1// SPDX-License-Identifier: GPL-2.0-only
2/*
3 * This is a module which is used for setting the MSS option in TCP packets.
4 *
5 * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
6 * Copyright (C) 2007 Patrick McHardy <kaber@trash.net>
7 */
8#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
9#include <linux/module.h>
10#include <linux/skbuff.h>
11#include <linux/ip.h>
12#include <linux/gfp.h>
13#include <linux/ipv6.h>
14#include <linux/tcp.h>
15#include <net/dst.h>
16#include <net/flow.h>
17#include <net/ipv6.h>
18#include <net/route.h>
19#include <net/tcp.h>
20
21#include <linux/netfilter_ipv4/ip_tables.h>
22#include <linux/netfilter_ipv6/ip6_tables.h>
23#include <linux/netfilter/x_tables.h>
24#include <linux/netfilter/xt_tcpudp.h>
25#include <linux/netfilter/xt_TCPMSS.h>
26
27MODULE_LICENSE("GPL");
28MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
29MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
30MODULE_ALIAS("ipt_TCPMSS");
31MODULE_ALIAS("ip6t_TCPMSS");
32
33static inline unsigned int
34optlen(const u_int8_t *opt, unsigned int offset)
35{
36	/* Beware zero-length options: make finite progress */
37	if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
38		return 1;
39	else
40		return opt[offset+1];
41}
42
43static u_int32_t tcpmss_reverse_mtu(struct net *net,
44				    const struct sk_buff *skb,
45				    unsigned int family)
46{
47	struct flowi fl;
48	struct rtable *rt = NULL;
49	u_int32_t mtu     = ~0U;
50
51	if (family == PF_INET) {
52		struct flowi4 *fl4 = &fl.u.ip4;
53		memset(fl4, 0, sizeof(*fl4));
54		fl4->daddr = ip_hdr(skb)->saddr;
55	} else {
56		struct flowi6 *fl6 = &fl.u.ip6;
57
58		memset(fl6, 0, sizeof(*fl6));
59		fl6->daddr = ipv6_hdr(skb)->saddr;
60	}
61
62	nf_route(net, (struct dst_entry **)&rt, &fl, false, family);
63	if (rt != NULL) {
64		mtu = dst_mtu(&rt->dst);
65		dst_release(&rt->dst);
66	}
67	return mtu;
68}
69
70static int
71tcpmss_mangle_packet(struct sk_buff *skb,
72		     const struct xt_action_param *par,
73		     unsigned int family,
74		     unsigned int tcphoff,
75		     unsigned int minlen)
76{
77	const struct xt_tcpmss_info *info = par->targinfo;
78	struct tcphdr *tcph;
79	int len, tcp_hdrlen;
80	unsigned int i;
81	__be16 oldval;
82	u16 newmss;
83	u8 *opt;
84
85	/* This is a fragment, no TCP header is available */
86	if (par->fragoff != 0)
87		return 0;
88
89	if (skb_ensure_writable(skb, skb->len))
90		return -1;
91
92	len = skb->len - tcphoff;
93	if (len < (int)sizeof(struct tcphdr))
94		return -1;
95
96	tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
97	tcp_hdrlen = tcph->doff * 4;
98
99	if (len < tcp_hdrlen || tcp_hdrlen < sizeof(struct tcphdr))
100		return -1;
101
102	if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
103		struct net *net = xt_net(par);
104		unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family);
105		unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu);
106
107		if (min_mtu <= minlen) {
108			net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
109					    min_mtu);
110			return -1;
111		}
112		newmss = min_mtu - minlen;
113	} else
114		newmss = info->mss;
115
116	opt = (u_int8_t *)tcph;
117	for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
118		if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
119			u_int16_t oldmss;
120
121			oldmss = (opt[i+2] << 8) | opt[i+3];
122
123			/* Never increase MSS, even when setting it, as
124			 * doing so results in problems for hosts that rely
125			 * on MSS being set correctly.
126			 */
127			if (oldmss <= newmss)
128				return 0;
129
130			opt[i+2] = (newmss & 0xff00) >> 8;
131			opt[i+3] = newmss & 0x00ff;
132
133			inet_proto_csum_replace2(&tcph->check, skb,
134						 htons(oldmss), htons(newmss),
135						 false);
136			return 0;
137		}
138	}
139
140	/* There is data after the header so the option can't be added
141	 * without moving it, and doing so may make the SYN packet
142	 * itself too large. Accept the packet unmodified instead.
143	 */
144	if (len > tcp_hdrlen)
145		return 0;
146
147	/* tcph->doff has 4 bits, do not wrap it to 0 */
148	if (tcp_hdrlen >= 15 * 4)
149		return 0;
150
151	/*
152	 * MSS Option not found ?! add it..
153	 */
154	if (skb_tailroom(skb) < TCPOLEN_MSS) {
155		if (pskb_expand_head(skb, 0,
156				     TCPOLEN_MSS - skb_tailroom(skb),
157				     GFP_ATOMIC))
158			return -1;
159		tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
160	}
161
162	skb_put(skb, TCPOLEN_MSS);
163
164	/*
165	 * IPv4: RFC 1122 states "If an MSS option is not received at
166	 * connection setup, TCP MUST assume a default send MSS of 536".
