// SPDX-License-Identifier: GPL-2.0-only
/* (C) 1999-2001 Paul `Rusty' Russell
 * (C) 2002-2004 Netfilter Core Team <coreteam@netfilter.org>
 */

#include <linux/module.h>
#include <net/ip.h>
#include <net/tcp.h>
#include <net/route.h>
#include <net/dst.h>
#include <net/netfilter/ipv4/nf_reject.h>
#include <linux/netfilter_ipv4.h>
#include <linux/netfilter_bridge.h>

static struct iphdr *nf_reject_iphdr_put(struct sk_buff *nskb,
                                         const struct sk_buff *oldskb,
                                         __u8 protocol, int ttl);
static void nf_reject_ip_tcphdr_put(struct sk_buff *nskb, const struct sk_buff *oldskb,
                                    const struct tcphdr *oth);
static const struct tcphdr *
nf_reject_ip_tcphdr_get(struct sk_buff *oldskb,
                        struct tcphdr *_oth, int hook);

static int nf_reject_iphdr_validate(struct sk_buff *skb)
{
        struct iphdr *iph;
        u32 len;

        if (!pskb_may_pull(skb, sizeof(struct iphdr)))
                return 0;

        iph = ip_hdr(skb);
        if (iph->ihl < 5 || iph->version != 4)
                return 0;

        len = ntohs(iph->tot_len);
        if (skb->len < len)
                return 0;
        else if (len < (iph->ihl*4))
                return 0;

        if (!pskb_may_pull(skb, iph->ihl*4))
                return 0;

        return 1;
}

struct sk_buff *nf_reject_skb_v4_tcp_reset(struct net *net,
                                           struct sk_buff *oldskb,
                                           const struct net_device *dev,
                                           int hook)
{
        const struct tcphdr *oth;
        struct sk_buff *nskb;
        struct iphdr *niph;
        struct tcphdr _oth;

        if (!nf_reject_iphdr_validate(oldskb))
                return NULL;

        oth = nf_reject_ip_tcphdr_get(oldskb, &_oth, hook);
        if (!oth)
                return NULL;

        nskb = alloc_skb(sizeof(struct iphdr) + sizeof(struct tcphdr) +
                         LL_MAX_HEADER, GFP_ATOMIC);
        if (!nskb)
                return NULL;

        nskb->dev = (struct net_device *)dev;

        skb_reserve(nskb, LL_MAX_HEADER);
        niph = nf_reject_iphdr_put(nskb, oldskb, IPPROTO_TCP,
                                   READ_ONCE(net->ipv4.sysctl_ip_default_ttl));
        nf_reject_ip_tcphdr_put(nskb, oldskb, oth);
        niph->tot_len = htons(nskb->len);
        ip_send_check(niph);

        return nskb;
}
EXPORT_SYMBOL_GPL(nf_reject_skb_v4_tcp_reset);

static bool nf_skb_is_icmp_unreach(const struct sk_buff *skb)
{
        const struct iphdr *iph = ip_hdr(skb);
        u8 *tp, _type;
        int thoff;

        if (iph->protocol != IPPROTO_ICMP)
                return false;

        thoff = skb_network_offset(skb) + sizeof(*iph);

        tp = skb_header_pointer(skb,
                                thoff + offsetof(struct icmphdr, type),
                                sizeof(_type), &_type);

        if (!tp)
                return false;

        return *tp == ICMP_DEST_UNREACH;
}

struct sk_buff *nf_reject_skb_v4_unreach(struct net *net,
                                         struct sk_buff *oldskb,
                                         const struct net_device *dev,
                                         int hook, u8 code)
{
        struct sk_buff *nskb;
        struct iphdr *niph;
        struct icmphdr *icmph;
        unsigned int len;
        int dataoff;
        __wsum csum;
        u8 proto;

        if (!nf_reject_iphdr_validate(oldskb))
                return NULL;

        /* IP header checks: fragment. */
        if (ip_hdr(oldskb)->frag_off & htons(IP_OFFSET))
                return NULL;

        /* don't reply to ICMP_DEST_UNREACH with ICMP_DEST_UNREACH. */
        if (nf_skb_is_icmp_unreach(oldskb))
                return NULL;

