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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
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/siphash.h>
17#include <linux/kernel.h>
18#include <linux/export.h>
19#include <net/secure_seq.h>
20#include <net/tcp.h>
21#include <net/route.h>
22
23static siphash_key_t syncookie_secret[2] __read_mostly;
24
25#define COOKIEBITS 24 /* Upper bits store count */
26#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
27
28/* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
29 * stores TCP options:
30 *
31 * MSB LSB
32 * | 31 ... 6 | 5 | 4 | 3 2 1 0 |
33 * | Timestamp | ECN | SACK | WScale |
34 *
35 * When we receive a valid cookie-ACK, we look at the echoed tsval (if
36 * any) to figure out which TCP options we should use for the rebuilt
37 * connection.
38 *
39 * A WScale setting of '0xf' (which is an invalid scaling value)
40 * means that original syn did not include the TCP window scaling option.
41 */
42#define TS_OPT_WSCALE_MASK 0xf
43#define TS_OPT_SACK BIT(4)
44#define TS_OPT_ECN BIT(5)
45/* There is no TS_OPT_TIMESTAMP:
46 * if ACK contains timestamp option, we already know it was
47 * requested/supported by the syn/synack exchange.
48 */
49#define TSBITS 6
50#define TSMASK (((__u32)1 << TSBITS) - 1)
51
52static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
53 u32 count, int c)
54{
55 net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
56 return siphash_4u32((__force u32)saddr, (__force u32)daddr,
57 (__force u32)sport << 16 | (__force u32)dport,
58 count, &syncookie_secret[c]);
59}
60
61
62/*
63 * when syncookies are in effect and tcp timestamps are enabled we encode
64 * tcp options in the lower bits of the timestamp value that will be
65 * sent in the syn-ack.
66 * Since subsequent timestamps use the normal tcp_time_stamp value, we
67 * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
68 */
69u64 cookie_init_timestamp(struct request_sock *req)
70{
71 struct inet_request_sock *ireq;
72 u32 ts, ts_now = tcp_time_stamp_raw();
73 u32 options = 0;
74
75 ireq = inet_rsk(req);
76
77 options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
78 if (ireq->sack_ok)
79 options |= TS_OPT_SACK;
80 if (ireq->ecn_ok)
81 options |= TS_OPT_ECN;
82
83 ts = ts_now & ~TSMASK;
84 ts |= options;
85 if (ts > ts_now) {
86 ts >>= TSBITS;
87 ts--;
88 ts <<= TSBITS;
89 ts |= options;
90 }
91 return (u64)ts * (USEC_PER_SEC / TCP_TS_HZ);
92}
93
94
95static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
96 __be16 dport, __u32 sseq, __u32 data)
97{
98 /*
99 * Compute the secure sequence number.
100 * The output should be:
101 * HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
102 * + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
103 * Where sseq is their sequence number and count increases every
104 * minute by 1.
105 * As an extra hack, we add a small "data" value that encodes the
106 * MSS into the second hash value.
107 */
108 u32 count = tcp_cookie_time();
109 return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
110 sseq + (count << COOKIEBITS) +
111 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
112 & COOKIEMASK));
113}
114
115/*
116 * This retrieves the small "data" value from the syncookie.
117 * If the syncookie is bad, the data returned will be out of
118 * range. This must be checked by the caller.
119 *
120 * The count value used to generate the cookie must be less than
121 * MAX_SYNCOOKIE_AGE minutes in the past.
122 * The return value (__u32)-1 if this test fails.
123 */
124static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
125 __be16 sport, __be16 dport, __u32 sseq)
126{
127 u32 diff, count = tcp_cookie_time();
128
129 /* Strip away the layers from the cookie */
130 cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
131
132 /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
133 diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
134 if (diff >= MAX_SYNCOOKIE_AGE)
135 return (__u32)-1;
136
137 return (cookie -
138 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
139 & COOKIEMASK; /* Leaving the data behind */
140}
141
142/*
143 * MSS Values are chosen based on the 2011 paper
144 * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson.
145 * Values ..
146 * .. lower than 536 are rare (< 0.2%)
147 * .. between 537 and 1299 account for less than < 1.5% of observed values
148 * .. in the 1300-1349 range account for about 15 to 20% of observed mss values
149 * .. exceeding 1460 are very rare (< 0.04%)
150 *
151 * 1460 is the single most frequently announced mss value (30 to 46% depending
152 * on monitor location). Table must be sorted.
