1/*
2 *  Syncookies implementation for the Linux kernel
3 *
4 *  Copyright (C) 1997 Andi Kleen
5 *  Based on ideas by D.J.Bernstein and Eric Schenk.
6 *
7 *	This program is free software; you can redistribute it and/or
8 *      modify it under the terms of the GNU General Public License
9 *      as published by the Free Software Foundation; either version
10 *      2 of the License, or (at your option) any later version.
11 */
12
13#include <linux/tcp.h>
14#include <linux/slab.h>
15#include <linux/random.h>
16#include <linux/cryptohash.h>
17#include <linux/kernel.h>
18#include <linux/export.h>
19#include <net/tcp.h>
20#include <net/route.h>
21
22extern int sysctl_tcp_syncookies;
23
24static u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS] __read_mostly;
25
26#define COOKIEBITS 24	/* Upper bits store count */
27#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
28
29/* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
30 * stores TCP options:
31 *
32 * MSB                               LSB
33 * | 31 ...   6 |  5  |  4   | 3 2 1 0 |
34 * |  Timestamp | ECN | SACK | WScale  |
35 *
36 * When we receive a valid cookie-ACK, we look at the echoed tsval (if
37 * any) to figure out which TCP options we should use for the rebuilt
38 * connection.
39 *
40 * A WScale setting of '0xf' (which is an invalid scaling value)
41 * means that original syn did not include the TCP window scaling option.
42 */
43#define TS_OPT_WSCALE_MASK	0xf
44#define TS_OPT_SACK		BIT(4)
45#define TS_OPT_ECN		BIT(5)
46/* There is no TS_OPT_TIMESTAMP:
47 * if ACK contains timestamp option, we already know it was
48 * requested/supported by the syn/synack exchange.
49 */
50#define TSBITS	6
51#define TSMASK	(((__u32)1 << TSBITS) - 1)
52
53static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
54		      ipv4_cookie_scratch);
55
56static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
57		       u32 count, int c)
58{
59	__u32 *tmp;
60
61	net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
62
63	tmp  = this_cpu_ptr(ipv4_cookie_scratch);
64	memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
65	tmp[0] = (__force u32)saddr;
66	tmp[1] = (__force u32)daddr;
67	tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
68	tmp[3] = count;
69	sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
70
71	return tmp[17];
72}
73
74
75/*
76 * when syncookies are in effect and tcp timestamps are enabled we encode
77 * tcp options in the lower bits of the timestamp value that will be
78 * sent in the syn-ack.
79 * Since subsequent timestamps use the normal tcp_time_stamp value, we
80 * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
81 */
82__u32 cookie_init_timestamp(struct request_sock *req)
83{
84	struct inet_request_sock *ireq;
85	u32 ts, ts_now = tcp_time_stamp;
86	u32 options = 0;
87
88	ireq = inet_rsk(req);
89
90	options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
91	if (ireq->sack_ok)
92		options |= TS_OPT_SACK;
93	if (ireq->ecn_ok)
94		options |= TS_OPT_ECN;
95
96	ts = ts_now & ~TSMASK;
97	ts |= options;
98	if (ts > ts_now) {
99		ts >>= TSBITS;
100		ts--;
101		ts <<= TSBITS;
102		ts |= options;
103	}
104	return ts;
105}
106
107
108static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
109				   __be16 dport, __u32 sseq, __u32 data)
110{
111	/*
112	 * Compute the secure sequence number.
113	 * The output should be:
114	 *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
115	 *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
116	 * Where sseq is their sequence number and count increases every
117	 * minute by 1.
118	 * As an extra hack, we add a small "data" value that encodes the
119	 * MSS into the second hash value.
120	 */
121	u32 count = tcp_cookie_time();
122	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
123		sseq + (count << COOKIEBITS) +
124		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
125		 & COOKIEMASK));
126}
127
128/*
129 * This retrieves the small "data" value from the syncookie.
130 * If the syncookie is bad, the data returned will be out of
131 * range.  This must be checked by the caller.
132 *
133 * The count value used to generate the cookie must be less than
134 * MAX_SYNCOOKIE_AGE minutes in the past.
135 * The return value (__u32)-1 if this test fails.
