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, cookie_scratch)[16 + 5 + SHA_WORKSPACE_WORDS];
  78
  79 static u32 cookie_hash(struct in6_addr *saddr, struct in6_addr *daddr,
  80                        __be16 sport, __be16 dport, u32 count, int c)
  81 {
  82         __u32 *tmp = __get_cpu_var(cookie_scratch);
  83
  84         /*
  85          * we have 320 bits of information to hash, copy in the remaining
  86          * 192 bits required for sha_transform, from the syncookie_secret
  87          * and overwrite the digest with the secret
  88          */
  89         memcpy(tmp + 10, syncookie_secret[c], 44);
  90         memcpy(tmp, saddr, 16);
  91         memcpy(tmp + 4, daddr, 16);
  92         tmp[8] = ((__force u32)sport << 16) + (__force u32)dport;
  93         tmp[9] = count;
  94         sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
  95
  96         return tmp[17];
  97 }
  98
  99 static __u32 secure_tcp_syn_cookie(struct in6_addr *saddr, struct in6_addr *daddr,
 100                                    __be16 sport, __be16 dport, __u32 sseq,
 101                                    __u32 count, __u32 data)
 102 {
 103         return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
 104                 sseq + (count << COOKIEBITS) +
 105                 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
 106                 & COOKIEMASK));
 107 }
 108
 109 static __u32 check_tcp_syn_cookie(__u32 cookie, struct in6_addr *saddr,
 110                                   struct in6_addr *daddr, __be16 sport,
 111                                   __be16 dport, __u32 sseq, __u32 count,
 112                                   __u32 maxdiff)
 113 {
 114         __u32 diff;
 115
 116         cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
 117
 118         diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
 119         if (diff >= maxdiff)
 120                 return (__u32)-1;
 121
 122         return (cookie -
 123                 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
 124                 & COOKIEMASK;
 125 }
 126
 127 __u32 cookie_v6_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
 128 {
 129         struct ipv6hdr *iph = ipv6_hdr(skb);
 130         const struct tcphdr *th = tcp_hdr(skb);
 131         int mssind;
 132         const __u16 mss = *mssp;
 133
 134         tcp_synq_overflow(sk);
 135
 136         for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
 137                 ;
 138         *mssp = msstab[mssind] + 1;
 139
 140         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
 141
 142         return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
 143                                      th->dest, ntohl(th->seq),
 144                                      jiffies / (HZ * 60), mssind);
 145 }
 146
 147 static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
 148 {
 149         struct ipv6hdr *iph = ipv6_hdr(skb);
 150         const struct tcphdr *th = tcp_hdr(skb);
 151         __u32 seq = ntohl(th->seq) - 1;
 152         __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
 153                                             th->source, th->dest, seq,
 154                                             jiffies / (HZ * 60), COUNTER_TRIES);
 155
 156         return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
 157 }
 158
 159 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 160 {
 161         struct inet_request_sock *ireq;
 162         struct inet6_request_sock *ireq6;
 163         struct tcp_request_sock *treq;
 164         struct ipv6_pinfo *np = inet6_sk(sk);
 165         struct tcp_sock *tp = tcp_sk(sk);
 166         const struct tcphdr *th = tcp_hdr(skb);
 167         __u32 cookie = ntohl(th->ack_seq) - 1;
 168         struct sock *ret = sk;
 169         struct request_sock *req;
 170         int mss;
 171         struct dst_entry *dst;
 172         __u8 rcv_wscale;
 173         struct tcp_options_received tcp_opt;
 174
 175         if (!sysctl_tcp_syncookies || !th->ack)
 176                 goto out;
 177
 178         if (tcp_synq_no_recent_overflow(sk) ||
 179                 (mss = cookie_check(skb, cookie)) == 0) {
