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 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         treq->txhash = net_tx_rndhash();
 213
 214         /*
 215          * We need to lookup the dst_entry to get the correct window size.
 216          * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
 217          * me if there is a preferred way.
 218          */
 219         {
 220                 struct in6_addr *final_p, final;
 221                 struct flowi6 fl6;
 222                 memset(&fl6, 0, sizeof(fl6));
 223                 fl6.flowi6_proto = IPPROTO_TCP;
 224                 fl6.daddr = ireq->ir_v6_rmt_addr;
 225                 final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
 226                 fl6.saddr = ireq->ir_v6_loc_addr;
 227                 fl6.flowi6_oif = ireq->ir_iif;
 228                 fl6.flowi6_mark = ireq->ir_mark;
 229                 fl6.fl6_dport = ireq->ir_rmt_port;
 230                 fl6.fl6_sport = inet_sk(sk)->inet_sport;
 231                 security_req_classify_flow(req, flowi6_to_flowi(&fl6));
 232
 233                 dst = ip6_dst_lookup_flow(sk, &fl6, final_p);
 234                 if (IS_ERR(dst))
 235                         goto out_free;
 236         }
 237
 238         req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
 239         tcp_select_initial_window(tcp_full_space(sk), req->mss,
 240                                   &req->rsk_rcv_wnd, &req->rsk_window_clamp,
 241                                   ireq->wscale_ok, &rcv_wscale,
 242                                   dst_metric(dst, RTAX_INITRWND));
 243
 244         ireq->rcv_wscale = rcv_wscale;
 245         ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst);
 246
 247         ret = tcp_get_cookie_sock(sk, skb, req, dst);
 248 out:
 249         return ret;
 250 out_free:
 251         reqsk_free(req);
 252         return NULL;
 253 }