1 /*
2 * IPv6 Syncookies implementation for the Linux kernel
3 *
4 * Authors:
5 * Glenn Griffin <ggriffin.kernel@gmail.com>
6 *
7 * Based on IPv4 implementation by Andi Kleen
8 * linux/net/ipv4/syncookies.c
9 *
10 * This program is free software; you can redistribute it and/or
11 * modify it under the terms of the GNU General Public License
12 * as published by the Free Software Foundation; either version
13 * 2 of the License, or (at your option) any later version.
14 *
15 */
16
17 #include <linux/tcp.h>
18 #include <linux/random.h>
19 #include <linux/siphash.h>
20 #include <linux/kernel.h>
21 #include <net/secure_seq.h>
22 #include <net/ipv6.h>
23 #include <net/tcp.h>
24
25 #define COOKIEBITS 24 /* Upper bits store count */
26 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
27
28 static siphash_key_t syncookie6_secret[2] __read_mostly;
29
30 /* RFC 2460, Section 8.3:
31 * [ipv6 tcp] MSS must be computed as the maximum packet size minus 60 [..]
32 *
33 * Due to IPV6_MIN_MTU=1280 the lowest possible MSS is 1220, which allows
34 * using higher values than ipv4 tcp syncookies.
35 * The other values are chosen based on ethernet (1500 and 9k MTU), plus
36 * one that accounts for common encap (PPPoe) overhead. Table must be sorted.
37 */
38 static __u16 const msstab[] = {
39 1280 - 60, /* IPV6_MIN_MTU - 60 */
40 1480 - 60,
41 1500 - 60,
42 9000 - 60,
43 };
44
cookie_hash(const struct in6_addr * saddr,const struct in6_addr * daddr,__be16 sport,__be16 dport,u32 count,int c)45 static u32 cookie_hash(const struct in6_addr *saddr,
46 const struct in6_addr *daddr,
47 __be16 sport, __be16 dport, u32 count, int c)
48 {
49 const struct {
50 struct in6_addr saddr;
51 struct in6_addr daddr;
52 u32 count;
53 __be16 sport;
54 __be16 dport;
55 } __aligned(SIPHASH_ALIGNMENT) combined = {
56 .saddr = *saddr,
57 .daddr = *daddr,
58 .count = count,
59 .sport = sport,
60 .dport = dport
61 };
62
63 net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret));
64 return siphash(&combined, offsetofend(typeof(combined), dport),
65 &syncookie6_secret[c]);
66 }
67
secure_tcp_syn_cookie(const struct in6_addr * saddr,const struct in6_addr * daddr,__be16 sport,__be16 dport,__u32 sseq,__u32 data)68 static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
69 const struct in6_addr *daddr,
70 __be16 sport, __be16 dport, __u32 sseq,
71 __u32 data)
72 {
73 u32 count = tcp_cookie_time();
74 return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
75 sseq + (count << COOKIEBITS) +
76 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
77 & COOKIEMASK));
78 }
79
check_tcp_syn_cookie(__u32 cookie,const struct in6_addr * saddr,const struct in6_addr * daddr,__be16 sport,__be16 dport,__u32 sseq)80 static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
81 const struct in6_addr *daddr, __be16 sport,
82 __be16 dport, __u32 sseq)
83 {
84 __u32 diff, count = tcp_cookie_time();
85
86 cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
87
88 diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
89 if (diff >= MAX_SYNCOOKIE_AGE)
90 return (__u32)-1;
91
92 return (cookie -
93 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
94 & COOKIEMASK;
95 }
96
__cookie_v6_init_sequence(const struct ipv6hdr * iph,const struct tcphdr * th,__u16 * mssp)97 u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph,
98 const struct tcphdr *th, __u16 *mssp)
99 {
100 int mssind;
101 const __u16 mss = *mssp;
102
103 for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
104 if (mss >= msstab[mssind])
105 break;
106
107 *mssp = msstab[mssind];
108
109 return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
110 th->dest, ntohl(th->seq), mssind);
111 }
112 EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence);
113
cookie_v6_init_sequence(const struct sk_buff * skb,__u16 * mssp)114 __u32 cookie_v6_init_sequence(const struct sk_buff *skb, __u16 *mssp)
115 {
116 const struct ipv6hdr *iph = ipv6_hdr(skb);
117 const struct tcphdr *th = tcp_hdr(skb);
118
119 return __cookie_v6_init_sequence(iph, th, mssp);
120 }
121
__cookie_v6_check(const struct ipv6hdr * iph,const struct tcphdr * th,__u32 cookie)122 int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th,
123 __u32 cookie)
124 {
125 __u32 seq = ntohl(th->seq) - 1;
126 __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
