// SPDX-License-Identifier: GPL-2.0-only
/* nf_nat_helper.c - generic support functions for NAT helpers
 *
 * (C) 2000-2002 Harald Welte <laforge@netfilter.org>
 * (C) 2003-2006 Netfilter Core Team <coreteam@netfilter.org>
 * (C) 2007-2012 Patrick McHardy <kaber@trash.net>
 */
#include <linux/module.h>
#include <linux/gfp.h>
#include <linux/types.h>
#include <linux/skbuff.h>
#include <linux/tcp.h>
#include <linux/udp.h>
#include <net/tcp.h>

#include <net/netfilter/nf_conntrack.h>
#include <net/netfilter/nf_conntrack_helper.h>
#include <net/netfilter/nf_conntrack_ecache.h>
#include <net/netfilter/nf_conntrack_expect.h>
#include <net/netfilter/nf_conntrack_seqadj.h>
#include <net/netfilter/nf_nat.h>
#include <net/netfilter/nf_nat_helper.h>

/* Frobs data inside this packet, which is linear. */
static void mangle_contents(struct sk_buff *skb,
                            unsigned int dataoff,
                            unsigned int match_offset,
                            unsigned int match_len,
                            const char *rep_buffer,
                            unsigned int rep_len)
{
        unsigned char *data;

        SKB_LINEAR_ASSERT(skb);
        data = skb_network_header(skb) + dataoff;

        /* move post-replacement */
        memmove(data + match_offset + rep_len,
                data + match_offset + match_len,
                skb_tail_pointer(skb) - (skb_network_header(skb) + dataoff +
                             match_offset + match_len));

        /* insert data from buffer */
        memcpy(data + match_offset, rep_buffer, rep_len);

        /* update skb info */
        if (rep_len > match_len) {
                pr_debug("nf_nat_mangle_packet: Extending packet by "
                         "%u from %u bytes\n", rep_len - match_len, skb->len);
                skb_put(skb, rep_len - match_len);
        } else {
                pr_debug("nf_nat_mangle_packet: Shrinking packet from "
                         "%u from %u bytes\n", match_len - rep_len, skb->len);
                __skb_trim(skb, skb->len + rep_len - match_len);
        }

        if (nf_ct_l3num((struct nf_conn *)skb_nfct(skb)) == NFPROTO_IPV4) {
                /* fix IP hdr checksum information */
                ip_hdr(skb)->tot_len = htons(skb->len);
                ip_send_check(ip_hdr(skb));
        } else
                ipv6_hdr(skb)->payload_len =
                        htons(skb->len - sizeof(struct ipv6hdr));
}

/* Unusual, but possible case. */
static bool enlarge_skb(struct sk_buff *skb, unsigned int extra)
{
        if (skb->len + extra > 65535)
                return false;

        if (pskb_expand_head(skb, 0, extra - skb_tailroom(skb), GFP_ATOMIC))
                return false;

        return true;
}

/* Generic function for mangling variable-length address changes inside
 * NATed TCP connections (like the PORT XXX,XXX,XXX,XXX,XXX,XXX
 * command in FTP).
 *
 * Takes care about all the nasty sequence number changes, checksumming,
 * skb enlargement, ...
 *
 * */
bool __nf_nat_mangle_tcp_packet(struct sk_buff *skb,
                                struct nf_conn *ct,
                                enum ip_conntrack_info ctinfo,
                                unsigned int protoff,
                                unsigned int match_offset,
                                unsigned int match_len,
                                const char *rep_buffer,
                                unsigned int rep_len, bool adjust)
{
        struct tcphdr *tcph;
        int oldlen, datalen;

        if (skb_ensure_writable(skb, skb->len))
                return false;

        if (rep_len > match_len &&
            rep_len - match_len > skb_tailroom(skb) &&
            !enlarge_skb(skb, rep_len - match_len))
                return false;

        tcph = (void *)skb->data + protoff;

        oldlen = skb->len - protoff;
        mangle_contents(skb, protoff + tcph->doff*4,
                        match_offset, match_len, rep_buffer, rep_len);

        datalen = skb->len - protoff;

        nf_nat_csum_recalc(skb, nf_ct_l3num(ct), IPPROTO_TCP,
                           tcph, &tcph->check, datalen, oldlen);

        if (adjust && rep_len != match_len)
                nf_ct_seqadj_set(ct, ctinfo, tcph->seq,
                                 (int)rep_len - (int)match_len);

        return true;
}
EXPORT_SYMBOL(__nf_nat_mangle_tcp_packet);

