syncookies.c

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/*
 *  IPv6 Syncookies implementation for the Linux kernel
 *
 *  Authors:
 *  Glenn Griffin   <[email protected]>
 *
 *  Based on IPv4 implementation by Andi Kleen
 *  linux/net/ipv4/syncookies.c
 *
 *  This program is free software; you can redistribute it and/or
 *      modify it under the terms of the GNU General Public License
 *      as published by the Free Software Foundation; either version
 *      2 of the License, or (at your option) any later version.
 *
 */

#include <linux/tcp.h>
#include <linux/random.h>
#include <linux/cryptohash.h>
#include <linux/kernel.h>
#include <net/ipv6.h>
#include <net/tcp.h>

#define COOKIEBITS 24   /* Upper bits store count */
#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)

/* Table must be sorted. */
static __u16 const msstab[] = {
    64,
    512,
    536,
    1280 - 60,
    1480 - 60,
    1500 - 60,
    4460 - 60,
    9000 - 60,
};

/*
 * This (misnamed) value is the age of syncookie which is permitted.
 * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
 * sysctl_tcp_retries1. It's a rather complicated formula (exponential
 * backoff) to compute at runtime so it's currently hardcoded here.
 */
#define COUNTER_TRIES 4

static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
                       struct request_sock *req,
                       struct dst_entry *dst)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct sock *child;

    child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
    if (child)
        inet_csk_reqsk_queue_add(sk, req, child);
    else
        reqsk_free(req);

    return child;
}

static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
              ipv6_cookie_scratch);

static u32 cookie_hash(const struct in6_addr *saddr, const struct in6_addr *daddr,
               __be16 sport, __be16 dport, u32 count, int c)
{
    __u32 *tmp = __get_cpu_var(ipv6_cookie_scratch);

    /*
     * we have 320 bits of information to hash, copy in the remaining
     * 192 bits required for sha_transform, from the syncookie_secret
     * and overwrite the digest with the secret
     */
    memcpy(tmp + 10, syncookie_secret[c], 44);
    memcpy(tmp, saddr, 16);
    memcpy(tmp + 4, daddr, 16);
    tmp[8] = ((__force u32)sport << 16) + (__force u32)dport;
    tmp[9] = count;
    sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);

    return tmp[17];
}

static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
                   const struct in6_addr *daddr,
                   __be16 sport, __be16 dport, __u32 sseq,
                   __u32 count, __u32 data)
{
    return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
        sseq + (count << COOKIEBITS) +
        ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
        & COOKIEMASK));
}

static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr,
                  const struct in6_addr *daddr, __be16 sport,
                  __be16 dport, __u32 sseq, __u32 count,
                  __u32 maxdiff)
{
    __u32 diff;

    cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;

    diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
    if (diff >= maxdiff)
        return (__u32)-1;

    return (cookie -
        cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
        & COOKIEMASK;
}

__u32 cookie_v6_init_sequence(struct sock *sk, const struct sk_buff *skb, __u16 *mssp)
{
    const struct ipv6hdr *iph = ipv6_hdr(skb);
    const struct tcphdr *th = tcp_hdr(skb);
    int mssind;
    const __u16 mss = *mssp;

    tcp_synq_overflow(sk);

    for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
        if (mss >= msstab[mssind])
            break;

    *mssp = msstab[mssind];

    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);

    return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
                     th->dest, ntohl(th->seq),
                     jiffies / (HZ * 60), mssind);
}

static inline int cookie_check(const struct sk_buff *skb, __u32 cookie)
{
    const struct ipv6hdr *iph = ipv6_hdr(skb);
    const struct tcphdr *th = tcp_hdr(skb);
    __u32 seq = ntohl(th->seq) - 1;
    __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
                        th->source, th->dest, seq,
                        jiffies / (HZ * 60), COUNTER_TRIES);

