// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright (c) 2013 Patrick McHardy <kaber@trash.net>
 */

#include <linux/netfilter_ipv4/ip_tables.h>
#include <linux/netfilter/x_tables.h>
#include <linux/netfilter/xt_SYNPROXY.h>

#include <net/netfilter/nf_synproxy.h>

static unsigned int
synproxy_tg4(struct sk_buff *skb, const struct xt_action_param *par)
{
        const struct xt_synproxy_info *info = par->targinfo;
        struct net *net = xt_net(par);
        struct synproxy_net *snet = synproxy_pernet(net);
        struct synproxy_options opts = {};
        struct tcphdr *th, _th;

        if (nf_ip_checksum(skb, xt_hooknum(par), par->thoff, IPPROTO_TCP))
                return NF_DROP;

        th = skb_header_pointer(skb, par->thoff, sizeof(_th), &_th);
        if (th == NULL)
                return NF_DROP;

        if (!synproxy_parse_options(skb, par->thoff, th, &opts))
                return NF_DROP;

        if (th->syn && !(th->ack || th->fin || th->rst)) {
                /* Initial SYN from client */
                this_cpu_inc(snet->stats->syn_received);

                if (th->ece && th->cwr)
                        opts.options |= XT_SYNPROXY_OPT_ECN;

                opts.options &= info->options;
                opts.mss_encode = opts.mss_option;
                opts.mss_option = info->mss;
                if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
                        synproxy_init_timestamp_cookie(info, &opts);
                else
                        opts.options &= ~(XT_SYNPROXY_OPT_WSCALE |
                                          XT_SYNPROXY_OPT_SACK_PERM |
                                          XT_SYNPROXY_OPT_ECN);

                synproxy_send_client_synack(net, skb, th, &opts);
                consume_skb(skb);
                return NF_STOLEN;
        } else if (th->ack && !(th->fin || th->rst || th->syn)) {
                /* ACK from client */
                if (synproxy_recv_client_ack(net, skb, th, &opts, ntohl(th->seq))) {
                        consume_skb(skb);
                        return NF_STOLEN;
                } else {
                        return NF_DROP;
                }
        }

        return XT_CONTINUE;
}

static int synproxy_tg4_check(const struct xt_tgchk_param *par)
{
        struct synproxy_net *snet = synproxy_pernet(par->net);
        const struct ipt_entry *e = par->entryinfo;
        int err;

        if (e->ip.proto != IPPROTO_TCP ||
            e->ip.invflags & XT_INV_PROTO)
                return -EINVAL;

        err = nf_ct_netns_get(par->net, par->family);
        if (err)
                return err;

        err = nf_synproxy_ipv4_init(snet, par->net);
        if (err) {
                nf_ct_netns_put(par->net, par->family);
                return err;
        }

        return err;
}

static void synproxy_tg4_destroy(const struct xt_tgdtor_param *par)
{
        struct synproxy_net *snet = synproxy_pernet(par->net);

        nf_synproxy_ipv4_fini(snet, par->net);
        nf_ct_netns_put(par->net, par->family);
}

static struct xt_target synproxy_tg4_reg __read_mostly = {
        .name           = "SYNPROXY",
        .family         = NFPROTO_IPV4,
        .hooks          = (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD),
        .target         = synproxy_tg4,
        .targetsize     = sizeof(struct xt_synproxy_info),
        .checkentry     = synproxy_tg4_check,
        .destroy        = synproxy_tg4_destroy,
        .me             = THIS_MODULE,
};

static int __init synproxy_tg4_init(void)
{
        return xt_register_target(&synproxy_tg4_reg);
}

static void __exit synproxy_tg4_exit(void)
{
        xt_unregister_target(&synproxy_tg4_reg);
}

module_init(synproxy_tg4_init);
module_exit(synproxy_tg4_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Patrick McHardy <kaber@trash.net>");
MODULE_DESCRIPTION("Intercept TCP connections and establish them using syncookies");