/* Generic function for mangling variable-length address changes inside
 * NATed UDP connections (like the CONNECT DATA XXXXX MESG XXXXX INDEX XXXXX
 * command in the Amanda protocol)
 *
 * Takes care about all the nasty sequence number changes, checksumming,
 * skb enlargement, ...
 *
 * XXX - This function could be merged with nf_nat_mangle_tcp_packet which
 *       should be fairly easy to do.
 */
bool
nf_nat_mangle_udp_packet(struct sk_buff *skb,
                         struct nf_conn *ct,
                         enum ip_conntrack_info ctinfo,
                         unsigned int protoff,
                         unsigned int match_offset,
                         unsigned int match_len,
                         const char *rep_buffer,
                         unsigned int rep_len)
{
        struct udphdr *udph;
        int datalen, oldlen;

        if (skb_ensure_writable(skb, skb->len))
                return false;

        if (rep_len > match_len &&
            rep_len - match_len > skb_tailroom(skb) &&
            !enlarge_skb(skb, rep_len - match_len))
                return false;

        udph = (void *)skb->data + protoff;

        oldlen = skb->len - protoff;
        mangle_contents(skb, protoff + sizeof(*udph),
                        match_offset, match_len, rep_buffer, rep_len);

        /* update the length of the UDP packet */
        datalen = skb->len - protoff;
        udph->len = htons(datalen);

        /* fix udp checksum if udp checksum was previously calculated */
        if (!udph->check && skb->ip_summed != CHECKSUM_PARTIAL)
                return true;

        nf_nat_csum_recalc(skb, nf_ct_l3num(ct), IPPROTO_UDP,
                           udph, &udph->check, datalen, oldlen);

        return true;
}
EXPORT_SYMBOL(nf_nat_mangle_udp_packet);

/* Setup NAT on this expected conntrack so it follows master. */
/* If we fail to get a free NAT slot, we'll get dropped on confirm */
void nf_nat_follow_master(struct nf_conn *ct,
                          struct nf_conntrack_expect *exp)
{
        struct nf_nat_range2 range;

        /* This must be a fresh one. */
        BUG_ON(ct->status & IPS_NAT_DONE_MASK);

        /* Change src to where master sends to */
        range.flags = NF_NAT_RANGE_MAP_IPS;
        range.min_addr = range.max_addr
                = ct->master->tuplehash[!exp->dir].tuple.dst.u3;
        nf_nat_setup_info(ct, &range, NF_NAT_MANIP_SRC);

        /* For DST manip, map port here to where it's expected. */
        range.flags = (NF_NAT_RANGE_MAP_IPS | NF_NAT_RANGE_PROTO_SPECIFIED);
        range.min_proto = range.max_proto = exp->saved_proto;
        range.min_addr = range.max_addr
                = ct->master->tuplehash[!exp->dir].tuple.src.u3;
        nf_nat_setup_info(ct, &range, NF_NAT_MANIP_DST);
}
EXPORT_SYMBOL(nf_nat_follow_master);

u16 nf_nat_exp_find_port(struct nf_conntrack_expect *exp, u16 port)
{
        static const unsigned int max_attempts = 128;
        int range, attempts_left;
        u16 min = port;

        range = USHRT_MAX - port;
        attempts_left = range;

        if (attempts_left > max_attempts)
                attempts_left = max_attempts;

        /* Try to get same port: if not, try to change it. */
        for (;;) {
                int res;

                exp->tuple.dst.u.tcp.port = htons(port);
                res = nf_ct_expect_related(exp, 0);
                if (res == 0)
                        return port;

                if (res != -EBUSY || (--attempts_left < 0))
                        break;

                port = min + get_random_u32_below(range);
        }

        return 0;
}
EXPORT_SYMBOL_GPL(nf_nat_exp_find_port);