2 * Syncookies implementation for the Linux kernel
4 * Copyright (C) 1997 Andi Kleen
5 * Based on ideas by D.J.Bernstein and Eric Schenk.
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.
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>
19 #include <net/route.h>
21 /* Timestamps: lowest 9 bits store TCP options */
23 #define TSMASK (((__u32)1 << TSBITS) - 1)
25 extern int sysctl_tcp_syncookies;
27 __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
28 EXPORT_SYMBOL(syncookie_secret);
30 static __init int init_syncookies(void)
32 get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
35 __initcall(init_syncookies);
37 #define COOKIEBITS 24 /* Upper bits store count */
38 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
40 static DEFINE_PER_CPU(__u32, cookie_scratch)[16 + 5 + SHA_WORKSPACE_WORDS];
42 static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
45 __u32 *tmp = __get_cpu_var(cookie_scratch);
47 memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
48 tmp[0] = (__force u32)saddr;
49 tmp[1] = (__force u32)daddr;
50 tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
52 sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
59 * when syncookies are in effect and tcp timestamps are enabled we encode
60 * tcp options in the lowest 9 bits of the timestamp value that will be
61 * sent in the syn-ack.
62 * Since subsequent timestamps use the normal tcp_time_stamp value, we
63 * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
65 __u32 cookie_init_timestamp(struct request_sock *req)
67 struct inet_request_sock *ireq;
68 u32 ts, ts_now = tcp_time_stamp;
72 if (ireq->wscale_ok) {
73 options = ireq->snd_wscale;
74 options |= ireq->rcv_wscale << 4;
76 options |= ireq->sack_ok << 8;
78 ts = ts_now & ~TSMASK;
90 static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
91 __be16 dport, __u32 sseq, __u32 count,
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
101 * As an extra hack, we add a small "data" value that encodes the
102 * MSS into the second hash value.
105 return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
106 sseq + (count << COOKIEBITS) +
107 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
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.
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.
120 static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
121 __be16 sport, __be16 dport, __u32 sseq,
122 __u32 count, __u32 maxdiff)
126 /* Strip away the layers from the cookie */
127 cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
129 /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
130 diff = (count - (cookie >> COOKIEBITS)) & ((__u32) - 1 >> COOKIEBITS);
135 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
136 & COOKIEMASK; /* Leaving the data behind */
140 * This table has to be sorted and terminated with (__u16)-1.
141 * XXX generate a better table.
142 * Unresolved Issues: HIPPI with a 64k MSS is not well supported.
144 static __u16 const msstab[] = {
155 /* The number doesn't include the -1 terminator */
156 #define NUM_MSS (ARRAY_SIZE(msstab) - 1)
159 * Generate a syncookie. mssp points to the mss, which is returned
160 * rounded down to the value encoded in the cookie.
162 __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
164 struct tcp_sock *tp = tcp_sk(sk);
165 const struct iphdr *iph = ip_hdr(skb);
166 const struct tcphdr *th = tcp_hdr(skb);
168 const __u16 mss = *mssp;
170 tp->last_synq_overflow = jiffies;
172 /* XXX sort msstab[] by probability? Binary search? */
173 for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
175 *mssp = msstab[mssind] + 1;
177 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
179 return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
180 th->source, th->dest, ntohl(th->seq),
181 jiffies / (HZ * 60), mssind);
185 * This (misnamed) value is the age of syncookie which is permitted.
186 * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
187 * sysctl_tcp_retries1. It's a rather complicated formula (exponential
188 * backoff) to compute at runtime so it's currently hardcoded here.
190 #define COUNTER_TRIES 4
192 * Check if a ack sequence number is a valid syncookie.
193 * Return the decoded mss if it is, or 0 if not.
195 static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
197 const struct iphdr *iph = ip_hdr(skb);
198 const struct tcphdr *th = tcp_hdr(skb);
199 __u32 seq = ntohl(th->seq) - 1;
200 __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
201 th->source, th->dest, seq,
205 return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
208 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
209 struct request_sock *req,
210 struct dst_entry *dst)
212 struct inet_connection_sock *icsk = inet_csk(sk);
215 child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
217 inet_csk_reqsk_queue_add(sk, req, child);
226 * when syncookies are in effect and tcp timestamps are enabled we stored
227 * additional tcp options in the timestamp.
