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_action_param *par,
  34                           unsigned int tcphoff, unsigned int minlen)
  35 {
  36         const struct xt_tcpoptstrip_target_info *info = par->targinfo;
  37         unsigned int optl, i, j;
  38         struct tcphdr *tcph;
  39         u_int16_t n, o;
  40         u_int8_t *opt;
  41         int len;
  42
  43         /* This is a fragment, no TCP header is available */
  44         if (par->fragoff != 0)
  45                 return XT_CONTINUE;
  46
  47         if (!skb_make_writable(skb, skb->len))
  48                 return NF_DROP;
  49
  50         len = skb->len - tcphoff;
  51         if (len < (int)sizeof(struct tcphdr))
  52                 return NF_DROP;
  53
  54         tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
  55         if (tcph->doff * 4 > len)
  56                 return NF_DROP;
  57
  58         opt  = (u_int8_t *)tcph;
  59
  60         /*
  61          * Walk through all TCP options - if we find some option to remove,
  62          * set all octets to %TCPOPT_NOP and adjust checksum.
  63          */
  64         for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) {
  65                 optl = optlen(opt, i);
  66
  67                 if (i + optl > tcp_hdrlen(skb))
  68                         break;
  69
  70                 if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
  71                         continue;
  72
  73                 for (j = 0; j < optl; ++j) {
  74                         o = opt[i+j];
  75                         n = TCPOPT_NOP;
  76                         if ((i + j) % 2 == 0) {
  77                                 o <<= 8;
  78                                 n <<= 8;
  79                         }
  80                         inet_proto_csum_replace2(&tcph->check, skb, htons(o),
  81                                                  htons(n), 0);
  82                 }
  83                 memset(opt + i, TCPOPT_NOP, optl);
  84         }
  85
  86         return XT_CONTINUE;
  87 }
  88
  89 static unsigned int
  90 tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_action_param *par)
  91 {
  92         return tcpoptstrip_mangle_packet(skb, par, ip_hdrlen(skb),
  93                sizeof(struct iphdr) + sizeof(struct tcphdr));
  94 }
  95
  96 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
  97 static unsigned int
  98 tcpoptstrip_tg6(struct sk_buff *skb, const struct xt_action_param *par)
  99 {
 100         struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 101         int tcphoff;
 102         u_int8_t nexthdr;
 103         __be16 frag_off;
 104
 105         nexthdr = ipv6h->nexthdr;
 106         tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
 107         if (tcphoff < 0)
 108                 return NF_DROP;
 109
 110         return tcpoptstrip_mangle_packet(skb, par, tcphoff,
 111                sizeof(*ipv6h) + sizeof(struct tcphdr));
 112 }
 113 #endif
 114
 115 static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
 116         {
 117                 .name       = "TCPOPTSTRIP",
 118                 .family     = NFPROTO_IPV4,
 119                 .table      = "mangle",
 120                 .proto      = IPPROTO_TCP,
 121                 .target     = tcpoptstrip_tg4,
 122                 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 123                 .me         = THIS_MODULE,
 124         },
 125 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
 126         {
 127                 .name       = "TCPOPTSTRIP",
 128                 .family     = NFPROTO_IPV6,
 129                 .table      = "mangle",
 130                 .proto      = IPPROTO_TCP,
 131                 .target     = tcpoptstrip_tg6,
 132                 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 133                 .me         = THIS_MODULE,
 134         },
 135 #endif
 136 };
 137
 138 static int __init tcpoptstrip_tg_init(void)
 139 {
 140         return xt_register_targets(tcpoptstrip_tg_reg,
 141                                    ARRAY_SIZE(tcpoptstrip_tg_reg));
 142 }
 143
 144 static void __exit tcpoptstrip_tg_exit(void)
 145 {
 146         xt_unregister_targets(tcpoptstrip_tg_reg,
 147                               ARRAY_SIZE(tcpoptstrip_tg_reg));
 148 }
 149
 150 module_init(tcpoptstrip_tg_init);
 151 module_exit(tcpoptstrip_tg_exit);
 152 MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
 153 MODULE_DESCRIPTION("Xtables: TCP option stripping");
 154 MODULE_LICENSE("GPL");
 155 MODULE_ALIAS("ipt_TCPOPTSTRIP");
 156 MODULE_ALIAS("ip6t_TCPOPTSTRIP");