// 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");