Loading...
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
26extern int sysctl_tcp_syncookies;
27
28static 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
33static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
34 ipv4_cookie_scratch);
35
36static 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
86static __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 */
115static __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 */
145static __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 */
156u32 __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}
171EXPORT_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 */
188int __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}
197EXPORT_SYMBOL_GPL(__cookie_v4_check);
198
199static 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 */
226bool 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}
255EXPORT_SYMBOL(cookie_check_timestamp);
256
257struct 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;
370out: return ret;
371}
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 bits store TCP options */
22#define TSBITS 6
23#define TSMASK (((__u32)1 << TSBITS) - 1)
24
25extern int sysctl_tcp_syncookies;
26
27__u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
28EXPORT_SYMBOL(syncookie_secret);
29
30static __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
40static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
41 ipv4_cookie_scratch);
42
43static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
44 u32 count, int c)
45{
46 __u32 *tmp = __get_cpu_var(ipv4_cookie_scratch);
47
48 memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
49 tmp[0] = (__force u32)saddr;
50 tmp[1] = (__force u32)daddr;
51 tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
52 tmp[3] = count;
53 sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
54
55 return tmp[17];
56}
57
58
59/*
60 * when syncookies are in effect and tcp timestamps are enabled we encode
61 * tcp options in the lower bits of the timestamp value that will be
62 * sent in the syn-ack.
63 * Since subsequent timestamps use the normal tcp_time_stamp value, we
64 * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
65 */
66__u32 cookie_init_timestamp(struct request_sock *req)
67{
68 struct inet_request_sock *ireq;
69 u32 ts, ts_now = tcp_time_stamp;
70 u32 options = 0;
71
72 ireq = inet_rsk(req);
73
74 options = ireq->wscale_ok ? ireq->snd_wscale : 0xf;
75 options |= ireq->sack_ok << 4;
76 options |= ireq->ecn_ok << 5;
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
90static __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 */
120static __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 * MSS Values are taken from the 2009 paper
141 * 'Measuring TCP Maximum Segment Size' by S. Alcock and R. Nelson:
142 * - values 1440 to 1460 accounted for 80% of observed mss values
143 * - values outside the 536-1460 range are rare (<0.2%).
144 *
145 * Table must be sorted.
146 */
147static __u16 const msstab[] = {
148 64,
149 512,
150 536,
151 1024,
152 1440,
153 1460,
154 4312,
155 8960,
156};
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 for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
172 if (mss >= msstab[mssind])
173 break;
174 *mssp = msstab[mssind];
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 */
194static 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 < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
205}
206
207static 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 store snd_wscale.
230 * next 2 bits indicate SACK and ECN support.
231 *
232 * return false if we decode an option that should not be.
233 */
234bool cookie_check_timestamp(struct tcp_options_received *tcp_opt, bool *ecn_ok)
235{
236 /* echoed timestamp, lowest bits contain options */
237 u32 options = tcp_opt->rcv_tsecr & TSMASK;
238
239 if (!tcp_opt->saw_tstamp) {
240 tcp_clear_options(tcp_opt);
