xref: /openbmc/linux/net/ipv4/tcp_minisocks.c (revision 92a2c6b2)
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 
30 int sysctl_tcp_syncookies __read_mostly = 1;
31 EXPORT_SYMBOL(sysctl_tcp_syncookies);
32 
33 int sysctl_tcp_abort_on_overflow __read_mostly;
34 
35 struct 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 EXPORT_SYMBOL_GPL(tcp_death_row);
51 
52 static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
53 {
54 	if (seq == s_win)
55 		return true;
56 	if (after(end_seq, s_win) && before(seq, e_win))
57 		return true;
58 	return seq == e_win && seq == end_seq;
59 }
60 
61 static enum tcp_tw_status
62 tcp_timewait_check_oow_rate_limit(struct inet_timewait_sock *tw,
63 				  const struct sk_buff *skb, int mib_idx)
64 {
65 	struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
66 
67 	if (!tcp_oow_rate_limited(twsk_net(tw), skb, mib_idx,
68 				  &tcptw->tw_last_oow_ack_time)) {
69 		/* Send ACK. Note, we do not put the bucket,
70 		 * it will be released by caller.
71 		 */
72 		return TCP_TW_ACK;
73 	}
74 
75 	/* We are rate-limiting, so just release the tw sock and drop skb. */
76 	inet_twsk_put(tw);
77 	return TCP_TW_SUCCESS;
78 }
79 
80 /*
81  * * Main purpose of TIME-WAIT state is to close connection gracefully,
82  *   when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
83  *   (and, probably, tail of data) and one or more our ACKs are lost.
84  * * What is TIME-WAIT timeout? It is associated with maximal packet
85  *   lifetime in the internet, which results in wrong conclusion, that
86  *   it is set to catch "old duplicate segments" wandering out of their path.
87  *   It is not quite correct. This timeout is calculated so that it exceeds
88  *   maximal retransmission timeout enough to allow to lose one (or more)
89  *   segments sent by peer and our ACKs. This time may be calculated from RTO.
90  * * When TIME-WAIT socket receives RST, it means that another end
91  *   finally closed and we are allowed to kill TIME-WAIT too.
92  * * Second purpose of TIME-WAIT is catching old duplicate segments.
93  *   Well, certainly it is pure paranoia, but if we load TIME-WAIT
94  *   with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
95  * * If we invented some more clever way to catch duplicates
96  *   (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
97  *
98  * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
99  * When you compare it to RFCs, please, read section SEGMENT ARRIVES
100  * from the very beginning.
101  *
102  * NOTE. With recycling (and later with fin-wait-2) TW bucket
103  * is _not_ stateless. It means, that strictly speaking we must
104  * spinlock it. I do not want! Well, probability of misbehaviour
105  * is ridiculously low and, seems, we could use some mb() tricks
106  * to avoid misread sequence numbers, states etc.  --ANK
107  *
108  * We don't need to initialize tmp_out.sack_ok as we don't use the results
109  */
110 enum tcp_tw_status
111 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
112 			   const struct tcphdr *th)
113 {
114 	struct tcp_options_received tmp_opt;
115 	struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
116 	bool paws_reject = false;
117 
118 	tmp_opt.saw_tstamp = 0;
119 	if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
120 		tcp_parse_options(skb, &tmp_opt, 0, NULL);
121 
122 		if (tmp_opt.saw_tstamp) {
123 			tmp_opt.rcv_tsecr	-= tcptw->tw_ts_offset;
124 			tmp_opt.ts_recent	= tcptw->tw_ts_recent;
125 			tmp_opt.ts_recent_stamp	= tcptw->tw_ts_recent_stamp;
126 			paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
127 		}
128 	}
129 
130 	if (tw->tw_substate == TCP_FIN_WAIT2) {
131 		/* Just repeat all the checks of tcp_rcv_state_process() */
132 
133 		/* Out of window, send ACK */
134 		if (paws_reject ||
135 		    !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
136 				   tcptw->tw_rcv_nxt,
137 				   tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
138 			return tcp_timewait_check_oow_rate_limit(
139 				tw, skb, LINUX_MIB_TCPACKSKIPPEDFINWAIT2);
140 
141 		if (th->rst)
142 			goto kill;
143 
144 		if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
145 			goto kill_with_rst;
146 
147 		/* Dup ACK? */
148 		if (!th->ack ||
149 		    !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
150 		    TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
151 			inet_twsk_put(tw);
152 			return TCP_TW_SUCCESS;
153 		}
154 
155 		/* New data or FIN. If new data arrive after half-duplex close,
156 		 * reset.
