1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /*
3  *  IPv6 Syncookies implementation for the Linux kernel
4  *
5  *  Authors:
6  *  Glenn Griffin	<ggriffin.kernel@gmail.com>
7  *
8  *  Based on IPv4 implementation by Andi Kleen
9  *  linux/net/ipv4/syncookies.c
10  */
11 
12 #include <linux/tcp.h>
13 #include <linux/random.h>
14 #include <linux/siphash.h>
15 #include <linux/kernel.h>
16 #include <net/secure_seq.h>
17 #include <net/ipv6.h>
18 #include <net/tcp.h>
19 
20 #define COOKIEBITS 24	/* Upper bits store count */
21 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
22 
23 static siphash_key_t syncookie6_secret[2] __read_mostly;
24 
25 /* RFC 2460, Section 8.3:
26  * [ipv6 tcp] MSS must be computed as the maximum packet size minus 60 [..]
27  *
28  * Due to IPV6_MIN_MTU=1280 the lowest possible MSS is 1220, which allows
29  * using higher values than ipv4 tcp syncookies.
30  * The other values are chosen based on ethernet (1500 and 9k MTU), plus
31  * one that accounts for common encap (PPPoe) overhead. Table must be sorted.
32  */
33 static __u16 const msstab[] = {
34 	1280 - 60, /* IPV6_MIN_MTU - 60 */
35 	1480 - 60,
36 	1500 - 60,
37 	9000 - 60,
38 };
39 
cookie_hash(const struct in6_addr * saddr,const struct in6_addr * daddr,__be16 sport,__be16 dport,u32 count,int c)40 static u32 cookie_hash(const struct in6_addr *saddr,
41 		       const struct in6_addr *daddr,
42 		       __be16 sport, __be16 dport, u32 count, int c)
43 {
44 	const struct {
45 		struct in6_addr saddr;
46 		struct in6_addr daddr;
47 		u32 count;
48 		__be16 sport;
49 		__be16 dport;
50 	} __aligned(SIPHASH_ALIGNMENT) combined = {
51 		.saddr = *saddr,
52 		.daddr = *daddr,
53 		.count = count,
54 		.sport = sport,
55 		.dport = dport
56 	};
57 
58 	net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret));
59 	return siphash(&combined, offsetofend(typeof(combined), dport),
60 		       &syncookie6_secret[c]);
61 }
62 
secure_tcp_syn_cookie(const struct in6_addr * saddr,const struct in6_addr * daddr,__be16 sport,__be16 dport,__u32 sseq,__u32 data)63 static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
64 				   const struct in6_addr *daddr,
65 				   __be16 sport, __be16 dport, __u32 sseq,
66 				   __u32 data)
67 {
68 	u32 count = tcp_cookie_time();
69 	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
70 		sseq + (count << COOKIEBITS) +
71 		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
72 		& COOKIEMASK));
73 }
74 
check_tcp_syn_cookie(__u32 cookie,const struct in6_addr * saddr,const struct in6_addr * daddr,__be16 sport,__be16 dport,__u32 sseq)75 static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
76 				  const struct in6_addr *daddr, __be16 sport,
77 				  __be16 dport, __u32 sseq)
78 {
79 	__u32 diff, count = tcp_cookie_time();
80 
81 	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
82 
83 	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
84 	if (diff >= MAX_SYNCOOKIE_AGE)
85 		return (__u32)-1;
86 
87 	return (cookie -
88 		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
89 		& COOKIEMASK;
90 }
91 
__cookie_v6_init_sequence(const struct ipv6hdr * iph,const struct tcphdr * th,__u16 * mssp)92 u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph,
93 			      const struct tcphdr *th, __u16 *mssp)
94 {
95 	int mssind;
96 	const __u16 mss = *mssp;
97 
98 	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
99 		if (mss >= msstab[mssind])
100 			break;
101 
102 	*mssp = msstab[mssind];
103 
104 	return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
105 				     th->dest, ntohl(th->seq), mssind);
106 }
107 EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence);
108 
cookie_v6_init_sequence(const struct sk_buff * skb,__u16 * mssp)109 __u32 cookie_v6_init_sequence(const struct sk_buff *skb, __u16 *mssp)
110 {
111 	const struct ipv6hdr *iph = ipv6_hdr(skb);
112 	const struct tcphdr *th = tcp_hdr(skb);
113 
114 	return __cookie_v6_init_sequence(iph, th, mssp);
115 }
116 
__cookie_v6_check(const struct ipv6hdr * iph,const struct tcphdr * th,__u32 cookie)117 int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th,
118 		      __u32 cookie)
119 {
120 	__u32 seq = ntohl(th->seq) - 1;
121 	__u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
122 					    th->source, th->dest, seq);
123 
124 	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
125 }
126 EXPORT_SYMBOL_GPL(__cookie_v6_check);
127 
cookie_v6_check(struct sock * sk,struct sk_buff * skb)128 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
129 {
