linux/net/ipv6/syncookies.c
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   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. */
  28static __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
  47static 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
  63static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
  64                      ipv6_cookie_scratch);
  65
  66static 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
  86static __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
  97static __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
 137static 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
 149struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 150{
 151        struct tcp_options_received tcp_opt;
 152        const u8 *hash_location;
 153        struct inet_request_sock *ireq;
 154        struct inet6_request_sock *ireq6;
 155        struct tcp_request_sock *treq;
 156        struct ipv6_pinfo *np = inet6_sk(sk);
 157        struct tcp_sock *tp = tcp_sk(sk);
 158        const struct tcphdr *th = tcp_hdr(skb);
 159        __u32 cookie = ntohl(th->ack_seq) - 1;
 160        struct sock *ret = sk;
 161        struct request_sock *req;
 162        int mss;
 163        struct dst_entry *dst;
 164        __u8 rcv_wscale;
 165        bool ecn_ok = false;
 166
 167        if (!sysctl_tcp_syncookies || !th->ack || th->rst)
 168                goto out;
 169
 170        if (tcp_synq_no_recent_overflow(sk) ||
 171                (mss = cookie_check(skb, cookie)) == 0) {
 172                NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 173                goto out;
 174        }
 175
 176        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 177
 178        /* check for timestamp cookie support */
 179        memset(&tcp_opt, 0, sizeof(tcp_opt));
 180        tcp_parse_options(skb, &tcp_opt, &hash_location, 0, NULL);
 181
 182        if (!cookie_check_timestamp(&tcp_opt, &ecn_ok))
 183                goto out;
 184
 185        ret = NULL;
 186        req = inet6_reqsk_alloc(&tcp6_request_sock_ops);
 187        if (!req)
 188                goto out;
 189
 190        ireq = inet_rsk(req);
 191        ireq6 = inet6_rsk(req);
 192        treq = tcp_rsk(req);
 193        treq->listener = NULL;
 194
 195        if (security_inet_conn_request(sk, skb, req))
 196                goto out_free;
 197
 198        req->mss = mss;
 199        ireq->rmt_port = th->source;
 200        ireq->loc_port = th->dest;
 201        ireq6->rmt_addr = ipv6_hdr(skb)->saddr;
 202        ireq6->loc_addr = ipv6_hdr(skb)->daddr;
 203        if (ipv6_opt_accepted(sk, skb) ||
 204            np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
 205            np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
 206                atomic_inc(&skb->users);
 207                ireq6->pktopts = skb;
 208        }
 209
 210        ireq6->iif = sk->sk_bound_dev_if;
 211        /* So that link locals have meaning */
 212        if (!sk->sk_bound_dev_if &&
 213            ipv6_addr_type(&ireq6->rmt_addr) & IPV6_ADDR_LINKLOCAL)
 214                ireq6->iif = inet6_iif(skb);
 215
 216        req->expires = 0UL;
 217        req->retrans = 0;
 218        ireq->ecn_ok            = ecn_ok;
 219        ireq->snd_wscale        = tcp_opt.snd_wscale;
 220        ireq->sack_ok           = tcp_opt.sack_ok;
 221        ireq->wscale_ok         = tcp_opt.wscale_ok;
 222        ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 223        req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 224        treq->snt_synack        = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 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 flowi6 fl6;
 236                memset(&fl6, 0, sizeof(fl6));
 237                fl6.flowi6_proto = IPPROTO_TCP;
 238                fl6.daddr = ireq6->rmt_addr;
 239                final_p = fl6_update_dst(&fl6, np->opt, &final);
 240                fl6.saddr = ireq6->loc_addr;
 241                fl6.flowi6_oif = sk->sk_bound_dev_if;
 242                fl6.flowi6_mark = sk->sk_mark;
 243                fl6.fl6_dport = inet_rsk(req)->rmt_port;
 244                fl6.fl6_sport = inet_sk(sk)->inet_sport;
 245                security_req_classify_flow(req, flowi6_to_flowi(&fl6));
 246
 247                dst = ip6_dst_lookup_flow(sk, &fl6, final_p, false);
 248                if (IS_ERR(dst))
 249                        goto out_free;
 250        }
 251
 252        req->window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
 253        tcp_select_initial_window(tcp_full_space(sk), req->mss,
 254                                  &req->rcv_wnd, &req->window_clamp,
 255                                  ireq->wscale_ok, &rcv_wscale,
 256                                  dst_metric(dst, RTAX_INITRWND));
 257
 258        ireq->rcv_wscale = rcv_wscale;
 259
 260        ret = get_cookie_sock(sk, skb, req, dst);
 261out:
 262        return ret;
 263out_free:
 264        reqsk_free(req);
 265        return NULL;
 266}
 267
 268
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