157 		 */
158 		if (!th->fin ||
159 		    TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) {
160 kill_with_rst:
161 			inet_twsk_deschedule(tw, &tcp_death_row);
162 			inet_twsk_put(tw);
163 			return TCP_TW_RST;
164 		}
165 
166 		/* FIN arrived, enter true time-wait state. */
167 		tw->tw_substate	  = TCP_TIME_WAIT;
168 		tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
169 		if (tmp_opt.saw_tstamp) {
170 			tcptw->tw_ts_recent_stamp = get_seconds();
171 			tcptw->tw_ts_recent	  = tmp_opt.rcv_tsval;
172 		}
173 
174 		if (tcp_death_row.sysctl_tw_recycle &&
175 		    tcptw->tw_ts_recent_stamp &&
176 		    tcp_tw_remember_stamp(tw))
177 			inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout,
178 					   TCP_TIMEWAIT_LEN);
179 		else
180 			inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
181 					   TCP_TIMEWAIT_LEN);
182 		return TCP_TW_ACK;
183 	}
184 
185 	/*
186 	 *	Now real TIME-WAIT state.
187 	 *
188 	 *	RFC 1122:
189 	 *	"When a connection is [...] on TIME-WAIT state [...]
190 	 *	[a TCP] MAY accept a new SYN from the remote TCP to
191 	 *	reopen the connection directly, if it:
192 	 *
193 	 *	(1)  assigns its initial sequence number for the new
194 	 *	connection to be larger than the largest sequence
195 	 *	number it used on the previous connection incarnation,
196 	 *	and
197 	 *
198 	 *	(2)  returns to TIME-WAIT state if the SYN turns out
199 	 *	to be an old duplicate".
200 	 */
201 
202 	if (!paws_reject &&
203 	    (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
204 	     (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
205 		/* In window segment, it may be only reset or bare ack. */
206 
207 		if (th->rst) {
208 			/* This is TIME_WAIT assassination, in two flavors.
209 			 * Oh well... nobody has a sufficient solution to this
210 			 * protocol bug yet.
211 			 */
212 			if (sysctl_tcp_rfc1337 == 0) {
213 kill:
214 				inet_twsk_deschedule(tw, &tcp_death_row);
215 				inet_twsk_put(tw);
216 				return TCP_TW_SUCCESS;
217 			}
218 		}
219 		inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
220 				   TCP_TIMEWAIT_LEN);
221 
222 		if (tmp_opt.saw_tstamp) {
223 			tcptw->tw_ts_recent	  = tmp_opt.rcv_tsval;
224 			tcptw->tw_ts_recent_stamp = get_seconds();
225 		}
226 
227 		inet_twsk_put(tw);
228 		return TCP_TW_SUCCESS;
229 	}
230 
231 	/* Out of window segment.
232 
233 	   All the segments are ACKed immediately.
234 
235 	   The only exception is new SYN. We accept it, if it is
236 	   not old duplicate and we are not in danger to be killed
237 	   by delayed old duplicates. RFC check is that it has
238 	   newer sequence number works at rates <40Mbit/sec.
239 	   However, if paws works, it is reliable AND even more,
240 	   we even may relax silly seq space cutoff.
241 
242 	   RED-PEN: we violate main RFC requirement, if this SYN will appear
243 	   old duplicate (i.e. we receive RST in reply to SYN-ACK),
244 	   we must return socket to time-wait state. It is not good,
245 	   but not fatal yet.
246 	 */
247 
248 	if (th->syn && !th->rst && !th->ack && !paws_reject &&
249 	    (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
250 	     (tmp_opt.saw_tstamp &&
251 	      (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
252 		u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
253 		if (isn == 0)
254 			isn++;
255 		TCP_SKB_CB(skb)->tcp_tw_isn = isn;
256 		return TCP_TW_SYN;
257 	}
258 
259 	if (paws_reject)
260 		NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
261 
262 	if (!th->rst) {
263 		/* In this case we must reset the TIMEWAIT timer.
264 		 *
265 		 * If it is ACKless SYN it may be both old duplicate
266 		 * and new good SYN with random sequence number <rcv_nxt.
