xref: /openbmc/linux/net/ipv4/syncookies.c (revision 6774def6)
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 static u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS] __read_mostly;
29 
30 #define COOKIEBITS 24	/* Upper bits store count */
31 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
32 
33 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
34 		      ipv4_cookie_scratch);
35 
36 static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
37 		       u32 count, int c)
38 {
39 	__u32 *tmp;
40 
41 	net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
42 
43 	tmp  = this_cpu_ptr(ipv4_cookie_scratch);
44 	memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
45 	tmp[0] = (__force u32)saddr;
46 	tmp[1] = (__force u32)daddr;
47 	tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
48 	tmp[3] = count;
49 	sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
50 
51 	return tmp[17];
52 }
53 
54 
55 /*
56  * when syncookies are in effect and tcp timestamps are enabled we encode
57  * tcp options in the lower bits of the timestamp value that will be
58  * sent in the syn-ack.
59  * Since subsequent timestamps use the normal tcp_time_stamp value, we
60  * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
61  */
62 __u32 cookie_init_timestamp(struct request_sock *req)
63 {
64 	struct inet_request_sock *ireq;
65 	u32 ts, ts_now = tcp_time_stamp;
66 	u32 options = 0;
67 
68 	ireq = inet_rsk(req);
69 
70 	options = ireq->wscale_ok ? ireq->snd_wscale : 0xf;
71 	options |= ireq->sack_ok << 4;
72 	options |= ireq->ecn_ok << 5;
73 
74 	ts = ts_now & ~TSMASK;
75 	ts |= options;
76 	if (ts > ts_now) {
77 		ts >>= TSBITS;
78 		ts--;
79 		ts <<= TSBITS;
80 		ts |= options;
81 	}
82 	return ts;
83 }
84 
85 
86 static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
87 				   __be16 dport, __u32 sseq, __u32 data)
88 {
89 	/*
90 	 * Compute the secure sequence number.
91 	 * The output should be:
92 	 *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
93 	 *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
94 	 * Where sseq is their sequence number and count increases every
95 	 * minute by 1.
96 	 * As an extra hack, we add a small "data" value that encodes the
97 	 * MSS into the second hash value.
98 	 */
99 	u32 count = tcp_cookie_time();
100 	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
101 		sseq + (count << COOKIEBITS) +
102 		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
103 		 & COOKIEMASK));
104 }
105 
106 /*
107  * This retrieves the small "data" value from the syncookie.
108  * If the syncookie is bad, the data returned will be out of
109  * range.  This must be checked by the caller.
110  *
111  * The count value used to generate the cookie must be less than
112  * MAX_SYNCOOKIE_AGE minutes in the past.
113  * The return value (__u32)-1 if this test fails.
114  */
115 static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
116 				  __be16 sport, __be16 dport, __u32 sseq)
117 {
118 	u32 diff, count = tcp_cookie_time();
119 
120 	/* Strip away the layers from the cookie */
121 	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
122 
123 	/* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
124 	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
125 	if (diff >= MAX_SYNCOOKIE_AGE)
126 		return (__u32)-1;
127 
128 	return (cookie -
129 		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
130 		& COOKIEMASK;	/* Leaving the data behind */
131 }
132 
133 /*
134  * MSS Values are chosen based on the 2011 paper
135  * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson.
136  * Values ..
137  *  .. lower than 536 are rare (< 0.2%)
138  *  .. between 537 and 1299 account for less than < 1.5% of observed values
139  *  .. in the 1300-1349 range account for about 15 to 20% of observed mss values
140  *  .. exceeding 1460 are very rare (< 0.04%)
141  *
142  *  1460 is the single most frequently announced mss value (30 to 46% depending
143  *  on monitor location).  Table must be sorted.
144  */
145 static __u16 const msstab[] = {
146 	536,
147 	1300,
148 	1440,	/* 1440, 1452: PPPoE */
149 	1460,
150 };
151 
152 /*
153  * Generate a syncookie.  mssp points to the mss, which is returned
154  * rounded down to the value encoded in the cookie.
