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1// SPDX-License-Identifier: GPL-2.0-or-later
2/*
3 * Syncookies implementation for the Linux kernel
4 *
5 * Copyright (C) 1997 Andi Kleen
6 * Based on ideas by D.J.Bernstein and Eric Schenk.
7 */
8
9#include <linux/tcp.h>
10#include <linux/siphash.h>
11#include <linux/kernel.h>
12#include <linux/export.h>
13#include <net/secure_seq.h>
14#include <net/tcp.h>
15#include <net/route.h>
16
17static siphash_aligned_key_t syncookie_secret[2];
18
19#define COOKIEBITS 24 /* Upper bits store count */
20#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
21
22/* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
23 * stores TCP options:
24 *
25 * MSB LSB
26 * | 31 ... 6 | 5 | 4 | 3 2 1 0 |
27 * | Timestamp | ECN | SACK | WScale |
28 *
29 * When we receive a valid cookie-ACK, we look at the echoed tsval (if
30 * any) to figure out which TCP options we should use for the rebuilt
31 * connection.
32 *
33 * A WScale setting of '0xf' (which is an invalid scaling value)
34 * means that original syn did not include the TCP window scaling option.
35 */
36#define TS_OPT_WSCALE_MASK 0xf
37#define TS_OPT_SACK BIT(4)
38#define TS_OPT_ECN BIT(5)
39/* There is no TS_OPT_TIMESTAMP:
40 * if ACK contains timestamp option, we already know it was
41 * requested/supported by the syn/synack exchange.
42 */
43#define TSBITS 6
44
45static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
46 u32 count, int c)
47{
48 net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
49 return siphash_4u32((__force u32)saddr, (__force u32)daddr,
50 (__force u32)sport << 16 | (__force u32)dport,
51 count, &syncookie_secret[c]);
52}
53
54/*
55 * when syncookies are in effect and tcp timestamps are enabled we encode
56 * tcp options in the lower bits of the timestamp value that will be
57 * sent in the syn-ack.
58 * Since subsequent timestamps use the normal tcp_time_stamp value, we
59 * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
60 */
61u64 cookie_init_timestamp(struct request_sock *req, u64 now)
62{
63 const struct inet_request_sock *ireq = inet_rsk(req);
64 u64 ts, ts_now = tcp_ns_to_ts(false, now);
65 u32 options = 0;
66
67 options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
68 if (ireq->sack_ok)
69 options |= TS_OPT_SACK;
70 if (ireq->ecn_ok)
71 options |= TS_OPT_ECN;
72
73 ts = (ts_now >> TSBITS) << TSBITS;
74 ts |= options;
75 if (ts > ts_now)
76 ts -= (1UL << TSBITS);
77
78 if (tcp_rsk(req)->req_usec_ts)
79 return ts * NSEC_PER_USEC;
80 return ts * NSEC_PER_MSEC;
81}
82
83
84static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
85 __be16 dport, __u32 sseq, __u32 data)
86{
87 /*
88 * Compute the secure sequence number.
89 * The output should be:
90 * HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
91 * + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
92 * Where sseq is their sequence number and count increases every
93 * minute by 1.
94 * As an extra hack, we add a small "data" value that encodes the
95 * MSS into the second hash value.
96 */
97 u32 count = tcp_cookie_time();
98 return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
99 sseq + (count << COOKIEBITS) +
100 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
101 & COOKIEMASK));
102}
103
104/*
105 * This retrieves the small "data" value from the syncookie.
106 * If the syncookie is bad, the data returned will be out of
107 * range. This must be checked by the caller.
108 *
109 * The count value used to generate the cookie must be less than
110 * MAX_SYNCOOKIE_AGE minutes in the past.
111 * The return value (__u32)-1 if this test fails.