267 		 * Do not reschedule in the last case.
268 		 */
269 		if (paws_reject || th->ack)
270 			inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
271 					   TCP_TIMEWAIT_LEN);
272 
273 		return tcp_timewait_check_oow_rate_limit(
274 			tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT);
275 	}
276 	inet_twsk_put(tw);
277 	return TCP_TW_SUCCESS;
278 }
279 EXPORT_SYMBOL(tcp_timewait_state_process);
280 
281 /*
282  * Move a socket to time-wait or dead fin-wait-2 state.
283  */
284 void tcp_time_wait(struct sock *sk, int state, int timeo)
285 {
286 	struct inet_timewait_sock *tw = NULL;
287 	const struct inet_connection_sock *icsk = inet_csk(sk);
288 	const struct tcp_sock *tp = tcp_sk(sk);
289 	bool recycle_ok = false;
290 
291 	if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp)
292 		recycle_ok = tcp_remember_stamp(sk);
293 
294 	if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets)
295 		tw = inet_twsk_alloc(sk, state);
296 
297 	if (tw != NULL) {
298 		struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
299 		const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
300 		struct inet_sock *inet = inet_sk(sk);
301 
302 		tw->tw_transparent	= inet->transparent;
303 		tw->tw_rcv_wscale	= tp->rx_opt.rcv_wscale;
304 		tcptw->tw_rcv_nxt	= tp->rcv_nxt;
305 		tcptw->tw_snd_nxt	= tp->snd_nxt;
306 		tcptw->tw_rcv_wnd	= tcp_receive_window(tp);
307 		tcptw->tw_ts_recent	= tp->rx_opt.ts_recent;
308 		tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
309 		tcptw->tw_ts_offset	= tp->tsoffset;
310 		tcptw->tw_last_oow_ack_time = 0;
311 
312 #if IS_ENABLED(CONFIG_IPV6)
313 		if (tw->tw_family == PF_INET6) {
314 			struct ipv6_pinfo *np = inet6_sk(sk);
315 
316 			tw->tw_v6_daddr = sk->sk_v6_daddr;
317 			tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
318 			tw->tw_tclass = np->tclass;
319 			tw->tw_flowlabel = np->flow_label >> 12;
320 			tw->tw_ipv6only = sk->sk_ipv6only;
321 		}
322 #endif
323 
324 #ifdef CONFIG_TCP_MD5SIG
325 		/*
326 		 * The timewait bucket does not have the key DB from the
327 		 * sock structure. We just make a quick copy of the
328 		 * md5 key being used (if indeed we are using one)
329 		 * so the timewait ack generating code has the key.
330 		 */
331 		do {
332 			struct tcp_md5sig_key *key;
333 			tcptw->tw_md5_key = NULL;
334 			key = tp->af_specific->md5_lookup(sk, sk);
335 			if (key != NULL) {
336 				tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
337 				if (tcptw->tw_md5_key && !tcp_alloc_md5sig_pool())
338 					BUG();
339 			}
340 		} while (0);
341 #endif
342 
343 		/* Linkage updates. */
344 		__inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
345 
346 		/* Get the TIME_WAIT timeout firing. */
347 		if (timeo < rto)
348 			timeo = rto;
349 
350 		if (recycle_ok) {
351 			tw->tw_timeout = rto;
352 		} else {
353 			tw->tw_timeout = TCP_TIMEWAIT_LEN;
354 			if (state == TCP_TIME_WAIT)
355 				timeo = TCP_TIMEWAIT_LEN;
356 		}
357 
358 		inet_twsk_schedule(tw, &tcp_death_row, timeo,
359 				   TCP_TIMEWAIT_LEN);
360 		inet_twsk_put(tw);
361 	} else {
362 		/* Sorry, if we're out of memory, just CLOSE this
363 		 * socket up.  We've got bigger problems than
364 		 * non-graceful socket closings.
