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Commit | Line | Data |
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1da177e4 LT |
1 | /* |
2 | * Syncookies implementation for the Linux kernel | |
3 | * | |
4 | * Copyright (C) 1997 Andi Kleen | |
e905a9ed | 5 | * Based on ideas by D.J.Bernstein and Eric Schenk. |
1da177e4 LT |
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. | |
1da177e4 LT |
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> | |
bc3b2d7f | 18 | #include <linux/export.h> |
1da177e4 | 19 | #include <net/tcp.h> |
86b08d86 | 20 | #include <net/route.h> |
1da177e4 | 21 | |
734f614b | 22 | /* Timestamps: lowest bits store TCP options */ |
172d69e6 | 23 | #define TSBITS 6 |
4dfc2817 FW |
24 | #define TSMASK (((__u32)1 << TSBITS) - 1) |
25 | ||
1da177e4 LT |
26 | extern int sysctl_tcp_syncookies; |
27 | ||
2051f11f | 28 | __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS]; |
c6aefafb | 29 | EXPORT_SYMBOL(syncookie_secret); |
1da177e4 LT |
30 | |
31 | static __init int init_syncookies(void) | |
32 | { | |
33 | get_random_bytes(syncookie_secret, sizeof(syncookie_secret)); | |
34 | return 0; | |
35 | } | |
c6aefafb | 36 | __initcall(init_syncookies); |
1da177e4 LT |
37 | |
38 | #define COOKIEBITS 24 /* Upper bits store count */ | |
39 | #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1) | |
40 | ||
245b2e70 TH |
41 | static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS], |
42 | ipv4_cookie_scratch); | |
11baab7a | 43 | |
714e85be | 44 | static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport, |
1da177e4 LT |
45 | u32 count, int c) |
46 | { | |
245b2e70 | 47 | __u32 *tmp = __get_cpu_var(ipv4_cookie_scratch); |
1da177e4 | 48 | |
2051f11f | 49 | memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c])); |
714e85be AV |
50 | tmp[0] = (__force u32)saddr; |
51 | tmp[1] = (__force u32)daddr; | |
52 | tmp[2] = ((__force u32)sport << 16) + (__force u32)dport; | |
1da177e4 LT |
53 | tmp[3] = count; |
54 | sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5); | |
55 | ||
56 | return tmp[17]; | |
57 | } | |
58 | ||
4dfc2817 FW |
59 | |
60 | /* | |
61 | * when syncookies are in effect and tcp timestamps are enabled we encode | |
734f614b | 62 | * tcp options in the lower bits of the timestamp value that will be |
4dfc2817 FW |
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); | |
734f614b FW |
74 | |
75 | options = ireq->wscale_ok ? ireq->snd_wscale : 0xf; | |
76 | options |= ireq->sack_ok << 4; | |
172d69e6 | 77 | options |= ireq->ecn_ok << 5; |
4dfc2817 FW |
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 | ||
714e85be | 91 | static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport, |
8c27bd75 | 92 | __be16 dport, __u32 sseq, __u32 data) |
1da177e4 LT |
93 | { |
94 | /* | |
95 | * Compute the secure sequence number. | |
96 | * The output should be: | |
e905a9ed | 97 | * HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24) |
1da177e4 LT |
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 | */ | |
8c27bd75 | 104 | u32 count = tcp_cookie_time(); |
1da177e4 LT |
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 | * | |
8c27bd75 FW |
116 | * The count value used to generate the cookie must be less than |
117 | * MAX_SYNCOOKIE_AGE minutes in the past. | |
118 | * The return value (__u32)-1 if this test fails. | |
1da177e4 | 119 | */ |
714e85be | 120 | static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr, |
