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. | |
e905a9ed | 11 | * |
1da177e4 | 12 | * $Id: syncookies.c,v 1.18 2002/02/01 22:01:04 davem Exp $ |
1da177e4 LT |
13 | */ |
14 | ||
15 | #include <linux/tcp.h> | |
16 | #include <linux/slab.h> | |
17 | #include <linux/random.h> | |
18 | #include <linux/cryptohash.h> | |
19 | #include <linux/kernel.h> | |
20 | #include <net/tcp.h> | |
21 | ||
22 | extern int sysctl_tcp_syncookies; | |
23 | ||
2051f11f | 24 | __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS]; |
c6aefafb | 25 | EXPORT_SYMBOL(syncookie_secret); |
1da177e4 LT |
26 | |
27 | static __init int init_syncookies(void) | |
28 | { | |
29 | get_random_bytes(syncookie_secret, sizeof(syncookie_secret)); | |
30 | return 0; | |
31 | } | |
c6aefafb | 32 | __initcall(init_syncookies); |
1da177e4 LT |
33 | |
34 | #define COOKIEBITS 24 /* Upper bits store count */ | |
35 | #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1) | |
36 | ||
11baab7a ED |
37 | static DEFINE_PER_CPU(__u32, cookie_scratch)[16 + 5 + SHA_WORKSPACE_WORDS]; |
38 | ||
714e85be | 39 | static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport, |
1da177e4 LT |
40 | u32 count, int c) |
41 | { | |
11baab7a | 42 | __u32 *tmp = __get_cpu_var(cookie_scratch); |
1da177e4 | 43 | |
2051f11f | 44 | memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c])); |
714e85be AV |
45 | tmp[0] = (__force u32)saddr; |
46 | tmp[1] = (__force u32)daddr; | |
47 | tmp[2] = ((__force u32)sport << 16) + (__force u32)dport; | |
1da177e4 LT |
48 | tmp[3] = count; |
49 | sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5); | |
50 | ||
51 | return tmp[17]; | |
52 | } | |
53 | ||
714e85be AV |
54 | static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport, |
55 | __be16 dport, __u32 sseq, __u32 count, | |
1da177e4 LT |
56 | __u32 data) |
57 | { | |
58 | /* | |
59 | * Compute the secure sequence number. | |
60 | * The output should be: | |
e905a9ed | 61 | * HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24) |
1da177e4 LT |
62 | * + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24). |
63 | * Where sseq is their sequence number and count increases every | |
64 | * minute by 1. | |
65 | * As an extra hack, we add a small "data" value that encodes the | |
66 | * MSS into the second hash value. | |
67 | */ | |
68 | ||
69 | return (cookie_hash(saddr, daddr, sport, dport, 0, 0) + | |
70 | sseq + (count << COOKIEBITS) + | |
71 | ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data) | |
72 | & COOKIEMASK)); | |
73 | } | |
74 | ||
75 | /* | |
76 | * This retrieves the small "data" value from the syncookie. | |
77 | * If the syncookie is bad, the data returned will be out of | |
78 | * range. This must be checked by the caller. | |
79 | * | |
80 | * The count value used to generate the cookie must be within | |
81 | * "maxdiff" if the current (passed-in) "count". The return value | |
82 | * is (__u32)-1 if this test fails. | |
83 | */ | |
714e85be AV |
84 | static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr, |
85 | __be16 sport, __be16 dport, __u32 sseq, | |
1da177e4 LT |
86 | __u32 count, __u32 maxdiff) |
87 | { | |
88 | __u32 diff; | |
89 | ||
90 | /* Strip away the layers from the cookie */ | |
91 | cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq; | |
92 | ||
93 | /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */ | |
94 | diff = (count - (cookie >> COOKIEBITS)) & ((__u32) - 1 >> COOKIEBITS); | |
95 | if (diff >= maxdiff) | |
96 | return (__u32)-1; | |
97 | ||
98 | return (cookie - | |
99 | cookie_hash(saddr, daddr, sport, dport, count - diff, 1)) | |