228 * This extracts these options from the timestamp echo.
230 * The lowest 4 bits are for snd_wscale
231 * The next 4 lsb are for rcv_wscale
232 * The next lsb is for sack_ok
234 void cookie_check_timestamp(struct tcp_options_received *tcp_opt)
236 /* echoed timestamp, 9 lowest bits contain options */
237 u32 options = tcp_opt->rcv_tsecr & TSMASK;
239 tcp_opt->snd_wscale = options & 0xf;
241 tcp_opt->rcv_wscale = options & 0xf;
243 tcp_opt->sack_ok = (options >> 4) & 0x1;
245 if (tcp_opt->sack_ok)
246 tcp_sack_reset(tcp_opt);
248 if (tcp_opt->snd_wscale || tcp_opt->rcv_wscale)
249 tcp_opt->wscale_ok = 1;
251 EXPORT_SYMBOL(cookie_check_timestamp);
253 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
254 struct ip_options *opt)
256 struct inet_request_sock *ireq;
257 struct tcp_request_sock *treq;
258 struct tcp_sock *tp = tcp_sk(sk);
259 const struct tcphdr *th = tcp_hdr(skb);
260 __u32 cookie = ntohl(th->ack_seq) - 1;
261 struct sock *ret = sk;
262 struct request_sock *req;
266 struct tcp_options_received tcp_opt;
268 if (!sysctl_tcp_syncookies || !th->ack)
271 if (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) ||
272 (mss = cookie_check(skb, cookie)) == 0) {
273 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
277 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
279 /* check for timestamp cookie support */
280 memset(&tcp_opt, 0, sizeof(tcp_opt));
281 tcp_parse_options(skb, &tcp_opt, 0);
283 if (tcp_opt.saw_tstamp)
284 cookie_check_timestamp(&tcp_opt);
287 req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
291 ireq = inet_rsk(req);
293 treq->rcv_isn = ntohl(th->seq) - 1;
294 treq->snt_isn = cookie;
296 ireq->loc_port = th->dest;
297 ireq->rmt_port = th->source;
298 ireq->loc_addr = ip_hdr(skb)->daddr;
299 ireq->rmt_addr = ip_hdr(skb)->saddr;
301 ireq->snd_wscale = tcp_opt.snd_wscale;
302 ireq->rcv_wscale = tcp_opt.rcv_wscale;
303 ireq->sack_ok = tcp_opt.sack_ok;
304 ireq->wscale_ok = tcp_opt.wscale_ok;
305 ireq->tstamp_ok = tcp_opt.saw_tstamp;
306 req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
308 /* We throwed the options of the initial SYN away, so we hope
309 * the ACK carries the same options again (see RFC1122 4.2.3.8)
311 if (opt && opt->optlen) {
312 int opt_size = sizeof(struct ip_options) + opt->optlen;
314 ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
315 if (ireq->opt != NULL && ip_options_echo(ireq->opt, skb)) {
321 if (security_inet_conn_request(sk, skb, req)) {
330 * We need to lookup the route here to get at the correct
331 * window size. We should better make sure that the window size
332 * hasn't changed since we received the original syn, but I see
333 * no easy way to do this.
336 struct flowi fl = { .nl_u = { .ip4_u =
337 { .daddr = ((opt && opt->srr) ?
340 .saddr = ireq->loc_addr,
341 .tos = RT_CONN_FLAGS(sk) } },
342 .proto = IPPROTO_TCP,
343 .flags = inet_sk_flowi_flags(sk),
346 .dport = th->source } } };
347 security_req_classify_flow(req, &fl);
348 if (ip_route_output_key(&init_net, &rt, &fl)) {
354 /* Try to redo what tcp_v4_send_synack did. */
355 req->window_clamp = tp->window_clamp ? :dst_metric(&rt->u.dst, RTAX_WINDOW);
357 tcp_select_initial_window(tcp_full_space(sk), req->mss,
358 &req->rcv_wnd, &req->window_clamp,
359 ireq->wscale_ok, &rcv_wscale);
361 ireq->rcv_wscale = rcv_wscale;
363 ret = get_cookie_sock(sk, skb, req, &rt->u.dst);