153 */
154static __u16 const msstab[] = {
155 536,
156 1300,
157 1440, /* 1440, 1452: PPPoE */
158 1460,
159};
160
161/*
162 * Generate a syncookie. mssp points to the mss, which is returned
163 * rounded down to the value encoded in the cookie.
164 */
165u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
166 u16 *mssp)
167{
168 int mssind;
169 const __u16 mss = *mssp;
170
171 for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
172 if (mss >= msstab[mssind])
173 break;
174 *mssp = msstab[mssind];
175
176 return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
177 th->source, th->dest, ntohl(th->seq),
178 mssind);
179}
180EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
181
182__u32 cookie_v4_init_sequence(const struct sk_buff *skb, __u16 *mssp)
183{
184 const struct iphdr *iph = ip_hdr(skb);
185 const struct tcphdr *th = tcp_hdr(skb);
186
187 return __cookie_v4_init_sequence(iph, th, mssp);
188}
189
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 */
194int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th,
195 u32 cookie)
196{
197 __u32 seq = ntohl(th->seq) - 1;
198 __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
199 th->source, th->dest, seq);
200
201 return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
202}
203EXPORT_SYMBOL_GPL(__cookie_v4_check);
204
205struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb,
206 struct request_sock *req,
207 struct dst_entry *dst, u32 tsoff)
208{
209 struct inet_connection_sock *icsk = inet_csk(sk);
210 struct sock *child;
211 bool own_req;
212
213 child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
214 NULL, &own_req);
215 if (child) {
216 refcount_set(&req->rsk_refcnt, 1);
217 tcp_sk(child)->tsoffset = tsoff;
218 sock_rps_save_rxhash(child, skb);
219 inet_csk_reqsk_queue_add(sk, req, child);
220 } else {
221 reqsk_free(req);
222 }
223 return child;
224}
225EXPORT_SYMBOL(tcp_get_cookie_sock);
226
227/*
228 * when syncookies are in effect and tcp timestamps are enabled we stored
229 * additional tcp options in the timestamp.
230 * This extracts these options from the timestamp echo.
231 *
232 * return false if we decode a tcp option that is disabled
233 * on the host.
234 */
235bool cookie_timestamp_decode(const struct net *net,
236 struct tcp_options_received *tcp_opt)
237{
238 /* echoed timestamp, lowest bits contain options */
239 u32 options = tcp_opt->rcv_tsecr;
240
241 if (!tcp_opt->saw_tstamp) {
242 tcp_clear_options(tcp_opt);
243 return true;
244 }
245
246 if (!net->ipv4.sysctl_tcp_timestamps)
247 return false;
248
249 tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;
250
251 if (tcp_opt->sack_ok && !net->ipv4.sysctl_tcp_sack)
252 return false;
253
254 if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK)
255 return true; /* no window scaling */
256
257 tcp_opt->wscale_ok = 1;
258 tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK;
259
260 return net->ipv4.sysctl_tcp_window_scaling != 0;
261}
262EXPORT_SYMBOL(cookie_timestamp_decode);
263
264bool cookie_ecn_ok(const struct tcp_options_received *tcp_opt,
265 const struct net *net, const struct dst_entry *dst)
266{
267 bool ecn_ok = tcp_opt->rcv_tsecr & TS_OPT_ECN;
268
269 if (!ecn_ok)
270 return false;
271
272 if (net->ipv4.sysctl_tcp_ecn)
273 return true;
274
275 return dst_feature(dst, RTAX_FEATURE_ECN);
276}
277EXPORT_SYMBOL(cookie_ecn_ok);
278
279/* On input, sk is a listener.
280 * Output is listener if incoming packet would not create a child
281 * NULL if memory could not be allocated.