8c27bd75 | 121 | __be16 sport, __be16 dport, __u32 sseq) |
1da177e4 | 122 | { |
8c27bd75 | 123 | u32 diff, count = tcp_cookie_time(); |
1da177e4 LT |
124 | |
125 | /* Strip away the layers from the cookie */ | |
126 | cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq; | |
127 | ||
128 | /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */ | |
129 | diff = (count - (cookie >> COOKIEBITS)) & ((__u32) - 1 >> COOKIEBITS); | |
8c27bd75 | 130 | if (diff >= MAX_SYNCOOKIE_AGE) |
1da177e4 LT |
131 | return (__u32)-1; |
132 | ||
133 | return (cookie - | |
134 | cookie_hash(saddr, daddr, sport, dport, count - diff, 1)) | |
135 | & COOKIEMASK; /* Leaving the data behind */ | |
136 | } | |
137 | ||
e905a9ed | 138 | /* |
08629354 FW |
139 | * MSS Values are chosen based on the 2011 paper |
140 | * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson. | |
141 | * Values .. | |
142 | * .. lower than 536 are rare (< 0.2%) | |
143 | * .. between 537 and 1299 account for less than < 1.5% of observed values | |
144 | * .. in the 1300-1349 range account for about 15 to 20% of observed mss values | |
145 | * .. exceeding 1460 are very rare (< 0.04%) | |
5918e2fb | 146 | * |
08629354 FW |
147 | * 1460 is the single most frequently announced mss value (30 to 46% depending |
148 | * on monitor location). Table must be sorted. | |
1da177e4 LT |
149 | */ |
150 | static __u16 const msstab[] = { | |
5918e2fb | 151 | 536, |
08629354 FW |
152 | 1300, |
153 | 1440, /* 1440, 1452: PPPoE */ | |
5918e2fb | 154 | 1460, |
1da177e4 | 155 | }; |
1da177e4 LT |
156 | |
157 | /* | |
158 | * Generate a syncookie. mssp points to the mss, which is returned | |
159 | * rounded down to the value encoded in the cookie. | |
160 | */ | |
0198230b PM |
161 | u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th, |
162 | u16 *mssp) | |
1da177e4 | 163 | { |
1da177e4 LT |
164 | int mssind; |
165 | const __u16 mss = *mssp; | |
166 | ||
5918e2fb FW |
167 | for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--) |
168 | if (mss >= msstab[mssind]) | |
169 | break; | |
170 | *mssp = msstab[mssind]; | |
1da177e4 | 171 | |
aa8223c7 ACM |
172 | return secure_tcp_syn_cookie(iph->saddr, iph->daddr, |
173 | th->source, th->dest, ntohl(th->seq), | |
8c27bd75 | 174 | mssind); |
1da177e4 | 175 | } |
0198230b PM |
176 | EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence); |
177 | ||
178 | __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp) | |
179 | { | |
180 | const struct iphdr *iph = ip_hdr(skb); | |
181 | const struct tcphdr *th = tcp_hdr(skb); | |
182 | ||
183 | tcp_synq_overflow(sk); | |
184 | NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT); | |
185 | ||
186 | return __cookie_v4_init_sequence(iph, th, mssp); | |
187 | } | |
1da177e4 | 188 | |
e905a9ed YH |
189 | /* |
190 | * Check if a ack sequence number is a valid syncookie. | |
1da177e4 LT |
191 | * Return the decoded mss if it is, or 0 if not. |
192 | */ | |
0198230b PM |
193 | int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th, |
194 | u32 cookie) | |
1da177e4 | 195 | { |
aa8223c7 ACM |
196 | __u32 seq = ntohl(th->seq) - 1; |
197 | __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr, | |
8c27bd75 | 198 | th->source, th->dest, seq); |
1da177e4 | 199 | |
5918e2fb | 200 | return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0; |
1da177e4 | 201 | } |
0198230b | 202 | EXPORT_SYMBOL_GPL(__cookie_v4_check); |
1da177e4 | 203 | |
