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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.

 *

 *  $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;

}