xref: /openbmc/linux/net/ipv4/syncookies.c (revision 95e9fd10) 
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 
22 /* Timestamps: lowest bits store TCP options */
23 #define TSBITS 6
24 #define TSMASK (((__u32)1 << TSBITS) - 1)
25 
26 extern int sysctl_tcp_syncookies;
27 
28 __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
29 EXPORT_SYMBOL(syncookie_secret);
30 
31 static __init int init_syncookies(void)
32 {
33 	get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
34 	return 0;
35 }
36 __initcall(init_syncookies);
37 
38 #define COOKIEBITS 24	/* Upper bits store count */
39 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
40 
41 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
42 		      ipv4_cookie_scratch);
43 
44 static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
45 		       u32 count, int c)
46 {
47 	__u32 *tmp = __get_cpu_var(ipv4_cookie_scratch);
48 
49 	memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
50 	tmp[0] = (__force u32)saddr;
51 	tmp[1] = (__force u32)daddr;
52 	tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
53 	tmp[3] = count;
54 	sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
55 
56 	return tmp[17];
57 }
58 
59 
60 /*
61  * when syncookies are in effect and tcp timestamps are enabled we encode
62  * tcp options in the lower bits of the timestamp value that will be
63  * sent in the syn-ack.
64  * Since subsequent timestamps use the normal tcp_time_stamp value, we
65  * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
66  */
67 __u32 cookie_init_timestamp(struct request_sock *req)
68 {
69 	struct inet_request_sock *ireq;
70 	u32 ts, ts_now = tcp_time_stamp;
71 	u32 options = 0;
72 
73 	ireq = inet_rsk(req);
74 
75 	options = ireq->wscale_ok ? ireq->snd_wscale : 0xf;
76 	options |= ireq->sack_ok << 4;
77 	options |= ireq->ecn_ok << 5;
78 
79 	ts = ts_now & ~TSMASK;
80 	ts |= options;
81 	if (ts > ts_now) {
82 		ts >>= TSBITS;
83 		ts--;
84 		ts <<= TSBITS;
85 		ts |= options;
86 	}
87 	return ts;
88 }
89 
90 
91 static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
92 				   __be16 dport, __u32 sseq, __u32 count,
93 				   __u32 data)
94 {
95 	/*
96 	 * Compute the secure sequence number.
97 	 * The output should be:
98 	 *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
99 	 *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
100 	 * Where sseq is their sequence number and count increases every
101 	 * minute by 1.
102 	 * As an extra hack, we add a small "data" value that encodes the
103 	 * MSS into the second hash value.
104 	 */
105 
106 	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
107 		sseq + (count << COOKIEBITS) +
108 		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
109 		 & COOKIEMASK));
110 }
111 
112 /*
113  * This retrieves the small "data" value from the syncookie.
114  * If the syncookie is bad, the data returned will be out of
115  * range.  This must be checked by the caller.
116  *
117  * The count value used to generate the cookie must be within
118  * "maxdiff" if the current (passed-in) "count".  The return value
119  * is (__u32)-1 if this test fails.
120  */
121 static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
122 				  __be16 sport, __be16 dport, __u32 sseq,
123 				  __u32 count, __u32 maxdiff)
124 {
125 	__u32 diff;
126 
127 	/* Strip away the layers from the cookie */
128 	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
129 
130 	/* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
131 	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) - 1 >> COOKIEBITS);
132 	if (diff >= maxdiff)
133 		return (__u32)-1;
134 
135 	return (cookie -
136 		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
137 		& COOKIEMASK;	/* Leaving the data behind */
138 }
139 
140 /*
141  * MSS Values are taken from the 2009 paper
142  * 'Measuring TCP Maximum Segment Size' by S. Alcock and R. Nelson:
143  *  - values 1440 to 1460 accounted for 80% of observed mss values
144  *  - values outside the 536-1460 range are rare (<0.2%).
145  *
146  * Table must be sorted.
147  */
148 static __u16 const msstab[] = {
149 	64,
150 	512,
151 	536,
152 	1024,
153 	1440,
154 	1460,
155 	4312,
156 	8960,
157 };
158 
159 /*
160  * Generate a syncookie.  mssp points to the mss, which is returned
161  * rounded down to the value encoded in the cookie.