136 */
137static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
138				  __be16 sport, __be16 dport, __u32 sseq)
139{
140	u32 diff, count = tcp_cookie_time();
141
142	/* Strip away the layers from the cookie */
143	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
144
145	/* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
146	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
147	if (diff >= MAX_SYNCOOKIE_AGE)
148		return (__u32)-1;
149
150	return (cookie -
151		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
152		& COOKIEMASK;	/* Leaving the data behind */
153}
154
155/*
156 * MSS Values are chosen based on the 2011 paper
157 * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson.
158 * Values ..
159 *  .. lower than 536 are rare (< 0.2%)
160 *  .. between 537 and 1299 account for less than < 1.5% of observed values
161 *  .. in the 1300-1349 range account for about 15 to 20% of observed mss values
162 *  .. exceeding 1460 are very rare (< 0.04%)
163 *
164 *  1460 is the single most frequently announced mss value (30 to 46% depending
165 *  on monitor location).  Table must be sorted.
166 */
167static __u16 const msstab[] = {
168	536,
169	1300,
170	1440,	/* 1440, 1452: PPPoE */
171	1460,
172};
173
174/*
175 * Generate a syncookie.  mssp points to the mss, which is returned
176 * rounded down to the value encoded in the cookie.
177 */
178u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
179			      u16 *mssp)
180{
181	int mssind;
182	const __u16 mss = *mssp;
183
184	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
185		if (mss >= msstab[mssind])
186			break;
187	*mssp = msstab[mssind];
188
189	return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
190				     th->source, th->dest, ntohl(th->seq),
191				     mssind);
192}
193EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
194
195__u32 cookie_v4_init_sequence(struct sock *sk, const struct sk_buff *skb,
196			      __u16 *mssp)
197{
198	const struct iphdr *iph = ip_hdr(skb);
199	const struct tcphdr *th = tcp_hdr(skb);
200
201	tcp_synq_overflow(sk);
202	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
203
204	return __cookie_v4_init_sequence(iph, th, mssp);
205}
206
207/*
208 * Check if a ack sequence number is a valid syncookie.
209 * Return the decoded mss if it is, or 0 if not.
210 */
211int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th,
212		      u32 cookie)
213{
214	__u32 seq = ntohl(th->seq) - 1;
215	__u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
216					    th->source, th->dest, seq);
217
218	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
219}
220EXPORT_SYMBOL_GPL(__cookie_v4_check);
221
222static struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
223				    struct request_sock *req,
224				    struct dst_entry *dst)
225{
226	struct inet_connection_sock *icsk = inet_csk(sk);
227	struct sock *child;
228
229	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
230	if (child) {
231		atomic_set(&req->rsk_refcnt, 1);
232		inet_csk_reqsk_queue_add(sk, req, child);
233	} else {
234		reqsk_free(req);
235	}
236	return child;
237}
238
239
240/*
241 * when syncookies are in effect and tcp timestamps are enabled we stored
242 * additional tcp options in the timestamp.
243 * This extracts these options from the timestamp echo.
244 *
245 * return false if we decode a tcp option that is disabled
246 * on the host.
247 */
248bool cookie_timestamp_decode(struct tcp_options_received *tcp_opt)
249{
250	/* echoed timestamp, lowest bits contain options */
251	u32 options = tcp_opt->rcv_tsecr;
252
253	if (!tcp_opt->saw_tstamp)  {
254		tcp_clear_options(tcp_opt);
255		return true;
256	}
257
258	if (!sysctl_tcp_timestamps)
259		return false;
260
261	tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;
262
263	if (tcp_opt->sack_ok && !sysctl_tcp_sack)
264		return false;
265
266	if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK)
267		return true; /* no window scaling */
268
269	tcp_opt->wscale_ok = 1;
270	tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK;
271
272	return sysctl_tcp_window_scaling != 0;