    return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
}

struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_options_received tcp_opt;
    const u8 *hash_location;
    struct inet_request_sock *ireq;
    struct inet6_request_sock *ireq6;
    struct tcp_request_sock *treq;
    struct ipv6_pinfo *np = inet6_sk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    const struct tcphdr *th = tcp_hdr(skb);
    __u32 cookie = ntohl(th->ack_seq) - 1;
    struct sock *ret = sk;
    struct request_sock *req;
    int mss;
    struct dst_entry *dst;
    __u8 rcv_wscale;
    bool ecn_ok = false;

    if (!sysctl_tcp_syncookies || !th->ack || th->rst)
        goto out;

    if (tcp_synq_no_recent_overflow(sk) ||
        (mss = cookie_check(skb, cookie)) == 0) {
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
        goto out;
    }

    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);

    /* check for timestamp cookie support */
    memset(&tcp_opt, 0, sizeof(tcp_opt));
    tcp_parse_options(skb, &tcp_opt, &hash_location, 0, NULL);

    if (!cookie_check_timestamp(&tcp_opt, &ecn_ok))
        goto out;

    ret = NULL;
    req = inet6_reqsk_alloc(&tcp6_request_sock_ops);
    if (!req)
        goto out;

    ireq = inet_rsk(req);
    ireq6 = inet6_rsk(req);
    treq = tcp_rsk(req);
    treq->listener = NULL;

    if (security_inet_conn_request(sk, skb, req))
        goto out_free;

    req->mss = mss;
    ireq->rmt_port = th->source;
    ireq->loc_port = th->dest;
    ireq6->rmt_addr = ipv6_hdr(skb)->saddr;
    ireq6->loc_addr = ipv6_hdr(skb)->daddr;
    if (ipv6_opt_accepted(sk, skb) ||
        np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
        np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
        atomic_inc(&skb->users);
        ireq6->pktopts = skb;
    }

    ireq6->iif = sk->sk_bound_dev_if;
    /* So that link locals have meaning */
    if (!sk->sk_bound_dev_if &&
        ipv6_addr_type(&ireq6->rmt_addr) & IPV6_ADDR_LINKLOCAL)
        ireq6->iif = inet6_iif(skb);

    req->expires = 0UL;
    req->retrans = 0;
    ireq->ecn_ok        = ecn_ok;
    ireq->snd_wscale    = tcp_opt.snd_wscale;
    ireq->sack_ok       = tcp_opt.sack_ok;
    ireq->wscale_ok     = tcp_opt.wscale_ok;
    ireq->tstamp_ok     = tcp_opt.saw_tstamp;
    req->ts_recent      = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
    treq->snt_synack    = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
    treq->rcv_isn = ntohl(th->seq) - 1;
    treq->snt_isn = cookie;

    /*
     * We need to lookup the dst_entry to get the correct window size.
     * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
     * me if there is a preferred way.
     */
    {
        struct in6_addr *final_p, final;
        struct flowi6 fl6;
        memset(&fl6, 0, sizeof(fl6));
        fl6.flowi6_proto = IPPROTO_TCP;
        fl6.daddr = ireq6->rmt_addr;
        final_p = fl6_update_dst(&fl6, np->opt, &final);
        fl6.saddr = ireq6->loc_addr;
        fl6.flowi6_oif = sk->sk_bound_dev_if;
        fl6.flowi6_mark = sk->sk_mark;
        fl6.fl6_dport = inet_rsk(req)->rmt_port;
        fl6.fl6_sport = inet_sk(sk)->inet_sport;
        security_req_classify_flow(req, flowi6_to_flowi(&fl6));

        dst = ip6_dst_lookup_flow(sk, &fl6, final_p, false);
        if (IS_ERR(dst))
            goto out_free;
    }

    req->window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
    tcp_select_initial_window(tcp_full_space(sk), req->mss,
                  &req->rcv_wnd, &req->window_clamp,
                  ireq->wscale_ok, &rcv_wscale,
                  dst_metric(dst, RTAX_INITRWND));

    ireq->rcv_wscale = rcv_wscale;

    ret = get_cookie_sock(sk, skb, req, dst);
out:
    return ret;
out_free:
    reqsk_free(req);
    return NULL;
}