162  */
163 __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
164 {
165 	const struct iphdr *iph = ip_hdr(skb);
166 	const struct tcphdr *th = tcp_hdr(skb);
167 	int mssind;
168 	const __u16 mss = *mssp;
169 
170 	tcp_synq_overflow(sk);
171 
172 	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
173 		if (mss >= msstab[mssind])
174 			break;
175 	*mssp = msstab[mssind];
176 
177 	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
178 
179 	return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
180 				     th->source, th->dest, ntohl(th->seq),
181 				     jiffies / (HZ * 60), mssind);
182 }
183 
184 /*
185  * This (misnamed) value is the age of syncookie which is permitted.
186  * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
187  * sysctl_tcp_retries1. It's a rather complicated formula (exponential
188  * backoff) to compute at runtime so it's currently hardcoded here.
189  */
190 #define COUNTER_TRIES 4
191 /*
192  * Check if a ack sequence number is a valid syncookie.
193  * Return the decoded mss if it is, or 0 if not.
194  */
195 static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
196 {
197 	const struct iphdr *iph = ip_hdr(skb);
198 	const struct tcphdr *th = tcp_hdr(skb);
199 	__u32 seq = ntohl(th->seq) - 1;
200 	__u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
201 					    th->source, th->dest, seq,
202 					    jiffies / (HZ * 60),
203 					    COUNTER_TRIES);
204 
205 	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
206 }
207 
208 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
209 					   struct request_sock *req,
210 					   struct dst_entry *dst)
211 {
212 	struct inet_connection_sock *icsk = inet_csk(sk);
213 	struct sock *child;
214 
215 	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
216 	if (child)
217 		inet_csk_reqsk_queue_add(sk, req, child);
218 	else
219 		reqsk_free(req);
220 
221 	return child;
222 }
223 
224 
225 /*
226  * when syncookies are in effect and tcp timestamps are enabled we stored
227  * additional tcp options in the timestamp.
228  * This extracts these options from the timestamp echo.
229  *
230  * The lowest 4 bits store snd_wscale.
231  * next 2 bits indicate SACK and ECN support.
232  *
233  * return false if we decode an option that should not be.
234  */
235 bool cookie_check_timestamp(struct tcp_options_received *tcp_opt, bool *ecn_ok)
236 {
237 	/* echoed timestamp, lowest bits contain options */
238 	u32 options = tcp_opt->rcv_tsecr & TSMASK;
239 
240 	if (!tcp_opt->saw_tstamp)  {
241 		tcp_clear_options(tcp_opt);
242 		return true;
243 	}
244 
245 	if (!sysctl_tcp_timestamps)
246 		return false;
247 
248 	tcp_opt->sack_ok = (options & (1 << 4)) ? TCP_SACK_SEEN : 0;
249 	*ecn_ok = (options >> 5) & 1;
250 	if (*ecn_ok && !sysctl_tcp_ecn)
251 		return false;
252 
253 	if (tcp_opt->sack_ok && !sysctl_tcp_sack)
254 		return false;
255 
256 	if ((options & 0xf) == 0xf)
257 		return true; /* no window scaling */
258 
259 	tcp_opt->wscale_ok = 1;
260 	tcp_opt->snd_wscale = options & 0xf;
261 	return sysctl_tcp_window_scaling != 0;
262 }
263 EXPORT_SYMBOL(cookie_check_timestamp);
264 
265 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
266 			     struct ip_options *opt)
267 {
268 	struct tcp_options_received tcp_opt;
269 	const u8 *hash_location;
270 	struct inet_request_sock *ireq;
271 	struct tcp_request_sock *treq;
272 	struct tcp_sock *tp = tcp_sk(sk);
273 	const struct tcphdr *th = tcp_hdr(skb);
