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 inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
  45                                            struct request_sock *req,
  46                                            struct dst_entry *dst)
  47 {
  48         struct inet_connection_sock *icsk = inet_csk(sk);
  49         struct sock *child;
  50
  51         child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
  52         if (child) {
  53                 atomic_set(&req->rsk_refcnt, 1);
  54                 inet_csk_reqsk_queue_add(sk, req, child);
  55         } else {
  56                 reqsk_free(req);
  57         }
  58         return child;
  59 }
  60
  61 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
  62                       ipv6_cookie_scratch);
  63
  64 static u32 cookie_hash(const struct in6_addr *saddr, const struct in6_addr *daddr,
  65                        __be16 sport, __be16 dport, u32 count, int c)
  66 {
  67         __u32 *tmp;
  68
  69         net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret));
  70
  71         tmp  = this_cpu_ptr(ipv6_cookie_scratch);
  72
  73         /*
  74          * we have 320 bits of information to hash, copy in the remaining
  75          * 192 bits required for sha_transform, from the syncookie6_secret
  76          * and overwrite the digest with the secret
  77          */
  78         memcpy(tmp + 10, syncookie6_secret[c], 44);
  79         memcpy(tmp, saddr, 16);
  80         memcpy(tmp + 4, daddr, 16);
  81         tmp[8] = ((__force u32)sport << 16) + (__force u32)dport;
  82         tmp[9] = count;
  83         sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
  84
  85         return tmp[17];
  86 }
  87
  88 static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
  89                                    const struct in6_addr *daddr,
  90                                    __be16 sport, __be16 dport, __u32 sseq,
  91                                    __u32 data)
  92 {
  93         u32 count = tcp_cookie_time();
  94         return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
  95                 sseq + (count << COOKIEBITS) +
  96                 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
  97                 & COOKIEMASK));
  98 }
  99
 100 static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
 101                                   const struct in6_addr *daddr, __be16 sport,
 102                                   __be16 dport, __u32 sseq)
 103 {
 104         __u32 diff, count = tcp_cookie_time();
 105
 106         cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
 107
 108         diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
 109         if (diff >= MAX_SYNCOOKIE_AGE)
 110                 return (__u32)-1;
 111
 112         return (cookie -
 113                 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
 114                 & COOKIEMASK;
 115 }
 116
 117 u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph,
 118                               const struct tcphdr *th, __u16 *mssp)
 119 {
 120         int mssind;
 121         const __u16 mss = *mssp;
 122
 123         for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
 124                 if (mss >= msstab[mssind])
 125                         break;
 126
 127         *mssp = msstab[mssind];
 128
 129         return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
 130                                      th->dest, ntohl(th->seq), mssind);
 131 }
 132 EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence);
 133
 134 __u32 cookie_v6_init_sequence(struct sock *sk, const struct sk_buff *skb, __u16 *mssp)
 135 {
 136         const struct ipv6hdr *iph = ipv6_hdr(skb);
 137         const struct tcphdr *th = tcp_hdr(skb);
 138
 139         tcp_synq_overflow(sk);
 140         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
 141
 142         return __cookie_v6_init_sequence(iph, th, mssp);
 143 }
 144
 145 int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th,
 146                       __u32 cookie)
 147 {
 148         __u32 seq = ntohl(th->seq) - 1;
 149         __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
 150                                             th->source, th->dest, seq);
 151
 152         return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
 153 }
 154 EXPORT_SYMBOL_GPL(__cookie_v6_check);
 155
 156 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 157 {
 158         struct tcp_options_received tcp_opt;
 159         struct inet_request_sock *ireq;
 160         struct tcp_request_sock *treq;
