// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright (c) 2011 Florian Westphal <fw@strlen.de>
 *
 * based on fib_frontend.c; Author: Alexey Kuznetsov, <kuznet@ms2.inr.ac.ru>
 */
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/netdevice.h>
#include <linux/ip.h>
#include <net/flow.h>
#include <net/ip.h>
#include <net/ip_fib.h>
#include <net/route.h>

#include <linux/netfilter/xt_rpfilter.h>
#include <linux/netfilter/x_tables.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Florian Westphal <fw@strlen.de>");
MODULE_DESCRIPTION("iptables: ipv4 reverse path filter match");

/* don't try to find route from mcast/bcast/zeronet */
static __be32 rpfilter_get_saddr(__be32 addr)
{
        if (ipv4_is_multicast(addr) || ipv4_is_lbcast(addr) ||
            ipv4_is_zeronet(addr))
                return 0;
        return addr;
}

static bool rpfilter_lookup_reverse(struct net *net, struct flowi4 *fl4,
                                const struct net_device *dev, u8 flags)
{
        struct fib_result res;

        if (fib_lookup(net, fl4, &res, FIB_LOOKUP_IGNORE_LINKSTATE))
                return false;

        if (res.type != RTN_UNICAST) {
                if (res.type != RTN_LOCAL || !(flags & XT_RPFILTER_ACCEPT_LOCAL))
                        return false;
        }
        return fib_info_nh_uses_dev(res.fi, dev) || flags & XT_RPFILTER_LOOSE;
}

static bool
rpfilter_is_loopback(const struct sk_buff *skb, const struct net_device *in)
{
        return skb->pkt_type == PACKET_LOOPBACK || in->flags & IFF_LOOPBACK;
}

static bool rpfilter_mt(const struct sk_buff *skb, struct xt_action_param *par)
{
        const struct xt_rpfilter_info *info;
        const struct iphdr *iph;
        struct flowi4 flow;
        bool invert;

        info = par->matchinfo;
        invert = info->flags & XT_RPFILTER_INVERT;

        if (rpfilter_is_loopback(skb, xt_in(par)))
                return true ^ invert;

        iph = ip_hdr(skb);
        if (ipv4_is_zeronet(iph->saddr)) {
                if (ipv4_is_lbcast(iph->daddr) ||
                    ipv4_is_local_multicast(iph->daddr))
                        return true ^ invert;
        }

        memset(&flow, 0, sizeof(flow));
        flow.flowi4_iif = LOOPBACK_IFINDEX;
        flow.daddr = iph->saddr;
        flow.saddr = rpfilter_get_saddr(iph->daddr);
        flow.flowi4_mark = info->flags & XT_RPFILTER_VALID_MARK ? skb->mark : 0;
        flow.flowi4_dscp = ip4h_dscp(iph);
        flow.flowi4_scope = RT_SCOPE_UNIVERSE;
        flow.flowi4_l3mdev = l3mdev_master_ifindex_rcu(xt_in(par));
        flow.flowi4_uid = sock_net_uid(xt_net(par), NULL);

        return rpfilter_lookup_reverse(xt_net(par), &flow, xt_in(par), info->flags) ^ invert;
}

static int rpfilter_check(const struct xt_mtchk_param *par)
{
        const struct xt_rpfilter_info *info = par->matchinfo;
        unsigned int options = ~XT_RPFILTER_OPTION_MASK;
        if (info->flags & options) {
                pr_info_ratelimited("unknown options\n");
                return -EINVAL;
        }

        if (strcmp(par->table, "mangle") != 0 &&
            strcmp(par->table, "raw") != 0) {
                pr_info_ratelimited("only valid in \'raw\' or \'mangle\' table, not \'%s\'\n",
                                    par->table);
                return -EINVAL;
        }

        return 0;
}

static struct xt_match rpfilter_mt_reg __read_mostly = {
        .name           = "rpfilter",
        .family         = NFPROTO_IPV4,
        .checkentry     = rpfilter_check,
        .match          = rpfilter_mt,
        .matchsize      = sizeof(struct xt_rpfilter_info),
        .hooks          = (1 << NF_INET_PRE_ROUTING),
        .me             = THIS_MODULE
};

static int __init rpfilter_mt_init(void)
{
        return xt_register_match(&rpfilter_mt_reg);
}

static void __exit rpfilter_mt_exit(void)
{
        xt_unregister_match(&rpfilter_mt_reg);
}

module_init(rpfilter_mt_init);
module_exit(rpfilter_mt_exit);