LXR linux/net/netfilter/xt

   1// SPDX-License-Identifier: GPL-2.0-only
   2/* Kernel module to match TCP MSS values. */
   3
   4/* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
   5 * Portions (C) 2005 by Harald Welte <laforge@netfilter.org>
   6 */
   7
   8#include <linux/module.h>
   9#include <linux/skbuff.h>
  10#include <net/tcp.h>
  11
  12#include <linux/netfilter/xt_tcpmss.h>
  13#include <linux/netfilter/x_tables.h>
  14
  15#include <linux/netfilter_ipv4/ip_tables.h>
  16#include <linux/netfilter_ipv6/ip6_tables.h>
  17
  18MODULE_LICENSE("GPL");
  19MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
  20MODULE_DESCRIPTION("Xtables: TCP MSS match");
  21MODULE_ALIAS("ipt_tcpmss");
  22MODULE_ALIAS("ip6t_tcpmss");
  23
  24static bool
  25tcpmss_mt(const struct sk_buff *skb, struct xt_action_param *par)
  26{
  27        const struct xt_tcpmss_match_info *info = par->matchinfo;
  28        const struct tcphdr *th;
  29        struct tcphdr _tcph;
  30        /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
  31        const u_int8_t *op;
  32        u8 _opt[15 * 4 - sizeof(_tcph)];
  33        unsigned int i, optlen;
  34
  35        /* If we don't have the whole header, drop packet. */
  36        th = skb_header_pointer(skb, par->thoff, sizeof(_tcph), &_tcph);
  37        if (th == NULL)
  38                goto dropit;
  39
  40        /* Malformed. */
  41        if (th->doff*4 < sizeof(*th))
  42                goto dropit;
  43
  44        optlen = th->doff*4 - sizeof(*th);
  45        if (!optlen)
  46                goto out;
  47
  48        /* Truncated options. */
  49        op = skb_header_pointer(skb, par->thoff + sizeof(*th), optlen, _opt);
  50        if (op == NULL)
  51                goto dropit;
  52
  53        for (i = 0; i < optlen; ) {
  54                if (op[i] == TCPOPT_MSS
  55                    && (optlen - i) >= TCPOLEN_MSS
  56                    && op[i+1] == TCPOLEN_MSS) {
  57                        u_int16_t mssval;
  58
  59                        mssval = (op[i+2] << 8) | op[i+3];
  60
  61                        return (mssval >= info->mss_min &&
  62                                mssval <= info->mss_max) ^ info->invert;
  63                }
  64                if (op[i] < 2)
  65                        i++;
  66                else
  67                        i += op[i+1] ? : 1;
  68        }
  69out:
  70        return info->invert;
  71
  72dropit:
  73        par->hotdrop = true;
  74        return false;
  75}
  76
  77static struct xt_match tcpmss_mt_reg[] __read_mostly = {
  78        {
  79                .name           = "tcpmss",
  80                .family         = NFPROTO_IPV4,
  81                .match          = tcpmss_mt,
  82                .matchsize      = sizeof(struct xt_tcpmss_match_info),
  83                .proto          = IPPROTO_TCP,
  84                .me             = THIS_MODULE,
  85        },
  86        {
  87                .name           = "tcpmss",
  88                .family         = NFPROTO_IPV6,
  89                .match          = tcpmss_mt,
  90                .matchsize      = sizeof(struct xt_tcpmss_match_info),
  91                .proto          = IPPROTO_TCP,
  92                .me             = THIS_MODULE,
  93        },
  94};
  95
  96static int __init tcpmss_mt_init(void)
  97{
  98        return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
  99}
 100
 101static void __exit tcpmss_mt_exit(void)
 102{
 103        xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
 104}
 105
 106module_init(tcpmss_mt_init);
 107module_exit(tcpmss_mt_exit);
 108