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
18MODULE_LICENSE("GPL");
19MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
20MODULE_DESCRIPTION("Xtables: TCP MSS match");
21MODULE_ALIAS("ipt_tcpmss");
22MODULE_ALIAS("ip6t_tcpmss");
23
24static bool
25tcpmss_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	}
69out:
70	return info->invert;
71
72dropit:
73	par->hotdrop = true;
74	return false;
75}
76
77static 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
96static int __init tcpmss_mt_init(void)
97{
98	return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
99}
100
101static void __exit tcpmss_mt_exit(void)
102{
103	xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
104}
105
106module_init(tcpmss_mt_init);
107module_exit(tcpmss_mt_exit);
108