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             tcp_hdr(skb)->doff * 4 > len)
  53                 return NF_DROP;
  54
  55         tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
  56         opt  = (u_int8_t *)tcph;
  57
  58         /*
  59          * Walk through all TCP options - if we find some option to remove,
  60          * set all octets to %TCPOPT_NOP and adjust checksum.
  61          */
  62         for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) {
  63                 optl = optlen(opt, i);
  64
  65                 if (i + optl > tcp_hdrlen(skb))
  66                         break;
  67
  68                 if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
  69                         continue;
  70
  71                 for (j = 0; j < optl; ++j) {
  72                         o = opt[i+j];
  73                         n = TCPOPT_NOP;
  74                         if ((i + j) % 2 == 0) {
  75                                 o <<= 8;
  76                                 n <<= 8;
  77                         }
  78                         inet_proto_csum_replace2(&tcph->check, skb, htons(o),
  79                                                  htons(n), 0);
  80                 }
  81                 memset(opt + i, TCPOPT_NOP, optl);
  82         }
  83
  84         return XT_CONTINUE;
  85 }
  86
  87 static unsigned int
  88 tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_action_param *par)
  89 {
  90         return tcpoptstrip_mangle_packet(skb, par, ip_hdrlen(skb),
  91                sizeof(struct iphdr) + sizeof(struct tcphdr));
  92 }
  93
  94 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
  95 static unsigned int
  96 tcpoptstrip_tg6(struct sk_buff *skb, const struct xt_action_param *par)
  97 {
  98         struct ipv6hdr *ipv6h = ipv6_hdr(skb);
  99         int tcphoff;
 100         u_int8_t nexthdr;
 101         __be16 frag_off;
 102
 103         nexthdr = ipv6h->nexthdr;
 104         tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
 105         if (tcphoff < 0)
 106                 return NF_DROP;
 107
 108         return tcpoptstrip_mangle_packet(skb, par, tcphoff,
 109                sizeof(*ipv6h) + sizeof(struct tcphdr));
 110 }
 111 #endif
 112
 113 static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
 114         {
 115                 .name       = "TCPOPTSTRIP",
 116                 .family     = NFPROTO_IPV4,
 117                 .table      = "mangle",
 118                 .proto      = IPPROTO_TCP,
 119                 .target     = tcpoptstrip_tg4,
 120                 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 121                 .me         = THIS_MODULE,
 122         },
 123 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
 124         {
 125                 .name       = "TCPOPTSTRIP",
 126                 .family     = NFPROTO_IPV6,
 127                 .table      = "mangle",
 128                 .proto      = IPPROTO_TCP,
 129                 .target     = tcpoptstrip_tg6,
 130                 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 131                 .me         = THIS_MODULE,
 132         },
 133 #endif
 134 };
 135
 136 static int __init tcpoptstrip_tg_init(void)
 137 {
 138         return xt_register_targets(tcpoptstrip_tg_reg,
 139                                    ARRAY_SIZE(tcpoptstrip_tg_reg));
 140 }
 141
 142 static void __exit tcpoptstrip_tg_exit(void)
 143 {
 144         xt_unregister_targets(tcpoptstrip_tg_reg,
 145                               ARRAY_SIZE(tcpoptstrip_tg_reg));
 146 }
 147
 148 module_init(tcpoptstrip_tg_init);
 149 module_exit(tcpoptstrip_tg_exit);
 150 MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
 151 MODULE_DESCRIPTION("Xtables: TCP option stripping");
 152 MODULE_LICENSE("GPL");
 153 MODULE_ALIAS("ipt_TCPOPTSTRIP");
 154 MODULE_ALIAS("ip6t_TCPOPTSTRIP");