xref: /openbmc/linux/net/ipv4/syncookies.c (revision 22fd411a) 
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 <net/tcp.h>
19 #include <net/route.h>
20 
21 /* Timestamps: lowest bits store TCP options */
22 #define TSBITS 6
23 #define TSMASK (((__u32)1 << TSBITS) - 1)
24 
25 extern int sysctl_tcp_syncookies;
26 
27 __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
28 EXPORT_SYMBOL(syncookie_secret);
29 
30 static __init int init_syncookies(void)
31 {
32 	get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
33 	return 0;
34 }
35 __initcall(init_syncookies);
36 
37 #define COOKIEBITS 24	/* Upper bits store count */
38 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
39 
40 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
41 		      ipv4_cookie_scratch);
42 
43 static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
44 		       u32 count, int c)
45 {
46 	__u32 *tmp = __get_cpu_var(ipv4_cookie_scratch);
47 
48 	memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
49 	tmp[0] = (__force u32)saddr;
50 	tmp[1] = (__force u32)daddr;
51 	tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
52 	tmp[3] = count;
53 	sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
54 
55 	return tmp[17];
56 }
57 
58 
59 /*
60  * when syncookies are in effect and tcp timestamps are enabled we encode
61  * tcp options in the lower bits of the timestamp value that will be
62  * sent in the syn-ack.
63  * Since subsequent timestamps use the normal tcp_time_stamp value, we
64  * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
65  */
66 __u32 cookie_init_timestamp(struct request_sock *req)
67 {
68 	struct inet_request_sock *ireq;
69 	u32 ts, ts_now = tcp_time_stamp;
70 	u32 options = 0;
71 
72 	ireq = inet_rsk(req);
73 
74 	options = ireq->wscale_ok ? ireq->snd_wscale : 0xf;
75 	options |= ireq->sack_ok << 4;
76 	options |= ireq->ecn_ok << 5;
77 
78 	ts = ts_now & ~TSMASK;
79 	ts |= options;
80 	if (ts > ts_now) {
81 		ts >>= TSBITS;
82 		ts--;
83 		ts <<= TSBITS;
84 		ts |= options;
85 	}
86 	return ts;
87 }
88 
89 
90 static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
91 				   __be16 dport, __u32 sseq, __u32 count,
92 				   __u32 data)
93 {
94 	/*
95 	 * Compute the secure sequence number.
96 	 * The output should be:
97 	 *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
98 	 *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
99 	 * Where sseq is their sequence number and count increases every
100 	 * minute by 1.
101 	 * As an extra hack, we add a small "data" value that encodes the
102 	 * MSS into the second hash value.
103 	 */
104 
105 	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
106 		sseq + (count << COOKIEBITS) +
107 		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
108 		 & COOKIEMASK));
109 }
110 
111 /*
112  * This retrieves the small "data" value from the syncookie.
113  * If the syncookie is bad, the data returned will be out of
114  * range.  This must be checked by the caller.
115  *
116  * The count value used to generate the cookie must be within
117  * "maxdiff" if the current (passed-in) "count".  The return value
118  * is (__u32)-1 if this test fails.
119  */
120 static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
121 				  __be16 sport, __be16 dport, __u32 sseq,
122 				  __u32 count, __u32 maxdiff)
123 {
124 	__u32 diff;
125 
126 	/* Strip away the layers from the cookie */
127 	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
128 
129 	/* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
130 	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) - 1 >> COOKIEBITS);
131 	if (diff >= maxdiff)
132 		return (__u32)-1;
133 
134 	return (cookie -
135 		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
136 		& COOKIEMASK;	/* Leaving the data behind */
137 }
138 
139 /*
140  * MSS Values are taken from the 2009 paper
141  * 'Measuring TCP Maximum Segment Size' by S. Alcock and R. Nelson:
142  *  - values 1440 to 1460 accounted for 80% of observed mss values
143  *  - values outside the 536-1460 range are rare (<0.2%).
144  *
145  * Table must be sorted.
