// SPDX-License-Identifier: GPL-2.0-only
/*
 * This is a module which is used for setting the MSS option in TCP packets.
 *
 * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
 * Copyright (C) 2007 Patrick McHardy <kaber@trash.net>
 */
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <linux/gfp.h>
#include <linux/ipv6.h>
#include <linux/tcp.h>
#include <net/dst.h>
#include <net/flow.h>
#include <net/ipv6.h>
#include <net/route.h>
#include <net/tcp.h>

#include <linux/netfilter_ipv4/ip_tables.h>
#include <linux/netfilter_ipv6/ip6_tables.h>
#include <linux/netfilter/x_tables.h>
#include <linux/netfilter/xt_tcpudp.h>
#include <linux/netfilter/xt_TCPMSS.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
MODULE_ALIAS("ipt_TCPMSS");
MODULE_ALIAS("ip6t_TCPMSS");

static inline unsigned int
optlen(const u_int8_t *opt, unsigned int offset)
{
        /* Beware zero-length options: make finite progress */
        if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
                return 1;
        else
                return opt[offset+1];
}

static u_int32_t tcpmss_reverse_mtu(struct net *net,
                                    const struct sk_buff *skb,
                                    unsigned int family)
{
        struct flowi fl;
        struct rtable *rt = NULL;
        u_int32_t mtu     = ~0U;

        if (family == PF_INET) {
                struct flowi4 *fl4 = &fl.u.ip4;
                memset(fl4, 0, sizeof(*fl4));
                fl4->daddr = ip_hdr(skb)->saddr;
        } else {
                struct flowi6 *fl6 = &fl.u.ip6;

                memset(fl6, 0, sizeof(*fl6));
                fl6->daddr = ipv6_hdr(skb)->saddr;
        }

        nf_route(net, (struct dst_entry **)&rt, &fl, false, family);
        if (rt != NULL) {
                mtu = dst_mtu(&rt->dst);
                dst_release(&rt->dst);
        }
        return mtu;
}

static int
tcpmss_mangle_packet(struct sk_buff *skb,
                     const struct xt_action_param *par,
                     unsigned int family,
                     unsigned int tcphoff,
                     unsigned int minlen)
{
        const struct xt_tcpmss_info *info = par->targinfo;
        struct tcphdr *tcph;
        int len, tcp_hdrlen;
        unsigned int i;
        __be16 oldval;
        u16 newmss;
        u8 *opt;

        /* This is a fragment, no TCP header is available */
        if (par->fragoff != 0)
                return 0;

        if (skb_ensure_writable(skb, skb->len))
                return -1;

        len = skb->len - tcphoff;
        if (len < (int)sizeof(struct tcphdr))
                return -1;

        tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
        tcp_hdrlen = tcph->doff * 4;

        if (len < tcp_hdrlen || tcp_hdrlen < sizeof(struct tcphdr))
                return -1;

        if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
                struct net *net = xt_net(par);
                unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family);
                unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu);

                if (min_mtu <= minlen) {
                        net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
                                            min_mtu);
                        return -1;
                }
                newmss = min_mtu - minlen;
        } else
                newmss = info->mss;

        opt = (u_int8_t *)tcph;
        for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
                if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
                        u_int16_t oldmss;

                        oldmss = (opt[i+2] << 8) | opt[i+3];

                        /* Never increase MSS, even when setting it, as
                         * doing so results in problems for hosts that rely
                         * on MSS being set correctly.
                         */
                        if (oldmss <= newmss)
                                return 0;

                        opt[i+2] = (newmss & 0xff00) >> 8;
                        opt[i+3] = newmss & 0x00ff;

                        inet_proto_csum_replace2(&tcph->check, skb,
                                                 htons(oldmss), htons(newmss),
                                                 false);
                        return 0;
                }
        }

        /* There is data after the header so the option can't be added
         * without moving it, and doing so may make the SYN packet
         * itself too large. Accept the packet unmodified instead.
         */
        if (len > tcp_hdrlen)
                return 0;

        /* tcph->doff has 4 bits, do not wrap it to 0 */
        if (tcp_hdrlen >= 15 * 4)
                return 0;

        /*
         * MSS Option not found ?! add it..
         */
        if (skb_tailroom(skb) < TCPOLEN_MSS) {
                if (pskb_expand_head(skb, 0,
                                     TCPOLEN_MSS - skb_tailroom(skb),
                                     GFP_ATOMIC))
                        return -1;
                tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
        }

        skb_put(skb, TCPOLEN_MSS);