1da177e4 | 204 | static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb, |
60236fdd | 205 | struct request_sock *req, |
1da177e4 LT |
206 | struct dst_entry *dst) |
207 | { | |
8292a17a | 208 | struct inet_connection_sock *icsk = inet_csk(sk); |
1da177e4 LT |
209 | struct sock *child; |
210 | ||
8292a17a | 211 | child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst); |
1da177e4 | 212 | if (child) |
463c84b9 | 213 | inet_csk_reqsk_queue_add(sk, req, child); |
1da177e4 | 214 | else |
60236fdd | 215 | reqsk_free(req); |
1da177e4 LT |
216 | |
217 | return child; | |
218 | } | |
219 | ||
4dfc2817 FW |
220 | |
221 | /* | |
222 | * when syncookies are in effect and tcp timestamps are enabled we stored | |
223 | * additional tcp options in the timestamp. | |
224 | * This extracts these options from the timestamp echo. | |
225 | * | |
734f614b | 226 | * The lowest 4 bits store snd_wscale. |
172d69e6 | 227 | * next 2 bits indicate SACK and ECN support. |
8c763681 FW |
228 | * |
229 | * return false if we decode an option that should not be. | |
4dfc2817 | 230 | */ |
5d134f1c HFS |
231 | bool cookie_check_timestamp(struct tcp_options_received *tcp_opt, |
232 | struct net *net, bool *ecn_ok) | |
4dfc2817 | 233 | { |
734f614b | 234 | /* echoed timestamp, lowest bits contain options */ |
4dfc2817 FW |
235 | u32 options = tcp_opt->rcv_tsecr & TSMASK; |
236 | ||
8c763681 FW |
237 | if (!tcp_opt->saw_tstamp) { |
238 | tcp_clear_options(tcp_opt); | |
239 | return true; | |
240 | } | |
241 | ||
242 | if (!sysctl_tcp_timestamps) | |
243 | return false; | |
244 | ||
ab56222a | 245 | tcp_opt->sack_ok = (options & (1 << 4)) ? TCP_SACK_SEEN : 0; |
172d69e6 | 246 | *ecn_ok = (options >> 5) & 1; |
5d134f1c | 247 | if (*ecn_ok && !net->ipv4.sysctl_tcp_ecn) |
172d69e6 | 248 | return false; |
4dfc2817 | 249 | |
8c763681 FW |
250 | if (tcp_opt->sack_ok && !sysctl_tcp_sack) |
251 | return false; | |
4dfc2817 | 252 | |
734f614b FW |
253 | if ((options & 0xf) == 0xf) |
254 | return true; /* no window scaling */ | |
255 | ||
256 | tcp_opt->wscale_ok = 1; | |
257 | tcp_opt->snd_wscale = options & 0xf; | |
258 | return sysctl_tcp_window_scaling != 0; | |
4dfc2817 FW |
259 | } |
260 | EXPORT_SYMBOL(cookie_check_timestamp); | |
261 | ||
1da177e4 LT |
262 | struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb, |
263 | struct ip_options *opt) | |
264 | { | |
4957faad | 265 | struct tcp_options_received tcp_opt; |
2e6599cb ACM |
266 | struct inet_request_sock *ireq; |
267 | struct tcp_request_sock *treq; | |
1da177e4 | 268 | struct tcp_sock *tp = tcp_sk(sk); |
aa8223c7 ACM |
269 | const struct tcphdr *th = tcp_hdr(skb); |
270 | __u32 cookie = ntohl(th->ack_seq) - 1; | |
1da177e4 | 271 | struct sock *ret = sk; |
e905a9ed YH |
272 | struct request_sock *req; |
273 | int mss; | |
274 | struct rtable *rt; | |
1da177e4 | 275 | __u8 rcv_wscale; |
f0e3d068 | 276 | bool ecn_ok = false; |
dfd25fff | 277 | struct flowi4 fl4; |
1da177e4 | 278 | |
af9b4738 | 279 | if (!sysctl_tcp_syncookies || !th->ack || th->rst) |
1da177e4 LT |
280 | goto out; |
281 | ||
a0f82f64 | 282 | if (tcp_synq_no_recent_overflow(sk) || |
0198230b | 283 | (mss = __cookie_v4_check(ip_hdr(skb), th, cookie)) == 0) { |
de0744af | 284 | NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED); |
1da177e4 LT |
285 | goto out; |
286 | } | |
287 | ||
de0744af | 288 | NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV); |
1da177e4 | 289 | |
bb5b7c11 DM |
290 | /* check for timestamp cookie support */ |
