net/netfilter/xt_TCPOPTSTRIP.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * A module for stripping a specific TCP option from TCP packets.
   4  *
   5  * Copyright (C) 2007 Sven Schnelle <svens@bitebene.org>
   6  * Copyright © CC Computer Consultants GmbH, 2007
   7  */
   8
   9 #include <linux/module.h>
  10 #include <linux/skbuff.h>
  11 #include <linux/ip.h>
  12 #include <linux/ipv6.h>
  13 #include <linux/tcp.h>
  14 #include <net/ipv6.h>
  15 #include <net/tcp.h>
  16 #include <linux/netfilter/x_tables.h>
  17 #include <linux/netfilter/xt_TCPOPTSTRIP.h>
  18
  19 static inline unsigned int optlen(const u_int8_t *opt, unsigned int offset)
  20 {
  21         /* Beware zero-length options: make finite progress */
  22         if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
  23                 return 1;
  24         else
  25                 return opt[offset+1];
  26 }
  27
  28 static unsigned int
  29 tcpoptstrip_mangle_packet(struct sk_buff *skb,
  30                           const struct xt_action_param *par,
  31                           unsigned int tcphoff, unsigned int minlen)
  32 {
  33         const struct xt_tcpoptstrip_target_info *info = par->targinfo;
  34         unsigned int optl, i, j;
  35         struct tcphdr *tcph;
  36         u_int16_t n, o;
  37         u_int8_t *opt;
  38         int len, tcp_hdrlen;
  39
  40         /* This is a fragment, no TCP header is available */
  41         if (par->fragoff != 0)
  42                 return XT_CONTINUE;
  43
  44         if (!skb_make_writable(skb, skb->len))
  45                 return NF_DROP;
  46
  47         len = skb->len - tcphoff;
  48         if (len < (int)sizeof(struct tcphdr))
  49                 return NF_DROP;
  50
  51         tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
  52         tcp_hdrlen = tcph->doff * 4;
  53
  54         if (len < tcp_hdrlen)
  55                 return NF_DROP;
  56
  57         opt  = (u_int8_t *)tcph;
  58
  59         /*
  60          * Walk through all TCP options - if we find some option to remove,
  61          * set all octets to %TCPOPT_NOP and adjust checksum.
  62          */
  63         for (i = sizeof(struct tcphdr); i < tcp_hdrlen - 1; i += optl) {
  64                 optl = optlen(opt, i);
  65
  66                 if (i + optl > tcp_hdrlen)
  67                         break;
  68
  69                 if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
  70                         continue;
  71
  72                 for (j = 0; j < optl; ++j) {
  73                         o = opt[i+j];
  74                         n = TCPOPT_NOP;
  75                         if ((i + j) % 2 == 0) {
  76                                 o <<= 8;
  77                                 n <<= 8;
  78                         }
  79                         inet_proto_csum_replace2(&tcph->check, skb, htons(o),
  80                                                  htons(n), false);
  81                 }
  82                 memset(opt + i, TCPOPT_NOP, optl);
  83         }
  84
  85         return XT_CONTINUE;
  86 }
  87
  88 static unsigned int
  89 tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_action_param *par)
  90 {
  91         return tcpoptstrip_mangle_packet(skb, par, ip_hdrlen(skb),
  92                sizeof(struct iphdr) + sizeof(struct tcphdr));
  93 }
  94
  95 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
  96 static unsigned int
  97 tcpoptstrip_tg6(struct sk_buff *skb, const struct xt_action_param *par)
  98 {
  99         struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 100         int tcphoff;
 101         u_int8_t nexthdr;
 102         __be16 frag_off;
 103
 104         nexthdr = ipv6h->nexthdr;
 105         tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
 106         if (tcphoff < 0)
 107                 return NF_DROP;
 108
 109         return tcpoptstrip_mangle_packet(skb, par, tcphoff,
 110                sizeof(*ipv6h) + sizeof(struct tcphdr));
 111 }
 112 #endif
 113
 114 static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
 115         {
 116                 .name       = "TCPOPTSTRIP",
 117                 .family     = NFPROTO_IPV4,
 118                 .table      = "mangle",
 119                 .proto      = IPPROTO_TCP,
 120                 .target     = tcpoptstrip_tg4,
 121                 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 122                 .me         = THIS_MODULE,
 123         },
 124 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
 125         {
 126                 .name       = "TCPOPTSTRIP",
 127                 .family     = NFPROTO_IPV6,
 128                 .table      = "mangle",
 129                 .proto      = IPPROTO_TCP,
 130                 .target     = tcpoptstrip_tg6,
 131                 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 132                 .me         = THIS_MODULE,
 133         },
 134 #endif
 135 };
 136
 137 static int __init tcpoptstrip_tg_init(void)
 138 {
 139         return xt_register_targets(tcpoptstrip_tg_reg,
 140                                    ARRAY_SIZE(tcpoptstrip_tg_reg));
 141 }
 142
 143 static void __exit tcpoptstrip_tg_exit(void)
 144 {
 145         xt_unregister_targets(tcpoptstrip_tg_reg,
 146                               ARRAY_SIZE(tcpoptstrip_tg_reg));
 147 }
 148
 149 module_init(tcpoptstrip_tg_init);
 150 module_exit(tcpoptstrip_tg_exit);
 151 MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
 152 MODULE_DESCRIPTION("Xtables: TCP option stripping");
 153 MODULE_LICENSE("GPL");
 154 MODULE_ALIAS("ipt_TCPOPTSTRIP");
 155 MODULE_ALIAS("ip6t_TCPOPTSTRIP");