock it. I do not want! Well, probability of misbehaviour
 86  * is ridiculously low and, seems, we could use some mb() tricks
 87  * to avoid misread sequence numbers, states etc.  --ANK
 88  *
 89  * We don't need to initialize tmp_out.sack_ok as we don't use the results
 90  */
 91 enum tcp_tw_status
 92 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
 93                            const struct tcphdr *th)
 94 {
 95         struct tcp_options_received tmp_opt;
 96         struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
 97         bool paws_reject = false;
 98 
 99         tmp_opt.saw_tstamp = 0;
100         if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
101                 tcp_parse_options(twsk_net(tw), skb, &tmp_opt, 0, NULL);
102 
103                 if (tmp_opt.saw_tstamp) {
104                         if (tmp_opt.rcv_tsecr)
105                                 tmp_opt.rcv_tsecr -= tcptw->tw_ts_offset;
106                         tmp_opt.ts_recent       = tcptw->tw_ts_recent;
107                         tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
108                         paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
109                 }
110         }
111 
112         if (tw->tw_substate == TCP_FIN_WAIT2) {
113                 /* Just repeat all the checks of tcp_rcv_state_process() */
114 
115                 /* Out of window, send ACK */
116                 if (paws_reject ||
117                     !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
118                                    tcptw->tw_rcv_nxt,
119                                    tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
120                         return tcp_timewait_check_oow_rate_limit(
121                                 tw, skb, LINUX_MIB_TCPACKSKIPPEDFINWAIT2);
122 
123                 if (th->rst)
124                         goto kill;
125 
126                 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
127                         return TCP_TW_RST;
128 
129                 /* Dup ACK? */
130                 if (!th->ack ||
131                     !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                         return TCP_TW_RST;
143 
144                 /* FIN arrived, enter true time-wait state. */
145                 tw->tw_substate   = TCP_TIME_WAIT;
146                 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
147                 if (tmp_opt.saw_tstamp) {
148                         tcptw->tw_ts_recent_stamp = ktime_get_seconds();
149                         tcptw->tw_ts_recent       = tmp_opt.rcv_tsval;
150                 }
151 
152                 inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
153                 return TCP_TW_ACK;
154         }
155 
156         /*
157          *      Now real TIME-WAIT state.
158          *
159          *      RFC 1122:
160          *      "When a connection is [...] on TIME-WAIT state [...]
161          *      [a TCP] MAY accept a new SYN from the remote TCP to
162          *      reopen the connection directly, if it:
163          *
164          *      (1)  assigns its initial sequence number for the new
165          *      connection to be larger than the largest sequence
166          *      number it used on the previous connection incarnation,
167          *      and
168          *
169          *      (2)  returns to TIME-WAIT state if the SYN turns out
170          *      to be an old duplicate".
171          */
172 
173         if (!paws_reject &&
174             (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
175              (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
176                 /* In window segment, it may be only reset or bare ack. */
177 
178                 if (th->rst) {
179                         /* This is TIME_WAIT assassination, in two flavors.
180                          * Oh well... nobody has a sufficient solution to this
181                          * protocol bug yet.
182                          */
183                         if (twsk_net(tw)->ipv4.sysctl_tcp_rfc1337 == 0) {
184 kill:
185                                 inet_twsk_deschedule_put(tw);
186                                 return TCP_TW_SUCCESS;
187                         }
188                 } else {
189                         inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
190                 }
191 
192                 if (tmp_opt.saw_tstamp) {
193                         tcptw->tw_ts_recent       = tmp_opt.rcv_tsval;
194                         tcptw->tw_ts_recent_stamp = ktime_get_seconds();
195                 }
196 
197                 inet_twsk_put(tw);
198                 return TCP_TW_SUCCESS;
199         }
200 
201         /* Out of window segment.
202 
203            All the segments are ACKed immediately.
204 
205            The only exception is new SYN. We accept it, if it is
206            not old duplicate and we are not in danger to be killed
207            by delayed old duplicates. RFC check is that it has
208            newer sequence number works at rates <40Mbit/sec.
