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)
32 {
33 const struct xt_tcpoptstrip_target_info *info = par->targinfo;
34 struct tcphdr *tcph, _th;
35 unsigned int optl, i, j;
36 u_int16_t n, o;
37 u_int8_t *opt;
38 int tcp_hdrlen;
39
40 /* This is a fragment, no TCP header is available */
41 if (par->fragoff != 0)
42 return XT_CONTINUE;
43
44 tcph = skb_header_pointer(skb, tcphoff, sizeof(_th), &_th);
45 if (!tcph)
46 return NF_DROP;
47
48 tcp_hdrlen = tcph->doff * 4;
49 if (tcp_hdrlen < sizeof(struct tcphdr))
50 return NF_DROP;
51
52 if (skb_ensure_writable(skb, tcphoff + tcp_hdrlen))
53 return NF_DROP;
54
55 /* must reload tcph, might have been moved */
56 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
57 opt = (u8 *)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 }
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 }
110 #endif
111
112 static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
113 {
114 .name = "TCPOPTSTRIP",
115 .family = NFPROTO_IPV4,
116 .table = "mangle",
117 .proto = IPPROTO_TCP,
118 .target = tcpoptstrip_tg4,
119 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
120 .me = THIS_MODULE,
121 },
122 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
123 {
124 .name = "TCPOPTSTRIP",
125 .family = NFPROTO_IPV6,
126 .table = "mangle",
127 .proto = IPPROTO_TCP,
128 .target = tcpoptstrip_tg6,
129 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
130 .me = THIS_MODULE,
131 },
132 #endif
133 };
134
135 static int __init tcpoptstrip_tg_init(void)
136 {
137 return xt_register_targets(tcpoptstrip_tg_reg,
138 ARRAY_SIZE(tcpoptstrip_tg_reg));
139 }
140
141 static void __exit tcpoptstrip_tg_exit(void)
142 {
143 xt_unregister_targets(tcpoptstrip_tg_reg,
144 ARRAY_SIZE(tcpoptstrip_tg_reg));
145 }
146
147 module_init(tcpoptstrip_tg_init);
148 module_exit(tcpoptstrip_tg_exit);
149 MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
150 MODULE_DESCRIPTION("Xtables: TCP option stripping");
151 MODULE_LICENSE("GPL");
152 MODULE_ALIAS("ipt_TCPOPTSTRIP");
153 MODULE_ALIAS("ip6t_TCPOPTSTRIP");
154
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