282 */
283struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
284{
285 struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt;
286 struct tcp_options_received tcp_opt;
287 struct inet_request_sock *ireq;
288 struct tcp_request_sock *treq;
289 struct tcp_sock *tp = tcp_sk(sk);
290 const struct tcphdr *th = tcp_hdr(skb);
291 __u32 cookie = ntohl(th->ack_seq) - 1;
292 struct sock *ret = sk;
293 struct request_sock *req;
294 int mss;
295 struct rtable *rt;
296 __u8 rcv_wscale;
297 struct flowi4 fl4;
298 u32 tsoff = 0;
299
300 if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst)
301 goto out;
302
303 if (tcp_synq_no_recent_overflow(sk))
304 goto out;
305
306 mss = __cookie_v4_check(ip_hdr(skb), th, cookie);
307 if (mss == 0) {
308 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
309 goto out;
310 }
311
312 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
313
314 /* check for timestamp cookie support */
315 memset(&tcp_opt, 0, sizeof(tcp_opt));
316 tcp_parse_options(sock_net(sk), skb, &tcp_opt, 0, NULL);
317
318 if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
319 tsoff = secure_tcp_ts_off(sock_net(sk),
320 ip_hdr(skb)->daddr,
321 ip_hdr(skb)->saddr);
322 tcp_opt.rcv_tsecr -= tsoff;
323 }
324
325 if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
326 goto out;
327
328 ret = NULL;
329 req = inet_reqsk_alloc(&tcp_request_sock_ops, sk, false); /* for safety */
330 if (!req)
331 goto out;
332
333 ireq = inet_rsk(req);
334 treq = tcp_rsk(req);
335 treq->rcv_isn = ntohl(th->seq) - 1;
336 treq->snt_isn = cookie;
337 treq->ts_off = 0;
338 treq->txhash = net_tx_rndhash();
339 req->mss = mss;
340 ireq->ir_num = ntohs(th->dest);
341 ireq->ir_rmt_port = th->source;
342 sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr);
343 sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr);
344 ireq->ir_mark = inet_request_mark(sk, skb);
345 ireq->snd_wscale = tcp_opt.snd_wscale;
346 ireq->sack_ok = tcp_opt.sack_ok;
347 ireq->wscale_ok = tcp_opt.wscale_ok;
348 ireq->tstamp_ok = tcp_opt.saw_tstamp;
349 req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
350 treq->snt_synack = 0;
351 treq->tfo_listener = false;
352 if (IS_ENABLED(CONFIG_SMC))
353 ireq->smc_ok = 0;
354
355 ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
356
357 /* We throwed the options of the initial SYN away, so we hope
358 * the ACK carries the same options again (see RFC1122 4.2.3.8)
359 */
360 RCU_INIT_POINTER(ireq->ireq_opt, tcp_v4_save_options(sock_net(sk), skb));
361
362 if (security_inet_conn_request(sk, skb, req)) {
363 reqsk_free(req);
364 goto out;
365 }
366
367 req->num_retrans = 0;
368
369 /*
370 * We need to lookup the route here to get at the correct
371 * window size. We should better make sure that the window size
372 * hasn't changed since we received the original syn, but I see
373 * no easy way to do this.
374 */
375 flowi4_init_output(&fl4, ireq->ir_iif, ireq->ir_mark,
376 RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
377 inet_sk_flowi_flags(sk),
378 opt->srr ? opt->faddr : ireq->ir_rmt_addr,
379 ireq->ir_loc_addr, th->source, th->dest, sk->sk_uid);
380 security_req_classify_flow(req, flowi4_to_flowi(&fl4));
381 rt = ip_route_output_key(sock_net(sk), &fl4);
382 if (IS_ERR(rt)) {
383 reqsk_free(req);
384 goto out;
385 }
386
387 /* Try to redo what tcp_v4_send_synack did. */
388 req->rsk_window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
389
390 tcp_select_initial_window(sk, tcp_full_space(sk), req->mss,
391 &req->rsk_rcv_wnd, &req->rsk_window_clamp,
392 ireq->wscale_ok, &rcv_wscale,
393 dst_metric(&rt->dst, RTAX_INITRWND));
394
395 ireq->rcv_wscale = rcv_wscale;
396 ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), &rt->dst);
397
398 ret = tcp_get_cookie_sock(sk, skb, req, &rt->dst, tsoff);
399 /* ip_queue_xmit() depends on our flow being setup
400 * Normal sockets get it right from inet_csk_route_child_sock()
401 */
402 if (ret)
403 inet_sk(ret)->cork.fl.u.ip4 = fl4;
404out: return ret;
405}