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(data: &combined, offsetofend(typeof(combined), dport),
60 key: &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, count: 0, c: 0) +
70 sseq + (count << COOKIEBITS) +
71 ((cookie_hash(saddr, daddr, sport, dport, count, c: 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, count: 0, c: 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: count - diff, c: 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(saddr: &iph->saddr, daddr: &iph->daddr, sport: th->source,
105 dport: th->dest, ntohl(th->seq), data: 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 __u32 cookie)
119{
120 __u32 seq = ntohl(th->seq) - 1;
121 __u32 mssind = check_tcp_syn_cookie(cookie, saddr: &iph->saddr, daddr: &iph->daddr,
122 sport: th->source, dport: th->dest, sseq: seq);
123
124 return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
125}
126EXPORT_SYMBOL_GPL(__cookie_v6_check);
127
128struct 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: 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 int l3index;
144
145 if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_syncookies) ||
146 !th->ack || th->rst)
147 goto out;
148
149 if (tcp_synq_no_recent_overflow(sk))
150 goto out;
151
152 mss = __cookie_v6_check(ipv6_hdr(skb), th, cookie);
153 if (mss == 0) {
154 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
155 goto out;
156 }
157
158 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
159
160 /* check for timestamp cookie support */
161 memset(&tcp_opt, 0, sizeof(tcp_opt));
162 tcp_parse_options(net: sock_net(sk), skb, opt_rx: &tcp_opt, estab: 0, NULL);
163
164 if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
165 tsoff = secure_tcpv6_ts_off(net: sock_net(sk),
166 saddr: ipv6_hdr(skb)->daddr.s6_addr32,
167 daddr: ipv6_hdr(skb)->saddr.s6_addr32);
168 tcp_opt.rcv_tsecr -= tsoff;
169 }
170
171 if (!cookie_timestamp_decode(net: sock_net(sk), opt: &tcp_opt))
172 goto out;
173
174 ret = NULL;
175 req = cookie_tcp_reqsk_alloc(ops: &tcp6_request_sock_ops,
176 af_ops: &tcp_request_sock_ipv6_ops, sk, skb);
177 if (!req)
178 goto out;
179
180 ireq = inet_rsk(sk: req);
181 treq = tcp_rsk(req);
182 treq->tfo_listener = false;
183
184 if (security_inet_conn_request(sk, skb, req))
185 goto out_free;
186
187 req->mss = mss;
188 ireq->ir_rmt_port = th->source;
189 ireq->ir_num = ntohs(th->dest);
190 ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
191 ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
192 if (ipv6_opt_accepted(sk, skb, opt: &TCP_SKB_CB(skb)->header.h6) ||
193 np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
194 np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
195 refcount_inc(r: &skb->users);
196 ireq->pktopts = skb;
197 }
198
199 ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
200 /* So that link locals have meaning */
201 if (!sk->sk_bound_dev_if &&
202 ipv6_addr_type(addr: &ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
203 ireq->ir_iif = tcp_v6_iif(skb);
204
205 ireq->ir_mark = inet_request_mark(sk, skb);
206
207 req->num_retrans = 0;
208 ireq->snd_wscale = tcp_opt.snd_wscale;
209 ireq->sack_ok = tcp_opt.sack_ok;
210 ireq->wscale_ok = tcp_opt.wscale_ok;
211 ireq->tstamp_ok = tcp_opt.saw_tstamp;
212 req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
213 treq->snt_synack = 0;
214 treq->rcv_isn = ntohl(th->seq) - 1;
215 treq->snt_isn = cookie;
216 treq->ts_off = 0;
217 treq->txhash = net_tx_rndhash();
218
219 l3index = l3mdev_master_ifindex_by_index(net: sock_net(sk), ifindex: ireq->ir_iif);
220 tcp_ao_syncookie(sk, skb, treq, AF_INET6, l3index);
221
222 if (IS_ENABLED(CONFIG_SMC))
223 ireq->smc_ok = 0;
224
225 /*
226 * We need to lookup the dst_entry to get the correct window size.
227 * This is taken from tcp_v6_syn_recv_sock. Somebody please enlighten
228 * me if there is a preferred way.
229 */
230 {
231 struct in6_addr *final_p, final;
232 struct flowi6 fl6;
233 memset(&fl6, 0, sizeof(fl6));
234 fl6.flowi6_proto = IPPROTO_TCP;
235 fl6.daddr = ireq->ir_v6_rmt_addr;
236 final_p = fl6_update_dst(fl6: &fl6, rcu_dereference(np->opt), orig: &final);
237 fl6.saddr = ireq->ir_v6_loc_addr;
238 fl6.flowi6_oif = ireq->ir_iif;
239 fl6.flowi6_mark = ireq->ir_mark;
240 fl6.fl6_dport = ireq->ir_rmt_port;
241 fl6.fl6_sport = inet_sk(sk)->inet_sport;
242 fl6.flowi6_uid = sk->sk_uid;
243 security_req_classify_flow(req, flic: flowi6_to_flowi_common(fl6: &fl6));
244
245 dst = ip6_dst_lookup_flow(net: sock_net(sk), sk, fl6: &fl6, final_dst: final_p);
246 if (IS_ERR(ptr: dst))
247 goto out_free;
248 }
249
250 req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
251 /* limit the window selection if the user enforce a smaller rx buffer */
252 full_space = tcp_full_space(sk);
253 if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
254 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
255 req->rsk_window_clamp = full_space;
256
257 tcp_select_initial_window(sk, space: full_space, mss: req->mss,
258 rcv_wnd: &req->rsk_rcv_wnd, window_clamp: &req->rsk_window_clamp,
259 wscale_ok: ireq->wscale_ok, rcv_wscale: &rcv_wscale,
260 init_rcv_wnd: dst_metric(dst, RTAX_INITRWND));
261
262 ireq->rcv_wscale = rcv_wscale;
263 ireq->ecn_ok = cookie_ecn_ok(opt: &tcp_opt, net: sock_net(sk), dst);
264
265 ret = tcp_get_cookie_sock(sk, skb, req, dst, tsoff);
266out:
267 return ret;
268out_free:
269 reqsk_free(req);
270 return NULL;
271}
272

source code of linux/net/ipv6/syncookies.c