365 		 */
366 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW);
367 	}
368 
369 	tcp_update_metrics(sk);
370 	tcp_done(sk);
371 }
372 
373 void tcp_twsk_destructor(struct sock *sk)
374 {
375 #ifdef CONFIG_TCP_MD5SIG
376 	struct tcp_timewait_sock *twsk = tcp_twsk(sk);
377 
378 	if (twsk->tw_md5_key)
379 		kfree_rcu(twsk->tw_md5_key, rcu);
380 #endif
381 }
382 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
383 
384 void tcp_openreq_init_rwin(struct request_sock *req,
385 			   struct sock *sk, struct dst_entry *dst)
386 {
387 	struct inet_request_sock *ireq = inet_rsk(req);
388 	struct tcp_sock *tp = tcp_sk(sk);
389 	__u8 rcv_wscale;
390 	int mss = dst_metric_advmss(dst);
391 
392 	if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss)
393 		mss = tp->rx_opt.user_mss;
394 
395 	/* Set this up on the first call only */
396 	req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW);
397 
398 	/* limit the window selection if the user enforce a smaller rx buffer */
399 	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
400 	    (req->window_clamp > tcp_full_space(sk) || req->window_clamp == 0))
401 		req->window_clamp = tcp_full_space(sk);
402 
403 	/* tcp_full_space because it is guaranteed to be the first packet */
404 	tcp_select_initial_window(tcp_full_space(sk),
405 		mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
406 		&req->rcv_wnd,
407 		&req->window_clamp,
408 		ireq->wscale_ok,
409 		&rcv_wscale,
410 		dst_metric(dst, RTAX_INITRWND));
411 	ireq->rcv_wscale = rcv_wscale;
412 }
413 EXPORT_SYMBOL(tcp_openreq_init_rwin);
414 
415 static void tcp_ecn_openreq_child(struct tcp_sock *tp,
416 				  const struct request_sock *req)
417 {
418 	tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
419 }
420 
421 void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
422 {
423 	struct inet_connection_sock *icsk = inet_csk(sk);
424 	u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
425 	bool ca_got_dst = false;
426 
427 	if (ca_key != TCP_CA_UNSPEC) {
428 		const struct tcp_congestion_ops *ca;
429 
430 		rcu_read_lock();
431 		ca = tcp_ca_find_key(ca_key);
432 		if (likely(ca && try_module_get(ca->owner))) {
433 			icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
434 			icsk->icsk_ca_ops = ca;
435 			ca_got_dst = true;
436 		}
437 		rcu_read_unlock();
438 	}
439 
440 	if (!ca_got_dst && !try_module_get(icsk->icsk_ca_ops->owner))
441 		tcp_assign_congestion_control(sk);
442 
443 	tcp_set_ca_state(sk, TCP_CA_Open);
444 }
445 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child);
446 
447 /* This is not only more efficient than what we used to do, it eliminates
448  * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
449  *
450  * Actually, we could lots of memory writes here. tp of listening
451  * socket contains all necessary default parameters.
452  */
453 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
454 {
455 	struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
456 
457 	if (newsk != NULL) {
458 		const struct inet_request_sock *ireq = inet_rsk(req);
459 		struct tcp_request_sock *treq = tcp_rsk(req);
460 		struct inet_connection_sock *newicsk = inet_csk(newsk);
461 		struct tcp_sock *newtp = tcp_sk(newsk);
462 
463 		/* Now setup tcp_sock */
464 		newtp->pred_flags = 0;
465 
466 		newtp->rcv_wup = newtp->copied_seq =
467 		newtp->rcv_nxt = treq->rcv_isn + 1;
468 
469 		newtp->snd_sml = newtp->snd_una =
470 		newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;
471 
472 		tcp_prequeue_init(newtp);
473 		INIT_LIST_HEAD(&newtp->tsq_node);
474 
475 		tcp_init_wl(newtp, treq->rcv_isn);
476 
477 		newtp->srtt_us = 0;
478 		newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT);
479 		newicsk->icsk_rto = TCP_TIMEOUT_INIT;
480 
481 		newtp->packets_out = 0;
482 		newtp->retrans_out = 0;
483 		newtp->sacked_out = 0;
484 		newtp->fackets_out = 0;
485 		newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
486 		tcp_enable_early_retrans(newtp);
487 		newtp->tlp_high_seq = 0;
488 		newtp->lsndtime = treq->snt_synack;
489 		newtp->last_oow_ack_time = 0;
490 		newtp->total_retrans = req->num_retrans;
491 
492 		/* So many TCP implementations out there (incorrectly) count the
493 		 * initial SYN frame in their delayed-ACK and congestion control
