xref: /linux/net/ipv4/syncookies.c (revision a1ff5a7d78a036d6c2178ee5acd6ba4946243800)
1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /*
3  *  Syncookies implementation for the Linux kernel
4  *
5  *  Copyright (C) 1997 Andi Kleen
6  *  Based on ideas by D.J.Bernstein and Eric Schenk.
7  */
8 
9 #include <linux/tcp.h>
10 #include <linux/siphash.h>
11 #include <linux/kernel.h>
12 #include <linux/export.h>
13 #include <net/secure_seq.h>
14 #include <net/tcp.h>
15 #include <net/route.h>
16 
17 static siphash_aligned_key_t syncookie_secret[2];
18 
19 #define COOKIEBITS 24	/* Upper bits store count */
20 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
21 
22 /* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
23  * stores TCP options:
24  *
25  * MSB                               LSB
26  * | 31 ...   6 |  5  |  4   | 3 2 1 0 |
27  * |  Timestamp | ECN | SACK | WScale  |
28  *
29  * When we receive a valid cookie-ACK, we look at the echoed tsval (if
30  * any) to figure out which TCP options we should use for the rebuilt
31  * connection.
32  *
33  * A WScale setting of '0xf' (which is an invalid scaling value)
34  * means that original syn did not include the TCP window scaling option.
35  */
36 #define TS_OPT_WSCALE_MASK	0xf
37 #define TS_OPT_SACK		BIT(4)
38 #define TS_OPT_ECN		BIT(5)
39 /* There is no TS_OPT_TIMESTAMP:
40  * if ACK contains timestamp option, we already know it was
41  * requested/supported by the syn/synack exchange.
42  */
43 #define TSBITS	6
44 
cookie_hash(__be32 saddr,__be32 daddr,__be16 sport,__be16 dport,u32 count,int c)45 static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
46 		       u32 count, int c)
47 {
48 	net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
49 	return siphash_4u32((__force u32)saddr, (__force u32)daddr,
50 			    (__force u32)sport << 16 | (__force u32)dport,
51 			    count, &syncookie_secret[c]);
52 }
53 
54 /*
55  * when syncookies are in effect and tcp timestamps are enabled we encode
56  * tcp options in the lower bits of the timestamp value that will be
57  * sent in the syn-ack.
58  * Since subsequent timestamps use the normal tcp_time_stamp value, we
59  * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
60  */
cookie_init_timestamp(struct request_sock * req,u64 now)61 u64 cookie_init_timestamp(struct request_sock *req, u64 now)
62 {
63 	const struct inet_request_sock *ireq = inet_rsk(req);
64 	u64 ts, ts_now = tcp_ns_to_ts(false, now);
65 	u32 options = 0;
66 
67 	options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
68 	if (ireq->sack_ok)
69 		options |= TS_OPT_SACK;
70 	if (ireq->ecn_ok)
71 		options |= TS_OPT_ECN;
72 
73 	ts = (ts_now >> TSBITS) << TSBITS;
74 	ts |= options;
75 	if (ts > ts_now)
76 		ts -= (1UL << TSBITS);
77 
78 	if (tcp_rsk(req)->req_usec_ts)
79 		return ts * NSEC_PER_USEC;
80 	return ts * NSEC_PER_MSEC;
81 }
82 
83 
secure_tcp_syn_cookie(__be32 saddr,__be32 daddr,__be16 sport,__be16 dport,__u32 sseq,__u32 data)84 static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
85 				   __be16 dport, __u32 sseq, __u32 data)
86 {
87 	/*
88 	 * Compute the secure sequence number.
89 	 * The output should be:
90 	 *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
91 	 *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
92 	 * Where sseq is their sequence number and count increases every
93 	 * minute by 1.
94 	 * As an extra hack, we add a small "data" value that encodes the
95 	 * MSS into the second hash value.
96 	 */
97 	u32 count = tcp_cookie_time();
98 	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
99 		sseq + (count << COOKIEBITS) +
100 		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
101 		 & COOKIEMASK));
102 }
103 
104 /*
105  * This retrieves the small "data" value from the syncookie.
