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
17static 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
45static 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 */
61u64 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
84static __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 */
113static __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 */
143static __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 */
154u32 __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}
169EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
170
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 */
183int __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}
194EXPORT_SYMBOL_GPL(__cookie_v4_check);
195
196struct 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}
225EXPORT_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 */
235bool 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}
262EXPORT_SYMBOL(cookie_timestamp_decode);
263
264static 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)
299struct 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}
313EXPORT_SYMBOL_GPL(cookie_bpf_check);
314#endif
315
316struct 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}
354EXPORT_SYMBOL_GPL(cookie_tcp_reqsk_alloc);
355
356static 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);
390out:
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 */
398struct 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 = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
466	/* limit the window selection if the user enforce a smaller rx buffer */
467	full_space = tcp_full_space(sk);
468	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
469	    (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
470		req->rsk_window_clamp = full_space;
471
472	tcp_select_initial_window(sk, full_space, req->mss,
473				  &req->rsk_rcv_wnd, &req->rsk_window_clamp,
474				  ireq->wscale_ok, &rcv_wscale,
475				  dst_metric(&rt->dst, RTAX_INITRWND));
476
477	/* req->syncookie is set true only if ACK is validated
478	 * by BPF kfunc, then, rcv_wscale is already configured.
479	 */
480	if (!req->syncookie)
481		ireq->rcv_wscale = rcv_wscale;
482	ireq->ecn_ok &= cookie_ecn_ok(net, &rt->dst);
483
484	ret = tcp_get_cookie_sock(sk, skb, req, &rt->dst);
485	/* ip_queue_xmit() depends on our flow being setup
486	 * Normal sockets get it right from inet_csk_route_child_sock()
487	 */
488	if (!ret) {
489		SKB_DR_SET(reason, NO_SOCKET);
490		goto out_drop;
491	}
492	inet_sk(ret)->cork.fl.u.ip4 = fl4;
493out:
494	return ret;
495out_free:
496	reqsk_free(req);
497out_drop:
498	kfree_skb_reason(skb, reason);
499	return NULL;
500}
501