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 
27 MODULE_LICENSE("GPL");
28 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
29 MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
30 MODULE_ALIAS("ipt_TCPMSS");
31 MODULE_ALIAS("ip6t_TCPMSS");
32 
33 static inline unsigned int
optlen(const u_int8_t * opt,unsigned int offset)34 optlen(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 
tcpmss_reverse_mtu(struct net * net,const struct sk_buff * skb,unsigned int family)43 static 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 
70 static int
tcpmss_mangle_packet(struct sk_buff * skb,const struct xt_action_param * par,unsigned int family,unsigned int tcphoff,unsigned int minlen)71 tcpmss_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 
195 static unsigned int
tcpmss_tg4(struct sk_buff * skb,const struct xt_action_param * par)196 tcpmss_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)
218 static unsigned int
tcpmss_tg6(struct sk_buff * skb,const struct xt_action_param * par)219 tcpmss_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, oldlen),
243 					     newlen);
244 		ipv6h->payload_len = newlen;
245 	}
246 	return XT_CONTINUE;
247 }
248 #endif
249 
250 /* Must specify -p tcp --syn */
find_syn_match(const struct xt_entry_match * m)251 static 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 
tcpmss_tg4_check(const struct xt_tgchk_param * par)263 static 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)
tcpmss_tg6_check(const struct xt_tgchk_param * par)287 static 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 
311 static 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 
tcpmss_tg_init(void)334 static int __init tcpmss_tg_init(void)
335 {
336 	return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
337 }
338 
tcpmss_tg_exit(void)339 static void __exit tcpmss_tg_exit(void)
340 {
341 	xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
342 }
343 
344 module_init(tcpmss_tg_init);
345 module_exit(tcpmss_tg_exit);
346