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 9 bits store TCP options */
22 #define TSBITS 9
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, cookie_scratch)[16 + 5 + SHA_WORKSPACE_WORDS];
41
42 static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
43 u32 count, int c)
44 {
45 __u32 *tmp = __get_cpu_var(cookie_scratch);
46
47 memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
48 tmp[0] = (__force u32)saddr;
49 tmp[1] = (__force u32)daddr;
50 tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
51 tmp[3] = count;
52 sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
53
54 return tmp[17];
55 }
56
57
58 /*
59 * when syncookies are in effect and tcp timestamps are enabled we encode
60 * tcp options in the lowest 9 bits of the timestamp value that will be
61 * sent in the syn-ack.
62 * Since subsequent timestamps use the normal tcp_time_stamp value, we
63 * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
64 */
65 __u32 cookie_init_timestamp(struct request_sock *req)
66 {
67 struct inet_request_sock *ireq;
68 u32 ts, ts_now = tcp_time_stamp;
69 u32 options = 0;
70
71 ireq = inet_rsk(req);
72 if (ireq->wscale_ok) {
73 options = ireq->snd_wscale;
74 options |= ireq->rcv_wscale << 4;
75 }
76 options |= ireq->sack_ok << 8;
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 * This table has to be sorted and terminated with (__u16)-1.
141 * XXX generate a better table.
142 * Unresolved Issues: HIPPI with a 64k MSS is not well supported.
143 */
144 static __u16 const msstab[] = {
145 64 - 1,
146 256 - 1,
147 512 - 1,
148 536 - 1,
149 1024 - 1,
150 1440 - 1,
151 1460 - 1,
152 4312 - 1,
153 (__u16)-1
154 };
155 /* The number doesn't include the -1 terminator */
156 #define NUM_MSS (ARRAY_SIZE(msstab) - 1)
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 /* XXX sort msstab[] by probability? Binary search? */
172 for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
173 ;
174 *mssp = msstab[mssind] + 1;
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 < NUM_MSS ? msstab[mssind] + 1 : 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 are for snd_wscale
230 * The next 4 lsb are for rcv_wscale
231 * The next lsb is for sack_ok
232 */
233 void cookie_check_timestamp(struct tcp_options_received *tcp_opt)
234 {
235 /* echoed timestamp, 9 lowest bits contain options */
236 u32 options = tcp_opt->rcv_tsecr & TSMASK;
237
238 tcp_opt->snd_wscale = options & 0xf;
239 options >>= 4;
240 tcp_opt->rcv_wscale = options & 0xf;
241
242 tcp_opt->sack_ok = (options >> 4) & 0x1;
243
244 if (tcp_opt->sack_ok)
245 tcp_sack_reset(tcp_opt);
246
247 if (tcp_opt->snd_wscale || tcp_opt->rcv_wscale)
248 tcp_opt->wscale_ok = 1;
249 }
250 EXPORT_SYMBOL(cookie_check_timestamp);
251
252 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
253 struct ip_options *opt)
254 {
255 struct inet_request_sock *ireq;
256 struct tcp_request_sock *treq;
257 struct tcp_sock *tp = tcp_sk(sk);
258 const struct tcphdr *th = tcp_hdr(skb);
259 __u32 cookie = ntohl(th->ack_seq) - 1;
260 struct sock *ret = sk;
261 struct request_sock *req;
262 int mss;
263 struct rtable *rt;
264 __u8 rcv_wscale;
265 struct tcp_options_received tcp_opt;
266
267 if (!sysctl_tcp_syncookies || !th->ack)
268 goto out;
269
270 if (tcp_synq_no_recent_overflow(sk) ||
271 (mss = cookie_check(skb, cookie)) == 0) {
272 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
273 goto out;
274 }
275
276 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
277
278 /* check for timestamp cookie support */
279 memset(&tcp_opt, 0, sizeof(tcp_opt));
280 tcp_parse_options(skb, &tcp_opt, 0);
281
282 if (tcp_opt.saw_tstamp)
283 cookie_check_timestamp(&tcp_opt);
284
285 ret = NULL;
286 req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
287 if (!req)
288 goto out;
289
290 ireq = inet_rsk(req);
291 treq = tcp_rsk(req);
292 treq->rcv_isn = ntohl(th->seq) - 1;
293 treq->snt_isn = cookie;
294 req->mss = mss;
295 ireq->loc_port = th->dest;
296 ireq->rmt_port = th->source;
297 ireq->loc_addr = ip_hdr(skb)->daddr;
298 ireq->rmt_addr = ip_hdr(skb)->saddr;
299 ireq->ecn_ok = 0;
300 ireq->snd_wscale = tcp_opt.snd_wscale;
301 ireq->rcv_wscale = tcp_opt.rcv_wscale;
302 ireq->sack_ok = tcp_opt.sack_ok;
303 ireq->wscale_ok = tcp_opt.wscale_ok;
304 ireq->tstamp_ok = tcp_opt.saw_tstamp;
305 req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
306
307 /* We throwed the options of the initial SYN away, so we hope
308 * the ACK carries the same options again (see RFC1122 4.2.3.8)
309 */
310 if (opt && opt->optlen) {
311 int opt_size = sizeof(struct ip_options) + opt->optlen;
312
313 ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
314 if (ireq->opt != NULL && ip_options_echo(ireq->opt, skb)) {
315 kfree(ireq->opt);
316 ireq->opt = NULL;
317 }
318 }
319
320 if (security_inet_conn_request(sk, skb, req)) {
321 reqsk_free(req);
322 goto out;
323 }
324
325 req->expires = 0UL;
326 req->retrans = 0;
327
328 /*
329 * We need to lookup the route here to get at the correct
330 * window size. We should better make sure that the window size
331 * hasn't changed since we received the original syn, but I see
332 * no easy way to do this.
333 */
334 {
335 struct flowi fl = { .nl_u = { .ip4_u =
336 { .daddr = ((opt && opt->srr) ?
337 opt->faddr :
338 ireq->rmt_addr),
339 .saddr = ireq->loc_addr,
340 .tos = RT_CONN_FLAGS(sk) } },
341 .proto = IPPROTO_TCP,
342 .flags = inet_sk_flowi_flags(sk),
343 .uli_u = { .ports =
344 { .sport = th->dest,
345 .dport = th->source } } };
346 security_req_classify_flow(req, &fl);
347 if (ip_route_output_key(&init_net, &rt, &fl)) {
348 reqsk_free(req);
349 goto out;
350 }
351 }
352
353 /* Try to redo what tcp_v4_send_synack did. */
354 req->window_clamp = tp->window_clamp ? :dst_metric(&rt->u.dst, RTAX_WINDOW);
355
356 tcp_select_initial_window(tcp_full_space(sk), req->mss,
357 &req->rcv_wnd, &req->window_clamp,
358 ireq->wscale_ok, &rcv_wscale);
359
360 ireq->rcv_wscale = rcv_wscale;
361
362 ret = get_cookie_sock(sk, skb, req, &rt->u.dst);
363 out: return ret;
364 }
365
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