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 /* Table must be sorted. */
  31 static __u16 const msstab[] = {
  32         64,
  33         512,
  34         536,
  35         1280 - 60,
  36         1480 - 60,
  37         1500 - 60,
  38         4460 - 60,
  39         9000 - 60,
  40 };
  41
  42 /*
  43  * This (misnamed) value is the age of syncookie which is permitted.
  44  * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
  45  * sysctl_tcp_retries1. It's a rather complicated formula (exponential
  46  * backoff) to compute at runtime so it's currently hardcoded here.
  47  */
  48 #define COUNTER_TRIES 4
  49
  50 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
  51                                            struct request_sock *req,
  52                                            struct dst_entry *dst)
  53 {
  54         struct inet_connection_sock *icsk = inet_csk(sk);
  55         struct sock *child;
  56
  57         child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
  58         if (child)
  59                 inet_csk_reqsk_queue_add(sk, req, child);
  60         else
  61                 reqsk_free(req);
  62
  63         return child;
  64 }
  65
  66 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
  67                       ipv6_cookie_scratch);
  68
  69 static u32 cookie_hash(struct in6_addr *saddr, struct in6_addr *daddr,
  70                        __be16 sport, __be16 dport, u32 count, int c)
  71 {
  72         __u32 *tmp = __get_cpu_var(ipv6_cookie_scratch);
  73
  74         /*
  75          * we have 320 bits of information to hash, copy in the remaining
  76          * 192 bits required for sha_transform, from the syncookie_secret
  77          * and overwrite the digest with the secret
  78          */
  79         memcpy(tmp + 10, syncookie_secret[c], 44);
  80         memcpy(tmp, saddr, 16);
  81         memcpy(tmp + 4, daddr, 16);
  82         tmp[8] = ((__force u32)sport << 16) + (__force u32)dport;
  83         tmp[9] = count;
  84         sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
  85
  86         return tmp[17];
  87 }
  88
  89 static __u32 secure_tcp_syn_cookie(struct in6_addr *saddr, struct in6_addr *daddr,
  90                                    __be16 sport, __be16 dport, __u32 sseq,
  91                                    __u32 count, __u32 data)
  92 {
  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, struct in6_addr *saddr,
 100                                   struct in6_addr *daddr, __be16 sport,
 101                                   __be16 dport, __u32 sseq, __u32 count,
 102                                   __u32 maxdiff)
 103 {
 104         __u32 diff;
 105
 106         cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
 107
 108         diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
 109         if (diff >= maxdiff)
 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(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
 118 {
 119         struct ipv6hdr *iph = ipv6_hdr(skb);
 120         const struct tcphdr *th = tcp_hdr(skb);
 121         int mssind;
 122         const __u16 mss = *mssp;
 123
 124         tcp_synq_overflow(sk);
 125
 126         for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
 127                 if (mss >= msstab[mssind])
 128                         break;
 129
 130         *mssp = msstab[mssind];
 131
 132         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
 133
 134         return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
 135                                      th->dest, ntohl(th->seq),
 136                                      jiffies / (HZ * 60), mssind);
 137 }
 138
 139 static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
 140 {
 141         struct ipv6hdr *iph = ipv6_hdr(skb);
 142         const struct tcphdr *th = tcp_hdr(skb);
 143         __u32 seq = ntohl(th->seq) - 1;
 144         __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
 145                                             th->source, th->dest, seq,
 146                                             jiffies / (HZ * 60), COUNTER_TRIES);
 147
 148         return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
 149 }
 150
 151 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 152 {
 153         struct tcp_options_received tcp_opt;
 154         u8 *hash_location;
 155         struct inet_request_sock *ireq;
 156         struct inet6_request_sock *ireq6;
 157         struct tcp_request_sock *treq;
 158         struct ipv6_pinfo *np = inet6_sk(sk);
