net/ipv6/syncookies.c

   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 /* RFC 2460, Section 8.3:
  28  * [ipv6 tcp] MSS must be computed as the maximum packet size minus 60 [..]
  29  *
  30  * Due to IPV6_MIN_MTU=1280 the lowest possible MSS is 1220, which allows
  31  * using higher values than ipv4 tcp syncookies.
  32  * The other values are chosen based on ethernet (1500 and 9k MTU), plus
  33  * one that accounts for common encap (PPPoe) overhead. Table must be sorted.
  34  */
  35 static __u16 const msstab[] = {
  36         1280 - 60, /* IPV6_MIN_MTU - 60 */
  37         1480 - 60,
  38         1500 - 60,
  39         9000 - 60,
  40 };
  41
  42 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
  43                                            struct request_sock *req,
  44                                            struct dst_entry *dst)
  45 {
  46         struct inet_connection_sock *icsk = inet_csk(sk);
  47         struct sock *child;
  48
  49         child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
  50         if (child)
  51                 inet_csk_reqsk_queue_add(sk, req, child);
  52         else
  53                 reqsk_free(req);
  54
  55         return child;
  56 }
  57
  58 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
  59                       ipv6_cookie_scratch);
  60
  61 static u32 cookie_hash(const struct in6_addr *saddr, const struct in6_addr *daddr,
  62                        __be16 sport, __be16 dport, u32 count, int c)
  63 {
  64         __u32 *tmp = __get_cpu_var(ipv6_cookie_scratch);
  65
  66         /*
  67          * we have 320 bits of information to hash, copy in the remaining
  68          * 192 bits required for sha_transform, from the syncookie_secret
  69          * and overwrite the digest with the secret
  70          */
  71         memcpy(tmp + 10, syncookie_secret[c], 44);
  72         memcpy(tmp, saddr, 16);
  73         memcpy(tmp + 4, daddr, 16);
  74         tmp[8] = ((__force u32)sport << 16) + (__force u32)dport;
  75         tmp[9] = count;
  76         sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
  77
  78         return tmp[17];
  79 }
  80
  81 static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
  82                                    const struct in6_addr *daddr,
  83                                    __be16 sport, __be16 dport, __u32 sseq,
  84                                    __u32 data)
  85 {
  86         u32 count = tcp_cookie_time();
  87         return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
  88                 sseq + (count << COOKIEBITS) +
  89                 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
  90                 & COOKIEMASK));
  91 }
  92
  93 static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
  94                                   const struct in6_addr *daddr, __be16 sport,
  95                                   __be16 dport, __u32 sseq)
  96 {
  97         __u32 diff, count = tcp_cookie_time();
  98
  99         cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
 100
 101         diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
 102         if (diff >= MAX_SYNCOOKIE_AGE)
 103                 return (__u32)-1;
 104
 105         return (cookie -
 106                 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
 107                 & COOKIEMASK;
 108 }
 109
 110 u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph,
 111                               const struct tcphdr *th, __u16 *mssp)
 112 {
 113         int mssind;
 114         const __u16 mss = *mssp;
 115
 116         for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
 117                 if (mss >= msstab[mssind])
 118                         break;
 119
 120         *mssp = msstab[mssind];
 121
 122         return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
 123                                      th->dest, ntohl(th->seq), mssind);
 124 }
 125 EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence);
 126
 127 __u32 cookie_v6_init_sequence(struct sock *sk, const struct sk_buff *skb, __u16 *mssp)
 128 {
 129         const struct ipv6hdr *iph = ipv6_hdr(skb);
 130         const struct tcphdr *th = tcp_hdr(skb);
 131
 132         tcp_synq_overflow(sk);
 133         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
 134
 135         return __cookie_v6_init_sequence(iph, th, mssp);
 136 }
 137
 138 int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th,
 139                       __u32 cookie)
 140 {
 141         __u32 seq = ntohl(th->seq) - 1;
 142         __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
 143                                             th->source, th->dest, seq);
 144
 145         return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
 146 }
