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/siphash.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 siphash_key_t syncookie6_secret[2] __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 u32 cookie_hash(const struct in6_addr *saddr,
  45                        const struct in6_addr *daddr,
  46                        __be16 sport, __be16 dport, u32 count, int c)
  47 {
  48         const struct {
  49                 struct in6_addr saddr;
  50                 struct in6_addr daddr;
  51                 u32 count;
  52                 __be16 sport;
  53                 __be16 dport;
  54         } __aligned(SIPHASH_ALIGNMENT) combined = {
  55                 .saddr = *saddr,
  56                 .daddr = *daddr,
  57                 .count = count,
  58                 .sport = sport,
  59                 .dport = dport
  60         };
  61
  62         net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret));
  63         return siphash(&combined, offsetofend(typeof(combined), dport),
  64                        &syncookie6_secret[c]);
  65 }
  66
  67 static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
  68                                    const struct in6_addr *daddr,
  69                                    __be16 sport, __be16 dport, __u32 sseq,
  70                                    __u32 data)
  71 {
  72         u32 count = tcp_cookie_time();
  73         return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
  74                 sseq + (count << COOKIEBITS) +
  75                 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
  76                 & COOKIEMASK));
  77 }
  78
  79 static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
  80                                   const struct in6_addr *daddr, __be16 sport,
  81                                   __be16 dport, __u32 sseq)
  82 {
  83         __u32 diff, count = tcp_cookie_time();
  84
  85         cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
  86
  87         diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
  88         if (diff >= MAX_SYNCOOKIE_AGE)
  89                 return (__u32)-1;
  90
  91         return (cookie -
  92                 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
  93                 & COOKIEMASK;
  94 }
  95
  96 u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph,
  97                               const struct tcphdr *th, __u16 *mssp)
  98 {
  99         int mssind;
 100         const __u16 mss = *mssp;
 101
 102         for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
 103                 if (mss >= msstab[mssind])
 104                         break;
 105
 106         *mssp = msstab[mssind];
 107
 108         return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
 109                                      th->dest, ntohl(th->seq), mssind);
 110 }
 111 EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence);
 112
 113 __u32 cookie_v6_init_sequence(const struct sk_buff *skb, __u16 *mssp)
 114 {
 115         const struct ipv6hdr *iph = ipv6_hdr(skb);
 116         const struct tcphdr *th = tcp_hdr(skb);
 117
 118         return __cookie_v6_init_sequence(iph, th, mssp);
 119 }
 120
 121 int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th,
 122                       __u32 cookie)
 123 {
 124         __u32 seq = ntohl(th->seq) - 1;
 125         __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
 126                                             th->source, th->dest, seq);
 127
 128         return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
 129 }
 130 EXPORT_SYMBOL_GPL(__cookie_v6_check);
 131
 132 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 133 {
 134         struct tcp_options_received tcp_opt;
 135         struct inet_request_sock *ireq;
 136         struct tcp_request_sock *treq;
 137         struct ipv6_pinfo *np = inet6_sk(sk);
 138         struct tcp_sock *tp = tcp_sk(sk);
 139         const struct tcphdr *th = tcp_hdr(skb);
 140         __u32 cookie = ntohl(th->ack_seq) - 1;
 141         struct sock *ret = sk;
 142         struct request_sock *req;
 143         int mss;
 144         struct dst_entry *dst;
 145         __u8 rcv_wscale;
 146
 147         if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst)
 148                 goto out;
 149
 150         if (tcp_synq_no_recent_overflow(sk))
 151                 goto out;
 152
 153         mss = __cookie_v6_check(ipv6_hdr(skb), th, cookie);
 154         if (mss == 0) {
 155                 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 156                 goto out;
 157         }
 158
 159         __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 160
 161         /* check for timestamp cookie support */
 162         memset(&tcp_opt, 0, sizeof(tcp_opt));
 163         tcp_parse_options(skb, &tcp_opt, 0, NULL);
 164
 165         if (!cookie_timestamp_decode(&tcp_opt))
 166                 goto out;
 167
 168         ret = NULL;
 169         req = inet_reqsk_alloc(&tcp6_request_sock_ops, sk, false);
 170         if (!req)
 171                 goto out;
 172
 173         ireq = inet_rsk(req);
 174         treq = tcp_rsk(req);
 175         treq->tfo_listener = false;
 176
 177         if (security_inet_conn_request(sk, skb, req))
 178                 goto out_free;
 179
 180         req->mss = mss;
 181         ireq->ir_rmt_port = th->source;
 182         ireq->ir_num = ntohs(th->dest);
 183         ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
 184         ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
 185         if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) ||
 186             np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
 187             np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
 188                 atomic_inc(&skb->users);
 189                 ireq->pktopts = skb;
 190         }
 191
 192         ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
 193         /* So that link locals have meaning */
 194         if (!sk->sk_bound_dev_if &&
 195             ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
 196                 ireq->ir_iif = tcp_v6_iif(skb);
 197
 198         ireq->ir_mark = inet_request_mark(sk, skb);
 199
 200         req->num_retrans = 0;
 201         ireq->snd_wscale        = tcp_opt.snd_wscale;
 202         ireq->sack_ok           = tcp_opt.sack_ok;
 203         ireq->wscale_ok         = tcp_opt.wscale_ok;
 204         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 205         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 206         treq->snt_synack.v64    = 0;
 207         treq->rcv_isn = ntohl(th->seq) - 1;
 208         treq->snt_isn = cookie;
 209         treq->ts_off = 0;
 210
 211         /*
 212          * We need to lookup the dst_entry to get the correct window size.
 213          * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
 214          * me if there is a preferred way.
 215          */
 216         {
 217                 struct in6_addr *final_p, final;
 218                 struct flowi6 fl6;
 219                 memset(&fl6, 0, sizeof(fl6));
 220                 fl6.flowi6_proto = IPPROTO_TCP;
 221                 fl6.daddr = ireq->ir_v6_rmt_addr;
 222                 final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
 223                 fl6.saddr = ireq->ir_v6_loc_addr;
 224                 fl6.flowi6_oif = ireq->ir_iif;
 225                 fl6.flowi6_mark = ireq->ir_mark;
 226                 fl6.fl6_dport = ireq->ir_rmt_port;
 227                 fl6.fl6_sport = inet_sk(sk)->inet_sport;
 228                 fl6.flowi6_uid = sk->sk_uid;
 229                 security_req_classify_flow(req, flowi6_to_flowi(&fl6));
 230
 231                 dst = ip6_dst_lookup_flow(sk, &fl6, final_p);
 232                 if (IS_ERR(dst))
 233                         goto out_free;
 234         }
 235
 236         req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
 237         tcp_select_initial_window(tcp_full_space(sk), req->mss,
 238                                   &req->rsk_rcv_wnd, &req->rsk_window_clamp,
 239                                   ireq->wscale_ok, &rcv_wscale,
 240                                   dst_metric(dst, RTAX_INITRWND));
 241
 242         ireq->rcv_wscale = rcv_wscale;
 243         ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst);
 244
 245         ret = tcp_get_cookie_sock(sk, skb, req, dst);
 246 out:
 247         return ret;
 248 out_free:
 249         reqsk_free(req);
 250         return NULL;
 251 }