 159         struct tcp_sock *tp = tcp_sk(sk);
 160         const struct tcphdr *th = tcp_hdr(skb);
 161         __u32 cookie = ntohl(th->ack_seq) - 1;
 162         struct sock *ret = sk;
 163         struct request_sock *req;
 164         int mss;
 165         struct dst_entry *dst;
 166         __u8 rcv_wscale;
 167         bool ecn_ok;
 168
 169         if (!sysctl_tcp_syncookies || !th->ack || th->rst)
 170                 goto out;
 171
 172         if (tcp_synq_no_recent_overflow(sk) ||
 173                 (mss = cookie_check(skb, cookie)) == 0) {
 174                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 175                 goto out;
 176         }
 177
 178         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 179
 180         /* check for timestamp cookie support */
 181         memset(&tcp_opt, 0, sizeof(tcp_opt));
 182         tcp_parse_options(skb, &tcp_opt, &hash_location, 0);
 183
 184         if (!cookie_check_timestamp(&tcp_opt, &ecn_ok))
 185                 goto out;
 186
 187         ret = NULL;
 188         req = inet6_reqsk_alloc(&tcp6_request_sock_ops);
 189         if (!req)
 190                 goto out;
 191
 192         ireq = inet_rsk(req);
 193         ireq6 = inet6_rsk(req);
 194         treq = tcp_rsk(req);
 195
 196         if (security_inet_conn_request(sk, skb, req))
 197                 goto out_free;
 198
 199         req->mss = mss;
 200         ireq->rmt_port = th->source;
 201         ireq->loc_port = th->dest;
 202         ipv6_addr_copy(&ireq6->rmt_addr, &ipv6_hdr(skb)->saddr);
 203         ipv6_addr_copy(&ireq6->loc_addr, &ipv6_hdr(skb)->daddr);
 204         if (ipv6_opt_accepted(sk, skb) ||
 205             np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
 206             np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
 207                 atomic_inc(&skb->users);
 208                 ireq6->pktopts = skb;
 209         }
 210
 211         ireq6->iif = sk->sk_bound_dev_if;
 212         /* So that link locals have meaning */
 213         if (!sk->sk_bound_dev_if &&
 214             ipv6_addr_type(&ireq6->rmt_addr) & IPV6_ADDR_LINKLOCAL)
 215                 ireq6->iif = inet6_iif(skb);
 216
 217         req->expires = 0UL;
 218         req->retrans = 0;
 219         ireq->ecn_ok            = ecn_ok;
 220         ireq->snd_wscale        = tcp_opt.snd_wscale;
 221         ireq->sack_ok           = tcp_opt.sack_ok;
 222         ireq->wscale_ok         = tcp_opt.wscale_ok;
 223         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 224         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 225         treq->rcv_isn = ntohl(th->seq) - 1;
 226         treq->snt_isn = cookie;
 227
 228         /*
 229          * We need to lookup the dst_entry to get the correct window size.
 230          * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
 231          * me if there is a preferred way.
 232          */
 233         {
 234                 struct in6_addr *final_p, final;
 235                 struct flowi fl;
 236                 memset(&fl, 0, sizeof(fl));
 237                 fl.proto = IPPROTO_TCP;
 238                 ipv6_addr_copy(&fl.fl6_dst, &ireq6->rmt_addr);
 239                 final_p = fl6_update_dst(&fl, np->opt, &final);
 240                 ipv6_addr_copy(&fl.fl6_src, &ireq6->loc_addr);
 241                 fl.oif = sk->sk_bound_dev_if;
 242                 fl.mark = sk->sk_mark;
 243                 fl.fl_ip_dport = inet_rsk(req)->rmt_port;
 244                 fl.fl_ip_sport = inet_sk(sk)->inet_sport;
 245                 security_req_classify_flow(req, &fl);
 246                 if (ip6_dst_lookup(sk, &dst, &fl))
 247                         goto out_free;
 248
 249                 if (final_p)
 250                         ipv6_addr_copy(&fl.fl6_dst, final_p);
 251                 if ((xfrm_lookup(sock_net(sk), &dst, &fl, sk, 0)) < 0)
 252                         goto out_free;
 253         }
 254
 255         req->window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
 256         tcp_select_initial_window(tcp_full_space(sk), req->mss,
 257                                   &req->rcv_wnd, &req->window_clamp,
 258                                   ireq->wscale_ok, &rcv_wscale,
 259                                   dst_metric(dst, RTAX_INITRWND));
 260
 261         ireq->rcv_wscale = rcv_wscale;
 262
 263         ret = get_cookie_sock(sk, skb, req, dst);
 264 out:
 265         return ret;
 266 out_free:
 267         reqsk_free(req);
 268         return NULL;
 269 }
 270