 147 EXPORT_SYMBOL_GPL(__cookie_v6_check);
 148
 149 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 150 {
 151         struct tcp_options_received tcp_opt;
 152         struct inet_request_sock *ireq;
 153         struct tcp_request_sock *treq;
 154         struct ipv6_pinfo *np = inet6_sk(sk);
 155         struct tcp_sock *tp = tcp_sk(sk);
 156         const struct tcphdr *th = tcp_hdr(skb);
 157         __u32 cookie = ntohl(th->ack_seq) - 1;
 158         struct sock *ret = sk;
 159         struct request_sock *req;
 160         int mss;
 161         struct dst_entry *dst;
 162         __u8 rcv_wscale;
 163         bool ecn_ok = false;
 164
 165         if (!sysctl_tcp_syncookies || !th->ack || th->rst)
 166                 goto out;
 167
 168         if (tcp_synq_no_recent_overflow(sk) ||
 169                 (mss = __cookie_v6_check(ipv6_hdr(skb), th, cookie)) == 0) {
 170                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 171                 goto out;
 172         }
 173
 174         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 175
 176         /* check for timestamp cookie support */
 177         memset(&tcp_opt, 0, sizeof(tcp_opt));
 178         tcp_parse_options(skb, &tcp_opt, 0, NULL);
 179
 180         if (!cookie_check_timestamp(&tcp_opt, sock_net(sk), &ecn_ok))
 181                 goto out;
 182
 183         ret = NULL;
 184         req = inet6_reqsk_alloc(&tcp6_request_sock_ops);
 185         if (!req)
 186                 goto out;
 187
 188         ireq = inet_rsk(req);
 189         treq = tcp_rsk(req);
 190         treq->listener = NULL;
 191
 192         if (security_inet_conn_request(sk, skb, req))
 193                 goto out_free;
 194
 195         req->mss = mss;
 196         ireq->ir_rmt_port = th->source;
 197         ireq->ir_loc_port = th->dest;
 198         ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
 199         ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
 200         if (ipv6_opt_accepted(sk, skb) ||
 201             np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
 202             np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
 203                 atomic_inc(&skb->users);
 204                 ireq->pktopts = skb;
 205         }
 206
 207         ireq->ir_iif = sk->sk_bound_dev_if;
 208         /* So that link locals have meaning */
 209         if (!sk->sk_bound_dev_if &&
 210             ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
 211                 ireq->ir_iif = inet6_iif(skb);
 212
 213         req->expires = 0UL;
 214         req->num_retrans = 0;
 215         ireq->ecn_ok            = ecn_ok;
 216         ireq->snd_wscale        = tcp_opt.snd_wscale;
 217         ireq->sack_ok           = tcp_opt.sack_ok;
 218         ireq->wscale_ok         = tcp_opt.wscale_ok;
 219         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 220         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 221         treq->snt_synack        = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
 222         treq->rcv_isn = ntohl(th->seq) - 1;
 223         treq->snt_isn = cookie;
 224
 225         /*
 226          * We need to lookup the dst_entry to get the correct window size.
 227          * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
 228          * me if there is a preferred way.
 229          */
 230         {
 231                 struct in6_addr *final_p, final;
 232                 struct flowi6 fl6;
 233                 memset(&fl6, 0, sizeof(fl6));
 234                 fl6.flowi6_proto = IPPROTO_TCP;
 235                 fl6.daddr = ireq->ir_v6_rmt_addr;
 236                 final_p = fl6_update_dst(&fl6, np->opt, &final);
 237                 fl6.saddr = ireq->ir_v6_loc_addr;
 238                 fl6.flowi6_oif = sk->sk_bound_dev_if;
 239                 fl6.flowi6_mark = sk->sk_mark;
 240                 fl6.fl6_dport = ireq->ir_rmt_port;
 241                 fl6.fl6_sport = inet_sk(sk)->inet_sport;
 242                 security_req_classify_flow(req, flowi6_to_flowi(&fl6));
 243
 244                 dst = ip6_dst_lookup_flow(sk, &fl6, final_p, false);
 245                 if (IS_ERR(dst))
 246                         goto out_free;
 247         }
 248
 249         req->window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
 250         tcp_select_initial_window(tcp_full_space(sk), req->mss,
 251                                   &req->rcv_wnd, &req->window_clamp,
 252                                   ireq->wscale_ok, &rcv_wscale,
 253                                   dst_metric(dst, RTAX_INITRWND));
 254
 255         ireq->rcv_wscale = rcv_wscale;
 256
 257         ret = get_cookie_sock(sk, skb, req, dst);
 258 out:
 259         return ret;
 260 out_free:
 261         reqsk_free(req);
 262         return NULL;
 263 }
 264