106  * If the syncookie is bad, the data returned will be out of
107  * range.  This must be checked by the caller.
108  *
109  * The count value used to generate the cookie must be less than
110  * MAX_SYNCOOKIE_AGE minutes in the past.
111  * The return value (__u32)-1 if this test fails.
112  */
check_tcp_syn_cookie(__u32 cookie,__be32 saddr,__be32 daddr,__be16 sport,__be16 dport,__u32 sseq)113 static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
114 				  __be16 sport, __be16 dport, __u32 sseq)
115 {
116 	u32 diff, count = tcp_cookie_time();
117 
118 	/* Strip away the layers from the cookie */
119 	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
120 
121 	/* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
122 	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
123 	if (diff >= MAX_SYNCOOKIE_AGE)
124 		return (__u32)-1;
125 
126 	return (cookie -
127 		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
128 		& COOKIEMASK;	/* Leaving the data behind */
129 }
130 
131 /*
132  * MSS Values are chosen based on the 2011 paper
133  * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson.
134  * Values ..
135  *  .. lower than 536 are rare (< 0.2%)
136  *  .. between 537 and 1299 account for less than < 1.5% of observed values
137  *  .. in the 1300-1349 range account for about 15 to 20% of observed mss values
138  *  .. exceeding 1460 are very rare (< 0.04%)
139  *
140  *  1460 is the single most frequently announced mss value (30 to 46% depending
141  *  on monitor location).  Table must be sorted.
142  */
143 static __u16 const msstab[] = {
144 	536,
145 	1300,
146 	1440,	/* 1440, 1452: PPPoE */
147 	1460,
148 };
149 
150 /*
151  * Generate a syncookie.  mssp points to the mss, which is returned
152  * rounded down to the value encoded in the cookie.
153  */
__cookie_v4_init_sequence(const struct iphdr * iph,const struct tcphdr * th,u16 * mssp)154 u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
155 			      u16 *mssp)
156 {
157 	int mssind;
158 	const __u16 mss = *mssp;
159 
160 	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
161 		if (mss >= msstab[mssind])
162 			break;
163 	*mssp = msstab[mssind];
164 
165 	return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
166 				     th->source, th->dest, ntohl(th->seq),
167 				     mssind);
168 }
169 EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
170 
cookie_v4_init_sequence(const struct sk_buff * skb,__u16 * mssp)171 __u32 cookie_v4_init_sequence(const struct sk_buff *skb, __u16 *mssp)
172 {
173 	const struct iphdr *iph = ip_hdr(skb);
174 	const struct tcphdr *th = tcp_hdr(skb);
175 
176 	return __cookie_v4_init_sequence(iph, th, mssp);
177 }
178 
179 /*
180  * Check if a ack sequence number is a valid syncookie.
181  * Return the decoded mss if it is, or 0 if not.
182  */
__cookie_v4_check(const struct iphdr * iph,const struct tcphdr * th)183 int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th)
184 {
185 	__u32 cookie = ntohl(th->ack_seq) - 1;
186 	__u32 seq = ntohl(th->seq) - 1;
187 	__u32 mssind;
188 
189 	mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
190 				      th->source, th->dest, seq);
191 
192 	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
193 }
194 EXPORT_SYMBOL_GPL(__cookie_v4_check);
195 
tcp_get_cookie_sock(struct sock * sk,struct sk_buff * skb,struct request_sock * req,struct dst_entry * dst)196 struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb,
197 				 struct request_sock *req,
198 				 struct dst_entry *dst)
199 {
200 	struct inet_connection_sock *icsk = inet_csk(sk);
201 	struct sock *child;
202 	bool own_req;
203 
204 	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
205 						 NULL, &own_req);
206 	if (child) {
207 		refcount_set(&req->rsk_refcnt, 1);
208 		sock_rps_save_rxhash(child, skb);
209 
210 		if (rsk_drop_req(req)) {
211 			reqsk_put(req);
212 			return child;
213 		}
214 
215 		if (inet_csk_reqsk_queue_add(sk, req, child))
216 			return child;
217 
218 		bh_unlock_sock(child);
219 		sock_put(child);
220 	}
221 	__reqsk_free(req);
222 
223 	return NULL;
224 }
225 EXPORT_SYMBOL(tcp_get_cookie_sock);
226 
227 /*
228  * when syncookies are in effect and tcp timestamps are enabled we stored
229  * additional tcp options in the timestamp.