291 | memset(&tcp_opt, 0, sizeof(tcp_opt)); | |
1a2c6181 | 292 | tcp_parse_options(skb, &tcp_opt, 0, NULL); |
bb5b7c11 | 293 | |
5d134f1c | 294 | if (!cookie_check_timestamp(&tcp_opt, sock_net(sk), &ecn_ok)) |
8c763681 | 295 | goto out; |
bb5b7c11 | 296 | |
1da177e4 | 297 | ret = NULL; |
ce4a7d0d | 298 | req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */ |
1da177e4 LT |
299 | if (!req) |
300 | goto out; | |
301 | ||
2e6599cb ACM |
302 | ireq = inet_rsk(req); |
303 | treq = tcp_rsk(req); | |
aa8223c7 | 304 | treq->rcv_isn = ntohl(th->seq) - 1; |
e905a9ed | 305 | treq->snt_isn = cookie; |
1da177e4 | 306 | req->mss = mss; |
a3116ac5 | 307 | ireq->loc_port = th->dest; |
aa8223c7 | 308 | ireq->rmt_port = th->source; |
eddc9ec5 ACM |
309 | ireq->loc_addr = ip_hdr(skb)->daddr; |
310 | ireq->rmt_addr = ip_hdr(skb)->saddr; | |
172d69e6 | 311 | ireq->ecn_ok = ecn_ok; |
bb5b7c11 | 312 | ireq->snd_wscale = tcp_opt.snd_wscale; |
bb5b7c11 DM |
313 | ireq->sack_ok = tcp_opt.sack_ok; |
314 | ireq->wscale_ok = tcp_opt.wscale_ok; | |
315 | ireq->tstamp_ok = tcp_opt.saw_tstamp; | |
316 | req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0; | |
9ad7c049 | 317 | treq->snt_synack = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0; |
8336886f | 318 | treq->listener = NULL; |
1da177e4 LT |
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) { | |
f6d8bd05 | 324 | int opt_size = sizeof(struct ip_options_rcu) + opt->optlen; |
1da177e4 | 325 | |
2e6599cb | 326 | ireq->opt = kmalloc(opt_size, GFP_ATOMIC); |
f6d8bd05 | 327 | if (ireq->opt != NULL && ip_options_echo(&ireq->opt->opt, skb)) { |
2e6599cb ACM |
328 | kfree(ireq->opt); |
329 | ireq->opt = NULL; | |
1da177e4 LT |
330 | } |
331 | } | |
332 | ||
284904aa PM |
333 | if (security_inet_conn_request(sk, skb, req)) { |
334 | reqsk_free(req); | |
335 | goto out; | |
336 | } | |
337 | ||
e905a9ed | 338 | req->expires = 0UL; |
e6c022a4 | 339 | req->num_retrans = 0; |
e905a9ed | 340 | |
1da177e4 LT |
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 | |
e905a9ed | 345 | * no easy way to do this. |
1da177e4 | 346 | */ |
d66954a0 DP |
347 | flowi4_init_output(&fl4, sk->sk_bound_dev_if, sk->sk_mark, |
348 | RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP, | |
dfd25fff ED |
349 | inet_sk_flowi_flags(sk), |
350 | (opt && opt->srr) ? opt->faddr : ireq->rmt_addr, | |
351 | ireq->loc_addr, th->source, th->dest); | |
352 | security_req_classify_flow(req, flowi4_to_flowi(&fl4)); | |
353 | rt = ip_route_output_key(sock_net(sk), &fl4); | |
354 | if (IS_ERR(rt)) { | |
355 | reqsk_free(req); | |
356 | goto out; | |
1da177e4 LT |
357 | } |
358 | ||
359 | /* Try to redo what tcp_v4_send_synack did. */ | |
d8d1f30b | 360 | req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW); |
4dfc2817 | 361 | |
1da177e4 | 362 | tcp_select_initial_window(tcp_full_space(sk), req->mss, |
e905a9ed | 363 | &req->rcv_wnd, &req->window_clamp, |
31d12926 | 364 | ireq->wscale_ok, &rcv_wscale, |
d8d1f30b | 365 | dst_metric(&rt->dst, RTAX_INITRWND)); |
4dfc2817 | 366 | |
e905a9ed | 367 | ireq->rcv_wscale = rcv_wscale; |
1da177e4 | 368 | |
d8d1f30b | 369 | ret = get_cookie_sock(sk, skb, req, &rt->dst); |
dfd25fff ED |
370 | /* ip_queue_xmit() depends on our flow being setup |
371 | * Normal sockets get it right from inet_csk_route_child_sock() | |
372 | */ | |
373 | if (ret) | |
374 | inet_sk(ret)->cork.fl.u.ip4 = fl4; | |
1da177e4 LT |
375 | out: return ret; |
376 | } |