274 	__u32 cookie = ntohl(th->ack_seq) - 1;
275 	struct sock *ret = sk;
276 	struct request_sock *req;
277 	int mss;
278 	struct rtable *rt;
279 	__u8 rcv_wscale;
280 	bool ecn_ok = false;
281 	struct flowi4 fl4;
282 
283 	if (!sysctl_tcp_syncookies || !th->ack || th->rst)
284 		goto out;
285 
286 	if (tcp_synq_no_recent_overflow(sk) ||
287 	    (mss = cookie_check(skb, cookie)) == 0) {
288 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
289 		goto out;
290 	}
291 
292 	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
293 
294 	/* check for timestamp cookie support */
295 	memset(&tcp_opt, 0, sizeof(tcp_opt));
296 	tcp_parse_options(skb, &tcp_opt, &hash_location, 0, NULL);
297 
298 	if (!cookie_check_timestamp(&tcp_opt, &ecn_ok))
299 		goto out;
300 
301 	ret = NULL;
302 	req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
303 	if (!req)
304 		goto out;
305 
306 	ireq = inet_rsk(req);
307 	treq = tcp_rsk(req);
308 	treq->rcv_isn		= ntohl(th->seq) - 1;
309 	treq->snt_isn		= cookie;
310 	req->mss		= mss;
311 	ireq->loc_port		= th->dest;
312 	ireq->rmt_port		= th->source;
313 	ireq->loc_addr		= ip_hdr(skb)->daddr;
314 	ireq->rmt_addr		= ip_hdr(skb)->saddr;
315 	ireq->ecn_ok		= ecn_ok;
316 	ireq->snd_wscale	= tcp_opt.snd_wscale;
317 	ireq->sack_ok		= tcp_opt.sack_ok;
318 	ireq->wscale_ok		= tcp_opt.wscale_ok;
319 	ireq->tstamp_ok		= tcp_opt.saw_tstamp;
320 	req->ts_recent		= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
321 	treq->snt_synack	= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
322 
323 	/* We throwed the options of the initial SYN away, so we hope
324 	 * the ACK carries the same options again (see RFC1122 4.2.3.8)
325 	 */
326 	if (opt && opt->optlen) {
327 		int opt_size = sizeof(struct ip_options_rcu) + opt->optlen;
328 
329 		ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
330 		if (ireq->opt != NULL && ip_options_echo(&ireq->opt->opt, skb)) {
331 			kfree(ireq->opt);
332 			ireq->opt = NULL;
333 		}
334 	}
335 
336 	if (security_inet_conn_request(sk, skb, req)) {
337 		reqsk_free(req);
338 		goto out;
339 	}
340 
341 	req->expires	= 0UL;
342 	req->retrans	= 0;
343 
344 	/*
345 	 * We need to lookup the route here to get at the correct
346 	 * window size. We should better make sure that the window size
347 	 * hasn't changed since we received the original syn, but I see
348 	 * no easy way to do this.
349 	 */
350 	flowi4_init_output(&fl4, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
351 			   RT_SCOPE_UNIVERSE, IPPROTO_TCP,
352 			   inet_sk_flowi_flags(sk),
353 			   (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
354 			   ireq->loc_addr, th->source, th->dest);
355 	security_req_classify_flow(req, flowi4_to_flowi(&fl4));
356 	rt = ip_route_output_key(sock_net(sk), &fl4);
357 	if (IS_ERR(rt)) {
358 		reqsk_free(req);
359 		goto out;
360 	}
361 
362 	/* Try to redo what tcp_v4_send_synack did. */
363 	req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
364 
365 	tcp_select_initial_window(tcp_full_space(sk), req->mss,
366 				  &req->rcv_wnd, &req->window_clamp,
367 				  ireq->wscale_ok, &rcv_wscale,
368 				  dst_metric(&rt->dst, RTAX_INITRWND));
369 
370 	ireq->rcv_wscale  = rcv_wscale;
371 
372 	ret = get_cookie_sock(sk, skb, req, &rt->dst);
373 	/* ip_queue_xmit() depends on our flow being setup
374 	 * Normal sockets get it right from inet_csk_route_child_sock()
375 	 */
376 	if (ret)
377 		inet_sk(ret)->cork.fl.u.ip4 = fl4;
378 out:	return ret;
379 }
380