209            However, if paws works, it is reliable AND even more,
210            we even may relax silly seq space cutoff.
211 
212            RED-PEN: we violate main RFC requirement, if this SYN will appear
213            old duplicate (i.e. we receive RST in reply to SYN-ACK),
214            we must return socket to time-wait state. It is not good,
215            but not fatal yet.
216          */
217 
218         if (th->syn && !th->rst && !th->ack && !paws_reject &&
219             (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
220              (tmp_opt.saw_tstamp &&
221               (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
222                 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
223                 if (isn == 0)
224                         isn++;
225                 TCP_SKB_CB(skb)->tcp_tw_isn = isn;
226                 return TCP_TW_SYN;
227         }
228 
229         if (paws_reject)
230                 __NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
231 
232         if (!th->rst) {
233                 /* In this case we must reset the TIMEWAIT timer.
234                  *
235                  * If it is ACKless SYN it may be both old duplicate
236                  * and new good SYN with random sequence number <rcv_nxt.
237                  * Do not reschedule in the last case.
238                  */
239                 if (paws_reject || th->ack)
240                         inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
241 
242                 return tcp_timewait_check_oow_rate_limit(
243                         tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT);
244         }
245         inet_twsk_put(tw);
246         return TCP_TW_SUCCESS;
247 }
248 EXPORT_SYMBOL(tcp_timewait_state_process);
249 
250 /*
251  * Move a socket to time-wait or dead fin-wait-2 state.
252  */
253 void tcp_time_wait(struct sock *sk, int state, int timeo)
254 {
255         const struct inet_connection_sock *icsk = inet_csk(sk);
256         const struct tcp_sock *tp = tcp_sk(sk);
257         struct inet_timewait_sock *tw;
258         struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row;
259 
260         tw = inet_twsk_alloc(sk, tcp_death_row, state);
261 
262         if (tw) {
263                 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
264                 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
265                 struct inet_sock *inet = inet_sk(sk);
266 
267                 tw->tw_transparent      = inet->transparent;
268                 tw->tw_mark             = sk->sk_mark;
269                 tw->tw_priority         = sk->sk_priority;
270                 tw->tw_rcv_wscale       = tp->rx_opt.rcv_wscale;
271                 tcptw->tw_rcv_nxt       = tp->rcv_nxt;
272                 tcptw->tw_snd_nxt       = tp->snd_nxt;
273                 tcptw->tw_rcv_wnd       = tcp_receive_window(tp);
274                 tcptw->tw_ts_recent     = tp->rx_opt.ts_recent;
275                 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
276                 tcptw->tw_ts_offset     = tp->tsoffset;
277                 tcptw->tw_last_oow_ack_time = 0;
278                 tcptw->tw_tx_delay      = tp->tcp_tx_delay;
279 #if IS_ENABLED(CONFIG_IPV6)
280                 if (tw->tw_family == PF_INET6) {
281                         struct ipv6_pinfo *np = inet6_sk(sk);
282 
283                         tw->tw_v6_daddr = sk->sk_v6_daddr;
284                         tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
285                         tw->tw_tclass = np->tclass;
286                         tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK);
287                         tw->tw_txhash = sk->sk_txhash;
288                         tw->tw_ipv6only = sk->sk_ipv6only;
289                 }
290 #endif
291 
292 #ifdef CONFIG_TCP_MD5SIG
293                 /*
294                  * The timewait bucket does not have the key DB from the
295                  * sock structure. We just make a quick copy of the
296                  * md5 key being used (if indeed we are using one)
297                  * so the timewait ack generating code has the key.