494 		 * algorithms that we must have the following bandaid to talk
495 		 * efficiently to them.  -DaveM
496 		 */
497 		newtp->snd_cwnd = TCP_INIT_CWND;
498 		newtp->snd_cwnd_cnt = 0;
499 
500 		tcp_init_xmit_timers(newsk);
501 		__skb_queue_head_init(&newtp->out_of_order_queue);
502 		newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
503 
504 		newtp->rx_opt.saw_tstamp = 0;
505 
506 		newtp->rx_opt.dsack = 0;
507 		newtp->rx_opt.num_sacks = 0;
508 
509 		newtp->urg_data = 0;
510 
511 		if (sock_flag(newsk, SOCK_KEEPOPEN))
512 			inet_csk_reset_keepalive_timer(newsk,
513 						       keepalive_time_when(newtp));
514 
515 		newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
516 		if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
517 			if (sysctl_tcp_fack)
518 				tcp_enable_fack(newtp);
519 		}
520 		newtp->window_clamp = req->window_clamp;
521 		newtp->rcv_ssthresh = req->rcv_wnd;
522 		newtp->rcv_wnd = req->rcv_wnd;
523 		newtp->rx_opt.wscale_ok = ireq->wscale_ok;
524 		if (newtp->rx_opt.wscale_ok) {
525 			newtp->rx_opt.snd_wscale = ireq->snd_wscale;
526 			newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
527 		} else {
528 			newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
529 			newtp->window_clamp = min(newtp->window_clamp, 65535U);
530 		}
531 		newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
532 				  newtp->rx_opt.snd_wscale);
533 		newtp->max_window = newtp->snd_wnd;
534 
535 		if (newtp->rx_opt.tstamp_ok) {
536 			newtp->rx_opt.ts_recent = req->ts_recent;
537 			newtp->rx_opt.ts_recent_stamp = get_seconds();
538 			newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
539 		} else {
540 			newtp->rx_opt.ts_recent_stamp = 0;
541 			newtp->tcp_header_len = sizeof(struct tcphdr);
542 		}
543 		newtp->tsoffset = 0;
544 #ifdef CONFIG_TCP_MD5SIG
545 		newtp->md5sig_info = NULL;	/*XXX*/
546 		if (newtp->af_specific->md5_lookup(sk, newsk))
547 			newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
548 #endif
549 		if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
550 			newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
551 		newtp->rx_opt.mss_clamp = req->mss;
552 		tcp_ecn_openreq_child(newtp, req);
553 		newtp->fastopen_rsk = NULL;
554 		newtp->syn_data_acked = 0;
555 
556 		TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS);
557 	}
558 	return newsk;
559 }
560 EXPORT_SYMBOL(tcp_create_openreq_child);
561 
562 /*
563  * Process an incoming packet for SYN_RECV sockets represented as a
564  * request_sock. Normally sk is the listener socket but for TFO it
565  * points to the child socket.
566  *
567  * XXX (TFO) - The current impl contains a special check for ack
568  * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
569  *
570  * We don't need to initialize tmp_opt.sack_ok as we don't use the results
571  */
572 
573 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
574 			   struct request_sock *req,
575 			   struct request_sock **prev,
576 			   bool fastopen)
577 {
578 	struct tcp_options_received tmp_opt;
579 	struct sock *child;
580 	const struct tcphdr *th = tcp_hdr(skb);
581 	__be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
582 	bool paws_reject = false;
583 
584 	BUG_ON(fastopen == (sk->sk_state == TCP_LISTEN));
585 
586 	tmp_opt.saw_tstamp = 0;
587 	if (th->doff > (sizeof(struct tcphdr)>>2)) {
588 		tcp_parse_options(skb, &tmp_opt, 0, NULL);
589 
590 		if (tmp_opt.saw_tstamp) {
591 			tmp_opt.ts_recent = req->ts_recent;
592 			/* We do not store true stamp, but it is not required,
593 			 * it can be estimated (approximately)
594 			 * from another data.
595 			 */
596 			tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
597 			paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
598 		}
599 	}
600 
601 	/* Check for pure retransmitted SYN. */
602 	if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
603 	    flg == TCP_FLAG_SYN &&
604 	    !paws_reject) {
605 		/*
606 		 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
607 		 * this case on figure 6 and figure 8, but formal
608 		 * protocol description says NOTHING.
609 		 * To be more exact, it says that we should send ACK,
610 		 * because this segment (at least, if it has no data)
611 		 * is out of window.