146  */
147 static __u16 const msstab[] = {
148 	64,
149 	512,
150 	536,
151 	1024,
152 	1440,
153 	1460,
154 	4312,
155 	8960,
156 };
157 
158 /*
159  * Generate a syncookie.  mssp points to the mss, which is returned
160  * rounded down to the value encoded in the cookie.
161  */
162 __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
163 {
164 	const struct iphdr *iph = ip_hdr(skb);
165 	const struct tcphdr *th = tcp_hdr(skb);
166 	int mssind;
167 	const __u16 mss = *mssp;
168 
169 	tcp_synq_overflow(sk);
170 
171 	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
172 		if (mss >= msstab[mssind])
173 			break;
174 	*mssp = msstab[mssind];
175 
176 	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
177 
178 	return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
179 				     th->source, th->dest, ntohl(th->seq),
180 				     jiffies / (HZ * 60), mssind);
181 }
182 
183 /*
184  * This (misnamed) value is the age of syncookie which is permitted.
185  * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
186  * sysctl_tcp_retries1. It's a rather complicated formula (exponential
187  * backoff) to compute at runtime so it's currently hardcoded here.
188  */
189 #define COUNTER_TRIES 4
190 /*
191  * Check if a ack sequence number is a valid syncookie.
192  * Return the decoded mss if it is, or 0 if not.
193  */
194 static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
195 {
196 	const struct iphdr *iph = ip_hdr(skb);
197 	const struct tcphdr *th = tcp_hdr(skb);
198 	__u32 seq = ntohl(th->seq) - 1;
199 	__u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
200 					    th->source, th->dest, seq,
201 					    jiffies / (HZ * 60),
202 					    COUNTER_TRIES);
203 
204 	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
205 }
206 
207 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
208 					   struct request_sock *req,
209 					   struct dst_entry *dst)
210 {
211 	struct inet_connection_sock *icsk = inet_csk(sk);
212 	struct sock *child;
213 
214 	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
215 	if (child)
216 		inet_csk_reqsk_queue_add(sk, req, child);
217 	else
218 		reqsk_free(req);
219 
220 	return child;
221 }
222 
223 
224 /*
225  * when syncookies are in effect and tcp timestamps are enabled we stored
226  * additional tcp options in the timestamp.
227  * This extracts these options from the timestamp echo.
228  *
229  * The lowest 4 bits store snd_wscale.
230  * next 2 bits indicate SACK and ECN support.
231  *
232  * return false if we decode an option that should not be.
233  */
234 bool cookie_check_timestamp(struct tcp_options_received *tcp_opt, bool *ecn_ok)
235 {
236 	/* echoed timestamp, lowest bits contain options */
237 	u32 options = tcp_opt->rcv_tsecr & TSMASK;
238 
239 	if (!tcp_opt->saw_tstamp)  {
240 		tcp_clear_options(tcp_opt);
241 		return true;
242 	}
243 
244 	if (!sysctl_tcp_timestamps)
245 		return false;
246 
247 	tcp_opt->sack_ok = (options >> 4) & 0x1;
248 	*ecn_ok = (options >> 5) & 1;
249 	if (*ecn_ok && !sysctl_tcp_ecn)
250 		return false;
251 
252 	if (tcp_opt->sack_ok && !sysctl_tcp_sack)
253 		return false;
254 
255 	if ((options & 0xf) == 0xf)
256 		return true; /* no window scaling */
257 
258 	tcp_opt->wscale_ok = 1;
259 	tcp_opt->snd_wscale = options & 0xf;
260 	return sysctl_tcp_window_scaling != 0;
261 }
262 EXPORT_SYMBOL(cookie_check_timestamp);
263 
264 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
265 			     struct ip_options *opt)