        /* RFC says return as much as we can without exceeding 576 bytes. */
        len = min_t(unsigned int, 536, oldskb->len);

        if (!pskb_may_pull(oldskb, len))
                return NULL;

        if (pskb_trim_rcsum(oldskb, ntohs(ip_hdr(oldskb)->tot_len)))
                return NULL;

        dataoff = ip_hdrlen(oldskb);
        proto = ip_hdr(oldskb)->protocol;

        if (!skb_csum_unnecessary(oldskb) &&
            nf_reject_verify_csum(oldskb, dataoff, proto) &&
            nf_ip_checksum(oldskb, hook, ip_hdrlen(oldskb), proto))
                return NULL;

        nskb = alloc_skb(sizeof(struct iphdr) + sizeof(struct icmphdr) +
                         LL_MAX_HEADER + len, GFP_ATOMIC);
        if (!nskb)
                return NULL;

        nskb->dev = (struct net_device *)dev;

        skb_reserve(nskb, LL_MAX_HEADER);
        niph = nf_reject_iphdr_put(nskb, oldskb, IPPROTO_ICMP,
                                   READ_ONCE(net->ipv4.sysctl_ip_default_ttl));

        skb_reset_transport_header(nskb);
        icmph = skb_put_zero(nskb, sizeof(struct icmphdr));
        icmph->type     = ICMP_DEST_UNREACH;
        icmph->code     = code;

        skb_put_data(nskb, skb_network_header(oldskb), len);

        csum = csum_partial((void *)icmph, len + sizeof(struct icmphdr), 0);
        icmph->checksum = csum_fold(csum);

        niph->tot_len   = htons(nskb->len);
        ip_send_check(niph);

        return nskb;
}
EXPORT_SYMBOL_GPL(nf_reject_skb_v4_unreach);

static const struct tcphdr *
nf_reject_ip_tcphdr_get(struct sk_buff *oldskb,
                        struct tcphdr *_oth, int hook)
{
        const struct tcphdr *oth;

        /* IP header checks: fragment. */
        if (ip_hdr(oldskb)->frag_off & htons(IP_OFFSET))
                return NULL;

        if (ip_hdr(oldskb)->protocol != IPPROTO_TCP)
                return NULL;

        oth = skb_header_pointer(oldskb, ip_hdrlen(oldskb),
                                 sizeof(struct tcphdr), _oth);
        if (oth == NULL)
                return NULL;

        /* No RST for RST. */
        if (oth->rst)
                return NULL;

        /* Check checksum */
        if (nf_ip_checksum(oldskb, hook, ip_hdrlen(oldskb), IPPROTO_TCP))
                return NULL;

        return oth;
}

static struct iphdr *nf_reject_iphdr_put(struct sk_buff *nskb,
                                         const struct sk_buff *oldskb,
                                         __u8 protocol, int ttl)
{
        struct iphdr *niph, *oiph = ip_hdr(oldskb);

        skb_reset_network_header(nskb);
        niph = skb_put(nskb, sizeof(struct iphdr));
        niph->version   = 4;
        niph->ihl       = sizeof(struct iphdr) / 4;
        niph->tos       = 0;
        niph->id        = 0;
        niph->frag_off  = htons(IP_DF);
        niph->protocol  = protocol;
        niph->check     = 0;
        niph->saddr     = oiph->daddr;
        niph->daddr     = oiph->saddr;
        niph->ttl       = ttl;

        nskb->protocol = htons(ETH_P_IP);

        return niph;
}

static void nf_reject_ip_tcphdr_put(struct sk_buff *nskb, const struct sk_buff *oldskb,
                                    const struct tcphdr *oth)
{
        struct iphdr *niph = ip_hdr(nskb);
        struct tcphdr *tcph;

        skb_reset_transport_header(nskb);
        tcph = skb_put_zero(nskb, sizeof(struct tcphdr));
        tcph->source    = oth->dest;
        tcph->dest      = oth->source;
        tcph->doff      = sizeof(struct tcphdr) / 4;

        if (oth->ack) {
                tcph->seq = oth->ack_seq;
        } else {
                tcph->ack_seq = htonl(ntohl(oth->seq) + oth->syn + oth->fin +
                                      oldskb->len - ip_hdrlen(oldskb) -
                                      (oth->doff << 2));
                tcph->ack = 1;
        }

