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 extern int sysctl_tcp_syncookies;
  25 extern __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
  26
  27 #define COOKIEBITS 24   /* Upper bits store count */
  28 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
  29
  30 /*
  31  * This table has to be sorted and terminated with (__u16)-1.
  32  * XXX generate a better table.
  33  * Unresolved Issues: HIPPI with a 64k MSS is not well supported.
  34  *
  35  * Taken directly from ipv4 implementation.
  36  * Should this list be modified for ipv6 use or is it close enough?
  37  * rfc 2460 8.3 suggests mss values 20 bytes less than ipv4 counterpart
  38  */
  39 static __u16 const msstab[] = {
  40         64 - 1,
  41         256 - 1,
  42         512 - 1,
  43         536 - 1,
  44         1024 - 1,
  45         1440 - 1,
  46         1460 - 1,
  47         4312 - 1,
  48         (__u16)-1
  49 };
  50 /* The number doesn't include the -1 terminator */
  51 #define NUM_MSS (ARRAY_SIZE(msstab) - 1)
  52
  53 /*
  54  * This (misnamed) value is the age of syncookie which is permitted.
  55  * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
  56  * sysctl_tcp_retries1. It's a rather complicated formula (exponential
  57  * backoff) to compute at runtime so it's currently hardcoded here.
  58  */
  59 #define COUNTER_TRIES 4
  60
  61 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
  62                                            struct request_sock *req,
  63                                            struct dst_entry *dst)
  64 {
  65         struct inet_connection_sock *icsk = inet_csk(sk);
  66         struct sock *child;
  67
  68         child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
  69         if (child)
  70                 inet_csk_reqsk_queue_add(sk, req, child);
  71         else
  72                 reqsk_free(req);
  73
  74         return child;
  75 }
  76
  77 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
  78                       ipv6_cookie_scratch);
  79
  80 static u32 cookie_hash(struct in6_addr *saddr, struct in6_addr *daddr,
  81                        __be16 sport, __be16 dport, u32 count, int c)
  82 {
  83         __u32 *tmp = __get_cpu_var(ipv6_cookie_scratch);
  84
  85         /*
  86          * we have 320 bits of information to hash, copy in the remaining
  87          * 192 bits required for sha_transform, from the syncookie_secret
  88          * and overwrite the digest with the secret
  89          */
  90         memcpy(tmp + 10, syncookie_secret[c], 44);
  91         memcpy(tmp, saddr, 16);
  92         memcpy(tmp + 4, daddr, 16);
  93         tmp[8] = ((__force u32)sport << 16) + (__force u32)dport;
  94         tmp[9] = count;
  95         sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
  96
  97         return tmp[17];
  98 }
  99
 100 static __u32 secure_tcp_syn_cookie(struct in6_addr *saddr, struct in6_addr *daddr,
 101                                    __be16 sport, __be16 dport, __u32 sseq,
 102                                    __u32 count, __u32 data)
 103 {
 104         return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
 105                 sseq + (count << COOKIEBITS) +
 106                 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
 107                 & COOKIEMASK));
 108 }
 109
 110 static __u32 check_tcp_syn_cookie(__u32 cookie, struct in6_addr *saddr,
 111                                   struct in6_addr *daddr, __be16 sport,
 112                                   __be16 dport, __u32 sseq, __u32 count,
 113                                   __u32 maxdiff)
 114 {
 115         __u32 diff;
 116
 117         cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
 118
 119         diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
 120         if (diff >= maxdiff)
 121                 return (__u32)-1;
 122
 123         return (cookie -
 124                 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
 125                 & COOKIEMASK;
 126 }
 127
 128 __u32 cookie_v6_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
 129 {
 130         struct ipv6hdr *iph = ipv6_hdr(skb);
 131         const struct tcphdr *th = tcp_hdr(skb);
 132         int mssind;
 133         const __u16 mss = *mssp;
 134
 135         tcp_synq_overflow(sk);
 136
 137         for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
 138                 ;
 139         *mssp = msstab[mssind] + 1;
 140
 141         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
 142
 143         return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
 144                                      th->dest, ntohl(th->seq),
 145                                      jiffies / (HZ * 60), mssind);
 146 }
 147
 148 static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
 149 {
 150         struct ipv6hdr *iph = ipv6_hdr(skb);
 151         const struct tcphdr *th = tcp_hdr(skb);
 152         __u32 seq = ntohl(th->seq) - 1;
 153         __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
 154                                             th->source, th->dest, seq,
