net/ipv4/netfilter/ipt_rpfilter.c

// 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/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;
	int ret __maybe_unused;

	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_tos = iph->tos & IPTOS_RT_MASK;
	flow.flowi4_scope = RT_SCOPE_UNIVERSE;
	flow.flowi4_oif = l3mdev_master_ifindex_rcu(xt_in(par));

	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);