112 */
113static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
114 __be16 sport, __be16 dport, __u32 sseq)
115{
116 u32 diff, count = tcp_cookie_time();
117
118 /* Strip away the layers from the cookie */
119 cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
120
121 /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
122 diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
123 if (diff >= MAX_SYNCOOKIE_AGE)
124 return (__u32)-1;
125
126 return (cookie -
127 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
128 & COOKIEMASK; /* Leaving the data behind */
129}
130
131/*
132 * MSS Values are chosen based on the 2011 paper
133 * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson.
134 * Values ..
135 * .. lower than 536 are rare (< 0.2%)
136 * .. between 537 and 1299 account for less than < 1.5% of observed values
137 * .. in the 1300-1349 range account for about 15 to 20% of observed mss values
138 * .. exceeding 1460 are very rare (< 0.04%)
139 *
140 * 1460 is the single most frequently announced mss value (30 to 46% depending
141 * on monitor location). Table must be sorted.
142 */
143static __u16 const msstab[] = {
144 536,
145 1300,
146 1440, /* 1440, 1452: PPPoE */
147 1460,
148};
149
150/*
151 * Generate a syncookie. mssp points to the mss, which is returned
152 * rounded down to the value encoded in the cookie.
153 */
154u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
155 u16 *mssp)
156{
157 int mssind;
158 const __u16 mss = *mssp;
159
160 for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
161 if (mss >= msstab[mssind])
162 break;
163 *mssp = msstab[mssind];
164
165 return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
166 th->source, th->dest, ntohl(th->seq),
167 mssind);
168}
169EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
170
171__u32 cookie_v4_init_sequence(const struct sk_buff *skb, __u16 *mssp)
172{
173 const struct iphdr *iph = ip_hdr(skb);
174 const struct tcphdr *th = tcp_hdr(skb);
175
176 return __cookie_v4_init_sequence(iph, th, mssp);
177}
178
179/*
180 * Check if a ack sequence number is a valid syncookie.
181 * Return the decoded mss if it is, or 0 if not.
182 */
183int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th)
184{
185 __u32 cookie = ntohl(th->ack_seq) - 1;
186 __u32 seq = ntohl(th->seq) - 1;
187 __u32 mssind;
188
189 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
190 th->source, th->dest, seq);
191
192 return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
193}
194EXPORT_SYMBOL_GPL(__cookie_v4_check);
195
196struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb,
197 struct request_sock *req,
198 struct dst_entry *dst)
199{
200 struct inet_connection_sock *icsk = inet_csk(sk);
201 struct sock *child;
202 bool own_req;
203
204 child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
205 NULL, &own_req);
206 if (child) {
207 refcount_set(&req->rsk_refcnt, 1);
208 sock_rps_save_rxhash(child, skb);
209
210 if (rsk_drop_req(req)) {
211 reqsk_put(req);
212 return child;
213 }
214
215 if (inet_csk_reqsk_queue_add(sk, req, child))
216 return child;
217
218 bh_unlock_sock(child);
219 sock_put(child);
220 }
221 __reqsk_free(req);
222
223 return NULL;
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 (!READ_ONCE(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 && !READ_ONCE(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 READ_ONCE(net->ipv4.sysctl_tcp_window_scaling) != 0;
261}
262EXPORT_SYMBOL(cookie_timestamp_decode);
263