 155                                             jiffies / (HZ * 60), COUNTER_TRIES);
 156
 157         return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
 158 }
 159
 160 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 161 {
 162         struct inet_request_sock *ireq;
 163         struct inet6_request_sock *ireq6;
 164         struct tcp_request_sock *treq;
 165         struct ipv6_pinfo *np = inet6_sk(sk);
 166         struct tcp_sock *tp = tcp_sk(sk);
 167         const struct tcphdr *th = tcp_hdr(skb);
 168         __u32 cookie = ntohl(th->ack_seq) - 1;
 169         struct sock *ret = sk;
 170         struct request_sock *req;
 171         int mss;
 172         struct dst_entry *dst;
 173         __u8 rcv_wscale;
 174         struct tcp_options_received tcp_opt;
 175
 176         if (!sysctl_tcp_syncookies || !th->ack)
 177                 goto out;
 178
 179         if (tcp_synq_no_recent_overflow(sk) ||
 180                 (mss = cookie_check(skb, cookie)) == 0) {
 181                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 182                 goto out;
 183         }
 184
 185         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 186
 187         /* check for timestamp cookie support */
 188         memset(&tcp_opt, 0, sizeof(tcp_opt));
 189         tcp_parse_options(skb, &tcp_opt, 0);
 190
 191         if (tcp_opt.saw_tstamp)
 192                 cookie_check_timestamp(&tcp_opt);
 193
 194         ret = NULL;
 195         req = inet6_reqsk_alloc(&tcp6_request_sock_ops);
 196         if (!req)
 197                 goto out;
 198
 199         ireq = inet_rsk(req);
 200         ireq6 = inet6_rsk(req);
 201         treq = tcp_rsk(req);
 202
 203         if (security_inet_conn_request(sk, skb, req))
 204                 goto out_free;
 205
 206         req->mss = mss;
 207         ireq->rmt_port = th->source;
 208         ireq->loc_port = th->dest;
 209         ipv6_addr_copy(&ireq6->rmt_addr, &ipv6_hdr(skb)->saddr);
 210         ipv6_addr_copy(&ireq6->loc_addr, &ipv6_hdr(skb)->daddr);
 211         if (ipv6_opt_accepted(sk, skb) ||
 212             np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
 213             np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
 214                 atomic_inc(&skb->users);
 215                 ireq6->pktopts = skb;
 216         }
 217
 218         ireq6->iif = sk->sk_bound_dev_if;
 219         /* So that link locals have meaning */
 220         if (!sk->sk_bound_dev_if &&
 221             ipv6_addr_type(&ireq6->rmt_addr) & IPV6_ADDR_LINKLOCAL)
 222                 ireq6->iif = inet6_iif(skb);
 223
 224         req->expires = 0UL;
 225         req->retrans = 0;
 226         ireq->ecn_ok            = 0;
 227         ireq->snd_wscale        = tcp_opt.snd_wscale;
 228         ireq->rcv_wscale        = tcp_opt.rcv_wscale;
 229         ireq->sack_ok           = tcp_opt.sack_ok;
 230         ireq->wscale_ok         = tcp_opt.wscale_ok;
 231         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 232         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 233         treq->rcv_isn = ntohl(th->seq) - 1;
 234         treq->snt_isn = cookie;
 235
 236         /*
 237          * We need to lookup the dst_entry to get the correct window size.
 238          * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
 239          * me if there is a preferred way.
 240          */
 241         {
 242                 struct in6_addr *final_p = NULL, final;
 243                 struct flowi fl;
 244                 memset(&fl, 0, sizeof(fl));
 245                 fl.proto = IPPROTO_TCP;
 246                 ipv6_addr_copy(&fl.fl6_dst, &ireq6->rmt_addr);
 247                 if (np->opt && np->opt->srcrt) {
 248                         struct rt0_hdr *rt0 = (struct rt0_hdr *) np->opt->srcrt;
 249                         ipv6_addr_copy(&final, &fl.fl6_dst);
 250                         ipv6_addr_copy(&fl.fl6_dst, rt0->addr);
 251                         final_p = &final;
 252                 }
 253                 ipv6_addr_copy(&fl.fl6_src, &ireq6->loc_addr);
 254                 fl.oif = sk->sk_bound_dev_if;
 255                 fl.fl_ip_dport = inet_rsk(req)->rmt_port;
 256                 fl.fl_ip_sport = inet_sk(sk)->sport;
 257                 security_req_classify_flow(req, &fl);
 258                 if (ip6_dst_lookup(sk, &dst, &fl))
 259                         goto out_free;
 260
 261                 if (final_p)
 262                         ipv6_addr_copy(&fl.fl6_dst, final_p);
 263                 if ((xfrm_lookup(sock_net(sk), &dst, &fl, sk, 0)) < 0)
 264                         goto out_free;
 265         }
 266
 267         req->window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
 268         tcp_select_initial_window(tcp_full_space(sk), req->mss,
 269                                   &req->rcv_wnd, &req->window_clamp,
 270                                   ireq->wscale_ok, &rcv_wscale);
 271
 272         ireq->rcv_wscale = rcv_wscale;
 273
 274         ret = get_cookie_sock(sk, skb, req, dst);
 275 out:
 276         return ret;
 277 out_free:
 278         reqsk_free(req);
 279         return NULL;
 280 }
 281