230  * This extracts these options from the timestamp echo.
231  *
232  * return false if we decode a tcp option that is disabled
233  * on the host.
234  */
cookie_timestamp_decode(const struct net * net,struct tcp_options_received * tcp_opt)235 bool cookie_timestamp_decode(const struct net *net,
236 			     struct tcp_options_received *tcp_opt)
237 {
238 	/* echoed timestamp, lowest bits contain options */
239 	u32 options = tcp_opt->rcv_tsecr;
240 
241 	if (!tcp_opt->saw_tstamp)  {
242 		tcp_clear_options(tcp_opt);
243 		return true;
244 	}
245 
246 	if (!READ_ONCE(net->ipv4.sysctl_tcp_timestamps))
247 		return false;
248 
249 	tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;
250 
251 	if (tcp_opt->sack_ok && !READ_ONCE(net->ipv4.sysctl_tcp_sack))
252 		return false;
253 
254 	if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK)
255 		return true; /* no window scaling */
256 
257 	tcp_opt->wscale_ok = 1;
258 	tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK;
259 
260 	return READ_ONCE(net->ipv4.sysctl_tcp_window_scaling) != 0;
261 }
262 EXPORT_SYMBOL(cookie_timestamp_decode);
263 
cookie_tcp_reqsk_init(struct sock * sk,struct sk_buff * skb,struct request_sock * req)264 static int cookie_tcp_reqsk_init(struct sock *sk, struct sk_buff *skb,
265 				 struct request_sock *req)
266 {
267 	struct inet_request_sock *ireq = inet_rsk(req);
268 	struct tcp_request_sock *treq = tcp_rsk(req);
269 	const struct tcphdr *th = tcp_hdr(skb);
270 
271 	req->num_retrans = 0;
272 
273 	ireq->ir_num = ntohs(th->dest);
274 	ireq->ir_rmt_port = th->source;
275 	ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
276 	ireq->ir_mark = inet_request_mark(sk, skb);
277 
278 	if (IS_ENABLED(CONFIG_SMC))
279 		ireq->smc_ok = 0;
280 
281 	treq->snt_synack = 0;
282 	treq->tfo_listener = false;
283 	treq->txhash = net_tx_rndhash();
284 	treq->rcv_isn = ntohl(th->seq) - 1;
285 	treq->snt_isn = ntohl(th->ack_seq) - 1;
286 	treq->syn_tos = TCP_SKB_CB(skb)->ip_dsfield;
287 	treq->req_usec_ts = false;
288 
289 #if IS_ENABLED(CONFIG_MPTCP)
290 	treq->is_mptcp = sk_is_mptcp(sk);
291 	if (treq->is_mptcp)
292 		return mptcp_subflow_init_cookie_req(req, sk, skb);
293 #endif
294 
295 	return 0;
296 }
297 
298 #if IS_ENABLED(CONFIG_BPF)
cookie_bpf_check(struct sock * sk,struct sk_buff * skb)299 struct request_sock *cookie_bpf_check(struct sock *sk, struct sk_buff *skb)
300 {
301 	struct request_sock *req = inet_reqsk(skb->sk);
302 
303 	skb->sk = NULL;
304 	skb->destructor = NULL;
305 
306 	if (cookie_tcp_reqsk_init(sk, skb, req)) {
307 		reqsk_free(req);
308 		req = NULL;
309 	}
310 
311 	return req;
