// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright (c) 2011, 2012 Patrick McHardy <kaber@trash.net>
 */

#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/ipv6.h>
#include <net/ipv6.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv6.h>
#include <linux/netfilter_ipv6/ip6t_NPT.h>
#include <linux/netfilter/x_tables.h>

static int ip6t_npt_checkentry(const struct xt_tgchk_param *par)
{
	struct ip6t_npt_tginfo *npt = par->targinfo;
	struct in6_addr pfx;
	__wsum src_sum, dst_sum;

	if (npt->src_pfx_len > 64 || npt->dst_pfx_len > 64)
		return -EINVAL;

	/* Ensure that LSB of prefix is zero */
	ipv6_addr_prefix(&pfx, &npt->src_pfx.in6, npt->src_pfx_len);
	if (!ipv6_addr_equal(&pfx, &npt->src_pfx.in6))
		return -EINVAL;
	ipv6_addr_prefix(&pfx, &npt->dst_pfx.in6, npt->dst_pfx_len);
	if (!ipv6_addr_equal(&pfx, &npt->dst_pfx.in6))
		return -EINVAL;

	src_sum = csum_partial(&npt->src_pfx.in6, sizeof(npt->src_pfx.in6), 0);
	dst_sum = csum_partial(&npt->dst_pfx.in6, sizeof(npt->dst_pfx.in6), 0);

	npt->adjustment = ~csum_fold(csum_sub(src_sum, dst_sum));
	return 0;
}

static bool ip6t_npt_map_pfx(const struct ip6t_npt_tginfo *npt,
			     struct in6_addr *addr)
{
	unsigned int pfx_len;
	unsigned int i, idx;
	__be32 mask;
	__sum16 sum;

	pfx_len = max(npt->src_pfx_len, npt->dst_pfx_len);
	for (i = 0; i < pfx_len; i += 32) {
		if (pfx_len - i >= 32)
			mask = 0;
		else
			mask = htonl((1 << (i - pfx_len + 32)) - 1);

		idx = i / 32;
		addr->s6_addr32[idx] &= mask;
		addr->s6_addr32[idx] |= ~mask & npt->dst_pfx.in6.s6_addr32[idx];
	}

	if (pfx_len <= 48)
		idx = 3;
	else {
		for (idx = 4; idx < ARRAY_SIZE(addr->s6_addr16); idx++) {
			if ((__force __sum16)addr->s6_addr16[idx] !=
			    CSUM_MANGLED_0)
				break;
		}
		if (idx == ARRAY_SIZE(addr->s6_addr16))
			return false;
	}

	sum = ~csum_fold(csum_add(csum_unfold((__force __sum16)addr->s6_addr16[idx]),
				  csum_unfold(npt->adjustment)));
	if (sum == CSUM_MANGLED_0)
		sum = 0;
	*(__force __sum16 *)&addr->s6_addr16[idx] = sum;

	return true;
}

static struct ipv6hdr *icmpv6_bounced_ipv6hdr(struct sk_buff *skb,
					      struct ipv6hdr *_bounced_hdr)
{
	if (ipv6_hdr(skb)->nexthdr != IPPROTO_ICMPV6)
		return NULL;

	if (!icmpv6_is_err(icmp6_hdr(skb)->icmp6_type))
		return NULL;

	return skb_header_pointer(skb,
				  skb_transport_offset(skb) + sizeof(struct icmp6hdr),
				  sizeof(struct ipv6hdr),
				  _bounced_hdr);
}

static unsigned int
ip6t_snpt_tg(struct sk_buff *skb, const struct xt_action_param *par)
{
	const struct ip6t_npt_tginfo *npt = par->targinfo;
	struct ipv6hdr _bounced_hdr;
	struct ipv6hdr *bounced_hdr;
	struct in6_addr bounced_pfx;

	if (!ip6t_npt_map_pfx(npt, &ipv6_hdr(skb)->saddr)) {
		icmpv6_send(skb, ICMPV6_PARAMPROB, ICMPV6_HDR_FIELD,
			    offsetof(struct ipv6hdr, saddr));
		return NF_DROP;
	}

	/* rewrite dst addr of bounced packet which was sent to dst range */
	bounced_hdr = icmpv6_bounced_ipv6hdr(skb, &_bounced_hdr);
	if (bounced_hdr) {
		ipv6_addr_prefix(&bounced_pfx, &bounced_hdr->daddr, npt->src_pfx_len);
		if (ipv6_addr_cmp(&bounced_pfx, &npt->src_pfx.in6) == 0)
			ip6t_npt_map_pfx(npt, &bounced_hdr->daddr);
	}

	return XT_CONTINUE;
}

static unsigned int
ip6t_dnpt_tg(struct sk_buff *skb, const struct xt_action_param *par)
{
	const struct ip6t_npt_tginfo *npt = par->targinfo;
	struct ipv6hdr _bounced_hdr;
	struct ipv6hdr *bounced_hdr;
	struct in6_addr bounced_pfx;

	if (!ip6t_npt_map_pfx(npt, &ipv6_hdr(skb)->daddr)) {
		icmpv6_send(skb, ICMPV6_PARAMPROB, ICMPV6_HDR_FIELD,
			    offsetof(struct ipv6hdr, daddr));
		return NF_DROP;
	}

	/* rewrite src addr of bounced packet which was sent from dst range */
	bounced_hdr = icmpv6_bounced_ipv6hdr(skb, &_bounced_hdr);
	if (bounced_hdr) {
		ipv6_addr_prefix(&bounced_pfx, &bounced_hdr->saddr, npt->src_pfx_len);
		if (ipv6_addr_cmp(&bounced_pfx, &npt->src_pfx.in6) == 0)
			ip6t_npt_map_pfx(npt, &bounced_hdr->saddr);
	}

	return XT_CONTINUE;
}

static struct xt_target ip6t_npt_target_reg[] __read_mostly = {
	{
		.name		= "SNPT",
		.table		= "mangle",
		.target		= ip6t_snpt_tg,
		.targetsize	= sizeof(struct ip6t_npt_tginfo),
		.usersize	= offsetof(struct ip6t_npt_tginfo, adjustment),
		.checkentry	= ip6t_npt_checkentry,
		.family		= NFPROTO_IPV6,
		.hooks		= (1 << NF_INET_LOCAL_IN) |
				  (1 << NF_INET_POST_ROUTING),
		.me		= THIS_MODULE,
	},
	{
		.name		= "DNPT",
		.table		= "mangle",
		.target		= ip6t_dnpt_tg,
		.targetsize	= sizeof(struct ip6t_npt_tginfo),
		.usersize	= offsetof(struct ip6t_npt_tginfo, adjustment),
		.checkentry	= ip6t_npt_checkentry,
		.family		= NFPROTO_IPV6,
		.hooks		= (1 << NF_INET_PRE_ROUTING) |
				  (1 << NF_INET_LOCAL_OUT),
		.me		= THIS_MODULE,
	},
};

static int __init ip6t_npt_init(void)
{
	return xt_register_targets(ip6t_npt_target_reg,
				   ARRAY_SIZE(ip6t_npt_target_reg));
}

static void __exit ip6t_npt_exit(void)
{
	xt_unregister_targets(ip6t_npt_target_reg,
			      ARRAY_SIZE(ip6t_npt_target_reg));
}

module_init(ip6t_npt_init);
module_exit(ip6t_npt_exit);

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("IPv6-to-IPv6 Network Prefix Translation (RFC 6296)");
MODULE_AUTHOR("Patrick McHardy <kaber@trash.net>");
MODULE_ALIAS("ip6t_SNPT");
MODULE_ALIAS("ip6t_DNPT");