        /*
         * IPv4: RFC 1122 states "If an MSS option is not received at
         * connection setup, TCP MUST assume a default send MSS of 536".
         * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
         * length IPv6 header of 60, ergo the default MSS value is 1220
         * Since no MSS was provided, we must use the default values
         */
        if (xt_family(par) == NFPROTO_IPV4)
                newmss = min(newmss, (u16)536);
        else
                newmss = min(newmss, (u16)1220);

        opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
        memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));

        inet_proto_csum_replace2(&tcph->check, skb,
                                 htons(len), htons(len + TCPOLEN_MSS), true);
        opt[0] = TCPOPT_MSS;
        opt[1] = TCPOLEN_MSS;
        opt[2] = (newmss & 0xff00) >> 8;
        opt[3] = newmss & 0x00ff;

        inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), false);

        oldval = ((__be16 *)tcph)[6];
        tcph->doff += TCPOLEN_MSS/4;
        inet_proto_csum_replace2(&tcph->check, skb,
                                 oldval, ((__be16 *)tcph)[6], false);
        return TCPOLEN_MSS;
}

static unsigned int
tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
{
        struct iphdr *iph = ip_hdr(skb);
        __be16 newlen;
        int ret;

        ret = tcpmss_mangle_packet(skb, par,
                                   PF_INET,
                                   iph->ihl * 4,
                                   sizeof(*iph) + sizeof(struct tcphdr));
        if (ret < 0)
                return NF_DROP;
        if (ret > 0) {
                iph = ip_hdr(skb);
                newlen = htons(ntohs(iph->tot_len) + ret);
                csum_replace2(&iph->check, iph->tot_len, newlen);
                iph->tot_len = newlen;
        }
        return XT_CONTINUE;
}

#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
static unsigned int
tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
{
        struct ipv6hdr *ipv6h = ipv6_hdr(skb);
        u8 nexthdr;
        __be16 frag_off, oldlen, newlen;
        int tcphoff;
        int ret;

        nexthdr = ipv6h->nexthdr;
        tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
        if (tcphoff < 0)
                return NF_DROP;
        ret = tcpmss_mangle_packet(skb, par,
                                   PF_INET6,
                                   tcphoff,
                                   sizeof(*ipv6h) + sizeof(struct tcphdr));
        if (ret < 0)
                return NF_DROP;
        if (ret > 0) {
                ipv6h = ipv6_hdr(skb);
                oldlen = ipv6h->payload_len;
                newlen = htons(ntohs(oldlen) + ret);
                if (skb->ip_summed == CHECKSUM_COMPLETE)
                        skb->csum = csum_add(csum_sub(skb->csum, (__force __wsum)oldlen),
                                             (__force __wsum)newlen);
                ipv6h->payload_len = newlen;
        }
        return XT_CONTINUE;
}
#endif

/* Must specify -p tcp --syn */
static inline bool find_syn_match(const struct xt_entry_match *m)
{
        const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;

        if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
            tcpinfo->flg_cmp & TCPHDR_SYN &&
            !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
                return true;

        return false;
}

static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
{
        const struct xt_tcpmss_info *info = par->targinfo;
        const struct ipt_entry *e = par->entryinfo;
        const struct xt_entry_match *ematch;

        if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
            (par->hook_mask & ~((1 << NF_INET_FORWARD) |
                           (1 << NF_INET_LOCAL_OUT) |
                           (1 << NF_INET_POST_ROUTING))) != 0) {
                pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
                return -EINVAL;
        }
        if (par->nft_compat)
                return 0;

        xt_ematch_foreach(ematch, e)
                if (find_syn_match(ematch))
                        return 0;
        pr_info_ratelimited("Only works on TCP SYN packets\n");
        return -EINVAL;
}

#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
{
        const struct xt_tcpmss_info *info = par->targinfo;
        const struct ip6t_entry *e = par->entryinfo;
        const struct xt_entry_match *ematch;

        if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
            (par->hook_mask & ~((1 << NF_INET_FORWARD) |
                           (1 << NF_INET_LOCAL_OUT) |
                           (1 << NF_INET_POST_ROUTING))) != 0) {
                pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
                return -EINVAL;
        }
        if (par->nft_compat)
                return 0;

        xt_ematch_foreach(ematch, e)
                if (find_syn_match(ematch))
                        return 0;
        pr_info_ratelimited("Only works on TCP SYN packets\n");
        return -EINVAL;
}
#endif

static struct xt_target tcpmss_tg_reg[] __read_mostly = {
        {
                .family         = NFPROTO_IPV4,
                .name           = "TCPMSS",
                .checkentry     = tcpmss_tg4_check,
                .target         = tcpmss_tg4,
                .targetsize     = sizeof(struct xt_tcpmss_info),
                .proto          = IPPROTO_TCP,
                .me             = THIS_MODULE,
        },
#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
        {
                .family         = NFPROTO_IPV6,
                .name           = "TCPMSS",
                .checkentry     = tcpmss_tg6_check,
                .target         = tcpmss_tg6,
                .targetsize     = sizeof(struct xt_tcpmss_info),
                .proto          = IPPROTO_TCP,
                .me             = THIS_MODULE,
        },
#endif
};

static int __init tcpmss_tg_init(void)
{
        return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
}

static void __exit tcpmss_tg_exit(void)
{
        xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
}

module_init(tcpmss_tg_init);
module_exit(tcpmss_tg_exit);