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 /* Table must be sorted. */
  28 static __u16 const msstab[] = {
  29         64,
  30         512,
  31         536,
  32         1280 - 60,
  33         1480 - 60,
  34         1500 - 60,
  35         4460 - 60,
  36         9000 - 60,
  37 };
  38
  39 /*
  40  * This (misnamed) value is the age of syncookie which is permitted.
  41  * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
  42  * sysctl_tcp_retries1. It's a rather complicated formula (exponential
  43  * backoff) to compute at runtime so it's currently hardcoded here.
  44  */
  45 #define COUNTER_TRIES 4
  46
  47 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
  48                                            struct request_sock *req,
  49                                            struct dst_entry *dst)
  50 {
  51         struct inet_connection_sock *icsk = inet_csk(sk);
  52         struct sock *child;
  53
  54         child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
  55         if (child)
  56                 inet_csk_reqsk_queue_add(sk, req, child);
  57         else
  58                 reqsk_free(req);
  59
  60         return child;
  61 }
  62
  63 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
  64                       ipv6_cookie_scratch);
  65
  66 static u32 cookie_hash(const struct in6_addr *saddr, const struct in6_addr *daddr,
  67                        __be16 sport, __be16 dport, u32 count, int c)
  68 {
  69         __u32 *tmp = __get_cpu_var(ipv6_cookie_scratch);
  70
  71         /*
  72          * we have 320 bits of information to hash, copy in the remaining
  73          * 192 bits required for sha_transform, from the syncookie_secret
  74          * and overwrite the digest with the secret
  75          */
  76         memcpy(tmp + 10, syncookie_secret[c], 44);
  77         memcpy(tmp, saddr, 16);
  78         memcpy(tmp + 4, daddr, 16);
  79         tmp[8] = ((__force u32)sport << 16) + (__force u32)dport;
  80         tmp[9] = count;
  81         sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
  82
  83         return tmp[17];
  84 }
  85
  86 static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
  87                                    const struct in6_addr *daddr,
  88                                    __be16 sport, __be16 dport, __u32 sseq,
  89                                    __u32 count, __u32 data)
  90 {
  91         return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
  92                 sseq + (count << COOKIEBITS) +
  93                 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
  94                 & COOKIEMASK));
  95 }
  96
  97 static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
  98                                   const struct in6_addr *daddr, __be16 sport,
  99                                   __be16 dport, __u32 sseq, __u32 count,
 100                                   __u32 maxdiff)
 101 {
 102         __u32 diff;
 103
 104         cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
 105
 106         diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
 107         if (diff >= maxdiff)
 108                 return (__u32)-1;
 109
 110         return (cookie -
 111                 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
 112                 & COOKIEMASK;
 113 }
 114
 115 __u32 cookie_v6_init_sequence(struct sock *sk, const struct sk_buff *skb, __u16 *mssp)
 116 {
 117         const struct ipv6hdr *iph = ipv6_hdr(skb);
 118         const struct tcphdr *th = tcp_hdr(skb);
 119         int mssind;
 120         const __u16 mss = *mssp;
 121
 122         tcp_synq_overflow(sk);
 123
 124         for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
 125                 if (mss >= msstab[mssind])
 126                         break;
 127
 128         *mssp = msstab[mssind];
 129
 130         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
 131
 132         return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
 133                                      th->dest, ntohl(th->seq),
 134                                      jiffies / (HZ * 60), mssind);
 135 }
 136
 137 static inline int cookie_check(const struct sk_buff *skb, __u32 cookie)
 138 {
 139         const struct ipv6hdr *iph = ipv6_hdr(skb);
 140         const struct tcphdr *th = tcp_hdr(skb);
 141         __u32 seq = ntohl(th->seq) - 1;
 142         __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
 143                                             th->source, th->dest, seq,
 144                                             jiffies / (HZ * 60), COUNTER_TRIES);
 145
 146         return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
 147 }
 148