264static int cookie_tcp_reqsk_init(struct sock *sk, struct sk_buff *skb,
265 struct request_sock *req)
266{
267 struct inet_request_sock *ireq = inet_rsk(req);
268 struct tcp_request_sock *treq = tcp_rsk(req);
269 const struct tcphdr *th = tcp_hdr(skb);
270
271 req->num_retrans = 0;
272
273 ireq->ir_num = ntohs(th->dest);
274 ireq->ir_rmt_port = th->source;
275 ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
276 ireq->ir_mark = inet_request_mark(sk, skb);
277
278 if (IS_ENABLED(CONFIG_SMC))
279 ireq->smc_ok = 0;
280
281 treq->snt_synack = 0;
282 treq->tfo_listener = false;
283 treq->txhash = net_tx_rndhash();
284 treq->rcv_isn = ntohl(th->seq) - 1;
285 treq->snt_isn = ntohl(th->ack_seq) - 1;
286 treq->syn_tos = TCP_SKB_CB(skb)->ip_dsfield;
287 treq->req_usec_ts = false;
288
289#if IS_ENABLED(CONFIG_MPTCP)
290 treq->is_mptcp = sk_is_mptcp(sk);
291 if (treq->is_mptcp)
292 return mptcp_subflow_init_cookie_req(req, sk, skb);
293#endif
294
295 return 0;
296}
297
298#if IS_ENABLED(CONFIG_BPF)
299struct request_sock *cookie_bpf_check(struct sock *sk, struct sk_buff *skb)
300{
301 struct request_sock *req = inet_reqsk(skb->sk);
302
303 skb->sk = NULL;
304 skb->destructor = NULL;
305
306 if (cookie_tcp_reqsk_init(sk, skb, req)) {
307 reqsk_free(req);
308 req = NULL;
309 }
310
311 return req;
312}
313EXPORT_SYMBOL_GPL(cookie_bpf_check);
314#endif
315
316struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
317 struct sock *sk, struct sk_buff *skb,
318 struct tcp_options_received *tcp_opt,
319 int mss, u32 tsoff)
320{
321 struct inet_request_sock *ireq;
322 struct tcp_request_sock *treq;
323 struct request_sock *req;
324
325 if (sk_is_mptcp(sk))
326 req = mptcp_subflow_reqsk_alloc(ops, sk, false);
327 else
328 req = inet_reqsk_alloc(ops, sk, false);
329
330 if (!req)
331 return NULL;
332
333 if (cookie_tcp_reqsk_init(sk, skb, req)) {
334 reqsk_free(req);
335 return NULL;
336 }
337
338 ireq = inet_rsk(req);
339 treq = tcp_rsk(req);
340
341 req->mss = mss;
342 req->ts_recent = tcp_opt->saw_tstamp ? tcp_opt->rcv_tsval : 0;
343
344 ireq->snd_wscale = tcp_opt->snd_wscale;
345 ireq->tstamp_ok = tcp_opt->saw_tstamp;
346 ireq->sack_ok = tcp_opt->sack_ok;
347 ireq->wscale_ok = tcp_opt->wscale_ok;
348 ireq->ecn_ok = !!(tcp_opt->rcv_tsecr & TS_OPT_ECN);
349
350 treq->ts_off = tsoff;
351
352 return req;
353}
354EXPORT_SYMBOL_GPL(cookie_tcp_reqsk_alloc);
355
356static struct request_sock *cookie_tcp_check(struct net *net, struct sock *sk,
357 struct sk_buff *skb)
358{
359 struct tcp_options_received tcp_opt;
360 u32 tsoff = 0;
361 int mss;
362
363 if (tcp_synq_no_recent_overflow(sk))
364 goto out;
365
366 mss = __cookie_v4_check(ip_hdr(skb), tcp_hdr(skb));
367 if (!mss) {
368 __NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESFAILED);
369 goto out;
370 }
371
372 __NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESRECV);
373
374 /* check for timestamp cookie support */
375 memset(&tcp_opt, 0, sizeof(tcp_opt));
376 tcp_parse_options(net, skb, &tcp_opt, 0, NULL);
377
378 if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
379 tsoff = secure_tcp_ts_off(net,
380 ip_hdr(skb)->daddr,
381 ip_hdr(skb)->saddr);
382 tcp_opt.rcv_tsecr -= tsoff;
383 }
384
385 if (!cookie_timestamp_decode(net, &tcp_opt))
386 goto out;
387
388 return cookie_tcp_reqsk_alloc(&tcp_request_sock_ops, sk, skb,
389 &tcp_opt, mss, tsoff);
390out:
391 return ERR_PTR(-EINVAL);
392}
393
394/* On input, sk is a listener.
395 * Output is listener if incoming packet would not create a child
396 * NULL if memory could not be allocated.