100 | & COOKIEMASK; /* Leaving the data behind */ | |
101 | } | |
102 | ||
e905a9ed | 103 | /* |
1da177e4 LT |
104 | * This table has to be sorted and terminated with (__u16)-1. |
105 | * XXX generate a better table. | |
106 | * Unresolved Issues: HIPPI with a 64k MSS is not well supported. | |
107 | */ | |
108 | static __u16 const msstab[] = { | |
109 | 64 - 1, | |
e905a9ed | 110 | 256 - 1, |
1da177e4 LT |
111 | 512 - 1, |
112 | 536 - 1, | |
e905a9ed | 113 | 1024 - 1, |
1da177e4 LT |
114 | 1440 - 1, |
115 | 1460 - 1, | |
116 | 4312 - 1, | |
117 | (__u16)-1 | |
118 | }; | |
119 | /* The number doesn't include the -1 terminator */ | |
120 | #define NUM_MSS (ARRAY_SIZE(msstab) - 1) | |
121 | ||
122 | /* | |
123 | * Generate a syncookie. mssp points to the mss, which is returned | |
124 | * rounded down to the value encoded in the cookie. | |
125 | */ | |
126 | __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp) | |
127 | { | |
128 | struct tcp_sock *tp = tcp_sk(sk); | |
aa8223c7 ACM |
129 | const struct iphdr *iph = ip_hdr(skb); |
130 | const struct tcphdr *th = tcp_hdr(skb); | |
1da177e4 LT |
131 | int mssind; |
132 | const __u16 mss = *mssp; | |
133 | ||
1da177e4 LT |
134 | tp->last_synq_overflow = jiffies; |
135 | ||
136 | /* XXX sort msstab[] by probability? Binary search? */ | |
137 | for (mssind = 0; mss > msstab[mssind + 1]; mssind++) | |
138 | ; | |
139 | *mssp = msstab[mssind] + 1; | |
140 | ||
141 | NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESSENT); | |
142 | ||
aa8223c7 ACM |
143 | return secure_tcp_syn_cookie(iph->saddr, iph->daddr, |
144 | th->source, th->dest, ntohl(th->seq), | |
1da177e4 LT |
145 | jiffies / (HZ * 60), mssind); |
146 | } | |
147 | ||
e905a9ed | 148 | /* |
1da177e4 LT |
149 | * This (misnamed) value is the age of syncookie which is permitted. |
150 | * Its ideal value should be dependent on TCP_TIMEOUT_INIT and | |
151 | * sysctl_tcp_retries1. It's a rather complicated formula (exponential | |
152 | * backoff) to compute at runtime so it's currently hardcoded here. | |
153 | */ | |
154 | #define COUNTER_TRIES 4 | |
e905a9ed YH |
155 | /* |
156 | * Check if a ack sequence number is a valid syncookie. | |
1da177e4 LT |
157 | * Return the decoded mss if it is, or 0 if not. |
158 | */ | |
159 | static inline int cookie_check(struct sk_buff *skb, __u32 cookie) | |
160 | { | |
aa8223c7 ACM |
161 | const struct iphdr *iph = ip_hdr(skb); |
162 | const struct tcphdr *th = tcp_hdr(skb); | |
163 | __u32 seq = ntohl(th->seq) - 1; | |
164 | __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr, | |
165 | th->source, th->dest, seq, | |
166 | jiffies / (HZ * 60), | |
167 | COUNTER_TRIES); | |
1da177e4 LT |
168 | |
169 | return mssind < NUM_MSS ? msstab[mssind] + 1 : 0; | |
170 | } | |
171 | ||
1da177e4 | 172 | static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb, |
60236fdd | 173 | struct request_sock *req, |
1da177e4 LT |
174 | struct dst_entry *dst) |
175 | { | |
8292a17a | 176 | struct inet_connection_sock *icsk = inet_csk(sk); |
1da177e4 LT |
177 | struct sock *child; |
178 | ||
8292a17a | 179 | child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst); |
1da177e4 | 180 | if (child) |
463c84b9 | 181 | inet_csk_reqsk_queue_add(sk, req, child); |
1da177e4 | 182 | else |
60236fdd | 183 | reqsk_free(req); |
1da177e4 LT |
184 | |
185 | return child; | |
186 | } | |
187 | ||
188 | struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb, | |
189 | struct ip_options *opt) | |
190 | { | |
2e6599cb ACM |
191 | struct inet_request_sock *ireq; |
192 | struct tcp_request_sock *treq; | |
1da177e4 | 193 | struct tcp_sock *tp = tcp_sk(sk); |