298                  */
299                 do {
300                         tcptw->tw_md5_key = NULL;
301                         if (static_branch_unlikely(&tcp_md5_needed)) {
302                                 struct tcp_md5sig_key *key;
303 
304                                 key = tp->af_specific->md5_lookup(sk, sk);
305                                 if (key) {
306                                         tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
307                                         BUG_ON(tcptw->tw_md5_key && !tcp_alloc_md5sig_pool());
308                                 }
309                         }
310                 } while (0);
311 #endif
312 
313                 /* Get the TIME_WAIT timeout firing. */
314                 if (timeo < rto)
315                         timeo = rto;
316 
317                 if (state == TCP_TIME_WAIT)
318                         timeo = TCP_TIMEWAIT_LEN;
319 
320                 /* tw_timer is pinned, so we need to make sure BH are disabled
321                  * in following section, otherwise timer handler could run before
322                  * we complete the initialization.
323                  */
324                 local_bh_disable();
325                 inet_twsk_schedule(tw, timeo);
326                 /* Linkage updates.
327                  * Note that access to tw after this point is illegal.
328                  */
329                 inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
330                 local_bh_enable();
331         } else {
332                 /* Sorry, if we're out of memory, just CLOSE this
333                  * socket up.  We've got bigger problems than
334                  * non-graceful socket closings.
335                  */
336                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW);
337         }
338 
339         tcp_update_metrics(sk);
340         tcp_done(sk);
341 }
342 EXPORT_SYMBOL(tcp_time_wait);
343 
344 void tcp_twsk_destructor(struct sock *sk)
345 {
346 #ifdef CONFIG_TCP_MD5SIG
347         if (static_branch_unlikely(&tcp_md5_needed)) {
348                 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
349 
350                 if (twsk->tw_md5_key)
351                         kfree_rcu(twsk->tw_md5_key, rcu);
352         }
353 #endif
354 }
355 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
356 
357 /* Warning : This function is called without sk_listener being locked.
358  * Be sure to read socket fields once, as their value could change under us.
359  */
360 void tcp_openreq_init_rwin(struct request_sock *req,
361                            const struct sock *sk_listener,
362                            const struct dst_entry *dst)
363 {
364         struct inet_request_sock *ireq = inet_rsk(req);
365         const struct tcp_sock *tp = tcp_sk(sk_listener);
366         int full_space = tcp_full_space(sk_listener);
367         u32 window_clamp;
368         __u8 rcv_wscale;
369         u32 rcv_wnd;
370         int mss;
371 
372         mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
373         window_clamp = READ_ONCE(tp->window_clamp);
374         /* Set this up on the first call only */
375         req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);
376 
377         /* limit the window selection if the user enforce a smaller rx buffer */
378         if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK &&
379             (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
380                 req->rsk_window_clamp = full_space;
381 
382         rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req);
383         if (rcv_wnd == 0)
384                 rcv_wnd = dst_metric(dst, RTAX_INITRWND);
385         else if (full_space < rcv_wnd * mss)
386                 full_space = rcv_wnd * mss;
387 
388         /* tcp_full_space because it is guaranteed to be the first packet */
389         tcp_select_initial_window(sk_listener, full_space,
390                 mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
391                 &req->rsk_rcv_wnd,
392                 &req->rsk_window_clamp,
393                 ireq->wscale_ok,
394                 &rcv_wscale,
395                 rcv_wnd);
396         ireq->rcv_wscale = rcv_wscale;
397 }
398 EXPORT_SYMBOL(tcp_openreq_init_rwin);
399 
400 static void tcp_ecn_openreq_child(struct tcp_sock *tp,
401                                   const struct request_sock *req)
402 {
403         tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
404 }
405 
406 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
407 {
408         struct inet_connection_sock *icsk = inet_csk(sk);
409         u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
410         bool ca_got_dst = false;
411 
412         if (ca_key != TCP_CA_UNSPEC) {
413                 const struct tcp_congestion_ops *ca;
414 
415                 rcu_read_lock();
416                 ca = tcp_ca_find_key(ca_key);
417                 if (likely(ca && try_module_get(ca->owner))) {
418                         icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
419                         icsk->icsk_ca_ops = ca;
420                         ca_got_dst = true;
421                 }
422                 rcu_read_unlock();
423         }
424 
425         /* If no valid choice made yet, assign current system default ca. */
426         if (!ca_got_dst &&
427             (!icsk->icsk_ca_setsockopt ||
428              !try_module_get(icsk->icsk_ca_ops->owner)))
429                 tcp_assign_congestion_control(sk);
430 
431         tcp_set_ca_state(sk, TCP_CA_Open);
432 }
433 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child);
434 
435 static void smc_check_reset_syn_req(struct tcp_sock *oldtp,
436                                     struct request_sock *req,
437                                     struct tcp_sock *newtp)
438 {
439 #if IS_ENABLED(CONFIG_SMC)
440         struct inet_request_sock *ireq;
441 
442         if (static_branch_unlikely(&tcp_have_smc)) {
443                 ireq = inet_rsk(req);
444                 if (oldtp->syn_smc && !ireq->smc_ok)
445                         newtp->syn_smc = 0;
446         }
447 #endif
448 }
449 
450 /* This is not only more efficient than what we used to do, it eliminates
451  * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
452  *
453  * Actually, we could lots of memory writes here. tp of listening
454  * socket contains all necessary default parameters.
