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_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 */ 33 static __u16 const msstab[] = { 34 1280 - 60, /* IPV6_MIN_MTU - 60 */ 35 1480 - 60, 36 1500 - 60, 37 9000 - 60, 38 }; 39 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 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 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 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); 108 109 __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 } 116 117 int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th) 118 { 119 __u32 cookie = ntohl(th->ack_seq) - 1; 120 __u32 seq = ntohl(th->seq) - 1; 121 __u32 mssind; 122 123 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr, 124 th->source, th->dest, seq); 125 126 return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0; 127 } 128 EXPORT_SYMBOL_GPL(__cookie_v6_check); 129 130 static struct request_sock *cookie_tcp_check(struct net *net, struct sock *sk, 131 struct sk_buff *skb) 132 { 133 struct tcp_options_received tcp_opt; 134 u32 tsoff = 0; 135 int mss; 136 137 if (tcp_synq_no_recent_overflow(sk)) 138 goto out; 139 140 mss = __cookie_v6_check(ipv6_hdr(skb), tcp_hdr(skb)); 141 if (!mss) { 142 __NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESFAILED); 143 goto out; 144 } 145 146 __NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESRECV); 147 148 /* check for timestamp cookie support */ 149 memset(&tcp_opt, 0, sizeof(tcp_opt)); 150 tcp_parse_options(net, skb, &tcp_opt, 0, NULL); 151 152 if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) { 153 tsoff = secure_tcpv6_ts_off(net, 154 ipv6_hdr(skb)->daddr.s6_addr32, 155 ipv6_hdr(skb)->saddr.s6_addr32); 156 tcp_opt.rcv_tsecr -= tsoff; 157 } 158 159 if (!cookie_timestamp_decode(net, &tcp_opt)) 160 goto out; 161 162 return cookie_tcp_reqsk_alloc(&tcp6_request_sock_ops, sk, skb, 163 &tcp_opt, mss, tsoff); 164 out: 165 return ERR_PTR(-EINVAL); 166 } 167 168 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb) 169 { 170 const struct tcphdr *th = tcp_hdr(skb); 171 struct ipv6_pinfo *np = inet6_sk(sk); 172 struct tcp_sock *tp = tcp_sk(sk); 173 struct inet_request_sock *ireq; 174 struct net *net = sock_net(sk); 175 struct request_sock *req; 176 struct dst_entry *dst; 177 struct sock *ret = sk; 178 __u8 rcv_wscale; 179 int full_space; 180 SKB_DR(reason); 181 182 if (!READ_ONCE(net->ipv4.sysctl_tcp_syncookies) || 183 !th->ack || th->rst) 184 goto out; 185 186 if (cookie_bpf_ok(skb)) { 187 req = cookie_bpf_check(sk, skb); 188 } else { 189 req = cookie_tcp_check(net, sk, skb); 190 if (IS_ERR(req)) 191 goto out; 192 } 193 if (!req) { 194 SKB_DR_SET(reason, NO_SOCKET); 195 goto out_drop; 196 } 197 198 ireq = inet_rsk(req); 199 200 ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr; 201 ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr; 202 203 if (security_inet_conn_request(sk, skb, req)) { 204 SKB_DR_SET(reason, SECURITY_HOOK); 205 goto out_free; 206 } 207 208 if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) || 209 np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo || 210 np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) { 211 refcount_inc(&skb->users); 212 ireq->pktopts = skb; 213 } 214 215 /* So that link locals have meaning */ 216 if (!sk->sk_bound_dev_if && 217 ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL) 218 ireq->ir_iif = tcp_v6_iif(skb); 219 220 tcp_ao_syncookie(sk, skb, req, AF_INET6); 221 222 /* 223 * We need to lookup the dst_entry to get the correct window size. 224 * This is taken from tcp_v6_syn_recv_sock. Somebody please enlighten 225 * me if there is a preferred way. 226 */ 227 { 228 struct in6_addr *final_p, final; 229 struct flowi6 fl6; 230 memset(&fl6, 0, sizeof(fl6)); 231 fl6.flowi6_proto = IPPROTO_TCP; 232 fl6.daddr = ireq->ir_v6_rmt_addr; 233 final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final); 234 fl6.saddr = ireq->ir_v6_loc_addr; 235 fl6.flowi6_oif = ireq->ir_iif; 236 fl6.flowi6_mark = ireq->ir_mark; 237 fl6.fl6_dport = ireq->ir_rmt_port; 238 fl6.fl6_sport = inet_sk(sk)->inet_sport; 239 fl6.flowi6_uid = sk->sk_uid; 240 security_req_classify_flow(req, flowi6_to_flowi_common(&fl6)); 241 242 dst = ip6_dst_lookup_flow(net, sk, &fl6, final_p); 243 if (IS_ERR(dst)) { 244 SKB_DR_SET(reason, IP_OUTNOROUTES); 245 goto out_free; 246 } 247 } 248 249 req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW); 250 /* limit the window selection if the user enforce a smaller rx buffer */ 251 full_space = tcp_full_space(sk); 252 if (sk->sk_userlocks & SOCK_RCVBUF_LOCK && 253 (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0)) 254 req->rsk_window_clamp = full_space; 255 256 tcp_select_initial_window(sk, full_space, req->mss, 257 &req->rsk_rcv_wnd, &req->rsk_window_clamp, 258 ireq->wscale_ok, &rcv_wscale, 259 dst_metric(dst, RTAX_INITRWND)); 260 261 /* req->syncookie is set true only if ACK is validated 262 * by BPF kfunc, then, rcv_wscale is already configured. 263 */ 264 if (!req->syncookie) 265 ireq->rcv_wscale = rcv_wscale; 266 ireq->ecn_ok &= cookie_ecn_ok(net, dst); 267 268 ret = tcp_get_cookie_sock(sk, skb, req, dst); 269 if (!ret) { 270 SKB_DR_SET(reason, NO_SOCKET); 271 goto out_drop; 272 } 273 out: 274 return ret; 275 out_free: 276 reqsk_free(req); 277 out_drop: 278 kfree_skb_reason(skb, reason); 279 return NULL; 280 } 281