syncookies.c

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/*
 *  Syncookies implementation for the Linux kernel
 *
 *  Copyright (C) 1997 Andi Kleen
 *  Based on ideas by D.J.Bernstein and Eric Schenk.
 *
 *  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/slab.h>
#include <linux/random.h>
#include <linux/cryptohash.h>
#include <linux/kernel.h>
#include <linux/export.h>
#include <net/tcp.h>
#include <net/route.h>

/* Timestamps: lowest bits store TCP options */
#define TSBITS 6
#define TSMASK (((__u32)1 << TSBITS) - 1)

extern int sysctl_tcp_syncookies;

__u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
EXPORT_SYMBOL(syncookie_secret);

static __init int init_syncookies(void)
{
    get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
    return 0;
}
__initcall(init_syncookies);

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

static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
              ipv4_cookie_scratch);

static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
               u32 count, int c)
{
    __u32 *tmp = __get_cpu_var(ipv4_cookie_scratch);

    memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
    tmp[0] = (__force u32)saddr;
    tmp[1] = (__force u32)daddr;
    tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
    tmp[3] = count;
    sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);

    return tmp[17];
}


/*
 * when syncookies are in effect and tcp timestamps are enabled we encode
 * tcp options in the lower bits of the timestamp value that will be
 * sent in the syn-ack.
 * Since subsequent timestamps use the normal tcp_time_stamp value, we
 * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
 */
__u32 cookie_init_timestamp(struct request_sock *req)
{
    struct inet_request_sock *ireq;
    u32 ts, ts_now = tcp_time_stamp;
    u32 options = 0;

    ireq = inet_rsk(req);

    options = ireq->wscale_ok ? ireq->snd_wscale : 0xf;
    options |= ireq->sack_ok << 4;
    options |= ireq->ecn_ok << 5;

    ts = ts_now & ~TSMASK;
    ts |= options;
    if (ts > ts_now) {
        ts >>= TSBITS;
        ts--;
        ts <<= TSBITS;
        ts |= options;
    }
    return ts;
}


static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
                   __be16 dport, __u32 sseq, __u32 count,
                   __u32 data)
{
    /*
     * Compute the secure sequence number.
     * The output should be:
     *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
     *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
     * Where sseq is their sequence number and count increases every
     * minute by 1.
     * As an extra hack, we add a small "data" value that encodes the
     * MSS into the second hash value.
     */

    return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
        sseq + (count << COOKIEBITS) +
        ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
         & COOKIEMASK));
}

/*
 * This retrieves the small "data" value from the syncookie.
 * If the syncookie is bad, the data returned will be out of
 * range.  This must be checked by the caller.
 *
 * The count value used to generate the cookie must be within
 * "maxdiff" if the current (passed-in) "count".  The return value
 * is (__u32)-1 if this test fails.
 */
static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
                  __be16 sport, __be16 dport, __u32 sseq,
                  __u32 count, __u32 maxdiff)
{
    __u32 diff;

    /* Strip away the layers from the cookie */
    cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;

    /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
    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;   /* Leaving the data behind */
}

/*
 * MSS Values are taken from the 2009 paper
 * 'Measuring TCP Maximum Segment Size' by S. Alcock and R. Nelson:
 *  - values 1440 to 1460 accounted for 80% of observed mss values
 *  - values outside the 536-1460 range are rare (<0.2%).
 *
 * Table must be sorted.
 */
static __u16 const msstab[] = {
    64,
    512,
    536,
    1024,
    1440,
    1460,
    4312,
    8960,
};

/*
 * Generate a syncookie.  mssp points to the mss, which is returned
 * rounded down to the value encoded in the cookie.
 */
__u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
{
    const struct iphdr *iph = ip_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);
}

/*
 * 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
/*
 * Check if a ack sequence number is a valid syncookie.
 * Return the decoded mss if it is, or 0 if not.
 */
static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
{
    const struct iphdr *iph = ip_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;
}