455  */
456 struct sock *tcp_create_openreq_child(const struct sock *sk,
457                                       struct request_sock *req,
458                                       struct sk_buff *skb)
459 {
460         struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
461         const struct inet_request_sock *ireq = inet_rsk(req);
462         struct tcp_request_sock *treq = tcp_rsk(req);
463         struct inet_connection_sock *newicsk;
464         struct tcp_sock *oldtp, *newtp;
465         u32 seq;
466 
467         if (!newsk)
468                 return NULL;
469 
470         newicsk = inet_csk(newsk);
471         newtp = tcp_sk(newsk);
472         oldtp = tcp_sk(sk);
473 
474         smc_check_reset_syn_req(oldtp, req, newtp);
475 
476         /* Now setup tcp_sock */
477         newtp->pred_flags = 0;
478 
479         seq = treq->rcv_isn + 1;
480         newtp->rcv_wup = seq;
481         WRITE_ONCE(newtp->copied_seq, seq);
482         WRITE_ONCE(newtp->rcv_nxt, seq);
483         newtp->segs_in = 1;
484 
485         seq = treq->snt_isn + 1;
486         newtp->snd_sml = newtp->snd_una = seq;
487         WRITE_ONCE(newtp->snd_nxt, seq);
488         newtp->snd_up = seq;
489 
490         INIT_LIST_HEAD(&newtp->tsq_node);
491         INIT_LIST_HEAD(&newtp->tsorted_sent_queue);
492 
493         tcp_init_wl(newtp, treq->rcv_isn);
494 
495         minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U);
496         newicsk->icsk_ack.lrcvtime = tcp_jiffies32;
497 
498         newtp->lsndtime = tcp_jiffies32;
499         newsk->sk_txhash = treq->txhash;
500         newtp->total_retrans = req->num_retrans;
501 
502         tcp_init_xmit_timers(newsk);
503         WRITE_ONCE(newtp->write_seq, newtp->pushed_seq = treq->snt_isn + 1);
504 
505         if (sock_flag(newsk, SOCK_KEEPOPEN))
506                 inet_csk_reset_keepalive_timer(newsk,
507                                                keepalive_time_when(newtp));
508 
509         newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
510         newtp->rx_opt.sack_ok = ireq->sack_ok;
511         newtp->window_clamp = req->rsk_window_clamp;
512         newtp->rcv_ssthresh = req->rsk_rcv_wnd;
513         newtp->rcv_wnd = req->rsk_rcv_wnd;
514         newtp->rx_opt.wscale_ok = ireq->wscale_ok;
515         if (newtp->rx_opt.wscale_ok) {
516                 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
517                 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
518         } else {
519                 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
520                 newtp->window_clamp = min(newtp->window_clamp, 65535U);
521         }
522         newtp->snd_wnd = ntohs(tcp_hdr(skb)->window) << newtp->rx_opt.snd_wscale;
523         newtp->max_window = newtp->snd_wnd;
524 
525         if (newtp->rx_opt.tstamp_ok) {
526                 newtp->rx_opt.ts_recent = req->ts_recent;
527                 newtp->rx_opt.ts_recent_stamp = ktime_get_seconds();
528                 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
529         } else {
530                 newtp->rx_opt.ts_recent_stamp = 0;
531                 newtp->tcp_header_len = sizeof(struct tcphdr);
532         }
533         if (req->num_timeout) {
534                 newtp->undo_marker = treq->snt_isn;
535                 newtp->retrans_stamp = div_u64(treq->snt_synack,
536                                                USEC_PER_SEC / TCP_TS_HZ);
537         }
538         newtp->tsoffset = treq->ts_off;
539 #ifdef CONFIG_TCP_MD5SIG
540         newtp->md5sig_info = NULL;      /*XXX*/
541         if (newtp->af_specific->md5_lookup(sk, newsk))
542                 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