        tcph->rst       = 1;
        tcph->check = ~tcp_v4_check(sizeof(struct tcphdr), niph->saddr,
                                    niph->daddr, 0);
        nskb->ip_summed = CHECKSUM_PARTIAL;
        nskb->csum_start = (unsigned char *)tcph - nskb->head;
        nskb->csum_offset = offsetof(struct tcphdr, check);
}

static int nf_reject_fill_skb_dst(struct sk_buff *skb_in)
{
        struct dst_entry *dst = NULL;
        struct flowi fl;

        memset(&fl, 0, sizeof(struct flowi));
        fl.u.ip4.daddr = ip_hdr(skb_in)->saddr;
        nf_ip_route(dev_net(skb_in->dev), &dst, &fl, false);
        if (!dst)
                return -1;

        skb_dst_set(skb_in, dst);
        return 0;
}

/* Send RST reply */
void nf_send_reset(struct net *net, struct sock *sk, struct sk_buff *oldskb,
                   int hook)
{
        const struct tcphdr *oth;
        struct sk_buff *nskb;
        struct tcphdr _oth;

        oth = nf_reject_ip_tcphdr_get(oldskb, &_oth, hook);
        if (!oth)
                return;

        if (!skb_dst(oldskb) && nf_reject_fill_skb_dst(oldskb) < 0)
                return;

        if (skb_rtable(oldskb)->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST))
                return;

        nskb = alloc_skb(sizeof(struct iphdr) + sizeof(struct tcphdr) +
                         LL_MAX_HEADER, GFP_ATOMIC);
        if (!nskb)
                return;

        /* ip_route_me_harder expects skb->dst to be set */
        skb_dst_set_noref(nskb, skb_dst(oldskb));

        nskb->mark = IP4_REPLY_MARK(net, oldskb->mark);

        skb_reserve(nskb, LL_MAX_HEADER);
        nf_reject_iphdr_put(nskb, oldskb, IPPROTO_TCP,
                            ip4_dst_hoplimit(skb_dst(nskb)));
        nf_reject_ip_tcphdr_put(nskb, oldskb, oth);
        if (ip_route_me_harder(net, sk, nskb, RTN_UNSPEC))
                goto free_nskb;

        /* "Never happens" */
        if (nskb->len > dst4_mtu(skb_dst(nskb)))
                goto free_nskb;

        nf_ct_attach(nskb, oldskb);
        nf_ct_set_closing(skb_nfct(oldskb));

#if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
        /* If we use ip_local_out for bridged traffic, the MAC source on
         * the RST will be ours, instead of the destination's.  This confuses
         * some routers/firewalls, and they drop the packet.  So we need to
         * build the eth header using the original destination's MAC as the
         * source, and send the RST packet directly.
         */
        if (nf_bridge_info_exists(oldskb)) {
                struct ethhdr *oeth = eth_hdr(oldskb);
                struct iphdr *niph = ip_hdr(nskb);
                struct net_device *br_indev;

                br_indev = nf_bridge_get_physindev(oldskb, net);
                if (!br_indev)
                        goto free_nskb;

                nskb->dev = br_indev;
                niph->tot_len = htons(nskb->len);
                ip_send_check(niph);
                if (dev_hard_header(nskb, nskb->dev, ntohs(nskb->protocol),
                                    oeth->h_source, oeth->h_dest, nskb->len) < 0)
                        goto free_nskb;
                dev_queue_xmit(nskb);
        } else
#endif
                ip_local_out(net, nskb->sk, nskb);

        return;

 free_nskb:
        kfree_skb(nskb);
}
EXPORT_SYMBOL_GPL(nf_send_reset);

void nf_send_unreach(struct sk_buff *skb_in, int code, int hook)
{
        struct iphdr *iph = ip_hdr(skb_in);
        int dataoff = ip_hdrlen(skb_in);
        u8 proto = iph->protocol;

        if (iph->frag_off & htons(IP_OFFSET))
                return;

        if (!skb_dst(skb_in) && nf_reject_fill_skb_dst(skb_in) < 0)
                return;

        if (skb_csum_unnecessary(skb_in) ||
            !nf_reject_verify_csum(skb_in, dataoff, proto)) {
                icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
                return;
        }

        if (nf_ip_checksum(skb_in, hook, dataoff, proto) == 0)
                icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
}
EXPORT_SYMBOL_GPL(nf_send_unreach);

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("IPv4 packet rejection core");