net/netfilter/xt_tcpmss.c

   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
  18 MODULE_LICENSE("GPL");
  19 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
  20 MODULE_DESCRIPTION("Xtables: TCP MSS match");
  21 MODULE_ALIAS("ipt_tcpmss");
  22 MODULE_ALIAS("ip6t_tcpmss");
  23
  24 static bool
  25 tcpmss_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         }
  69 out:
  70         return info->invert;
  71
  72 dropit:
  73         par->hotdrop = true;
  74         return false;
  75 }
  76
  77 static 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
  96 static int __init tcpmss_mt_init(void)
  97 {
  98         return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
  99 }
 100
 101 static void __exit tcpmss_mt_exit(void)
 102 {
 103         xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
 104 }
 105
 106 module_init(tcpmss_mt_init);
 107 module_exit(tcpmss_mt_exit);