/*
 *  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.
 *
 *  $Id: syncookies.c,v 1.18 2002/02/01 22:01:04 davem Exp $
 *
 *  Missing: IPv6 support.
 */

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

extern int sysctl_tcp_syncookies;

static __u32 syncookie_secret[2][16-3+SHA_DIGEST_WORDS];

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

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

static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
                       u32 count, int c)
{
        __u32 tmp[16 + 5 + SHA_WORKSPACE_WORDS];

        memcpy(tmp + 3, 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];
}

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 */
}

/*
 * This table has to be sorted and terminated with (__u16)-1.
 * XXX generate a better table.
 * Unresolved Issues: HIPPI with a 64k MSS is not well supported.
 */
static __u16 const msstab[] = {
        64 - 1,
        256 - 1,
        512 - 1,
        536 - 1,
        1024 - 1,
        1440 - 1,
        1460 - 1,
        4312 - 1,
        (__u16)-1
};
/* The number doesn't include the -1 terminator */
#define NUM_MSS (ARRAY_SIZE(msstab) - 1)

/*
 * 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)
{
        struct tcp_sock *tp = tcp_sk(sk);
        const struct iphdr *iph = ip_hdr(skb);
        const struct tcphdr *th = tcp_hdr(skb);
        int mssind;
        const __u16 mss = *mssp;

        tp->last_synq_overflow = jiffies;

        /* XXX sort msstab[] by probability?  Binary search? */
        for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
                ;
        *mssp = msstab[mssind] + 1;

        NET_INC_STATS_BH(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 < NUM_MSS ? msstab[mssind] + 1 : 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;
}

struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
                             struct ip_options *opt)
{
        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;

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

        if (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) ||
            (mss = cookie_check(skb, cookie)) == 0) {
                NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESFAILED);
                goto out;
        }

        NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESRECV);

        ret = NULL;
        req = reqsk_alloc(&tcp_request_sock_ops); /* for safety */
        if (!req)
                goto out;

        if (security_inet_conn_request(sk, skb, req)) {
                reqsk_free(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->rmt_port          = th->source;
        ireq->loc_addr          = ip_hdr(skb)->daddr;
        ireq->rmt_addr          = ip_hdr(skb)->saddr;
        ireq->opt               = 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) + opt->optlen;

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

        ireq->snd_wscale = ireq->rcv_wscale = ireq->tstamp_ok = 0;
        ireq->wscale_ok  = ireq->sack_ok = 0;
        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.
         */
        {
                struct flowi fl = { .nl_u = { .ip4_u =
                                              { .daddr = ((opt && opt->srr) ?
                                                          opt->faddr :
                                                          ireq->rmt_addr),
                                                .saddr = ireq->loc_addr,
                                                .tos = RT_CONN_FLAGS(sk) } },
                                    .proto = IPPROTO_TCP,
                                    .uli_u = { .ports =
                                               { .sport = th->dest,
                                                 .dport = th->source } } };
                security_req_classify_flow(req, &fl);
                if (ip_route_output_key(&rt, &fl)) {
                        reqsk_free(req);
                        goto out;
                }
        }

        /* Try to redo what tcp_v4_send_synack did. */
        req->window_clamp = dst_metric(&rt->u.dst, RTAX_WINDOW);
        tcp_select_initial_window(tcp_full_space(sk), req->mss,
                                  &req->rcv_wnd, &req->window_clamp,
                                  0, &rcv_wscale);
        /* BTW win scale with syncookies is 0 by definition */
        ireq->rcv_wscale  = rcv_wscale;

        ret = get_cookie_sock(sk, skb, req, &rt->u.dst);
out:    return ret;
}