612 		 *
613 		 *  CONCLUSION: RFC793 (even with RFC1122) DOES NOT
614 		 *  describe SYN-RECV state. All the description
615 		 *  is wrong, we cannot believe to it and should
616 		 *  rely only on common sense and implementation
617 		 *  experience.
618 		 *
619 		 * Enforce "SYN-ACK" according to figure 8, figure 6
620 		 * of RFC793, fixed by RFC1122.
621 		 *
622 		 * Note that even if there is new data in the SYN packet
623 		 * they will be thrown away too.
624 		 *
625 		 * Reset timer after retransmitting SYNACK, similar to
626 		 * the idea of fast retransmit in recovery.
627 		 */
628 		if (!tcp_oow_rate_limited(sock_net(sk), skb,
629 					  LINUX_MIB_TCPACKSKIPPEDSYNRECV,
630 					  &tcp_rsk(req)->last_oow_ack_time) &&
631 
632 		    !inet_rtx_syn_ack(sk, req))
633 			req->expires = min(TCP_TIMEOUT_INIT << req->num_timeout,
634 					   TCP_RTO_MAX) + jiffies;
635 		return NULL;
636 	}
637 
638 	/* Further reproduces section "SEGMENT ARRIVES"
639 	   for state SYN-RECEIVED of RFC793.
640 	   It is broken, however, it does not work only
641 	   when SYNs are crossed.
642 
643 	   You would think that SYN crossing is impossible here, since
644 	   we should have a SYN_SENT socket (from connect()) on our end,
645 	   but this is not true if the crossed SYNs were sent to both
646 	   ends by a malicious third party.  We must defend against this,
647 	   and to do that we first verify the ACK (as per RFC793, page
648 	   36) and reset if it is invalid.  Is this a true full defense?
649 	   To convince ourselves, let us consider a way in which the ACK
650 	   test can still pass in this 'malicious crossed SYNs' case.
651 	   Malicious sender sends identical SYNs (and thus identical sequence
652 	   numbers) to both A and B:
653 
654 		A: gets SYN, seq=7
655 		B: gets SYN, seq=7
656 
657 	   By our good fortune, both A and B select the same initial
658 	   send sequence number of seven :-)
659 
660 		A: sends SYN|ACK, seq=7, ack_seq=8
661 		B: sends SYN|ACK, seq=7, ack_seq=8
662 
663 	   So we are now A eating this SYN|ACK, ACK test passes.  So
664 	   does sequence test, SYN is truncated, and thus we consider
665 	   it a bare ACK.
666 
667 	   If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
668 	   bare ACK.  Otherwise, we create an established connection.  Both
669 	   ends (listening sockets) accept the new incoming connection and try
670 	   to talk to each other. 8-)
671 
672 	   Note: This case is both harmless, and rare.  Possibility is about the
673 	   same as us discovering intelligent life on another plant tomorrow.
674 
675 	   But generally, we should (RFC lies!) to accept ACK
676 	   from SYNACK both here and in tcp_rcv_state_process().
677 	   tcp_rcv_state_process() does not, hence, we do not too.
678 
679 	   Note that the case is absolutely generic:
680 	   we cannot optimize anything here without
681 	   violating protocol. All the checks must be made
682 	   before attempt to create socket.
683 	 */
684 
685 	/* RFC793 page 36: "If the connection is in any non-synchronized state ...
686 	 *                  and the incoming segment acknowledges something not yet
687 	 *                  sent (the segment carries an unacceptable ACK) ...
688 	 *                  a reset is sent."
689 	 *
690 	 * Invalid ACK: reset will be sent by listening socket.
691 	 * Note that the ACK validity check for a Fast Open socket is done
692 	 * elsewhere and is checked directly against the child socket rather
693 	 * than req because user data may have been sent out.
694 	 */
695 	if ((flg & TCP_FLAG_ACK) && !fastopen &&
696 	    (TCP_SKB_CB(skb)->ack_seq !=
697 	     tcp_rsk(req)->snt_isn + 1))
698 		return sk;
699 
700 	/* Also, it would be not so bad idea to check rcv_tsecr, which
701 	 * is essentially ACK extension and too early or too late values
702 	 * should cause reset in unsynchronized states.
