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/cryptohash.h> 20 #include <linux/kernel.h> 21 #include <net/ipv6.h> 22 #include <net/tcp.h> 23 24 #define COOKIEBITS 24 /* Upper bits store count */ 25 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1) 26 27 static u32 syncookie6_secret[2][16-4+SHA_DIGEST_WORDS] __read_mostly; 28 29 /* RFC 2460, Section 8.3: 30 * [ipv6 tcp] MSS must be computed as the maximum packet size minus 60 [..] 31 * 32 * Due to IPV6_MIN_MTU=1280 the lowest possible MSS is 1220, which allows 33 * using higher values than ipv4 tcp syncookies. 34 * The other values are chosen based on ethernet (1500 and 9k MTU), plus 35 * one that accounts for common encap (PPPoe) overhead. Table must be sorted. 36 */ 37 static __u16 const msstab[] = { 38 1280 - 60, /* IPV6_MIN_MTU - 60 */ 39 1480 - 60, 40 1500 - 60, 41 9000 - 60, 42 }; 43 44 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS], ipv6_cookie_scratch); 45 46 static u32 cookie_hash(const struct in6_addr *saddr, const struct in6_addr *daddr, 47 __be16 sport, __be16 dport, u32 count, int c) 48 { 49 __u32 *tmp; 50 51 net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret)); 52 53 tmp = this_cpu_ptr(ipv6_cookie_scratch); 54 55 /* 56 * we have 320 bits of information to hash, copy in the remaining 57 * 192 bits required for sha_transform, from the syncookie6_secret 58 * and overwrite the digest with the secret 59 */ 60 memcpy(tmp + 10, syncookie6_secret[c], 44); 61 memcpy(tmp, saddr, 16); 62 memcpy(tmp + 4, daddr, 16); 63 tmp[8] = ((__force u32)sport << 16) + (__force u32)dport; 64 tmp[9] = count; 65 sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5); 66 67 return tmp[17]; 68 } 69 70 static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr, 71 const struct in6_addr *daddr, 72 __be16 sport, __be16 dport, __u32 sseq, 73 __u32 data) 74 { 75 u32 count = tcp_cookie_time(); 76 return (cookie_hash(saddr, daddr, sport, dport, 0, 0) + 77 sseq + (count << COOKIEBITS) + 78 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data) 79 & COOKIEMASK)); 80 } 81 82 static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr, 83 const struct in6_addr *daddr, __be16 sport, 84 __be16 dport, __u32 sseq) 85 { 86 __u32 diff, count = tcp_cookie_time(); 87 88 cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq; 89 90 diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS); 91 if (diff >= MAX_SYNCOOKIE_AGE) 92 return (__u32)-1; 93 94 return (cookie - 95 cookie_hash(saddr, daddr, sport, dport, count - diff, 1)) 96 & COOKIEMASK; 97 } 98 99 u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph, 100 const struct tcphdr *th, __u16 *mssp) 101 { 102 int mssind; 103 const __u16 mss = *mssp; 104 105 for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--) 106 if (mss >= msstab[mssind]) 107 break; 108 109 *mssp = msstab[mssind]; 110 111 return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source, 112 th->dest, ntohl(th->seq), mssind); 113 } 114 EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence); 115 116 __u32 cookie_v6_init_sequence(const struct sk_buff *skb, __u16 *mssp) 117 { 118 const struct ipv6hdr *iph = ipv6_hdr(skb); 119 const struct tcphdr *th = tcp_hdr(skb); 120 121 return __cookie_v6_init_sequence(iph, th, mssp); 122 } 123 124 int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th, 125 __u32 cookie) 126 { 127 __u32 seq = ntohl(th->seq) - 1; 128 __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr, 129 th->source, th->dest, seq); 130 131 return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0; 132 } 133 EXPORT_SYMBOL_GPL(__cookie_v6_check); 134 135 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb) 