266 {
267 	struct tcp_options_received tcp_opt;
268 	u8 *hash_location;
269 	struct inet_request_sock *ireq;
270 	struct tcp_request_sock *treq;
271 	struct tcp_sock *tp = tcp_sk(sk);
272 	const struct tcphdr *th = tcp_hdr(skb);
273 	__u32 cookie = ntohl(th->ack_seq) - 1;
274 	struct sock *ret = sk;
275 	struct request_sock *req;
276 	int mss;
277 	struct rtable *rt;
278 	__u8 rcv_wscale;
279 	bool ecn_ok;
280 
281 	if (!sysctl_tcp_syncookies || !th->ack || th->rst)
282 		goto out;
283 
284 	if (tcp_synq_no_recent_overflow(sk) ||
285 	    (mss = cookie_check(skb, cookie)) == 0) {
286 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
287 		goto out;
288 	}
289 
290 	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
291 
292 	/* check for timestamp cookie support */
293 	memset(&tcp_opt, 0, sizeof(tcp_opt));
294 	tcp_parse_options(skb, &tcp_opt, &hash_location, 0);
295 
296 	if (!cookie_check_timestamp(&tcp_opt, &ecn_ok))
297 		goto out;
298 
299 	ret = NULL;
300 	req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
301 	if (!req)
302 		goto out;
303 
304 	ireq = inet_rsk(req);
305 	treq = tcp_rsk(req);
306 	treq->rcv_isn		= ntohl(th->seq) - 1;
307 	treq->snt_isn		= cookie;
308 	req->mss		= mss;
309 	ireq->loc_port		= th->dest;
310 	ireq->rmt_port		= th->source;
311 	ireq->loc_addr		= ip_hdr(skb)->daddr;
312 	ireq->rmt_addr		= ip_hdr(skb)->saddr;
313 	ireq->ecn_ok		= ecn_ok;
314 	ireq->snd_wscale	= tcp_opt.snd_wscale;
315 	ireq->sack_ok		= tcp_opt.sack_ok;
316 	ireq->wscale_ok		= tcp_opt.wscale_ok;
317 	ireq->tstamp_ok		= tcp_opt.saw_tstamp;
318 	req->ts_recent		= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
319 
320 	/* We throwed the options of the initial SYN away, so we hope
321 	 * the ACK carries the same options again (see RFC1122 4.2.3.8)
322 	 */
323 	if (opt && opt->optlen) {
324 		int opt_size = sizeof(struct ip_options) + opt->optlen;
325 
326 		ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
327 		if (ireq->opt != NULL && ip_options_echo(ireq->opt, skb)) {
328 			kfree(ireq->opt);
329 			ireq->opt = NULL;
330 		}
331 	}
332 
333 	if (security_inet_conn_request(sk, skb, req)) {
334 		reqsk_free(req);
335 		goto out;
336 	}
337 
338 	req->expires	= 0UL;
339 	req->retrans	= 0;
340 
341 	/*
342 	 * We need to lookup the route here to get at the correct
343 	 * window size. We should better make sure that the window size
344 	 * hasn't changed since we received the original syn, but I see
345 	 * no easy way to do this.
346 	 */
347 	{
348 		struct flowi fl = { .mark = sk->sk_mark,
349 				    .fl4_dst = ((opt && opt->srr) ?
350 						opt->faddr : ireq->rmt_addr),
351 				    .fl4_src = ireq->loc_addr,
352 				    .fl4_tos = RT_CONN_FLAGS(sk),
353 				    .proto = IPPROTO_TCP,
354 				    .flags = inet_sk_flowi_flags(sk),
355 				    .fl_ip_sport = th->dest,
356 				    .fl_ip_dport = th->source };
357 		security_req_classify_flow(req, &fl);
358 		if (ip_route_output_key(sock_net(sk), &rt, &fl)) {
359 			reqsk_free(req);
360 			goto out;
361 		}
362 	}
363 
364 	/* Try to redo what tcp_v4_send_synack did. */
365 	req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
366 
367 	tcp_select_initial_window(tcp_full_space(sk), req->mss,
368 				  &req->rcv_wnd, &req->window_clamp,
369 				  ireq->wscale_ok, &rcv_wscale,
370 				  dst_metric(&rt->dst, RTAX_INITRWND));
371 
372 	ireq->rcv_wscale  = rcv_wscale;
373 
374 	ret = get_cookie_sock(sk, skb, req, &rt->dst);
375 out:	return ret;
376 }
377