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;
}


/*
 * when syncookies are in effect and tcp timestamps are enabled we stored
 * additional tcp options in the timestamp.
 * This extracts these options from the timestamp echo.
 *
 * The lowest 4 bits store snd_wscale.
 * next 2 bits indicate SACK and ECN support.
 *
 * return false if we decode an option that should not be.
 */
bool cookie_check_timestamp(struct tcp_options_received *tcp_opt, bool *ecn_ok)
{
    /* echoed timestamp, lowest bits contain options */
    u32 options = tcp_opt->rcv_tsecr & TSMASK;

    if (!tcp_opt->saw_tstamp)  {
        tcp_clear_options(tcp_opt);
        return true;
    }

    if (!sysctl_tcp_timestamps)
        return false;

    tcp_opt->sack_ok = (options & (1 << 4)) ? TCP_SACK_SEEN : 0;
    *ecn_ok = (options >> 5) & 1;
    if (*ecn_ok && !sysctl_tcp_ecn)
        return false;

    if (tcp_opt->sack_ok && !sysctl_tcp_sack)
        return false;

    if ((options & 0xf) == 0xf)
        return true; /* no window scaling */

    tcp_opt->wscale_ok = 1;
    tcp_opt->snd_wscale = options & 0xf;
    return sysctl_tcp_window_scaling != 0;
}
EXPORT_SYMBOL(cookie_check_timestamp);

struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
                 struct ip_options *opt)
{
    struct tcp_options_received tcp_opt;
    const u8 *hash_location;
    struct inet_request_sock *ireq;
    struct tcp_request_sock *treq;
    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 rtable *rt;
    __u8 rcv_wscale;
    bool ecn_ok = false;
    struct flowi4 fl4;

    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 = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
    if (!req)
        goto out;

    ireq = inet_rsk(req);
    treq = tcp_rsk(req);
    treq->rcv_isn       = ntohl(th->seq) - 1;
    treq->snt_isn       = cookie;
    req->mss        = mss;
    ireq->loc_port      = th->dest;
    ireq->rmt_port      = th->source;
    ireq->loc_addr      = ip_hdr(skb)->daddr;
    ireq->rmt_addr      = ip_hdr(skb)->saddr;
    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->listener      = NULL;

    /* We throwed the options of the initial SYN away, so we hope
     * the ACK carries the same options again (see RFC1122 4.2.3.8)
     */
    if (opt && opt->optlen) {
        int opt_size = sizeof(struct ip_options_rcu) + opt->optlen;

        ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
        if (ireq->opt != NULL && ip_options_echo(&ireq->opt->opt, skb)) {
            kfree(ireq->opt);
            ireq->opt = NULL;
        }
    }

    if (security_inet_conn_request(sk, skb, req)) {
        reqsk_free(req);
        goto out;
    }

    req->expires    = 0UL;
    req->retrans    = 0;

    /*
     * We need to lookup the route here to get at the correct
     * window size. We should better make sure that the window size
     * hasn't changed since we received the original syn, but I see
     * no easy way to do this.
     */
    flowi4_init_output(&fl4, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
               RT_SCOPE_UNIVERSE, IPPROTO_TCP,
               inet_sk_flowi_flags(sk),
               (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
               ireq->loc_addr, th->source, th->dest);
    security_req_classify_flow(req, flowi4_to_flowi(&fl4));
    rt = ip_route_output_key(sock_net(sk), &fl4);
    if (IS_ERR(rt)) {
        reqsk_free(req);
        goto out;
    }

    /* Try to redo what tcp_v4_send_synack did. */
    req->window_clamp = tp->window_clamp ? :dst_metric(&rt->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(&rt->dst, RTAX_INITRWND));

    ireq->rcv_wscale  = rcv_wscale;

    ret = get_cookie_sock(sk, skb, req, &rt->dst);
    /* ip_queue_xmit() depends on our flow being setup
     * Normal sockets get it right from inet_csk_route_child_sock()
     */
    if (ret)
        inet_sk(ret)->cork.fl.u.ip4 = fl4;
out:    return ret;
}