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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]; |
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 = __get_cpu_var(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, struct sk_buff *skb, __u16 *mssp) |
174 | { |
175 | const struct iphdr *iph = ip_hdr(skb); |
176 | const struct tcphdr *th = tcp_hdr(skb); |
177 | |
178 | tcp_synq_overflow(sk); |
179 | NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT); |
180 | |
181 | return __cookie_v4_init_sequence(iph, th, mssp); |
182 | } |
183 | |
184 | /* |
185 | * Check if a ack sequence number is a valid syncookie. |
186 | * Return the decoded mss if it is, or 0 if not. |
187 | */ |
188 | int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th, |
189 | u32 cookie) |
190 | { |
191 | __u32 seq = ntohl(th->seq) - 1; |
192 | __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr, |
193 | th->source, th->dest, seq); |
194 | |
195 | return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0; |
196 | } |
197 | EXPORT_SYMBOL_GPL(__cookie_v4_check); |
198 | |
199 | static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb, |
200 | struct request_sock *req, |
201 | struct dst_entry *dst) |
202 | { |
203 | struct inet_connection_sock *icsk = inet_csk(sk); |
204 | struct sock *child; |
205 | |
206 | child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst); |
207 | if (child) |
208 | inet_csk_reqsk_queue_add(sk, req, child); |
209 | else |
210 | reqsk_free(req); |
211 | |
212 | return child; |
213 | } |
214 | |
215 | |
216 | /* |
217 | * when syncookies are in effect and tcp timestamps are enabled we stored |
218 | * additional tcp options in the timestamp. |
219 | * This extracts these options from the timestamp echo. |
220 | * |
221 | * The lowest 4 bits store snd_wscale. |
222 | * next 2 bits indicate SACK and ECN support. |
223 | * |
224 | * return false if we decode an option that should not be. |
225 | */ |
226 | bool cookie_check_timestamp(struct tcp_options_received *tcp_opt, |
227 | struct net *net, bool *ecn_ok) |
228 | { |
229 | /* echoed timestamp, lowest bits contain options */ |
230 | u32 options = tcp_opt->rcv_tsecr & TSMASK; |
231 | |
232 | if (!tcp_opt->saw_tstamp) { |
233 | tcp_clear_options(tcp_opt); |
234 | return true; |
235 | } |
236 | |
237 | if (!sysctl_tcp_timestamps) |
238 | return false; |
239 | |
240 | tcp_opt->sack_ok = (options & (1 << 4)) ? TCP_SACK_SEEN : 0; |
241 | *ecn_ok = (options >> 5) & 1; |
242 | if (*ecn_ok && !net->ipv4.sysctl_tcp_ecn) |
243 | return false; |
244 | |
245 | if (tcp_opt->sack_ok && !sysctl_tcp_sack) |
246 | return false; |
247 | |
248 | if ((options & 0xf) == 0xf) |
249 | return true; /* no window scaling */ |
250 | |
251 | tcp_opt->wscale_ok = 1; |
252 | tcp_opt->snd_wscale = options & 0xf; |
253 | return sysctl_tcp_window_scaling != 0; |
254 | } |
255 | EXPORT_SYMBOL(cookie_check_timestamp); |
256 | |
257 | struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb, |
258 | struct ip_options *opt) |
259 | { |
260 | struct tcp_options_received tcp_opt; |
261 | struct inet_request_sock *ireq; |
262 | struct tcp_request_sock *treq; |
263 | struct tcp_sock *tp = tcp_sk(sk); |
264 | const struct tcphdr *th = tcp_hdr(skb); |
265 | __u32 cookie = ntohl(th->ack_seq) - 1; |
266 | struct sock *ret = sk; |
267 | struct request_sock *req; |
268 | int mss; |
269 | struct rtable *rt; |
270 | __u8 rcv_wscale; |
271 | bool ecn_ok = false; |
272 | struct flowi4 fl4; |
273 | |
274 | if (!sysctl_tcp_syncookies || !th->ack || th->rst) |
