xref: /openbmc/linux/net/ipv6/syncookies.c (revision 4800cd83)
1 /*
2  *  IPv6 Syncookies implementation for the Linux kernel
3  *
4  *  Authors:
5  *  Glenn Griffin	<ggriffin.kernel@gmail.com>
6  *
7  *  Based on IPv4 implementation by Andi Kleen
8  *  linux/net/ipv4/syncookies.c
9  *
10  *	This program is free software; you can redistribute it and/or
11  *      modify it under the terms of the GNU General Public License
12  *      as published by the Free Software Foundation; either version
13  *      2 of the License, or (at your option) any later version.
14  *
15  */
16 
17 #include <linux/tcp.h>
18 #include <linux/random.h>
19 #include <linux/cryptohash.h>
20 #include <linux/kernel.h>
21 #include <net/ipv6.h>
22 #include <net/tcp.h>
23 
24 extern int sysctl_tcp_syncookies;
25 extern __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
26 
27 #define COOKIEBITS 24	/* Upper bits store count */
28 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
29 
30 /* Table must be sorted. */
31 static __u16 const msstab[] = {
32 	64,
33 	512,
34 	536,
35 	1280 - 60,
36 	1480 - 60,
37 	1500 - 60,
38 	4460 - 60,
39 	9000 - 60,
40 };
41 
42 /*
43  * This (misnamed) value is the age of syncookie which is permitted.
44  * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
45  * sysctl_tcp_retries1. It's a rather complicated formula (exponential
46  * backoff) to compute at runtime so it's currently hardcoded here.
47  */
48 #define COUNTER_TRIES 4
49 
50 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
51 					   struct request_sock *req,
52 					   struct dst_entry *dst)
53 {
54 	struct inet_connection_sock *icsk = inet_csk(sk);
55 	struct sock *child;
56 
57 	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
58 	if (child)
59 		inet_csk_reqsk_queue_add(sk, req, child);
60 	else
61 		reqsk_free(req);
62 
63 	return child;
64 }
65 
66 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
67 		      ipv6_cookie_scratch);
68 
69 static u32 cookie_hash(struct in6_addr *saddr, struct in6_addr *daddr,
70 		       __be16 sport, __be16 dport, u32 count, int c)
71 {
72 	__u32 *tmp = __get_cpu_var(ipv6_cookie_scratch);
73 
74 	/*
75 	 * we have 320 bits of information to hash, copy in the remaining
76 	 * 192 bits required for sha_transform, from the syncookie_secret
77 	 * and overwrite the digest with the secret
78 	 */
79 	memcpy(tmp + 10, syncookie_secret[c], 44);
80 	memcpy(tmp, saddr, 16);
81 	memcpy(tmp + 4, daddr, 16);
82 	tmp[8] = ((__force u32)sport << 16) + (__force u32)dport;
83 	tmp[9] = count;
84 	sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
85 
86 	return tmp[17];
87 }
88 
89 static __u32 secure_tcp_syn_cookie(struct in6_addr *saddr, struct in6_addr *daddr,
90 				   __be16 sport, __be16 dport, __u32 sseq,
91 				   __u32 count, __u32 data)
92 {
93 	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
94 		sseq + (count << COOKIEBITS) +
95 		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
96 		& COOKIEMASK));
97 }
98 
99 static __u32 check_tcp_syn_cookie(__u32 cookie, struct in6_addr *saddr,
100 				  struct in6_addr *daddr, __be16 sport,
101 				  __be16 dport, __u32 sseq, __u32 count,
102 				  __u32 maxdiff)
103 {
104 	__u32 diff;
105 
106 	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
107 
108 	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
109 	if (diff >= maxdiff)
110 		return (__u32)-1;
111 
112 	return (cookie -
113 		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
114 		& COOKIEMASK;
115 }
116 
117 __u32 cookie_v6_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
118 {
119 	struct ipv6hdr *iph = ipv6_hdr(skb);
120 	const struct tcphdr *th = tcp_hdr(skb);
121 	int mssind;
122 	const __u16 mss = *mssp;
123 
124 	tcp_synq_overflow(sk);
125 
126 	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
127 		if (mss >= msstab[mssind])
128 			break;
129 
130 	*mssp = msstab[mssind];
131 
132 	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
133 
134 	return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
135 				     th->dest, ntohl(th->seq),
136 				     jiffies / (HZ * 60), mssind);
137 }
138 
139 static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
