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 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 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 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 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); 113 114 __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 } 121 122 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); 132 133 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(); 220 221 /* 222 * We need to lookup the dst_entry to get the correct window size. 223 * This is taken from tcp_v6_syn_recv_sock. Somebody please enlighten 224 * me if there is a preferred way. 225 */ 226 { 227 struct in6_addr *final_p, final; 228 struct flowi6 fl6; 229 memset(&fl6, 0, sizeof(fl6)); 230 fl6.flowi6_proto = IPPROTO_TCP; 231 fl6.daddr = ireq->ir_v6_rmt_addr; 232 final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final); 233 fl6.saddr = ireq->ir_v6_loc_addr; 234 fl6.flowi6_oif = ireq->ir_iif; 235 fl6.flowi6_mark = ireq->ir_mark; 236 fl6.fl6_dport = ireq->ir_rmt_port; 237 fl6.fl6_sport = inet_sk(sk)->inet_sport; 238 fl6.flowi6_uid = sk->sk_uid; 239 security_req_classify_flow(req, flowi6_to_flowi(&fl6)); 240 241 dst = ip6_dst_lookup_flow(sk, &fl6, final_p); 242 if (IS_ERR(dst)) 243 goto out_free; 244 } 245 246 req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW); 247 tcp_select_initial_window(sk, tcp_full_space(sk), req->mss, 248 &req->rsk_rcv_wnd, &req->rsk_window_clamp, 249 ireq->wscale_ok, &rcv_wscale, 250 dst_metric(dst, RTAX_INITRWND)); 251 252 ireq->rcv_wscale = rcv_wscale; 253 ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst); 254 255 ret = tcp_get_cookie_sock(sk, skb, req, dst, tsoff); 256 out: 257 return ret; 258 out_free: 259 reqsk_free(req); 260 return NULL; 261 } 262