/*
 *  IPv6 Syncookies implementation for the Linux kernel
 *
 *  Authors:
 *  Glenn Griffin       <ggriffin.kernel@gmail.com>
 *
 *  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/siphash.h>
#include <linux/kernel.h>
#include <net/secure_seq.h>
#include <net/ipv6.h>
#include <net/tcp.h>

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

static siphash_key_t syncookie6_secret[2] __read_mostly;

/* RFC 2460, Section 8.3:
 * [ipv6 tcp] MSS must be computed as the maximum packet size minus 60 [..]
 *
 * Due to IPV6_MIN_MTU=1280 the lowest possible MSS is 1220, which allows
 * using higher values than ipv4 tcp syncookies.
 * The other values are chosen based on ethernet (1500 and 9k MTU), plus
 * one that accounts for common encap (PPPoe) overhead. Table must be sorted.
 */
static __u16 const msstab[] = {
        1280 - 60, /* IPV6_MIN_MTU - 60 */
        1480 - 60,
        1500 - 60,
        9000 - 60,
};

static u32 cookie_hash(const struct in6_addr *saddr,
                       const struct in6_addr *daddr,
                       __be16 sport, __be16 dport, u32 count, int c)
{
        const struct {
                struct in6_addr saddr;
                struct in6_addr daddr;
                u32 count;
                __be16 sport;
                __be16 dport;
        } __aligned(SIPHASH_ALIGNMENT) combined = {
                .saddr = *saddr,
                .daddr = *daddr,
                .count = count,
                .sport = sport,
                .dport = dport
        };

        net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret));
        return siphash(&combined, offsetofend(typeof(combined), dport),
                       &syncookie6_secret[c]);
}

static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr,
                                   const struct in6_addr *daddr,
                                   __be16 sport, __be16 dport, __u32 sseq,
                                   __u32 data)
{
        u32 count = tcp_cookie_time();
        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 diff, count = tcp_cookie_time();

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

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

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

u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph,
                              const struct tcphdr *th, __u16 *mssp)
{
        int mssind;
        const __u16 mss = *mssp;

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

        *mssp = msstab[mssind];

        return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
                                     th->dest, ntohl(th->seq), mssind);
}
EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence);

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

        return __cookie_v6_init_sequence(iph, th, mssp);
}

int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th,
                      __u32 cookie)
{
        __u32 seq = ntohl(th->seq) - 1;
        __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr,
                                            th->source, th->dest, seq);

        return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
}
EXPORT_SYMBOL_GPL(__cookie_v6_check);

struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_options_received tcp_opt;
        struct inet_request_sock *ireq;
        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;
        u32 tsoff = 0;

        if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst)
                goto out;

        if (tcp_synq_no_recent_overflow(sk))
                goto out;

        mss = __cookie_v6_check(ipv6_hdr(skb), th, cookie);
        if (mss == 0) {
                __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
                goto out;
        }

        __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);

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

        if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
                tsoff = secure_tcpv6_ts_off(sock_net(sk),
                                            ipv6_hdr(skb)->daddr.s6_addr32,
                                            ipv6_hdr(skb)->saddr.s6_addr32);
                tcp_opt.rcv_tsecr -= tsoff;
        }

        if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
                goto out;

        ret = NULL;
        req = inet_reqsk_alloc(&tcp6_request_sock_ops, sk, false);
        if (!req)
                goto out;

        ireq = inet_rsk(req);
        treq = tcp_rsk(req);
        treq->tfo_listener = false;

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

        req->mss = mss;
        ireq->ir_rmt_port = th->source;
        ireq->ir_num = ntohs(th->dest);
        ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr;
        ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
        if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) ||
            np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
            np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
                refcount_inc(&skb->users);
                ireq->pktopts = skb;
        }

        ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
        /* So that link locals have meaning */
        if (!sk->sk_bound_dev_if &&
            ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL)
                ireq->ir_iif = tcp_v6_iif(skb);

        ireq->ir_mark = inet_request_mark(sk, skb);

        req->num_retrans = 0;
        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        = 0;
        treq->rcv_isn = ntohl(th->seq) - 1;
        treq->snt_isn = cookie;
        treq->ts_off = 0;
        treq->txhash = net_tx_rndhash();
        if (IS_ENABLED(CONFIG_SMC))
                ireq->smc_ok = 0;

        /*
         * 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 = ireq->ir_v6_rmt_addr;
                final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final);
                fl6.saddr = ireq->ir_v6_loc_addr;
                fl6.flowi6_oif = ireq->ir_iif;
                fl6.flowi6_mark = ireq->ir_mark;
                fl6.fl6_dport = ireq->ir_rmt_port;
                fl6.fl6_sport = inet_sk(sk)->inet_sport;
                fl6.flowi6_uid = sk->sk_uid;
                security_req_classify_flow(req, flowi6_to_flowi(&fl6));

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

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

        ireq->rcv_wscale = rcv_wscale;
        ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst);

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