130 	struct tcp_options_received tcp_opt;
131 	struct inet_request_sock *ireq;
132 	struct tcp_request_sock *treq;
133 	struct ipv6_pinfo *np = inet6_sk(sk);
134 	struct tcp_sock *tp = tcp_sk(sk);
135 	const struct tcphdr *th = tcp_hdr(skb);
136 	__u32 cookie = ntohl(th->ack_seq) - 1;
137 	struct sock *ret = sk;
138 	struct request_sock *req;
139 	int full_space, mss;
140 	struct dst_entry *dst;
141 	__u8 rcv_wscale;
142 	u32 tsoff = 0;
143 
144 	if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst)
145 		goto out;
146 
147 	if (tcp_synq_no_recent_overflow(sk))
148 		goto out;
149 
150 	mss = __cookie_v6_check(ipv6_hdr(skb), th, cookie);
151 	if (mss == 0) {
152 		__NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
153 		goto out;
154 	}
155 
156 	__NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
157 
158 	/* check for timestamp cookie support */
159 	memset(&tcp_opt, 0, sizeof(tcp_opt));
160 	tcp_parse_options(sock_net(sk), skb, &tcp_opt, 0, NULL);
161 
162 	if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
163 		tsoff = secure_tcpv6_ts_off(sock_net(sk),
164 					    ipv6_hdr(skb)->daddr.s6_addr32,
165 					    ipv6_hdr(skb)->saddr.s6_addr32);
166 		tcp_opt.rcv_tsecr -= tsoff;
167 	}
168 
169 	if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
170 		goto out;
171 
172 	ret = NULL;
173 	req = cookie_tcp_reqsk_alloc(&tcp6_request_sock_ops, sk, skb);
174 	if (!req)
175 		goto out;
176 
177 	ireq = inet_rsk(req);
178 	treq = tcp_rsk(req);
179 	treq->tfo_listener = false;
180 
181 	if (security_inet_conn_request(sk, skb, req))
182 		goto out_free;
183 
184 	req->mss = mss;
185 	ireq->ir_rmt_port = th->source;
186 	ireq->ir_num = ntohs(th->dest);
187 	ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
188 	ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
189 	if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) ||
190 	    np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
191 	    np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
192 		refcount_inc(&skb->users);
193 		ireq->pktopts = skb;
194 	}
195 
196 	ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
197 	/* So that link locals have meaning */
198 	if (!sk->sk_bound_dev_if &&
199 	    ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
200 		ireq->ir_iif = tcp_v6_iif(skb);
201 
202 	ireq->ir_mark = inet_request_mark(sk, skb);
203 
204 	req->num_retrans = 0;
205 	ireq->snd_wscale	= tcp_opt.snd_wscale;
206 	ireq->sack_ok		= tcp_opt.sack_ok;
207 	ireq->wscale_ok		= tcp_opt.wscale_ok;
208 	ireq->tstamp_ok		= tcp_opt.saw_tstamp;
209 	req->ts_recent		= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
210 	treq->snt_synack	= 0;
211 	treq->rcv_isn = ntohl(th->seq) - 1;
212 	treq->snt_isn = cookie;
213 	treq->ts_off = 0;
214 	treq->txhash = net_tx_rndhash();
215 	if (IS_ENABLED(CONFIG_SMC))
216 		ireq->smc_ok = 0;
217 
218 	/*
219 	 * We need to lookup the dst_entry to get the correct window size.
220 	 * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
221 	 * me if there is a preferred way.
222 	 */
223 	{
224 		struct in6_addr *final_p, final;
225 		struct flowi6 fl6;
226 		memset(&fl6, 0, sizeof(fl6));
227 		fl6.flowi6_proto = IPPROTO_TCP;
228 		fl6.daddr = ireq->ir_v6_rmt_addr;
229 		final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
230 		fl6.saddr = ireq->ir_v6_loc_addr;
231 		fl6.flowi6_oif = ireq->ir_iif;
232 		fl6.flowi6_mark = ireq->ir_mark;
233 		fl6.fl6_dport = ireq->ir_rmt_port;
234 		fl6.fl6_sport = inet_sk(sk)->inet_sport;
235 		fl6.flowi6_uid = sk->sk_uid;
236 		security_req_classify_flow(req, flowi6_to_flowi_common(&fl6));
237 
238 		dst = ip6_dst_lookup_flow(sock_net(sk), sk, &fl6, final_p);
239 		if (IS_ERR(dst))
240 			goto out_free;
241 	}
242 
243 	req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
244 	/* limit the window selection if the user enforce a smaller rx buffer */
245 	full_space = tcp_full_space(sk);
246 	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
247 	    (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
248 		req->rsk_window_clamp = full_space;
249 
250 	tcp_select_initial_window(sk, full_space, req->mss,
251 				  &req->rsk_rcv_wnd, &req->rsk_window_clamp,
252 				  ireq->wscale_ok, &rcv_wscale,
253 				  dst_metric(dst, RTAX_INITRWND));
254 
255 	ireq->rcv_wscale = rcv_wscale;
256 	ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst);
257 
258 	ret = tcp_get_cookie_sock(sk, skb, req, dst, tsoff);
259 out:
260 	return ret;
261 out_free:
262 	reqsk_free(req);
263 	return NULL;
264 }
265