167	 * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
168	 * length IPv6 header of 60, ergo the default MSS value is 1220
169	 * Since no MSS was provided, we must use the default values
170	 */
171	if (xt_family(par) == NFPROTO_IPV4)
172		newmss = min(newmss, (u16)536);
173	else
174		newmss = min(newmss, (u16)1220);
175
176	opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
177	memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));
178
179	inet_proto_csum_replace2(&tcph->check, skb,
180				 htons(len), htons(len + TCPOLEN_MSS), true);
181	opt[0] = TCPOPT_MSS;
182	opt[1] = TCPOLEN_MSS;
183	opt[2] = (newmss & 0xff00) >> 8;
184	opt[3] = newmss & 0x00ff;
185
186	inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), false);
187
188	oldval = ((__be16 *)tcph)[6];
189	tcph->doff += TCPOLEN_MSS/4;
190	inet_proto_csum_replace2(&tcph->check, skb,
191				 oldval, ((__be16 *)tcph)[6], false);
192	return TCPOLEN_MSS;
193}
194
195static unsigned int
196tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
197{
198	struct iphdr *iph = ip_hdr(skb);
199	__be16 newlen;
200	int ret;
201
202	ret = tcpmss_mangle_packet(skb, par,
203				   PF_INET,
204				   iph->ihl * 4,
205				   sizeof(*iph) + sizeof(struct tcphdr));
206	if (ret < 0)
207		return NF_DROP;
208	if (ret > 0) {
209		iph = ip_hdr(skb);
210		newlen = htons(ntohs(iph->tot_len) + ret);
211		csum_replace2(&iph->check, iph->tot_len, newlen);
212		iph->tot_len = newlen;
213	}
214	return XT_CONTINUE;
215}
216
217#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
218static unsigned int
219tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
220{
221	struct ipv6hdr *ipv6h = ipv6_hdr(skb);
222	u8 nexthdr;
223	__be16 frag_off, oldlen, newlen;
224	int tcphoff;
225	int ret;
226
227	nexthdr = ipv6h->nexthdr;
228	tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
229	if (tcphoff < 0)
230		return NF_DROP;
231	ret = tcpmss_mangle_packet(skb, par,
232				   PF_INET6,
233				   tcphoff,
234				   sizeof(*ipv6h) + sizeof(struct tcphdr));
235	if (ret < 0)
236		return NF_DROP;
237	if (ret > 0) {
238		ipv6h = ipv6_hdr(skb);
239		oldlen = ipv6h->payload_len;
240		newlen = htons(ntohs(oldlen) + ret);
241		if (skb->ip_summed == CHECKSUM_COMPLETE)
242			skb->csum = csum_add(csum_sub(skb->csum, (__force __wsum)oldlen),
243					     (__force __wsum)newlen);
244		ipv6h->payload_len = newlen;
245	}
246	return XT_CONTINUE;
247}
248#endif
249
250/* Must specify -p tcp --syn */
251static inline bool find_syn_match(const struct xt_entry_match *m)
252{
253	const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
254
255	if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
256	    tcpinfo->flg_cmp & TCPHDR_SYN &&
257	    !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
258		return true;
259
260	return false;
261}
262
263static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
264{
265	const struct xt_tcpmss_info *info = par->targinfo;
266	const struct ipt_entry *e = par->entryinfo;
267	const struct xt_entry_match *ematch;
268
269	if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
270	    (par->hook_mask & ~((1 << NF_INET_FORWARD) |
271			   (1 << NF_INET_LOCAL_OUT) |
272			   (1 << NF_INET_POST_ROUTING))) != 0) {
273		pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
274		return -EINVAL;
275	}
276	if (par->nft_compat)
277		return 0;
278
279	xt_ematch_foreach(ematch, e)
280		if (find_syn_match(ematch))
281			return 0;
282	pr_info_ratelimited("Only works on TCP SYN packets\n");
283	return -EINVAL;
284}
285
286#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
287static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
288{
289	const struct xt_tcpmss_info *info = par->targinfo;
290	const struct ip6t_entry *e = par->entryinfo;
291	const struct xt_entry_match *ematch;
292
293	if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
294	    (par->hook_mask & ~((1 << NF_INET_FORWARD) |
295			   (1 << NF_INET_LOCAL_OUT) |
296			   (1 << NF_INET_POST_ROUTING))) != 0) {
297		pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
298		return -EINVAL;
299	}
300	if (par->nft_compat)
301		return 0;
302
303	xt_ematch_foreach(ematch, e)
304		if (find_syn_match(ematch))
305			return 0;
306	pr_info_ratelimited("Only works on TCP SYN packets\n");
307	return -EINVAL;
308}
309#endif
310
311static struct xt_target tcpmss_tg_reg[] __read_mostly = {
312	{
313		.family		= NFPROTO_IPV4,
314		.name		= "TCPMSS",
315		.checkentry	= tcpmss_tg4_check,
316		.target		= tcpmss_tg4,
317		.targetsize	= sizeof(struct xt_tcpmss_info),
318		.proto		= IPPROTO_TCP,
319		.me		= THIS_MODULE,
320	},
321#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
322	{
323		.family		= NFPROTO_IPV6,
324		.name		= "TCPMSS",
325		.checkentry	= tcpmss_tg6_check,
326		.target		= tcpmss_tg6,
327		.targetsize	= sizeof(struct xt_tcpmss_info),
328		.proto		= IPPROTO_TCP,
329		.me		= THIS_MODULE,
330	},
331#endif
332};
333
334static int __init tcpmss_tg_init(void)
335{
336	return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
337}
338
339static void __exit tcpmss_tg_exit(void)
340{
341	xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
342}
343
344module_init(tcpmss_tg_init);
345module_exit(tcpmss_tg_exit);
346