127 th->source, th->dest, seq);
128
129 return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
130 }
131 EXPORT_SYMBOL_GPL(__cookie_v6_check);
132
cookie_v6_check(struct sock * sk,struct sk_buff * skb)133 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
134 {
135 struct tcp_options_received tcp_opt;
136 struct inet_request_sock *ireq;
137 struct tcp_request_sock *treq;
138 struct ipv6_pinfo *np = inet6_sk(sk);
139 struct tcp_sock *tp = tcp_sk(sk);
140 const struct tcphdr *th = tcp_hdr(skb);
141 __u32 cookie = ntohl(th->ack_seq) - 1;
142 struct sock *ret = sk;
143 struct request_sock *req;
144 int mss;
145 struct dst_entry *dst;
146 __u8 rcv_wscale;
147 u32 tsoff = 0;
148
149 if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst)
150 goto out;
151
152 if (tcp_synq_no_recent_overflow(sk))
153 goto out;
154
155 mss = __cookie_v6_check(ipv6_hdr(skb), th, cookie);
156 if (mss == 0) {
157 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
158 goto out;
159 }
160
161 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
162
163 /* check for timestamp cookie support */
164 memset(&tcp_opt, 0, sizeof(tcp_opt));
165 tcp_parse_options(sock_net(sk), skb, &tcp_opt, 0, NULL);
166
167 if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
168 tsoff = secure_tcpv6_ts_off(sock_net(sk),
169 ipv6_hdr(skb)->daddr.s6_addr32,
170 ipv6_hdr(skb)->saddr.s6_addr32);
171 tcp_opt.rcv_tsecr -= tsoff;
172 }
173
174 if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
175 goto out;
176
177 ret = NULL;
178 req = inet_reqsk_alloc(&tcp6_request_sock_ops, sk, false);
179 if (!req)
180 goto out;
181
182 ireq = inet_rsk(req);
183 treq = tcp_rsk(req);
184 treq->tfo_listener = false;
185
186 if (security_inet_conn_request(sk, skb, req))
187 goto out_free;
188
189 req->mss = mss;
190 ireq->ir_rmt_port = th->source;
191 ireq->ir_num = ntohs(th->dest);
192 ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
193 ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
194 if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) ||
195 np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
196 np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
197 refcount_inc(&skb->users);
198 ireq->pktopts = skb;
199 }
200
201 ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
202 /* So that link locals have meaning */
203 if (!sk->sk_bound_dev_if &&
204 ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
205 ireq->ir_iif = tcp_v6_iif(skb);
206
207 ireq->ir_mark = inet_request_mark(sk, skb);
208
209 req->num_retrans = 0;
210 ireq->snd_wscale = tcp_opt.snd_wscale;
211 ireq->sack_ok = tcp_opt.sack_ok;
212 ireq->wscale_ok = tcp_opt.wscale_ok;
213 ireq->tstamp_ok = tcp_opt.saw_tstamp;
214 req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
215 treq->snt_synack = 0;
216 treq->rcv_isn = ntohl(th->seq) - 1;
217 treq->snt_isn = cookie;
218 treq->ts_off = 0;
219 treq->txhash = net_tx_rndhash();
220 if (IS_ENABLED(CONFIG_SMC))
221 ireq->smc_ok = 0;
222
223 /*
224 * We need to lookup the dst_entry to get the correct window size.
225 * This is taken from tcp_v6_syn_recv_sock. Somebody please enlighten
226 * me if there is a preferred way.
227 */
228 {
229 struct in6_addr *final_p, final;
230 struct flowi6 fl6;
231 memset(&fl6, 0, sizeof(fl6));
232 fl6.flowi6_proto = IPPROTO_TCP;
233 fl6.daddr = ireq->ir_v6_rmt_addr;
234 final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
235 fl6.saddr = ireq->ir_v6_loc_addr;
236 fl6.flowi6_oif = ireq->ir_iif;
237 fl6.flowi6_mark = ireq->ir_mark;
238 fl6.fl6_dport = ireq->ir_rmt_port;
239 fl6.fl6_sport = inet_sk(sk)->inet_sport;
240 fl6.flowi6_uid = sk->sk_uid;
241 security_req_classify_flow(req, flowi6_to_flowi(&fl6));
242
243 dst = ip6_dst_lookup_flow(sk, &fl6, final_p);
244 if (IS_ERR(dst))
245 goto out_free;
246 }
247
248 req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
249 tcp_select_initial_window(sk, tcp_full_space(sk), req->mss,
250 &req->rsk_rcv_wnd, &req->rsk_window_clamp,
251 ireq->wscale_ok, &rcv_wscale,
252 dst_metric(dst, RTAX_INITRWND));
253
254 ireq->rcv_wscale = rcv_wscale;
255 ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst);
256
257 ret = tcp_get_cookie_sock(sk, skb, req, dst, tsoff);
258 out:
259 return ret;
260 out_free:
261 reqsk_free(req);
262 return NULL;
263 }
264
