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