312 }
313 EXPORT_SYMBOL_GPL(cookie_bpf_check);
314 #endif
315 
cookie_tcp_reqsk_alloc(const struct request_sock_ops * ops,struct sock * sk,struct sk_buff * skb,struct tcp_options_received * tcp_opt,int mss,u32 tsoff)316 struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
317 					    struct sock *sk, struct sk_buff *skb,
318 					    struct tcp_options_received *tcp_opt,
319 					    int mss, u32 tsoff)
320 {
321 	struct inet_request_sock *ireq;
322 	struct tcp_request_sock *treq;
323 	struct request_sock *req;
324 
325 	if (sk_is_mptcp(sk))
326 		req = mptcp_subflow_reqsk_alloc(ops, sk, false);
327 	else
328 		req = inet_reqsk_alloc(ops, sk, false);
329 
330 	if (!req)
331 		return NULL;
332 
333 	if (cookie_tcp_reqsk_init(sk, skb, req)) {
334 		reqsk_free(req);
335 		return NULL;
336 	}
337 
338 	ireq = inet_rsk(req);
339 	treq = tcp_rsk(req);
340 
341 	req->mss = mss;
342 	req->ts_recent = tcp_opt->saw_tstamp ? tcp_opt->rcv_tsval : 0;
343 
344 	ireq->snd_wscale = tcp_opt->snd_wscale;
345 	ireq->tstamp_ok = tcp_opt->saw_tstamp;
346 	ireq->sack_ok = tcp_opt->sack_ok;
347 	ireq->wscale_ok = tcp_opt->wscale_ok;
348 	ireq->ecn_ok = !!(tcp_opt->rcv_tsecr & TS_OPT_ECN);
349 
350 	treq->ts_off = tsoff;
351 
352 	return req;
353 }
354 EXPORT_SYMBOL_GPL(cookie_tcp_reqsk_alloc);
355 
cookie_tcp_check(struct net * net,struct sock * sk,struct sk_buff * skb)356 static struct request_sock *cookie_tcp_check(struct net *net, struct sock *sk,
357 					     struct sk_buff *skb)
358 {
359 	struct tcp_options_received tcp_opt;
360 	u32 tsoff = 0;
361 	int mss;
362 
363 	if (tcp_synq_no_recent_overflow(sk))
364 		goto out;
365 
366 	mss = __cookie_v4_check(ip_hdr(skb), tcp_hdr(skb));
367 	if (!mss) {
368 		__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESFAILED);
369 		goto out;
370 	}
371 
372 	__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESRECV);
373 
374 	/* check for timestamp cookie support */
375 	memset(&tcp_opt, 0, sizeof(tcp_opt));
376 	tcp_parse_options(net, skb, &tcp_opt, 0, NULL);
377 
378 	if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
379 		tsoff = secure_tcp_ts_off(net,
380 					  ip_hdr(skb)->daddr,
381 					  ip_hdr(skb)->saddr);
382 		tcp_opt.rcv_tsecr -= tsoff;
383 	}
384 
385 	if (!cookie_timestamp_decode(net, &tcp_opt))
386 		goto out;
387 
388 	return cookie_tcp_reqsk_alloc(&tcp_request_sock_ops, sk, skb,
389 				      &tcp_opt, mss, tsoff);
390 out:
391 	return ERR_PTR(-EINVAL);
392 }
393 
394 /* On input, sk is a listener.
395  * Output is listener if incoming packet would not create a child
396  *           NULL if memory could not be allocated.