136 { 137 struct tcp_options_received tcp_opt; 138 struct inet_request_sock *ireq; 139 struct tcp_request_sock *treq; 140 struct ipv6_pinfo *np = inet6_sk(sk); 141 struct tcp_sock *tp = tcp_sk(sk); 142 const struct tcphdr *th = tcp_hdr(skb); 143 __u32 cookie = ntohl(th->ack_seq) - 1; 144 struct sock *ret = sk; 145 struct request_sock *req; 146 int mss; 147 struct dst_entry *dst; 148 __u8 rcv_wscale; 149 150 if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst) 151 goto out; 152 153 if (tcp_synq_no_recent_overflow(sk)) 154 goto out; 155 156 mss = __cookie_v6_check(ipv6_hdr(skb), th, cookie); 157 if (mss == 0) { 158 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED); 159 goto out; 160 } 161 162 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV); 163 164 /* check for timestamp cookie support */ 165 memset(&tcp_opt, 0, sizeof(tcp_opt)); 166 tcp_parse_options(skb, &tcp_opt, 0, NULL); 167 168 if (!cookie_timestamp_decode(&tcp_opt)) 169 goto out; 170 171 ret = NULL; 172 req = inet_reqsk_alloc(&tcp6_request_sock_ops, sk, false); 173 if (!req) 174 goto out; 175 176 ireq = inet_rsk(req); 177 treq = tcp_rsk(req); 178 treq->tfo_listener = false; 179 180 if (security_inet_conn_request(sk, skb, req)) 181 goto out_free; 182 183 req->mss = mss; 184 ireq->ir_rmt_port = th->source; 185 ireq->ir_num = ntohs(th->dest); 186 ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr; 187 ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr; 188 if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) || 189 np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo || 190 np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) { 191 atomic_inc(&skb->users); 192 ireq->pktopts = skb; 193 } 194 195 ireq->ir_iif = inet_request_bound_dev_if(sk, skb); 196 /* So that link locals have meaning */ 197 if (!sk->sk_bound_dev_if && 198 ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL) 199 ireq->ir_iif = tcp_v6_iif(skb); 200 201 ireq->ir_mark = inet_request_mark(sk, skb); 202 203 req->num_retrans = 0; 204 ireq->snd_wscale = tcp_opt.snd_wscale; 205 ireq->sack_ok = tcp_opt.sack_ok; 206 ireq->wscale_ok = tcp_opt.wscale_ok; 207 ireq->tstamp_ok = tcp_opt.saw_tstamp; 208 req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0; 209 treq->snt_synack.v64 = 0; 210 treq->rcv_isn = ntohl(th->seq) - 1; 211 treq->snt_isn = cookie; 212 213 /* 214 * We need to lookup the dst_entry to get the correct window size. 215 * This is taken from tcp_v6_syn_recv_sock. Somebody please enlighten 216 * me if there is a preferred way. 217 */ 218 { 219 struct in6_addr *final_p, final; 220 struct flowi6 fl6; 221 memset(&fl6, 0, sizeof(fl6)); 222 fl6.flowi6_proto = IPPROTO_TCP; 223 fl6.daddr = ireq->ir_v6_rmt_addr; 224 final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final); 225 fl6.saddr = ireq->ir_v6_loc_addr; 226 fl6.flowi6_oif = ireq->ir_iif; 227 fl6.flowi6_mark = ireq->ir_mark; 228 fl6.fl6_dport = ireq->ir_rmt_port; 229 fl6.fl6_sport = inet_sk(sk)->inet_sport; 230 security_req_classify_flow(req, flowi6_to_flowi(&fl6)); 231 232 dst = ip6_dst_lookup_flow(sk, &fl6, final_p); 233 if (IS_ERR(dst)) 234 goto out_free; 235 } 236 237 req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW); 238 tcp_select_initial_window(tcp_full_space(sk), req->mss, 239 &req->rsk_rcv_wnd, &req->rsk_window_clamp, 240 ireq->wscale_ok, &rcv_wscale, 241 dst_metric(dst, RTAX_INITRWND)); 242 243 ireq->rcv_wscale = rcv_wscale; 244 ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst); 245 246 ret = tcp_get_cookie_sock(sk, skb, req, dst); 247 out: 248 return ret; 249 out_free: 250 reqsk_free(req); 251 return NULL; 252 } 253