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1// SPDX-License-Identifier: GPL-2.0-or-later
2/*
3 * IPv6 Syncookies implementation for the Linux kernel
4 *
5 * Authors:
6 * Glenn Griffin <[email protected]>
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 ip6_ecmp_set_mp_hash(net, &fl6, tcp_rsk(req)->txhash);
249
250 dst = ip6_dst_lookup_flow(net, sk, &fl6, final_p);
251 if (IS_ERR(dst)) {
252 SKB_DR_SET(reason, IP_OUTNOROUTES);
253 goto out_free;
254 }
255 }
256
257 req->rsk_window_clamp = READ_ONCE(tp->window_clamp) ? :dst_metric(dst, RTAX_WINDOW);
258 /* limit the window selection if the user enforce a smaller rx buffer */
259 full_space = tcp_full_space(sk);
260 if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
261 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
262 req->rsk_window_clamp = full_space;
263
264 tcp_select_initial_window(sk, full_space, req->mss,
265 &req->rsk_rcv_wnd, &req->rsk_window_clamp,
266 ireq->wscale_ok, &rcv_wscale,
267 dst_metric(dst, RTAX_INITRWND));
268
269 /* req->syncookie is set true only if ACK is validated
270 * by BPF kfunc, then, rcv_wscale is already configured.
271 */
272 if (!req->syncookie)
273 ireq->rcv_wscale = rcv_wscale;
274 ireq->ecn_ok &= cookie_ecn_ok(net, dst);
275 tcp_rsk(req)->accecn_ok = ireq->ecn_ok && cookie_accecn_ok(th);
276
277 ret = tcp_get_cookie_sock(sk, skb, req, dst);
278 if (!ret) {
279 SKB_DR_SET(reason, NO_SOCKET);
280 goto out_drop;
281 }
282out:
283 return ret;
284out_free:
285 reqsk_free(req);
286out_drop:
287 sk_skb_reason_drop(sk, skb, reason);
288 return NULL;
289}