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, dst: (struct dst_entry **)&rt, fl: &fl, strict: false, family);
63 if (rt != NULL) {
64 mtu = dst_mtu(dst: &rt->dst);
65 dst_release(dst: &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, write_len: 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, offset: 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(sum: &tcph->check, skb,
134 htons(oldmss), htons(newmss),
135 pseudohdr: 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, nhead: 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(sum: &tcph->check, skb,
180 htons(len), htons(len + TCPOLEN_MSS), pseudohdr: 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(sum: &tcph->check, skb, from: 0, to: *((__be32 *)opt), pseudohdr: false);
187
188 oldval = ((__be16 *)tcph)[6];
189 tcph->doff += TCPOLEN_MSS/4;
190 inet_proto_csum_replace2(sum: &tcph->check, skb,
191 from: oldval, to: ((__be16 *)tcph)[6], pseudohdr: 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 tcphoff: iph->ihl * 4,
205 minlen: 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(sum: &iph->check, old: iph->tot_len, new: 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, start: sizeof(*ipv6h), nexthdrp: &nexthdr, frag_offp: &frag_off);
229 if (tcphoff < 0)
230 return NF_DROP;
231 ret = tcpmss_mangle_packet(skb, par,
232 PF_INET6,
233 tcphoff,
234 minlen: 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: csum_sub(csum: skb->csum, addend: (__force __wsum)oldlen),
243 addend: (__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(m: 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(m: 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(target: tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
337}
338
339static void __exit tcpmss_tg_exit(void)
340{
341 xt_unregister_targets(target: tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
342}
343
344module_init(tcpmss_tg_init);
345module_exit(tcpmss_tg_exit);
346

source code of linux/net/netfilter/xt_TCPMSS.c