 180                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 181                 goto out;
 182         }
 183
 184         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 185
 186         /* check for timestamp cookie support */
 187         memset(&tcp_opt, 0, sizeof(tcp_opt));
 188         tcp_parse_options(skb, &tcp_opt, 0);
 189
 190         if (tcp_opt.saw_tstamp)
 191                 cookie_check_timestamp(&tcp_opt);
 192
 193         ret = NULL;
 194         req = inet6_reqsk_alloc(&tcp6_request_sock_ops);
 195         if (!req)
 196                 goto out;
 197
 198         ireq = inet_rsk(req);
 199         ireq6 = inet6_rsk(req);
 200         treq = tcp_rsk(req);
 201
 202         if (security_inet_conn_request(sk, skb, req))
 203                 goto out_free;
 204
 205         req->mss = mss;
 206         ireq->rmt_port = th->source;
 207         ireq->loc_port = th->dest;
 208         ipv6_addr_copy(&ireq6->rmt_addr, &ipv6_hdr(skb)->saddr);
 209         ipv6_addr_copy(&ireq6->loc_addr, &ipv6_hdr(skb)->daddr);
 210         if (ipv6_opt_accepted(sk, skb) ||
 211             np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
 212             np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
 213                 atomic_inc(&skb->users);
 214                 ireq6->pktopts = skb;
 215         }
 216
 217         ireq6->iif = sk->sk_bound_dev_if;
 218         /* So that link locals have meaning */
 219         if (!sk->sk_bound_dev_if &&
 220             ipv6_addr_type(&ireq6->rmt_addr) & IPV6_ADDR_LINKLOCAL)
 221                 ireq6->iif = inet6_iif(skb);
 222
 223         req->expires = 0UL;
 224         req->retrans = 0;
 225         ireq->ecn_ok            = 0;
 226         ireq->snd_wscale        = tcp_opt.snd_wscale;
 227         ireq->rcv_wscale        = tcp_opt.rcv_wscale;
 228         ireq->sack_ok           = tcp_opt.sack_ok;
 229         ireq->wscale_ok         = tcp_opt.wscale_ok;
 230         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 231         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 232         treq->rcv_isn = ntohl(th->seq) - 1;
 233         treq->snt_isn = cookie;
 234
 235         /*
 236          * We need to lookup the dst_entry to get the correct window size.
 237          * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
 238          * me if there is a preferred way.
 239          */
 240         {
 241                 struct in6_addr *final_p = NULL, final;
 242                 struct flowi fl;
 243                 memset(&fl, 0, sizeof(fl));
 244                 fl.proto = IPPROTO_TCP;
 245                 ipv6_addr_copy(&fl.fl6_dst, &ireq6->rmt_addr);
 246                 if (np->opt && np->opt->srcrt) {
 247                         struct rt0_hdr *rt0 = (struct rt0_hdr *) np->opt->srcrt;
 248                         ipv6_addr_copy(&final, &fl.fl6_dst);
 249                         ipv6_addr_copy(&fl.fl6_dst, rt0->addr);
 250                         final_p = &final;
 251                 }
 252                 ipv6_addr_copy(&fl.fl6_src, &ireq6->loc_addr);
 253                 fl.oif = sk->sk_bound_dev_if;
 254                 fl.fl_ip_dport = inet_rsk(req)->rmt_port;
 255                 fl.fl_ip_sport = inet_sk(sk)->sport;
 256                 security_req_classify_flow(req, &fl);
 257                 if (ip6_dst_lookup(sk, &dst, &fl))
 258                         goto out_free;
 259
 260                 if (final_p)
 261                         ipv6_addr_copy(&fl.fl6_dst, final_p);
 262                 if ((xfrm_lookup(sock_net(sk), &dst, &fl, sk, 0)) < 0)
 263                         goto out_free;
 264         }
 265
 266         req->window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
 267         tcp_select_initial_window(tcp_full_space(sk), req->mss,
 268                                   &req->rcv_wnd, &req->window_clamp,
 269                                   ireq->wscale_ok, &rcv_wscale);
 270
 271         ireq->rcv_wscale = rcv_wscale;
 272
 273         ret = get_cookie_sock(sk, skb, req, dst);
 274 out:
 275         return ret;
 276 out_free:
 277         reqsk_free(req);
 278         return NULL;
 279 }
 280