net/netfilter/xt_TCPOPTSTRIP.c

   1 /*
   2  * A module for stripping a specific TCP option from TCP packets.
   3  *
   4  * Copyright (C) 2007 Sven Schnelle <svens@bitebene.org>
   5  * Copyright © CC Computer Consultants GmbH, 2007
   6  *
   7  * This program is free software; you can redistribute it and/or modify
   8  * it under the terms of the GNU General Public License version 2 as
   9  * published by the Free Software Foundation.
  10  */
  11
  12 #include <linux/module.h>
  13 #include <linux/skbuff.h>
  14 #include <linux/ip.h>
  15 #include <linux/ipv6.h>
  16 #include <linux/tcp.h>
  17 #include <net/ipv6.h>
  18 #include <net/tcp.h>
  19 #include <linux/netfilter/x_tables.h>
  20 #include <linux/netfilter/xt_TCPOPTSTRIP.h>
  21
  22 static inline unsigned int optlen(const u_int8_t *opt, unsigned int offset)
  23 {
  24         /* Beware zero-length options: make finite progress */
  25         if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
  26                 return 1;
  27         else
  28                 return opt[offset+1];
  29 }
  30
  31 static unsigned int
  32 tcpoptstrip_mangle_packet(struct sk_buff *skb,
  33                           const struct xt_tcpoptstrip_target_info *info,
  34                           unsigned int tcphoff, unsigned int minlen)
  35 {
  36         unsigned int optl, i, j;
  37         struct tcphdr *tcph;
  38         u_int16_t n, o;
  39         u_int8_t *opt;
  40
  41         if (!skb_make_writable(skb, skb->len))
  42                 return NF_DROP;
  43
  44         tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
  45         opt  = (u_int8_t *)tcph;
  46
  47         /*
  48          * Walk through all TCP options - if we find some option to remove,
  49          * set all octets to %TCPOPT_NOP and adjust checksum.
  50          */
  51         for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) {
  52                 optl = optlen(opt, i);
  53
  54                 if (i + optl > tcp_hdrlen(skb))
  55                         break;
  56
  57                 if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
  58                         continue;
  59
  60                 for (j = 0; j < optl; ++j) {
  61                         o = opt[i+j];
  62                         n = TCPOPT_NOP;
  63                         if ((i + j) % 2 == 0) {
  64                                 o <<= 8;
  65                                 n <<= 8;
  66                         }
  67                         inet_proto_csum_replace2(&tcph->check, skb, htons(o),
  68                                                  htons(n), 0);
  69                 }
  70                 memset(opt + i, TCPOPT_NOP, optl);
  71         }
  72
  73         return XT_CONTINUE;
  74 }
  75
  76 static unsigned int
  77 tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_action_param *par)
  78 {
  79         return tcpoptstrip_mangle_packet(skb, par->targinfo, ip_hdrlen(skb),
  80                sizeof(struct iphdr) + sizeof(struct tcphdr));
  81 }
  82
  83 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
  84 static unsigned int
  85 tcpoptstrip_tg6(struct sk_buff *skb, const struct xt_action_param *par)
  86 {
  87         struct ipv6hdr *ipv6h = ipv6_hdr(skb);
  88         int tcphoff;
  89         u_int8_t nexthdr;
  90         __be16 frag_off;
  91
  92         nexthdr = ipv6h->nexthdr;
  93         tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
  94         if (tcphoff < 0)
  95                 return NF_DROP;
  96
  97         return tcpoptstrip_mangle_packet(skb, par->targinfo, tcphoff,
  98                sizeof(*ipv6h) + sizeof(struct tcphdr));
  99 }
 100 #endif
 101
 102 static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
 103         {
 104                 .name       = "TCPOPTSTRIP",
 105                 .family     = NFPROTO_IPV4,
 106                 .table      = "mangle",
 107                 .proto      = IPPROTO_TCP,
 108                 .target     = tcpoptstrip_tg4,
 109                 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 110                 .me         = THIS_MODULE,
 111         },
 112 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
 113         {
 114                 .name       = "TCPOPTSTRIP",
 115                 .family     = NFPROTO_IPV6,
 116                 .table      = "mangle",
 117                 .proto      = IPPROTO_TCP,
 118                 .target     = tcpoptstrip_tg6,
 119                 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 120                 .me         = THIS_MODULE,
 121         },
 122 #endif
 123 };
 124
 125 static int __init tcpoptstrip_tg_init(void)
 126 {
 127         return xt_register_targets(tcpoptstrip_tg_reg,
 128                                    ARRAY_SIZE(tcpoptstrip_tg_reg));
 129 }
 130
 131 static void __exit tcpoptstrip_tg_exit(void)
 132 {
 133         xt_unregister_targets(tcpoptstrip_tg_reg,
 134                               ARRAY_SIZE(tcpoptstrip_tg_reg));
 135 }
 136
 137 module_init(tcpoptstrip_tg_init);
 138 module_exit(tcpoptstrip_tg_exit);
 139 MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
 140 MODULE_DESCRIPTION("Xtables: TCP option stripping");
 141 MODULE_LICENSE("GPL");
 142 MODULE_ALIAS("ipt_TCPOPTSTRIP");
 143 MODULE_ALIAS("ip6t_TCPOPTSTRIP");