543 #endif
544         if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
545                 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
546         newtp->rx_opt.mss_clamp = req->mss;
547         tcp_ecn_openreq_child(newtp, req);
548         newtp->fastopen_req = NULL;
549         RCU_INIT_POINTER(newtp->fastopen_rsk, NULL);
550 
551         __TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS);
552 
553         return newsk;
554 }
555 EXPORT_SYMBOL(tcp_create_openreq_child);
556 
557 /*
558  * Process an incoming packet for SYN_RECV sockets represented as a
559  * request_sock. Normally sk is the listener socket but for TFO it
560  * points to the child socket.
561  *
562  * XXX (TFO) - The current impl contains a special check for ack
563  * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
564  *
565  * We don't need to initialize tmp_opt.sack_ok as we don't use the results
566  */
567 
568 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
569                            struct request_sock *req,
570                            bool fastopen, bool *req_stolen)
571 {
572         struct tcp_options_received tmp_opt;
573         struct sock *child;
574         const struct tcphdr *th = tcp_hdr(skb);
575         __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
576         bool paws_reject = false;
577         bool own_req;
578 
579         tmp_opt.saw_tstamp = 0;
580         if (th->doff > (sizeof(struct tcphdr)>>2)) {
581                 tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL);
582 
583                 if (tmp_opt.saw_tstamp) {
584                         tmp_opt.ts_recent = req->ts_recent;
585                         if (tmp_opt.rcv_tsecr)
586                                 tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off;
587                         /* We do not store true stamp, but it is not required,
588                          * it can be estimated (approximately)
589                          * from another data.
590                          */
591                         tmp_opt.ts_recent_stamp = ktime_get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
592                         paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
593                 }
594         }
595 
596         /* Check for pure retransmitted SYN. */
597         if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
598             flg == TCP_FLAG_SYN &&
599             !paws_reject) {
600                 /*
601                  * RFC793 draws (Incorrectly! It was fixed in RFC1122)
602                  * this case on figure 6 and figure 8, but formal
603                  * protocol description says NOTHING.
604                  * To be more exact, it says that we should send ACK,
605                  * because this segment (at least, if it has no data)
606                  * is out of window.
607                  *
608                  *  CONCLUSION: RFC793 (even with RFC1122) DOES NOT
609                  *  describe SYN-RECV state. All the description
610                  *  is wrong, we cannot believe to it and should
611                  *  rely only on common sense and implementation
612                  *  experience.
613                  *
614                  * Enforce "SYN-ACK" according to figure 8, figure 6
615                  * of RFC793, fixed by RFC1122.
616                  *
617                  * Note that even if there is new data in the SYN packet
618                  * they will be thrown away too.
619                  *
620                  * Reset timer after retransmitting SYNACK, similar to
621                  * the idea of fast retransmit in recovery.