273}
274EXPORT_SYMBOL(cookie_timestamp_decode);
275
276bool cookie_ecn_ok(const struct tcp_options_received *tcp_opt,
277		   const struct net *net, const struct dst_entry *dst)
278{
279	bool ecn_ok = tcp_opt->rcv_tsecr & TS_OPT_ECN;
280
281	if (!ecn_ok)
282		return false;
283
284	if (net->ipv4.sysctl_tcp_ecn)
285		return true;
286
287	return dst_feature(dst, RTAX_FEATURE_ECN);
288}
289EXPORT_SYMBOL(cookie_ecn_ok);
290
291struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
292{
293	struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt;
294	struct tcp_options_received tcp_opt;
295	struct inet_request_sock *ireq;
296	struct tcp_request_sock *treq;
297	struct tcp_sock *tp = tcp_sk(sk);
298	const struct tcphdr *th = tcp_hdr(skb);
299	__u32 cookie = ntohl(th->ack_seq) - 1;
300	struct sock *ret = sk;
301	struct request_sock *req;
302	int mss;
303	struct rtable *rt;
304	__u8 rcv_wscale;
305	struct flowi4 fl4;
306
307	if (!sysctl_tcp_syncookies || !th->ack || th->rst)
308		goto out;
309
310	if (tcp_synq_no_recent_overflow(sk))
311		goto out;
312
313	mss = __cookie_v4_check(ip_hdr(skb), th, cookie);
314	if (mss == 0) {
315		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
316		goto out;
317	}
318
319	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
320
321	/* check for timestamp cookie support */
322	memset(&tcp_opt, 0, sizeof(tcp_opt));
323	tcp_parse_options(skb, &tcp_opt, 0, NULL);
324
325	if (!cookie_timestamp_decode(&tcp_opt))
326		goto out;
327
328	ret = NULL;
329	req = inet_reqsk_alloc(&tcp_request_sock_ops, sk); /* for safety */
330	if (!req)
331		goto out;
332
333	ireq = inet_rsk(req);
334	treq = tcp_rsk(req);
335	treq->rcv_isn		= ntohl(th->seq) - 1;
336	treq->snt_isn		= cookie;
337	req->mss		= mss;
338	ireq->ir_num		= ntohs(th->dest);
339	ireq->ir_rmt_port	= th->source;
340	sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr);
341	sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr);
342	ireq->ir_mark		= inet_request_mark(sk, skb);
343	ireq->snd_wscale	= tcp_opt.snd_wscale;
344	ireq->sack_ok		= tcp_opt.sack_ok;
345	ireq->wscale_ok		= tcp_opt.wscale_ok;
346	ireq->tstamp_ok		= tcp_opt.saw_tstamp;
347	req->ts_recent		= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
348	treq->snt_synack	= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
349	treq->tfo_listener	= false;
350
351	ireq->ir_iif = sk->sk_bound_dev_if;
352
353	/* We throwed the options of the initial SYN away, so we hope
354	 * the ACK carries the same options again (see RFC1122 4.2.3.8)
355	 */
356	ireq->opt = tcp_v4_save_options(skb);
357
358	if (security_inet_conn_request(sk, skb, req)) {
359		reqsk_free(req);
360		goto out;
361	}
362
363	req->num_retrans = 0;
364
365	/*
366	 * We need to lookup the route here to get at the correct
367	 * window size. We should better make sure that the window size
368	 * hasn't changed since we received the original syn, but I see
369	 * no easy way to do this.
370	 */
371	flowi4_init_output(&fl4, sk->sk_bound_dev_if, ireq->ir_mark,
372			   RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
373			   inet_sk_flowi_flags(sk),
374			   opt->srr ? opt->faddr : ireq->ir_rmt_addr,
375			   ireq->ir_loc_addr, th->source, th->dest);
376	security_req_classify_flow(req, flowi4_to_flowi(&fl4));
377	rt = ip_route_output_key(sock_net(sk), &fl4);
378	if (IS_ERR(rt)) {
379		reqsk_free(req);
380		goto out;
381	}
382
383	/* Try to redo what tcp_v4_send_synack did. */
384	req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
385
386	tcp_select_initial_window(tcp_full_space(sk), req->mss,
387				  &req->rcv_wnd, &req->window_clamp,
388				  ireq->wscale_ok, &rcv_wscale,
389				  dst_metric(&rt->dst, RTAX_INITRWND));
390
391	ireq->rcv_wscale  = rcv_wscale;
392	ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), &rt->dst);
393
394	ret = get_cookie_sock(sk, skb, req, &rt->dst);
395	/* ip_queue_xmit() depends on our flow being setup
396	 * Normal sockets get it right from inet_csk_route_child_sock()
397	 */
398	if (ret)
399		inet_sk(ret)->cork.fl.u.ip4 = fl4;
400out:	return ret;
401}
402