1 // SPDX-License-Identifier: GPL-2.0-only
2 /* Kernel module to match TCP MSS values. */
3 
4 /* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
5  * Portions (C) 2005 by Harald Welte <laforge@netfilter.org>
6  */
7 
8 #include <linux/module.h>
9 #include <linux/skbuff.h>
10 #include <net/tcp.h>
11 
12 #include <linux/netfilter/xt_tcpmss.h>
13 #include <linux/netfilter/x_tables.h>
14 
15 #include <linux/netfilter_ipv4/ip_tables.h>
16 #include <linux/netfilter_ipv6/ip6_tables.h>
17 
18 MODULE_LICENSE("GPL");
19 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
20 MODULE_DESCRIPTION("Xtables: TCP MSS match");
21 MODULE_ALIAS("ipt_tcpmss");
22 MODULE_ALIAS("ip6t_tcpmss");
23 
24 static bool
tcpmss_mt(const struct sk_buff * skb,struct xt_action_param * par)25 tcpmss_mt(const struct sk_buff *skb, struct xt_action_param *par)
26 {
27 	const struct xt_tcpmss_match_info *info = par->matchinfo;
28 	const struct tcphdr *th;
29 	struct tcphdr _tcph;
30 	/* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
31 	const u_int8_t *op;
32 	u8 _opt[15 * 4 - sizeof(_tcph)];
33 	unsigned int i, optlen;
34 
35 	/* If we don't have the whole header, drop packet. */
36 	th = skb_header_pointer(skb, par->thoff, sizeof(_tcph), &_tcph);
37 	if (th == NULL)
38 		goto dropit;
39 
40 	/* Malformed. */
41 	if (th->doff*4 < sizeof(*th))
42 		goto dropit;
43 
44 	optlen = th->doff*4 - sizeof(*th);
45 	if (!optlen)
46 		goto out;
47 
48 	/* Truncated options. */
49 	op = skb_header_pointer(skb, par->thoff + sizeof(*th), optlen, _opt);
50 	if (op == NULL)
51 		goto dropit;
52 
53 	for (i = 0; i < optlen; ) {
54 		if (op[i] == TCPOPT_MSS
55 		    && (optlen - i) >= TCPOLEN_MSS
56 		    && op[i+1] == TCPOLEN_MSS) {
57 			u_int16_t mssval;
58 
59 			mssval = (op[i+2] << 8) | op[i+3];
60 
61 			return (mssval >= info->mss_min &&
62 				mssval <= info->mss_max) ^ info->invert;
63 		}
64 		if (op[i] < 2)
65 			i++;
66 		else
67 			i += op[i+1] ? : 1;
68 	}
69 out:
70 	return info->invert;
71 
72 dropit:
73 	par->hotdrop = true;
74 	return false;
75 }
76 
77 static struct xt_match tcpmss_mt_reg[] __read_mostly = {
78 	{
79 		.name		= "tcpmss",
80 		.family		= NFPROTO_IPV4,
81 		.match		= tcpmss_mt,
82 		.matchsize	= sizeof(struct xt_tcpmss_match_info),
83 		.proto		= IPPROTO_TCP,
84 		.me		= THIS_MODULE,
85 	},
86 	{
87 		.name		= "tcpmss",
88 		.family		= NFPROTO_IPV6,
89 		.match		= tcpmss_mt,
90 		.matchsize	= sizeof(struct xt_tcpmss_match_info),
91 		.proto		= IPPROTO_TCP,
92 		.me		= THIS_MODULE,
93 	},
94 };
95 
tcpmss_mt_init(void)96 static int __init tcpmss_mt_init(void)
97 {
98 	return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
99 }
100 
tcpmss_mt_exit(void)101 static void __exit tcpmss_mt_exit(void)
102 {
103 	xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
104 }
105 
106 module_init(tcpmss_mt_init);
107 module_exit(tcpmss_mt_exit);
108