aa8223c7 ACM |
194 | const struct tcphdr *th = tcp_hdr(skb); |
195 | __u32 cookie = ntohl(th->ack_seq) - 1; | |
1da177e4 | 196 | struct sock *ret = sk; |
e905a9ed YH |
197 | struct request_sock *req; |
198 | int mss; | |
199 | struct rtable *rt; | |
1da177e4 LT |
200 | __u8 rcv_wscale; |
201 | ||
aa8223c7 | 202 | if (!sysctl_tcp_syncookies || !th->ack) |
1da177e4 LT |
203 | goto out; |
204 | ||
e905a9ed | 205 | if (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) || |
1da177e4 | 206 | (mss = cookie_check(skb, cookie)) == 0) { |
e905a9ed | 207 | NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESFAILED); |
1da177e4 LT |
208 | goto out; |
209 | } | |
210 | ||
211 | NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESRECV); | |
212 | ||
1da177e4 | 213 | ret = NULL; |
60236fdd | 214 | req = reqsk_alloc(&tcp_request_sock_ops); /* for safety */ |
1da177e4 LT |
215 | if (!req) |
216 | goto out; | |
217 | ||
4237c75c VY |
218 | if (security_inet_conn_request(sk, skb, req)) { |
219 | reqsk_free(req); | |
220 | goto out; | |
221 | } | |
2e6599cb ACM |
222 | ireq = inet_rsk(req); |
223 | treq = tcp_rsk(req); | |
aa8223c7 | 224 | treq->rcv_isn = ntohl(th->seq) - 1; |
e905a9ed | 225 | treq->snt_isn = cookie; |
1da177e4 | 226 | req->mss = mss; |
aa8223c7 | 227 | ireq->rmt_port = th->source; |
eddc9ec5 ACM |
228 | ireq->loc_addr = ip_hdr(skb)->daddr; |
229 | ireq->rmt_addr = ip_hdr(skb)->saddr; | |
2e6599cb | 230 | ireq->opt = NULL; |
1da177e4 LT |
231 | |
232 | /* We throwed the options of the initial SYN away, so we hope | |
233 | * the ACK carries the same options again (see RFC1122 4.2.3.8) | |
234 | */ | |
235 | if (opt && opt->optlen) { | |
236 | int opt_size = sizeof(struct ip_options) + opt->optlen; | |
237 | ||
2e6599cb ACM |
238 | ireq->opt = kmalloc(opt_size, GFP_ATOMIC); |
239 | if (ireq->opt != NULL && ip_options_echo(ireq->opt, skb)) { | |
240 | kfree(ireq->opt); | |
241 | ireq->opt = NULL; | |
1da177e4 LT |
242 | } |
243 | } | |
244 | ||
2e6599cb | 245 | ireq->snd_wscale = ireq->rcv_wscale = ireq->tstamp_ok = 0; |
e905a9ed YH |
246 | ireq->wscale_ok = ireq->sack_ok = 0; |
247 | req->expires = 0UL; | |
248 | req->retrans = 0; | |
249 | ||
1da177e4 LT |
250 | /* |
251 | * We need to lookup the route here to get at the correct | |
252 | * window size. We should better make sure that the window size | |
253 | * hasn't changed since we received the original syn, but I see | |
e905a9ed | 254 | * no easy way to do this. |
1da177e4 LT |
255 | */ |
256 | { | |
257 | struct flowi fl = { .nl_u = { .ip4_u = | |
258 | { .daddr = ((opt && opt->srr) ? | |
259 | opt->faddr : | |
2e6599cb ACM |
260 | ireq->rmt_addr), |
261 | .saddr = ireq->loc_addr, | |
1da177e4 LT |
262 | .tos = RT_CONN_FLAGS(sk) } }, |
263 | .proto = IPPROTO_TCP, | |
264 | .uli_u = { .ports = | |
aa8223c7 ACM |
265 | { .sport = th->dest, |
266 | .dport = th->source } } }; | |
4237c75c | 267 | security_req_classify_flow(req, &fl); |
f206351a | 268 | if (ip_route_output_key(&init_net, &rt, &fl)) { |
60236fdd | 269 | reqsk_free(req); |
e905a9ed | 270 | goto out; |
1da177e4 LT |
271 | } |
272 | } | |
273 | ||
274 | /* Try to redo what tcp_v4_send_synack did. */ | |
275 | req->window_clamp = dst_metric(&rt->u.dst, RTAX_WINDOW); | |
276 | tcp_select_initial_window(tcp_full_space(sk), req->mss, | |
e905a9ed | 277 | &req->rcv_wnd, &req->window_clamp, |
1da177e4 LT |
278 | 0, &rcv_wscale); |
279 | /* BTW win scale with syncookies is 0 by definition */ | |
e905a9ed | 280 | ireq->rcv_wscale = rcv_wscale; |
1da177e4 LT |
281 | |
282 | ret = get_cookie_sock(sk, skb, req, &rt->u.dst); | |
283 | out: return ret; | |
284 | } |