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
23static siphash_aligned_key_t syncookie6_secret[2];
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 */
33static __u16 const msstab[] = {
34	1280 - 60, /* IPV6_MIN_MTU - 60 */
35	1480 - 60,
36	1500 - 60,
37	9000 - 60,
38};
39
40static 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
63static __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
75static __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
92u32 __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}
107EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence);
108
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
117int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th)
118{
119	__u32 cookie = ntohl(th->ack_seq) - 1;
120	__u32 seq = ntohl(th->seq) - 1;
121	__u32 mssind;
122
123	mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
124				      th->source, th->dest, seq);
125
126	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
127}
128EXPORT_SYMBOL_GPL(__cookie_v6_check);
129
130static struct request_sock *cookie_tcp_check(struct net *net, struct sock *sk,
131					     struct sk_buff *skb)
132{
133	struct tcp_options_received tcp_opt;
134	u32 tsoff = 0;
135	int mss;
136
137	if (tcp_synq_no_recent_overflow(sk))
138		goto out;
139
140	mss = __cookie_v6_check(ipv6_hdr(skb), tcp_hdr(skb));
141	if (!mss) {
142		__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESFAILED);
143		goto out;
144	}
145
146	__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESRECV);
147
148	/* check for timestamp cookie support */
149	memset(&tcp_opt, 0, sizeof(tcp_opt));
150	tcp_parse_options(net, skb, &tcp_opt, 0, NULL);
151
152	if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
153		tsoff = secure_tcpv6_ts_off(net,
154					    ipv6_hdr(skb)->daddr.s6_addr32,
155					    ipv6_hdr(skb)->saddr.s6_addr32);
156		tcp_opt.rcv_tsecr -= tsoff;
157	}
158
159	if (!cookie_timestamp_decode(net, &tcp_opt))
160		goto out;
161
162	return cookie_tcp_reqsk_alloc(&tcp6_request_sock_ops, sk, skb,
163				      &tcp_opt, mss, tsoff);
164out:
165	return ERR_PTR(-EINVAL);
166}
167
168struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
169{
170	const struct tcphdr *th = tcp_hdr(skb);
171	struct ipv6_pinfo *np = inet6_sk(sk);
172	struct tcp_sock *tp = tcp_sk(sk);
173	struct inet_request_sock *ireq;
174	struct net *net = sock_net(sk);
175	struct request_sock *req;
176	struct dst_entry *dst;
177	struct sock *ret = sk;
178	__u8 rcv_wscale;
179	int full_space;
180	SKB_DR(reason);
181
182	if (!READ_ONCE(net->ipv4.sysctl_tcp_syncookies) ||
183	    !th->ack || th->rst)
184		goto out;
185
186	if (cookie_bpf_ok(skb)) {
187		req = cookie_bpf_check(sk, skb);
188	} else {
189		req = cookie_tcp_check(net, sk, skb);
190		if (IS_ERR(req))
191			goto out;
192	}
193	if (!req) {
194		SKB_DR_SET(reason, NO_SOCKET);
195		goto out_drop;
196	}
197
198	ireq = inet_rsk(req);
199
200	ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
201	ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
202
203	if (security_inet_conn_request(sk, skb, req)) {
204		SKB_DR_SET(reason, SECURITY_HOOK);
205		goto out_free;
206	}
207
208	if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) ||
209	    np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
210	    np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
211		refcount_inc(&skb->users);
212		ireq->pktopts = skb;
213	}
214
215	/* So that link locals have meaning */
216	if (!sk->sk_bound_dev_if &&
217	    ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
218		ireq->ir_iif = tcp_v6_iif(skb);
219
220	tcp_ao_syncookie(sk, skb, req, AF_INET6);
221
222	/*
223	 * We need to lookup the dst_entry to get the correct window size.
224	 * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
225	 * me if there is a preferred way.
226	 */
227	{
228		struct in6_addr *final_p, final;
229		struct flowi6 fl6;
230		memset(&fl6, 0, sizeof(fl6));
231		fl6.flowi6_proto = IPPROTO_TCP;
232		fl6.daddr = ireq->ir_v6_rmt_addr;
233		final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
234		fl6.saddr = ireq->ir_v6_loc_addr;
235		fl6.flowi6_oif = ireq->ir_iif;
236		fl6.flowi6_mark = ireq->ir_mark;
237		fl6.fl6_dport = ireq->ir_rmt_port;
238		fl6.fl6_sport = inet_sk(sk)->inet_sport;
239		fl6.flowi6_uid = sk->sk_uid;
240		security_req_classify_flow(req, flowi6_to_flowi_common(&fl6));
241
242		dst = ip6_dst_lookup_flow(net, sk, &fl6, final_p);
243		if (IS_ERR(dst)) {
244			SKB_DR_SET(reason, IP_OUTNOROUTES);
245			goto out_free;
246		}
247	}
248
249	req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
250	/* limit the window selection if the user enforce a smaller rx buffer */
251	full_space = tcp_full_space(sk);
252	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
253	    (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
254		req->rsk_window_clamp = full_space;
255
256	tcp_select_initial_window(sk, full_space, req->mss,
257				  &req->rsk_rcv_wnd, &req->rsk_window_clamp,
258				  ireq->wscale_ok, &rcv_wscale,
259				  dst_metric(dst, RTAX_INITRWND));
260
261	/* req->syncookie is set true only if ACK is validated
262	 * by BPF kfunc, then, rcv_wscale is already configured.
263	 */
264	if (!req->syncookie)
265		ireq->rcv_wscale = rcv_wscale;
266	ireq->ecn_ok &= cookie_ecn_ok(net, dst);
267
268	ret = tcp_get_cookie_sock(sk, skb, req, dst);
269	if (!ret) {
270		SKB_DR_SET(reason, NO_SOCKET);
271		goto out_drop;
272	}
273out:
274	return ret;
275out_free:
276	reqsk_free(req);
277out_drop:
278	kfree_skb_reason(skb, reason);
279	return NULL;
280}
281