syncookies.c

  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#include <net/tcp_ecn.h>
 20
 21#define COOKIEBITS 24	/* Upper bits store count */
 22#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
 23
 24static siphash_aligned_key_t syncookie6_secret[2];
 25
 26/* RFC 2460, Section 8.3:
 27 * [ipv6 tcp] MSS must be computed as the maximum packet size minus 60 [..]
 28 *
 29 * Due to IPV6_MIN_MTU=1280 the lowest possible MSS is 1220, which allows
 30 * using higher values than ipv4 tcp syncookies.
 31 * The other values are chosen based on ethernet (1500 and 9k MTU), plus
 32 * one that accounts for common encap (PPPoe) overhead. Table must be sorted.
 33 */
 34static __u16 const msstab[] = {
 35	1280 - 60, /* IPV6_MIN_MTU - 60 */
 36	1480 - 60,
 37	1500 - 60,
 38	9000 - 60,
 39};
 40
 41static u32 cookie_hash(const struct in6_addr *saddr,
 42		       const struct in6_addr *daddr,
 43		       __be16 sport, __be16 dport, u32 count, int c)
 44{
 45	const struct {
 46		struct in6_addr saddr;
 47		struct in6_addr daddr;
 48		u32 count;
 49		__be16 sport;
 50		__be16 dport;
 51	} __aligned(SIPHASH_ALIGNMENT) combined = {
 52		.saddr = *saddr,
 53		.daddr = *daddr,
 54		.count = count,
 55		.sport = sport,
 56		.dport = dport
 57	};
 58
 59	net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret));
 60	return siphash(&combined, offsetofend(typeof(combined), dport),
 61		       &syncookie6_secret[c]);
 62}
 63
 64static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
 65				   const struct in6_addr *daddr,
 66				   __be16 sport, __be16 dport, __u32 sseq,
 67				   __u32 data)
 68{
 69	u32 count = tcp_cookie_time();
 70	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
 71		sseq + (count << COOKIEBITS) +
 72		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
 73		& COOKIEMASK));
 74}
 75
 76static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
 77				  const struct in6_addr *daddr, __be16 sport,
 78				  __be16 dport, __u32 sseq)
 79{
 80	__u32 diff, count = tcp_cookie_time();
 81
 82	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
 83
 84	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
 85	if (diff >= MAX_SYNCOOKIE_AGE)
 86		return (__u32)-1;
 87
 88	return (cookie -
 89		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
 90		& COOKIEMASK;
 91}
 92
 93u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph,
 94			      const struct tcphdr *th, __u16 *mssp)
 95{
 96	int mssind;
 97	const __u16 mss = *mssp;
 98
 99	for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
100		if (mss >= msstab[mssind])
101			break;
102
103	*mssp = msstab[mssind];
104
105	return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
106				     th->dest, ntohl(th->seq), mssind);
107}
108EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence);
109
110__u32 cookie_v6_init_sequence(const struct sk_buff *skb, __u16 *mssp)
111{
112	const struct ipv6hdr *iph = ipv6_hdr(skb);
113	const struct tcphdr *th = tcp_hdr(skb);
114
115	return __cookie_v6_init_sequence(iph, th, mssp);
116}
117
118int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th)
119{
120	__u32 cookie = ntohl(th->ack_seq) - 1;
121	__u32 seq = ntohl(th->seq) - 1;
122	__u32 mssind;
123
124	mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
125				      th->source, th->dest, seq);
126
127	return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
128}
129EXPORT_SYMBOL_GPL(__cookie_v6_check);
130
131static struct request_sock *cookie_tcp_check(struct net *net, struct sock *sk,
132					     struct sk_buff *skb)
133{
134	struct tcp_options_received tcp_opt;
135	u32 tsoff = 0;
136	int mss;
137
138	if (tcp_synq_no_recent_overflow(sk))
139		goto out;
140
141	mss = __cookie_v6_check(ipv6_hdr(skb), tcp_hdr(skb));
142	if (!mss) {
143		__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESFAILED);
144		goto out;
145	}
146
147	__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESRECV);
148
149	/* check for timestamp cookie support */
