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
  24extern int sysctl_tcp_syncookies;
  25extern __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 */
  39static __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
  61static 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
  77static DEFINE_PER_CPU(__u32, cookie_scratch)[16 + 5 + SHA_WORKSPACE_WORDS];
  78
  79static 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
  99static __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
 109static __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_sk(sk)->last_synq_overflow = jiffies;
 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
 147static 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
 159struct 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 (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) ||
 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(&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);
 274out:
 275        return ret;
 276out_free:
 277        reqsk_free(req);
 278        return NULL;
 279}
 280
 281
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