 149 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 150 {
 151         struct tcp_options_received tcp_opt;
 152         struct inet_request_sock *ireq;
 153         struct inet6_request_sock *ireq6;
 154         struct tcp_request_sock *treq;
 155         struct ipv6_pinfo *np = inet6_sk(sk);
 156         struct tcp_sock *tp = tcp_sk(sk);
 157         const struct tcphdr *th = tcp_hdr(skb);
 158         __u32 cookie = ntohl(th->ack_seq) - 1;
 159         struct sock *ret = sk;
 160         struct request_sock *req;
 161         int mss;
 162         struct dst_entry *dst;
 163         __u8 rcv_wscale;
 164         bool ecn_ok = false;
 165
 166         if (!sysctl_tcp_syncookies || !th->ack || th->rst)
 167                 goto out;
 168
 169         if (tcp_synq_no_recent_overflow(sk) ||
 170                 (mss = cookie_check(skb, cookie)) == 0) {
 171                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 172                 goto out;
 173         }
 174
 175         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 176
 177         /* check for timestamp cookie support */
 178         memset(&tcp_opt, 0, sizeof(tcp_opt));
 179         tcp_parse_options(skb, &tcp_opt, 0, NULL);
 180
 181         if (!cookie_check_timestamp(&tcp_opt, sock_net(sk), &ecn_ok))
 182                 goto out;
 183
 184         ret = NULL;
 185         req = inet6_reqsk_alloc(&tcp6_request_sock_ops);
 186         if (!req)
 187                 goto out;
 188
 189         ireq = inet_rsk(req);
 190         ireq6 = inet6_rsk(req);
 191         treq = tcp_rsk(req);
 192         treq->listener = NULL;
 193
 194         if (security_inet_conn_request(sk, skb, req))
 195                 goto out_free;
 196
 197         req->mss = mss;
 198         ireq->rmt_port = th->source;
 199         ireq->loc_port = th->dest;
 200         ireq6->rmt_addr = ipv6_hdr(skb)->saddr;
 201         ireq6->loc_addr = ipv6_hdr(skb)->daddr;
 202         if (ipv6_opt_accepted(sk, skb) ||
 203             np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
 204             np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
 205                 atomic_inc(&skb->users);
 206                 ireq6->pktopts = skb;
 207         }
 208
 209         ireq6->iif = sk->sk_bound_dev_if;
 210         /* So that link locals have meaning */
 211         if (!sk->sk_bound_dev_if &&
 212             ipv6_addr_type(&ireq6->rmt_addr) & IPV6_ADDR_LINKLOCAL)
 213                 ireq6->iif = inet6_iif(skb);
 214
 215         req->expires = 0UL;
 216         req->num_retrans = 0;
 217         ireq->ecn_ok            = ecn_ok;
 218         ireq->snd_wscale        = tcp_opt.snd_wscale;
 219         ireq->sack_ok           = tcp_opt.sack_ok;
 220         ireq->wscale_ok         = tcp_opt.wscale_ok;
 221         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 222         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 223         treq->snt_synack        = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
 224         treq->rcv_isn = ntohl(th->seq) - 1;
 225         treq->snt_isn = cookie;
 226
 227         /*
 228          * We need to lookup the dst_entry to get the correct window size.
 229          * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
 230          * me if there is a preferred way.
 231          */
 232         {
 233                 struct in6_addr *final_p, final;
 234                 struct flowi6 fl6;
 235                 memset(&fl6, 0, sizeof(fl6));
 236                 fl6.flowi6_proto = IPPROTO_TCP;
 237                 fl6.daddr = ireq6->rmt_addr;
 238                 final_p = fl6_update_dst(&fl6, np->opt, &final);
 239                 fl6.saddr = ireq6->loc_addr;
 240                 fl6.flowi6_oif = sk->sk_bound_dev_if;
 241                 fl6.flowi6_mark = sk->sk_mark;
 242                 fl6.fl6_dport = inet_rsk(req)->rmt_port;
 243                 fl6.fl6_sport = inet_sk(sk)->inet_sport;
 244                 security_req_classify_flow(req, flowi6_to_flowi(&fl6));
 245
 246                 dst = ip6_dst_lookup_flow(sk, &fl6, final_p, false);
 247                 if (IS_ERR(dst))
 248                         goto out_free;
 249         }
 250
 251         req->window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
 252         tcp_select_initial_window(tcp_full_space(sk), req->mss,
 253                                   &req->rcv_wnd, &req->window_clamp,
 254                                   ireq->wscale_ok, &rcv_wscale,
 255                                   dst_metric(dst, RTAX_INITRWND));
 256
 257         ireq->rcv_wscale = rcv_wscale;
 258
 259         ret = get_cookie_sock(sk, skb, req, dst);
 260 out:
 261         return ret;
 262 out_free:
 263         reqsk_free(req);
 264         return NULL;
 265 }
 266