397 */
398struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
399{
400 struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt;
401 const struct tcphdr *th = tcp_hdr(skb);
402 struct tcp_sock *tp = tcp_sk(sk);
403 struct inet_request_sock *ireq;
404 struct net *net = sock_net(sk);
405 struct request_sock *req;
406 struct sock *ret = sk;
407 struct flowi4 fl4;
408 struct rtable *rt;
409 __u8 rcv_wscale;
410 int full_space;
411 SKB_DR(reason);
412
413 if (!READ_ONCE(net->ipv4.sysctl_tcp_syncookies) ||
414 !th->ack || th->rst)
415 goto out;
416
417 if (cookie_bpf_ok(skb)) {
418 req = cookie_bpf_check(sk, skb);
419 } else {
420 req = cookie_tcp_check(net, sk, skb);
421 if (IS_ERR(req))
422 goto out;
423 }
424 if (!req) {
425 SKB_DR_SET(reason, NO_SOCKET);
426 goto out_drop;
427 }
428
429 ireq = inet_rsk(req);
430
431 sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr);
432 sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr);
433
434 /* We throwed the options of the initial SYN away, so we hope
435 * the ACK carries the same options again (see RFC1122 4.2.3.8)
436 */
437 RCU_INIT_POINTER(ireq->ireq_opt, tcp_v4_save_options(net, skb));
438
439 if (security_inet_conn_request(sk, skb, req)) {
440 SKB_DR_SET(reason, SECURITY_HOOK);
441 goto out_free;
442 }
443
444 tcp_ao_syncookie(sk, skb, req, AF_INET);
445
446 /*
447 * We need to lookup the route here to get at the correct
448 * window size. We should better make sure that the window size
449 * hasn't changed since we received the original syn, but I see
450 * no easy way to do this.
451 */
452 flowi4_init_output(&fl4, ireq->ir_iif, ireq->ir_mark,
453 ip_sock_rt_tos(sk), ip_sock_rt_scope(sk),
454 IPPROTO_TCP, inet_sk_flowi_flags(sk),
455 opt->srr ? opt->faddr : ireq->ir_rmt_addr,
456 ireq->ir_loc_addr, th->source, th->dest, sk->sk_uid);
457 security_req_classify_flow(req, flowi4_to_flowi_common(&fl4));
458 rt = ip_route_output_key(net, &fl4);
459 if (IS_ERR(rt)) {
460 SKB_DR_SET(reason, IP_OUTNOROUTES);
461 goto out_free;
462 }
463
464 /* Try to redo what tcp_v4_send_synack did. */
465 req->rsk_window_clamp = READ_ONCE(tp->window_clamp) ? :
466 dst_metric(&rt->dst, RTAX_WINDOW);
467 /* limit the window selection if the user enforce a smaller rx buffer */
468 full_space = tcp_full_space(sk);
469 if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
470 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
471 req->rsk_window_clamp = full_space;
472
473 tcp_select_initial_window(sk, full_space, req->mss,
474 &req->rsk_rcv_wnd, &req->rsk_window_clamp,
475 ireq->wscale_ok, &rcv_wscale,
476 dst_metric(&rt->dst, RTAX_INITRWND));
477
478 /* req->syncookie is set true only if ACK is validated
479 * by BPF kfunc, then, rcv_wscale is already configured.
480 */
481 if (!req->syncookie)
482 ireq->rcv_wscale = rcv_wscale;
483 ireq->ecn_ok &= cookie_ecn_ok(net, &rt->dst);
484
485 ret = tcp_get_cookie_sock(sk, skb, req, &rt->dst);
486 /* ip_queue_xmit() depends on our flow being setup
487 * Normal sockets get it right from inet_csk_route_child_sock()
488 */
489 if (!ret) {
490 SKB_DR_SET(reason, NO_SOCKET);
491 goto out_drop;
492 }
493 inet_sk(ret)->cork.fl.u.ip4 = fl4;
494out:
495 return ret;
496out_free:
497 reqsk_free(req);
498out_drop:
499 sk_skb_reason_drop(sk, skb, reason);
500 return NULL;
501}
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}