155  */
156 u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
157 			      u16 *mssp)
158 {
159 	int mssind;
160 	const __u16 mss = *mssp;
161 
162 	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
163 		if (mss >= msstab[mssind])
164 			break;
165 	*mssp = msstab[mssind];
166 
167 	return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
168 				     th->source, th->dest, ntohl(th->seq),
169 				     mssind);
170 }
171 EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
172 
173 __u32 cookie_v4_init_sequence(struct sock *sk, const struct sk_buff *skb,
174 			      __u16 *mssp)
175 {
176 	const struct iphdr *iph = ip_hdr(skb);
177 	const struct tcphdr *th = tcp_hdr(skb);
178 
179 	tcp_synq_overflow(sk);
180 	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
181 
182 	return __cookie_v4_init_sequence(iph, th, mssp);
183 }
184 
185 /*
186  * Check if a ack sequence number is a valid syncookie.
187  * Return the decoded mss if it is, or 0 if not.
188  */
189 int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th,
190 		      u32 cookie)
191 {
192 	__u32 seq = ntohl(th->seq) - 1;
193 	__u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
194 					    th->source, th->dest, seq);
195 
196 	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
197 }
198 EXPORT_SYMBOL_GPL(__cookie_v4_check);
199 
200 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
201 					   struct request_sock *req,
202 					   struct dst_entry *dst)
203 {
204 	struct inet_connection_sock *icsk = inet_csk(sk);
205 	struct sock *child;
206 
207 	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
208 	if (child)
209 		inet_csk_reqsk_queue_add(sk, req, child);
210 	else
211 		reqsk_free(req);
212 
213 	return child;
214 }
215 
216 
217 /*
218  * when syncookies are in effect and tcp timestamps are enabled we stored
219  * additional tcp options in the timestamp.
220  * This extracts these options from the timestamp echo.
221  *
222  * The lowest 4 bits store snd_wscale.
223  * next 2 bits indicate SACK and ECN support.
224  *
225  * return false if we decode an option that should not be.
226  */
227 bool cookie_check_timestamp(struct tcp_options_received *tcp_opt,
228 			struct net *net, bool *ecn_ok)
229 {
230 	/* echoed timestamp, lowest bits contain options */
231 	u32 options = tcp_opt->rcv_tsecr & TSMASK;
232 
233 	if (!tcp_opt->saw_tstamp)  {
234 		tcp_clear_options(tcp_opt);
235 		return true;
236 	}
237 
238 	if (!sysctl_tcp_timestamps)
239 		return false;
240 
241 	tcp_opt->sack_ok = (options & (1 << 4)) ? TCP_SACK_SEEN : 0;
242 	*ecn_ok = (options >> 5) & 1;
243 	if (*ecn_ok && !net->ipv4.sysctl_tcp_ecn)
244 		return false;
245 
246 	if (tcp_opt->sack_ok && !sysctl_tcp_sack)
247 		return false;
248 
249 	if ((options & 0xf) == 0xf)
250 		return true; /* no window scaling */
251 
252 	tcp_opt->wscale_ok = 1;
253 	tcp_opt->snd_wscale = options & 0xf;
254 	return sysctl_tcp_window_scaling != 0;
255 }
256 EXPORT_SYMBOL(cookie_check_timestamp);
257 
258 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
259 {
260 	struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt;
261 	struct tcp_options_received tcp_opt;
262 	struct inet_request_sock *ireq;
263 	struct tcp_request_sock *treq;
264 	struct tcp_sock *tp = tcp_sk(sk);
265 	const struct tcphdr *th = tcp_hdr(skb);