140 {
141 	struct ipv6hdr *iph = ipv6_hdr(skb);
142 	const struct tcphdr *th = tcp_hdr(skb);
143 	__u32 seq = ntohl(th->seq) - 1;
144 	__u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
145 					    th->source, th->dest, seq,
146 					    jiffies / (HZ * 60), COUNTER_TRIES);
147 
148 	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
149 }
150 
151 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
152 {
153 	struct tcp_options_received tcp_opt;
154 	u8 *hash_location;
155 	struct inet_request_sock *ireq;
156 	struct inet6_request_sock *ireq6;
157 	struct tcp_request_sock *treq;
158 	struct ipv6_pinfo *np = inet6_sk(sk);
159 	struct tcp_sock *tp = tcp_sk(sk);
160 	const struct tcphdr *th = tcp_hdr(skb);
161 	__u32 cookie = ntohl(th->ack_seq) - 1;
162 	struct sock *ret = sk;
163 	struct request_sock *req;
164 	int mss;
165 	struct dst_entry *dst;
166 	__u8 rcv_wscale;
167 	bool ecn_ok;
168 
169 	if (!sysctl_tcp_syncookies || !th->ack || th->rst)
170 		goto out;
171 
172 	if (tcp_synq_no_recent_overflow(sk) ||
173 		(mss = cookie_check(skb, cookie)) == 0) {
174 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
175 		goto out;
176 	}
177 
178 	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
179 
180 	/* check for timestamp cookie support */
181 	memset(&tcp_opt, 0, sizeof(tcp_opt));
182 	tcp_parse_options(skb, &tcp_opt, &hash_location, 0);
183 
184 	if (!cookie_check_timestamp(&tcp_opt, &ecn_ok))
185 		goto out;
186 
187 	ret = NULL;
188 	req = inet6_reqsk_alloc(&tcp6_request_sock_ops);
189 	if (!req)
190 		goto out;
191 
192 	ireq = inet_rsk(req);
193 	ireq6 = inet6_rsk(req);
194 	treq = tcp_rsk(req);
195 
196 	if (security_inet_conn_request(sk, skb, req))
197 		goto out_free;
198 
199 	req->mss = mss;
200 	ireq->rmt_port = th->source;
201 	ireq->loc_port = th->dest;
202 	ipv6_addr_copy(&ireq6->rmt_addr, &ipv6_hdr(skb)->saddr);
203 	ipv6_addr_copy(&ireq6->loc_addr, &ipv6_hdr(skb)->daddr);
204 	if (ipv6_opt_accepted(sk, skb) ||
205 	    np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
206 	    np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
207 		atomic_inc(&skb->users);
208 		ireq6->pktopts = skb;
209 	}
210 
211 	ireq6->iif = sk->sk_bound_dev_if;
212 	/* So that link locals have meaning */
213 	if (!sk->sk_bound_dev_if &&
214 	    ipv6_addr_type(&ireq6->rmt_addr) & IPV6_ADDR_LINKLOCAL)
215 		ireq6->iif = inet6_iif(skb);
216 
217 	req->expires = 0UL;
218 	req->retrans = 0;
219 	ireq->ecn_ok		= ecn_ok;
220 	ireq->snd_wscale	= tcp_opt.snd_wscale;
221 	ireq->sack_ok		= tcp_opt.sack_ok;
222 	ireq->wscale_ok		= tcp_opt.wscale_ok;
223 	ireq->tstamp_ok		= tcp_opt.saw_tstamp;
224 	req->ts_recent		= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
225 	treq->rcv_isn = ntohl(th->seq) - 1;
226 	treq->snt_isn = cookie;
227 
228 	/*
229 	 * We need to lookup the dst_entry to get the correct window size.
230 	 * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
231 	 * me if there is a preferred way.
232 	 */
233 	{
234 		struct in6_addr *final_p, final;
235 		struct flowi fl;
236 		memset(&fl, 0, sizeof(fl));
237 		fl.proto = IPPROTO_TCP;
238 		ipv6_addr_copy(&fl.fl6_dst, &ireq6->rmt_addr);
239 		final_p = fl6_update_dst(&fl, np->opt, &final);
240 		ipv6_addr_copy(&fl.fl6_src, &ireq6->loc_addr);
241 		fl.oif = sk->sk_bound_dev_if;
242 		fl.mark = sk->sk_mark;
243 		fl.fl_ip_dport = inet_rsk(req)->rmt_port;
244 		fl.fl_ip_sport = inet_sk(sk)->inet_sport;
245 		security_req_classify_flow(req, &fl);
246 		if (ip6_dst_lookup(sk, &dst, &fl))
247 			goto out_free;
248 
249 		if (final_p)
250 			ipv6_addr_copy(&fl.fl6_dst, final_p);
251 		if ((xfrm_lookup(sock_net(sk), &dst, &fl, sk, 0)) < 0)
252 			goto out_free;
253 	}
254 
255 	req->window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
256 	tcp_select_initial_window(tcp_full_space(sk), req->mss,
257 				  &req->rcv_wnd, &req->window_clamp,
258 				  ireq->wscale_ok, &rcv_wscale,
259 				  dst_metric(dst, RTAX_INITRWND));
260 
261 	ireq->rcv_wscale = rcv_wscale;
262 
263 	ret = get_cookie_sock(sk, skb, req, dst);
264 out:
265 	return ret;
266 out_free:
267 	reqsk_free(req);
268 	return NULL;
269 }
270 
271