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/siphash.h>
  20 #include <linux/kernel.h>
  21 #include <net/secure_seq.h>
  22 #include <net/ipv6.h>
  23 #include <net/tcp.h>
  24
  25 #define COOKIEBITS 24   /* Upper bits store count */
  26 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
  27
  28 static siphash_key_t syncookie6_secret[2] __read_mostly;
  29
  30 /* RFC 2460, Section 8.3:
  31  * [ipv6 tcp] MSS must be computed as the maximum packet size minus 60 [..]
  32  *
  33  * Due to IPV6_MIN_MTU=1280 the lowest possible MSS is 1220, which allows
  34  * using higher values than ipv4 tcp syncookies.
  35  * The other values are chosen based on ethernet (1500 and 9k MTU), plus
  36  * one that accounts for common encap (PPPoe) overhead. Table must be sorted.
  37  */
  38 static __u16 const msstab[] = {
  39         1280 - 60, /* IPV6_MIN_MTU - 60 */
  40         1480 - 60,
  41         1500 - 60,
  42         9000 - 60,
  43 };
  44
  45 static u32 cookie_hash(const struct in6_addr *saddr,
  46                        const struct in6_addr *daddr,
  47                        __be16 sport, __be16 dport, u32 count, int c)
  48 {
  49         const struct {
  50                 struct in6_addr saddr;
  51                 struct in6_addr daddr;
  52                 u32 count;
  53                 __be16 sport;
  54                 __be16 dport;
  55         } __aligned(SIPHASH_ALIGNMENT) combined = {
  56                 .saddr = *saddr,
  57                 .daddr = *daddr,
  58                 .count = count,
  59                 .sport = sport,
  60                 .dport = dport
  61         };
  62
  63         net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret));
  64         return siphash(&combined, offsetofend(typeof(combined), dport),
  65                        &syncookie6_secret[c]);
  66 }
  67
  68 static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
  69                                    const struct in6_addr *daddr,
  70                                    __be16 sport, __be16 dport, __u32 sseq,
  71                                    __u32 data)
  72 {
  73         u32 count = tcp_cookie_time();
  74         return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
  75                 sseq + (count << COOKIEBITS) +
  76                 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
  77                 & COOKIEMASK));
  78 }
  79
  80 static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
  81                                   const struct in6_addr *daddr, __be16 sport,
  82                                   __be16 dport, __u32 sseq)
  83 {
  84         __u32 diff, count = tcp_cookie_time();
  85
  86         cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
  87
  88         diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
  89         if (diff >= MAX_SYNCOOKIE_AGE)
  90                 return (__u32)-1;
  91
  92         return (cookie -
  93                 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
  94                 & COOKIEMASK;
  95 }
  96
  97 u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph,
  98                               const struct tcphdr *th, __u16 *mssp)
  99 {
 100         int mssind;
 101         const __u16 mss = *mssp;
 102
 103         for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
 104                 if (mss >= msstab[mssind])
 105                         break;
 106
 107         *mssp = msstab[mssind];
 108
 109         return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
 110                                      th->dest, ntohl(th->seq), mssind);
 111 }
 112 EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence);
 113
 114 __u32 cookie_v6_init_sequence(const struct sk_buff *skb, __u16 *mssp)
 115 {
 116         const struct ipv6hdr *iph = ipv6_hdr(skb);
 117         const struct tcphdr *th = tcp_hdr(skb);
 118
 119         return __cookie_v6_init_sequence(iph, th, mssp);
 120 }
 121
 122 int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th,
 123                       __u32 cookie)
 124 {
 125         __u32 seq = ntohl(th->seq) - 1;
 126         __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
 127                                             th->source, th->dest, seq);
 128
 129         return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
 130 }
 131 EXPORT_SYMBOL_GPL(__cookie_v6_check);
 132
 133 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 134 {
 135         struct tcp_options_received tcp_opt;
 136         struct inet_request_sock *ireq;
 137         struct tcp_request_sock *treq;
 138         struct ipv6_pinfo *np = inet6_sk(sk);
 139         struct tcp_sock *tp = tcp_sk(sk);
 140         const struct tcphdr *th = tcp_hdr(skb);
 141         __u32 cookie = ntohl(th->ack_seq) - 1;
 142         struct sock *ret = sk;
 143         struct request_sock *req;
 144         int mss;
 145         struct dst_entry *dst;