266 	__u32 cookie = ntohl(th->ack_seq) - 1;
267 	struct sock *ret = sk;
268 	struct request_sock *req;
269 	int mss;
270 	struct rtable *rt;
271 	__u8 rcv_wscale;
272 	bool ecn_ok = false;
273 	struct flowi4 fl4;
274 
275 	if (!sysctl_tcp_syncookies || !th->ack || th->rst)
276 		goto out;
277 
278 	if (tcp_synq_no_recent_overflow(sk) ||
279 	    (mss = __cookie_v4_check(ip_hdr(skb), th, cookie)) == 0) {
280 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
281 		goto out;
282 	}
283 
284 	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
285 
286 	/* check for timestamp cookie support */
287 	memset(&tcp_opt, 0, sizeof(tcp_opt));
288 	tcp_parse_options(skb, &tcp_opt, 0, NULL);
289 
290 	if (!cookie_check_timestamp(&tcp_opt, sock_net(sk), &ecn_ok))
291 		goto out;
292 
293 	ret = NULL;
294 	req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
295 	if (!req)
296 		goto out;
297 
298 	ireq = inet_rsk(req);
299 	treq = tcp_rsk(req);
300 	treq->rcv_isn		= ntohl(th->seq) - 1;
301 	treq->snt_isn		= cookie;
302 	req->mss		= mss;
303 	ireq->ir_num		= ntohs(th->dest);
304 	ireq->ir_rmt_port	= th->source;
305 	ireq->ir_loc_addr	= ip_hdr(skb)->daddr;
306 	ireq->ir_rmt_addr	= ip_hdr(skb)->saddr;
307 	ireq->ir_mark		= inet_request_mark(sk, skb);
308 	ireq->ecn_ok		= ecn_ok;
309 	ireq->snd_wscale	= tcp_opt.snd_wscale;
310 	ireq->sack_ok		= tcp_opt.sack_ok;
311 	ireq->wscale_ok		= tcp_opt.wscale_ok;
312 	ireq->tstamp_ok		= tcp_opt.saw_tstamp;
313 	req->ts_recent		= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
314 	treq->snt_synack	= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
315 	treq->listener		= NULL;
316 
317 	/* We throwed the options of the initial SYN away, so we hope
318 	 * the ACK carries the same options again (see RFC1122 4.2.3.8)
319 	 */
320 	ireq->opt = tcp_v4_save_options(skb);
321 
322 	if (security_inet_conn_request(sk, skb, req)) {
323 		reqsk_free(req);
324 		goto out;
325 	}
326 
327 	req->expires	= 0UL;
328 	req->num_retrans = 0;
329 
330 	/*
331 	 * We need to lookup the route here to get at the correct
332 	 * window size. We should better make sure that the window size
333 	 * hasn't changed since we received the original syn, but I see
334 	 * no easy way to do this.
335 	 */
336 	flowi4_init_output(&fl4, sk->sk_bound_dev_if, ireq->ir_mark,
337 			   RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
338 			   inet_sk_flowi_flags(sk),
339 			   opt->srr ? opt->faddr : ireq->ir_rmt_addr,
340 			   ireq->ir_loc_addr, th->source, th->dest);
341 	security_req_classify_flow(req, flowi4_to_flowi(&fl4));
342 	rt = ip_route_output_key(sock_net(sk), &fl4);
343 	if (IS_ERR(rt)) {
344 		reqsk_free(req);
345 		goto out;
346 	}
347 
348 	/* Try to redo what tcp_v4_send_synack did. */
349 	req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
350 
351 	tcp_select_initial_window(tcp_full_space(sk), req->mss,
352 				  &req->rcv_wnd, &req->window_clamp,
353 				  ireq->wscale_ok, &rcv_wscale,
354 				  dst_metric(&rt->dst, RTAX_INITRWND));
355 
356 	ireq->rcv_wscale  = rcv_wscale;
357 
358 	ret = get_cookie_sock(sk, skb, req, &rt->dst);
359 	/* ip_queue_xmit() depends on our flow being setup
360 	 * Normal sockets get it right from inet_csk_route_child_sock()
361 	 */
362 	if (ret)
363 		inet_sk(ret)->cork.fl.u.ip4 = fl4;
364 out:	return ret;
365 }
366