 161         struct ipv6_pinfo *np = inet6_sk(sk);
 162         struct tcp_sock *tp = tcp_sk(sk);
 163         const struct tcphdr *th = tcp_hdr(skb);
 164         __u32 cookie = ntohl(th->ack_seq) - 1;
 165         struct sock *ret = sk;
 166         struct request_sock *req;
 167         int mss;
 168         struct dst_entry *dst;
 169         __u8 rcv_wscale;
 170
 171         if (!sysctl_tcp_syncookies || !th->ack || th->rst)
 172                 goto out;
 173
 174         if (tcp_synq_no_recent_overflow(sk))
 175                 goto out;
 176
 177         mss = __cookie_v6_check(ipv6_hdr(skb), th, cookie);
 178         if (mss == 0) {
 179                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 180                 goto out;
 181         }
 182
 183         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 184
 185         /* check for timestamp cookie support */
 186         memset(&tcp_opt, 0, sizeof(tcp_opt));
 187         tcp_parse_options(skb, &tcp_opt, 0, NULL);
 188
 189         if (!cookie_timestamp_decode(&tcp_opt))
 190                 goto out;
 191
 192         ret = NULL;
 193         req = inet_reqsk_alloc(&tcp6_request_sock_ops, sk);
 194         if (!req)
 195                 goto out;
 196
 197         ireq = inet_rsk(req);
 198         treq = tcp_rsk(req);
 199         treq->tfo_listener = false;
 200
 201         if (security_inet_conn_request(sk, skb, req))
 202                 goto out_free;
 203
 204         req->mss = mss;
 205         ireq->ir_rmt_port = th->source;
 206         ireq->ir_num = ntohs(th->dest);
 207         ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
 208         ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
 209         if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) ||
 210             np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
 211             np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
 212                 atomic_inc(&skb->users);
 213                 ireq->pktopts = skb;
 214         }
 215
 216         ireq->ir_iif = sk->sk_bound_dev_if;
 217         /* So that link locals have meaning */
 218         if (!sk->sk_bound_dev_if &&
 219             ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
 220                 ireq->ir_iif = tcp_v6_iif(skb);
 221
 222         ireq->ir_mark = inet_request_mark(sk, skb);
 223
 224         req->num_retrans = 0;
 225         ireq->snd_wscale        = tcp_opt.snd_wscale;
 226         ireq->sack_ok           = tcp_opt.sack_ok;
 227         ireq->wscale_ok         = tcp_opt.wscale_ok;
 228         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 229         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 230         treq->snt_synack        = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
 231         treq->rcv_isn = ntohl(th->seq) - 1;
 232         treq->snt_isn = cookie;
 233
 234         /*
 235          * We need to lookup the dst_entry to get the correct window size.
 236          * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
 237          * me if there is a preferred way.
 238          */
 239         {
 240                 struct in6_addr *final_p, final;
 241                 struct flowi6 fl6;
 242                 memset(&fl6, 0, sizeof(fl6));
 243                 fl6.flowi6_proto = IPPROTO_TCP;
 244                 fl6.daddr = ireq->ir_v6_rmt_addr;
 245                 final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
 246                 fl6.saddr = ireq->ir_v6_loc_addr;
 247                 fl6.flowi6_oif = sk->sk_bound_dev_if;
 248                 fl6.flowi6_mark = ireq->ir_mark;
 249                 fl6.fl6_dport = ireq->ir_rmt_port;
 250                 fl6.fl6_sport = inet_sk(sk)->inet_sport;
 251                 security_req_classify_flow(req, flowi6_to_flowi(&fl6));
 252
 253                 dst = ip6_dst_lookup_flow(sk, &fl6, final_p);
 254                 if (IS_ERR(dst))
 255                         goto out_free;
 256         }
 257
 258         req->window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
 259         tcp_select_initial_window(tcp_full_space(sk), req->mss,
 260                                   &req->rcv_wnd, &req->window_clamp,
 261                                   ireq->wscale_ok, &rcv_wscale,
 262                                   dst_metric(dst, RTAX_INITRWND));
 263
 264         ireq->rcv_wscale = rcv_wscale;
 265         ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst);
 266
 267         ret = get_cookie_sock(sk, skb, req, dst);
 268 out:
 269         return ret;
 270 out_free:
 271         reqsk_free(req);
 272         return NULL;
 273 }