275 | goto out; |
276 | |
277 | if (tcp_synq_no_recent_overflow(sk) || |
278 | (mss = __cookie_v4_check(ip_hdr(skb), th, cookie)) == 0) { |
279 | NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED); |
280 | goto out; |
281 | } |
282 | |
283 | NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV); |
284 | |
285 | /* check for timestamp cookie support */ |
286 | memset(&tcp_opt, 0, sizeof(tcp_opt)); |
287 | tcp_parse_options(skb, &tcp_opt, 0, NULL); |
288 | |
289 | if (!cookie_check_timestamp(&tcp_opt, sock_net(sk), &ecn_ok)) |
290 | goto out; |
291 | |
292 | ret = NULL; |
293 | req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */ |
294 | if (!req) |
295 | goto out; |
296 | |
297 | ireq = inet_rsk(req); |
298 | treq = tcp_rsk(req); |
299 | treq->rcv_isn = ntohl(th->seq) - 1; |
300 | treq->snt_isn = cookie; |
301 | req->mss = mss; |
302 | ireq->ir_num = ntohs(th->dest); |
303 | ireq->ir_rmt_port = th->source; |
304 | ireq->ir_loc_addr = ip_hdr(skb)->daddr; |
305 | ireq->ir_rmt_addr = ip_hdr(skb)->saddr; |
306 | ireq->ecn_ok = ecn_ok; |
307 | ireq->snd_wscale = tcp_opt.snd_wscale; |
308 | ireq->sack_ok = tcp_opt.sack_ok; |
309 | ireq->wscale_ok = tcp_opt.wscale_ok; |
310 | ireq->tstamp_ok = tcp_opt.saw_tstamp; |
311 | req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0; |
312 | treq->snt_synack = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0; |
313 | treq->listener = NULL; |
314 | |
315 | /* We throwed the options of the initial SYN away, so we hope |
316 | * the ACK carries the same options again (see RFC1122 4.2.3.8) |
317 | */ |
318 | if (opt && opt->optlen) { |
319 | int opt_size = sizeof(struct ip_options_rcu) + opt->optlen; |
320 | |
321 | ireq->opt = kmalloc(opt_size, GFP_ATOMIC); |
322 | if (ireq->opt != NULL && ip_options_echo(&ireq->opt->opt, skb)) { |
323 | kfree(ireq->opt); |
324 | ireq->opt = NULL; |
325 | } |
326 | } |
327 | |
328 | if (security_inet_conn_request(sk, skb, req)) { |
329 | reqsk_free(req); |
330 | goto out; |
331 | } |
332 | |
333 | req->expires = 0UL; |
334 | req->num_retrans = 0; |
335 | |
336 | /* |
337 | * We need to lookup the route here to get at the correct |
338 | * window size. We should better make sure that the window size |
339 | * hasn't changed since we received the original syn, but I see |
340 | * no easy way to do this. |
341 | */ |
342 | flowi4_init_output(&fl4, sk->sk_bound_dev_if, sk->sk_mark, |
343 | RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP, |
344 | inet_sk_flowi_flags(sk), |
345 | (opt && opt->srr) ? opt->faddr : ireq->ir_rmt_addr, |
346 | ireq->ir_loc_addr, th->source, th->dest); |
347 | security_req_classify_flow(req, flowi4_to_flowi(&fl4)); |
348 | rt = ip_route_output_key(sock_net(sk), &fl4); |
349 | if (IS_ERR(rt)) { |
350 | reqsk_free(req); |
351 | goto out; |
352 | } |
353 | |
354 | /* Try to redo what tcp_v4_send_synack did. */ |
355 | req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW); |
356 | |
357 | tcp_select_initial_window(tcp_full_space(sk), req->mss, |
358 | &req->rcv_wnd, &req->window_clamp, |
359 | ireq->wscale_ok, &rcv_wscale, |
360 | dst_metric(&rt->dst, RTAX_INITRWND)); |
361 | |
362 | ireq->rcv_wscale = rcv_wscale; |
363 | |
364 | ret = get_cookie_sock(sk, skb, req, &rt->dst); |
365 | /* ip_queue_xmit() depends on our flow being setup |
366 | * Normal sockets get it right from inet_csk_route_child_sock() |
367 | */ |
368 | if (ret) |
369 | inet_sk(ret)->cork.fl.u.ip4 = fl4; |
370 | out: return ret; |
371 | } |
372 |
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