622                  */
623                 if (!tcp_oow_rate_limited(sock_net(sk), skb,
624                                           LINUX_MIB_TCPACKSKIPPEDSYNRECV,
625                                           &tcp_rsk(req)->last_oow_ack_time) &&
626 
627                     !inet_rtx_syn_ack(sk, req)) {
628                         unsigned long expires = jiffies;
629 
630                         expires += min(TCP_TIMEOUT_INIT << req->num_timeout,
631                                        TCP_RTO_MAX);
632                         if (!fastopen)
633                                 mod_timer_pending(&req->rsk_timer, expires);
634                         else
635                                 req->rsk_timer.expires = expires;
636                 }
637                 return NULL;
638         }
639 
640         /* Further reproduces section "SEGMENT ARRIVES"
641            for state SYN-RECEIVED of RFC793.
642            It is broken, however, it does not work only
643            when SYNs are crossed.
644 
645            You would think that SYN crossing is impossible here, since
646            we should have a SYN_SENT socket (from connect()) on our end,
647            but this is not true if the crossed SYNs were sent to both
648            ends by a malicious third party.  We must defend against this,
649            and to do that we first verify the ACK (as per RFC793, page
650            36) and reset if it is invalid.  Is this a true full defense?
651            To convince ourselves, let us consider a way in which the ACK
652            test can still pass in this 'malicious crossed SYNs' case.
653            Malicious sender sends identical SYNs (and thus identical sequence
654            numbers) to both A and B:
655 
656                 A: gets SYN, seq=7
657                 B: gets SYN, seq=7
658 
659            By our good fortune, both A and B select the same initial
660            send sequence number of seven :-)
661 
662                 A: sends SYN|ACK, seq=7, ack_seq=8
663                 B: sends SYN|ACK, seq=7, ack_seq=8
664 
665            So we are now A eating this SYN|ACK, ACK test passes.  So
666            does sequence test, SYN is truncated, and thus we consider
667            it a bare ACK.
668 
669            If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
670            bare ACK.  Otherwise, we create an established connection.  Both
671            ends (listening sockets) accept the new incoming connection and try
672            to talk to each other. 8-)
673 
674            Note: This case is both harmless, and rare.  Possibility is about the
675            same as us discovering intelligent life on another plant tomorrow.
676 
677            But generally, we should (RFC lies!) to accept ACK
678            from SYNACK both here and in tcp_rcv_state_process().
679            tcp_rcv_state_process() does not, hence, we do not too.
680 
681            Note that the case is absolutely generic:
682            we cannot optimize anything here without
683            violating protocol. All the checks must be made
684            before attempt to create socket.
685          */
686 
687         /* RFC793 page 36: "If the connection is in any non-synchronized state ...
688          *                  and the incoming segment acknowledges something not yet
689          *                  sent (the segment carries an unacceptable ACK) ...
690          *                  a reset is sent."
691          *
692          * Invalid ACK: reset will be sent by listening socket.
693          * Note that the ACK validity check for a Fast Open socket is done
694          * elsewhere and is checked directly against the child socket rather
695          * than req because user data may have been sent out.
696          */
697         if ((flg & TCP_FLAG_ACK) && !fastopen &&
698             (TCP_SKB_CB(skb)->ack_seq !=
699              tcp_rsk(req)->snt_isn + 1))
700                 return sk;
701 
702         /* Also, it would be not so bad idea to check rcv_tsecr, which
703          * is essentially ACK extension and too early or too late values
704          * should cause reset in unsynchronized states.
705          */
706 
707         /* RFC793: "first check sequence number". */
708 
709         if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
710                                           tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) {
711                 /* Out of window: send ACK and drop. */
712                 if (!(flg & TCP_FLAG_RST) &&
713                     !tcp_oow_rate_limited(sock_net(sk), skb,
714                                           LINUX_MIB_TCPACKSKIPPEDSYNRECV,
715                                           &tcp_rsk(req)->last_oow_ack_time))
716                         req->rsk_ops->send_ack(sk, skb, req);
717                 if (paws_reject)
718                         __NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
719                 return NULL;
720         }
721 
722         /* In sequence, PAWS is OK. */
723 
724         if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
725                 req->ts_recent = tmp_opt.rcv_tsval;
726 
727         if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
728                 /* Truncate SYN, it is out of window starting
729                    at tcp_rsk(req)->rcv_isn + 1. */
730                 flg &= ~TCP_FLAG_SYN;
731         }
732 
733         /* RFC793: "second check the RST bit" and
734          *         "fourth, check the SYN bit"
735          */
736         if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
737                 __TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
738                 goto embryonic_reset;
739         }
740 
741         /* ACK sequence verified above, just make sure ACK is
742          * set.  If ACK not set, just silently drop the packet.