241 return true;
242 }
243
244 if (!sysctl_tcp_timestamps)
245 return false;
246
247 tcp_opt->sack_ok = (options >> 4) & 0x1;
248 *ecn_ok = (options >> 5) & 1;
249 if (*ecn_ok && !sysctl_tcp_ecn)
250 return false;
251
252 if (tcp_opt->sack_ok && !sysctl_tcp_sack)
253 return false;
254
255 if ((options & 0xf) == 0xf)
256 return true; /* no window scaling */
257
258 tcp_opt->wscale_ok = 1;
259 tcp_opt->snd_wscale = options & 0xf;
260 return sysctl_tcp_window_scaling != 0;
261}
262EXPORT_SYMBOL(cookie_check_timestamp);
263
264struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
265 struct ip_options *opt)
266{
267 struct tcp_options_received tcp_opt;
268 u8 *hash_location;
269 struct inet_request_sock *ireq;
270 struct tcp_request_sock *treq;
271 struct tcp_sock *tp = tcp_sk(sk);
272 const struct tcphdr *th = tcp_hdr(skb);
273 __u32 cookie = ntohl(th->ack_seq) - 1;
274 struct sock *ret = sk;
275 struct request_sock *req;
276 int mss;
277 struct rtable *rt;
278 __u8 rcv_wscale;
279 bool ecn_ok = false;
280
281 if (!sysctl_tcp_syncookies || !th->ack || th->rst)
282 goto out;
283
284 if (tcp_synq_no_recent_overflow(sk) ||
285 (mss = cookie_check(skb, cookie)) == 0) {
286 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
287 goto out;
288 }
289
290 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
291
292 /* check for timestamp cookie support */
293 memset(&tcp_opt, 0, sizeof(tcp_opt));
294 tcp_parse_options(skb, &tcp_opt, &hash_location, 0);
295
296 if (!cookie_check_timestamp(&tcp_opt, &ecn_ok))
297 goto out;
298
299 ret = NULL;
300 req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
301 if (!req)
302 goto out;
303
304 ireq = inet_rsk(req);
305 treq = tcp_rsk(req);
306 treq->rcv_isn = ntohl(th->seq) - 1;
307 treq->snt_isn = cookie;
308 req->mss = mss;
309 ireq->loc_port = th->dest;
310 ireq->rmt_port = th->source;
311 ireq->loc_addr = ip_hdr(skb)->daddr;
312 ireq->rmt_addr = ip_hdr(skb)->saddr;
313 ireq->ecn_ok = ecn_ok;
314 ireq->snd_wscale = tcp_opt.snd_wscale;
315 ireq->sack_ok = tcp_opt.sack_ok;
316 ireq->wscale_ok = tcp_opt.wscale_ok;
317 ireq->tstamp_ok = tcp_opt.saw_tstamp;
318 req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
319 treq->snt_synack = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
320
321 /* We throwed the options of the initial SYN away, so we hope
322 * the ACK carries the same options again (see RFC1122 4.2.3.8)
323 */
324 if (opt && opt->optlen) {
325 int opt_size = sizeof(struct ip_options_rcu) + opt->optlen;
326
327 ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
328 if (ireq->opt != NULL && ip_options_echo(&ireq->opt->opt, skb)) {
329 kfree(ireq->opt);
330 ireq->opt = NULL;
331 }
332 }
333
334 if (security_inet_conn_request(sk, skb, req)) {
335 reqsk_free(req);
336 goto out;
337 }
338
339 req->expires = 0UL;
340 req->retrans = 0;
341
342 /*
343 * We need to lookup the route here to get at the correct
344 * window size. We should better make sure that the window size
345 * hasn't changed since we received the original syn, but I see
346 * no easy way to do this.
347 */
348 {
349 struct flowi4 fl4;
350
351 flowi4_init_output(&fl4, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
352 RT_SCOPE_UNIVERSE, IPPROTO_TCP,
353 inet_sk_flowi_flags(sk),
354 (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
355 ireq->loc_addr, th->source, th->dest);
356 security_req_classify_flow(req, flowi4_to_flowi(&fl4));
357 rt = ip_route_output_key(sock_net(sk), &fl4);
358 if (IS_ERR(rt)) {
359 reqsk_free(req);
360 goto out;
361 }
362 }
363
364 /* Try to redo what tcp_v4_send_synack did. */
365 req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
366
367 tcp_select_initial_window(tcp_full_space(sk), req->mss,
368 &req->rcv_wnd, &req->window_clamp,
369 ireq->wscale_ok, &rcv_wscale,
370 dst_metric(&rt->dst, RTAX_INITRWND));
371
372 ireq->rcv_wscale = rcv_wscale;
373
374 ret = get_cookie_sock(sk, skb, req, &rt->dst);
375out: return ret;
376}