 146         __u8 rcv_wscale;
 147         u32 tsoff = 0;
 148
 149         if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst)
 150                 goto out;
 151
 152         if (tcp_synq_no_recent_overflow(sk))
 153                 goto out;
 154
 155         mss = __cookie_v6_check(ipv6_hdr(skb), th, cookie);
 156         if (mss == 0) {
 157                 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 158                 goto out;
 159         }
 160
 161         __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 162
 163         /* check for timestamp cookie support */
 164         memset(&tcp_opt, 0, sizeof(tcp_opt));
 165         tcp_parse_options(sock_net(sk), skb, &tcp_opt, 0, NULL);
 166
 167         if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
 168                 tsoff = secure_tcpv6_ts_off(sock_net(sk),
 169                                             ipv6_hdr(skb)->daddr.s6_addr32,
 170                                             ipv6_hdr(skb)->saddr.s6_addr32);
 171                 tcp_opt.rcv_tsecr -= tsoff;
 172         }
 173
 174         if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
 175                 goto out;
 176
 177         ret = NULL;
 178         req = inet_reqsk_alloc(&tcp6_request_sock_ops, sk, false);
 179         if (!req)
 180                 goto out;
 181
 182         ireq = inet_rsk(req);
 183         treq = tcp_rsk(req);
 184         treq->tfo_listener = false;
 185
 186         if (security_inet_conn_request(sk, skb, req))
 187                 goto out_free;
 188
 189         req->mss = mss;
 190         ireq->ir_rmt_port = th->source;
 191         ireq->ir_num = ntohs(th->dest);
 192         ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
 193         ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
 194         if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) ||
 195             np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
 196             np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
 197                 refcount_inc(&skb->users);
 198                 ireq->pktopts = skb;
 199         }
 200
 201         ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
 202         /* So that link locals have meaning */
 203         if (!sk->sk_bound_dev_if &&
 204             ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
 205                 ireq->ir_iif = tcp_v6_iif(skb);
 206
 207         ireq->ir_mark = inet_request_mark(sk, skb);
 208
 209         req->num_retrans = 0;
 210         ireq->snd_wscale        = tcp_opt.snd_wscale;
 211         ireq->sack_ok           = tcp_opt.sack_ok;
 212         ireq->wscale_ok         = tcp_opt.wscale_ok;
 213         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 214         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 215         treq->snt_synack        = 0;
 216         treq->rcv_isn = ntohl(th->seq) - 1;
 217         treq->snt_isn = cookie;
 218         treq->ts_off = 0;
 219         treq->txhash = net_tx_rndhash();
 220         if (IS_ENABLED(CONFIG_SMC))
 221                 ireq->smc_ok = 0;
 222
 223         /*
 224          * We need to lookup the dst_entry to get the correct window size.
 225          * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
 226          * me if there is a preferred way.
 227          */
 228         {
 229                 struct in6_addr *final_p, final;
 230                 struct flowi6 fl6;
 231                 memset(&fl6, 0, sizeof(fl6));
 232                 fl6.flowi6_proto = IPPROTO_TCP;
 233                 fl6.daddr = ireq->ir_v6_rmt_addr;
 234                 final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
 235                 fl6.saddr = ireq->ir_v6_loc_addr;
 236                 fl6.flowi6_oif = ireq->ir_iif;
 237                 fl6.flowi6_mark = ireq->ir_mark;
 238                 fl6.fl6_dport = ireq->ir_rmt_port;
 239                 fl6.fl6_sport = inet_sk(sk)->inet_sport;
 240                 fl6.flowi6_uid = sk->sk_uid;
 241                 security_req_classify_flow(req, flowi6_to_flowi(&fl6));
 242
 243                 dst = ip6_dst_lookup_flow(sk, &fl6, final_p);
 244                 if (IS_ERR(dst))
 245                         goto out_free;
 246         }
 247
 248         req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
 249         tcp_select_initial_window(sk, tcp_full_space(sk), req->mss,
 250                                   &req->rsk_rcv_wnd, &req->rsk_window_clamp,
 251                                   ireq->wscale_ok, &rcv_wscale,
 252                                   dst_metric(dst, RTAX_INITRWND));
 253
 254         ireq->rcv_wscale = rcv_wscale;
 255         ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst);
 256
 257         ret = tcp_get_cookie_sock(sk, skb, req, dst, tsoff);
 258 out:
 259         return ret;
 260 out_free:
 261         reqsk_free(req);
 262         return NULL;
 263 }