743          *
744          * XXX (TFO) - if we ever allow "data after SYN", the
745          * following check needs to be removed.
746          */
747         if (!(flg & TCP_FLAG_ACK))
748                 return NULL;
749 
750         /* For Fast Open no more processing is needed (sk is the
751          * child socket).
752          */
753         if (fastopen)
754                 return sk;
755 
756         /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
757         if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
758             TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
759                 inet_rsk(req)->acked = 1;
760                 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
761                 return NULL;
762         }
763 
764         /* OK, ACK is valid, create big socket and
765          * feed this segment to it. It will repeat all
766          * the tests. THIS SEGMENT MUST MOVE SOCKET TO
767          * ESTABLISHED STATE. If it will be dropped after
768          * socket is created, wait for troubles.
769          */
770         child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
771                                                          req, &own_req);
772         if (!child)
773                 goto listen_overflow;
774 
775         sock_rps_save_rxhash(child, skb);
776         tcp_synack_rtt_meas(child, req);
777         *req_stolen = !own_req;
778         return inet_csk_complete_hashdance(sk, child, req, own_req);
779 
780 listen_overflow:
781         if (!sock_net(sk)->ipv4.sysctl_tcp_abort_on_overflow) {
782                 inet_rsk(req)->acked = 1;
783                 return NULL;
784         }
785 
786 embryonic_reset:
787         if (!(flg & TCP_FLAG_RST)) {
788                 /* Received a bad SYN pkt - for TFO We try not to reset
789                  * the local connection unless it's really necessary to
790                  * avoid becoming vulnerable to outside attack aiming at
791                  * resetting legit local connections.
792                  */
793                 req->rsk_ops->send_reset(sk, skb);
794         } else if (fastopen) { /* received a valid RST pkt */
795                 reqsk_fastopen_remove(sk, req, true);
796                 tcp_reset(sk);
797         }
798         if (!fastopen) {
799                 inet_csk_reqsk_queue_drop(sk, req);
800                 __NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
801         }
802         return NULL;
803 }
804 EXPORT_SYMBOL(tcp_check_req);
805 
806 /*
807  * Queue segment on the new socket if the new socket is active,
808  * otherwise we just shortcircuit this and continue with
809  * the new socket.
810  *
811  * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
812  * when entering. But other states are possible due to a race condition
813  * where after __inet_lookup_established() fails but before the listener
814  * locked is obtained, other packets cause the same connection to
815  * be created.
816  */
817 
818 int tcp_child_process(struct sock *parent, struct sock *child,
819                       struct sk_buff *skb)
820 {
821         int ret = 0;
822         int state = child->sk_state;
823 
824         /* record NAPI ID of child */
825         sk_mark_napi_id(child, skb);
826 
827         tcp_segs_in(tcp_sk(child), skb);
828         if (!sock_owned_by_user(child)) {
829                 ret = tcp_rcv_state_process(child, skb);
830                 /* Wakeup parent, send SIGIO */
831                 if (state == TCP_SYN_RECV && child->sk_state != state)
832                         parent->sk_data_ready(parent);
833         } else {
834                 /* Alas, it is possible again, because we do lookup
835                  * in main socket hash table and lock on listening
836                  * socket does not protect us more.
837                  */
838                 __sk_add_backlog(child, skb);
839         }
840 
841         bh_unlock_sock(child);
842         sock_put(child);
843         return ret;
844 }
845 EXPORT_SYMBOL(tcp_child_process);
846 

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