150	memset(&tcp_opt, 0, sizeof(tcp_opt));
151	tcp_parse_options(net, skb, &tcp_opt, 0, NULL);
152
153	if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
154		union tcp_seq_and_ts_off st;
155
156		st = secure_tcpv6_seq_and_ts_off(net,
157						 ipv6_hdr(skb)->daddr.s6_addr32,
158						 ipv6_hdr(skb)->saddr.s6_addr32,
159						 tcp_hdr(skb)->dest,
160						 tcp_hdr(skb)->source);
161		tsoff = st.ts_off;
162		tcp_opt.rcv_tsecr -= tsoff;
163	}
164
165	if (!cookie_timestamp_decode(net, &tcp_opt))
166		goto out;
167
168	return cookie_tcp_reqsk_alloc(&tcp6_request_sock_ops, sk, skb,
169				      &tcp_opt, mss, tsoff);
170out:
171	return ERR_PTR(-EINVAL);
172}
173
174struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
175{
176	const struct tcphdr *th = tcp_hdr(skb);
177	struct ipv6_pinfo *np = inet6_sk(sk);
178	struct tcp_sock *tp = tcp_sk(sk);
179	struct inet_request_sock *ireq;
180	struct net *net = sock_net(sk);
181	struct request_sock *req;
182	struct dst_entry *dst;
183	struct sock *ret = sk;
184	__u8 rcv_wscale;
185	int full_space;
186	SKB_DR(reason);
187
188	if (!READ_ONCE(net->ipv4.sysctl_tcp_syncookies) ||
189	    !th->ack || th->rst)
190		goto out;
191
192	if (cookie_bpf_ok(skb)) {
193		req = cookie_bpf_check(sk, skb);
194	} else {
195		req = cookie_tcp_check(net, sk, skb);
196		if (IS_ERR(req))
197			goto out;
198	}
199	if (!req) {
200		SKB_DR_SET(reason, NO_SOCKET);
201		goto out_drop;
202	}
203
204	ireq = inet_rsk(req);
205
206	ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
207	ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
208
209	if (security_inet_conn_request(sk, skb, req)) {
210		SKB_DR_SET(reason, SECURITY_HOOK);
211		goto out_free;
212	}
213
214	if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) ||
215	    np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
216	    np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
217		refcount_inc(&skb->users);
218		ireq->pktopts = skb;
219	}
220
221	/* So that link locals have meaning */
222	if (!sk->sk_bound_dev_if &&
223	    ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
224		ireq->ir_iif = tcp_v6_iif(skb);
225
226	tcp_ao_syncookie(sk, skb, req, AF_INET6);
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 flowi6 fl6;
236		memset(&fl6, 0, sizeof(fl6));
237		fl6.flowi6_proto = IPPROTO_TCP;
238		fl6.daddr = ireq->ir_v6_rmt_addr;
239		final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
240		fl6.saddr = ireq->ir_v6_loc_addr;
241		fl6.flowi6_oif = ireq->ir_iif;
242		fl6.flowi6_mark = ireq->ir_mark;
243		fl6.fl6_dport = ireq->ir_rmt_port;
244		fl6.fl6_sport = inet_sk(sk)->inet_sport;
245		fl6.flowi6_uid = sk_uid(sk);
246		security_req_classify_flow(req, flowi6_to_flowi_common(&fl6));
247
248		dst = ip6_dst_lookup_flow(net, sk, &fl6, final_p);
249		if (IS_ERR(dst)) {
250			SKB_DR_SET(reason, IP_OUTNOROUTES);
251			goto out_free;
252		}
253	}
254
255	req->rsk_window_clamp = READ_ONCE(tp->window_clamp) ? :dst_metric(dst, RTAX_WINDOW);
256	/* limit the window selection if the user enforce a smaller rx buffer */
257	full_space = tcp_full_space(sk);
258	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
259	    (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
260		req->rsk_window_clamp = full_space;
261
262	tcp_select_initial_window(sk, full_space, req->mss,
263				  &req->rsk_rcv_wnd, &req->rsk_window_clamp,
264				  ireq->wscale_ok, &rcv_wscale,
265				  dst_metric(dst, RTAX_INITRWND));
266
267	/* req->syncookie is set true only if ACK is validated
268	 * by BPF kfunc, then, rcv_wscale is already configured.
269	 */
270	if (!req->syncookie)
271		ireq->rcv_wscale = rcv_wscale;
272	ireq->ecn_ok &= cookie_ecn_ok(net, dst);
273	tcp_rsk(req)->accecn_ok = ireq->ecn_ok && cookie_accecn_ok(th);
274
275	ret = tcp_get_cookie_sock(sk, skb, req, dst);
276	if (!ret) {
277		SKB_DR_SET(reason, NO_SOCKET);
278		goto out_drop;
279	}
280out:
281	return ret;
282out_free:
283	reqsk_free(req);
284out_drop:
285	sk_skb_reason_drop(sk, skb, reason);
286	return NULL;
287}