397  */
cookie_v4_check(struct sock * sk,struct sk_buff * skb)398 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
399 {
400 	struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt;
401 	const struct tcphdr *th = tcp_hdr(skb);
402 	struct tcp_sock *tp = tcp_sk(sk);
403 	struct inet_request_sock *ireq;
404 	struct net *net = sock_net(sk);
405 	struct request_sock *req;
406 	struct sock *ret = sk;
407 	struct flowi4 fl4;
408 	struct rtable *rt;
409 	__u8 rcv_wscale;
410 	int full_space;
411 	SKB_DR(reason);
412 
413 	if (!READ_ONCE(net->ipv4.sysctl_tcp_syncookies) ||
414 	    !th->ack || th->rst)
415 		goto out;
416 
417 	if (cookie_bpf_ok(skb)) {
418 		req = cookie_bpf_check(sk, skb);
419 	} else {
420 		req = cookie_tcp_check(net, sk, skb);
421 		if (IS_ERR(req))
422 			goto out;
423 	}
424 	if (!req) {
425 		SKB_DR_SET(reason, NO_SOCKET);
426 		goto out_drop;
427 	}
428 
429 	ireq = inet_rsk(req);
430 
431 	sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr);
432 	sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr);
433 
434 	/* We throwed the options of the initial SYN away, so we hope
435 	 * the ACK carries the same options again (see RFC1122 4.2.3.8)
436 	 */
437 	RCU_INIT_POINTER(ireq->ireq_opt, tcp_v4_save_options(net, skb));
438 
439 	if (security_inet_conn_request(sk, skb, req)) {
440 		SKB_DR_SET(reason, SECURITY_HOOK);
441 		goto out_free;
442 	}
443 
444 	tcp_ao_syncookie(sk, skb, req, AF_INET);
445 
446 	/*
447 	 * We need to lookup the route here to get at the correct
448 	 * window size. We should better make sure that the window size
449 	 * hasn't changed since we received the original syn, but I see
450 	 * no easy way to do this.
451 	 */
452 	flowi4_init_output(&fl4, ireq->ir_iif, ireq->ir_mark,
453 			   ip_sock_rt_tos(sk), ip_sock_rt_scope(sk),
454 			   IPPROTO_TCP, inet_sk_flowi_flags(sk),
455 			   opt->srr ? opt->faddr : ireq->ir_rmt_addr,
456 			   ireq->ir_loc_addr, th->source, th->dest, sk->sk_uid);
457 	security_req_classify_flow(req, flowi4_to_flowi_common(&fl4));
458 	rt = ip_route_output_key(net, &fl4);
459 	if (IS_ERR(rt)) {
460 		SKB_DR_SET(reason, IP_OUTNOROUTES);
461 		goto out_free;
462 	}
463 
464 	/* Try to redo what tcp_v4_send_synack did. */
465 	req->rsk_window_clamp = READ_ONCE(tp->window_clamp) ? :
466 				dst_metric(&rt->dst, RTAX_WINDOW);
467 	/* limit the window selection if the user enforce a smaller rx buffer */
468 	full_space = tcp_full_space(sk);
469 	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
470 	    (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
471 		req->rsk_window_clamp = full_space;
472 
473 	tcp_select_initial_window(sk, full_space, req->mss,
474 				  &req->rsk_rcv_wnd, &req->rsk_window_clamp,
475 				  ireq->wscale_ok, &rcv_wscale,
476 				  dst_metric(&rt->dst, RTAX_INITRWND));
477 
478 	/* req->syncookie is set true only if ACK is validated
479 	 * by BPF kfunc, then, rcv_wscale is already configured.
480 	 */
481 	if (!req->syncookie)
482 		ireq->rcv_wscale = rcv_wscale;
483 	ireq->ecn_ok &= cookie_ecn_ok(net, &rt->dst);
484 
485 	ret = tcp_get_cookie_sock(sk, skb, req, &rt->dst);
486 	/* ip_queue_xmit() depends on our flow being setup
487 	 * Normal sockets get it right from inet_csk_route_child_sock()
488 	 */
489 	if (!ret) {
490 		SKB_DR_SET(reason, NO_SOCKET);
491 		goto out_drop;
492 	}
493 	inet_sk(ret)->cork.fl.u.ip4 = fl4;
494 out:
495 	return ret;
496 out_free:
497 	reqsk_free(req);
498 out_drop:
499 	sk_skb_reason_drop(sk, skb, reason);
500 	return NULL;
501 }
502