703 	 */
704 
705 	/* RFC793: "first check sequence number". */
706 
707 	if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
708 					  tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rcv_wnd)) {
709 		/* Out of window: send ACK and drop. */
710 		if (!(flg & TCP_FLAG_RST))
711 			req->rsk_ops->send_ack(sk, skb, req);
712 		if (paws_reject)
713 			NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
714 		return NULL;
715 	}
716 
717 	/* In sequence, PAWS is OK. */
718 
719 	if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
720 		req->ts_recent = tmp_opt.rcv_tsval;
721 
722 	if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
723 		/* Truncate SYN, it is out of window starting
724 		   at tcp_rsk(req)->rcv_isn + 1. */
725 		flg &= ~TCP_FLAG_SYN;
726 	}
727 
728 	/* RFC793: "second check the RST bit" and
729 	 *	   "fourth, check the SYN bit"
730 	 */
731 	if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
732 		TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
733 		goto embryonic_reset;
734 	}
735 
736 	/* ACK sequence verified above, just make sure ACK is
737 	 * set.  If ACK not set, just silently drop the packet.
738 	 *
739 	 * XXX (TFO) - if we ever allow "data after SYN", the
740 	 * following check needs to be removed.
741 	 */
742 	if (!(flg & TCP_FLAG_ACK))
743 		return NULL;
744 
745 	/* For Fast Open no more processing is needed (sk is the
746 	 * child socket).
747 	 */
748 	if (fastopen)
749 		return sk;
750 
751 	/* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
752 	if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
753 	    TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
754 		inet_rsk(req)->acked = 1;
755 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
756 		return NULL;
757 	}
758 
759 	/* OK, ACK is valid, create big socket and
760 	 * feed this segment to it. It will repeat all
761 	 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
762 	 * ESTABLISHED STATE. If it will be dropped after
763 	 * socket is created, wait for troubles.
764 	 */
765 	child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL);
766 	if (child == NULL)
767 		goto listen_overflow;
768 
769 	inet_csk_reqsk_queue_unlink(sk, req, prev);
770 	inet_csk_reqsk_queue_removed(sk, req);
771 
772 	inet_csk_reqsk_queue_add(sk, req, child);
773 	return child;
774 
775 listen_overflow:
776 	if (!sysctl_tcp_abort_on_overflow) {
777 		inet_rsk(req)->acked = 1;
778 		return NULL;
779 	}
780 
781 embryonic_reset:
782 	if (!(flg & TCP_FLAG_RST)) {
783 		/* Received a bad SYN pkt - for TFO We try not to reset
784 		 * the local connection unless it's really necessary to
785 		 * avoid becoming vulnerable to outside attack aiming at
786 		 * resetting legit local connections.
787 		 */
788 		req->rsk_ops->send_reset(sk, skb);
789 	} else if (fastopen) { /* received a valid RST pkt */
790 		reqsk_fastopen_remove(sk, req, true);
791 		tcp_reset(sk);
792 	}
793 	if (!fastopen) {
794 		inet_csk_reqsk_queue_drop(sk, req, prev);
795 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
796 	}
797 	return NULL;
798 }
799 EXPORT_SYMBOL(tcp_check_req);
800 
801 /*
802  * Queue segment on the new socket if the new socket is active,
803  * otherwise we just shortcircuit this and continue with
804  * the new socket.
805  *
806  * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
807  * when entering. But other states are possible due to a race condition
808  * where after __inet_lookup_established() fails but before the listener
809  * locked is obtained, other packets cause the same connection to
810  * be created.
811  */
812 
813 int tcp_child_process(struct sock *parent, struct sock *child,
814 		      struct sk_buff *skb)
815 {
816 	int ret = 0;
817 	int state = child->sk_state;
818 
819 	if (!sock_owned_by_user(child)) {
820 		ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
821 					    skb->len);
822 		/* Wakeup parent, send SIGIO */
823 		if (state == TCP_SYN_RECV && child->sk_state != state)
824 			parent->sk_data_ready(parent);
825 	} else {
826 		/* Alas, it is possible again, because we do lookup
827 		 * in main socket hash table and lock on listening
828 		 * socket does not protect us more.
829 		 */
830 		__sk_add_backlog(child, skb);
831 	}
832 
833 	bh_unlock_sock(child);
834 	sock_put(child);
835 	return ret;
836 }
837 EXPORT_SYMBOL(tcp_child_process);
838