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(&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
