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11#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
12#include <linux/module.h>
13#include <linux/skbuff.h>
14#include <linux/ip.h>
15#include <linux/gfp.h>
16#include <linux/ipv6.h>
17#include <linux/tcp.h>
18#include <net/dst.h>
19#include <net/flow.h>
20#include <net/ipv6.h>
21#include <net/route.h>
22#include <net/tcp.h>
23
24#include <linux/netfilter_ipv4/ip_tables.h>
25#include <linux/netfilter_ipv6/ip6_tables.h>
26#include <linux/netfilter/x_tables.h>
27#include <linux/netfilter/xt_tcpudp.h>
28#include <linux/netfilter/xt_TCPMSS.h>
29
30MODULE_LICENSE("GPL");
31MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
32MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
33MODULE_ALIAS("ipt_TCPMSS");
34MODULE_ALIAS("ip6t_TCPMSS");
35
36static inline unsigned int
37optlen(const u_int8_t *opt, unsigned int offset)
38{
39
40 if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
41 return 1;
42 else
43 return opt[offset+1];
44}
45
46static int
47tcpmss_mangle_packet(struct sk_buff *skb,
48 const struct xt_action_param *par,
49 unsigned int in_mtu,
50 unsigned int tcphoff,
51 unsigned int minlen)
52{
53 const struct xt_tcpmss_info *info = par->targinfo;
54 struct tcphdr *tcph;
55 unsigned int tcplen, i;
56 __be16 oldval;
57 u16 newmss;
58 u8 *opt;
59
60
61 if (par->fragoff != 0)
62 return XT_CONTINUE;
63
64 if (!skb_make_writable(skb, skb->len))
65 return -1;
66
67 tcplen = skb->len - tcphoff;
68 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
69
70
71 if (tcplen < tcph->doff*4)
72 return -1;
73
74 if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
75 if (dst_mtu(skb_dst(skb)) <= minlen) {
76 net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
77 dst_mtu(skb_dst(skb)));
78 return -1;
79 }
80 if (in_mtu <= minlen) {
81 net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
82 in_mtu);
83 return -1;
84 }
85 newmss = min(dst_mtu(skb_dst(skb)), in_mtu) - minlen;
86 } else
87 newmss = info->mss;
88
89 opt = (u_int8_t *)tcph;
90 for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
91 if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
92 opt[i+1] == TCPOLEN_MSS) {
93 u_int16_t oldmss;
94
95 oldmss = (opt[i+2] << 8) | opt[i+3];
96
97
98
99
100
101 if (oldmss <= newmss)
102 return 0;
103
104 opt[i+2] = (newmss & 0xff00) >> 8;
105 opt[i+3] = newmss & 0x00ff;
106
107 inet_proto_csum_replace2(&tcph->check, skb,
108 htons(oldmss), htons(newmss),
109 0);
110 return 0;
111 }
112 }
113
114
115
116
117 if (tcplen > tcph->doff*4)
118 return 0;
119
120
121
122
123 if (skb_tailroom(skb) < TCPOLEN_MSS) {
124 if (pskb_expand_head(skb, 0,
125 TCPOLEN_MSS - skb_tailroom(skb),
126 GFP_ATOMIC))
127 return -1;
128 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
129 }
130
131 skb_put(skb, TCPOLEN_MSS);
132
133
134
135
136
137
138
139
140 if (par->family == NFPROTO_IPV4)
141 newmss = min(newmss, (u16)536);
142 else
143 newmss = min(newmss, (u16)1220);
144
145 opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
146 memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
147
148 inet_proto_csum_replace2(&tcph->check, skb,
149 htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1);
150 opt[0] = TCPOPT_MSS;
151 opt[1] = TCPOLEN_MSS;
152 opt[2] = (newmss & 0xff00) >> 8;
153 opt[3] = newmss & 0x00ff;
154
155 inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0);
156
157 oldval = ((__be16 *)tcph)[6];
158 tcph->doff += TCPOLEN_MSS/4;
159 inet_proto_csum_replace2(&tcph->check, skb,
160 oldval, ((__be16 *)tcph)[6], 0);
161 return TCPOLEN_MSS;
162}
163
164static u_int32_t tcpmss_reverse_mtu(const struct sk_buff *skb,
165 unsigned int family)
166{
167 struct flowi fl;
168 const struct nf_afinfo *ai;
169 struct rtable *rt = NULL;
170 u_int32_t mtu = ~0U;
171
172 if (family == PF_INET) {
173 struct flowi4 *fl4 = &fl.u.ip4;
174 memset(fl4, 0, sizeof(*fl4));
175 fl4->daddr = ip_hdr(skb)->saddr;
176 } else {
177 struct flowi6 *fl6 = &fl.u.ip6;
178
179 memset(fl6, 0, sizeof(*fl6));
180 fl6->daddr = ipv6_hdr(skb)->saddr;
181 }
182 rcu_read_lock();
183 ai = nf_get_afinfo(family);
184 if (ai != NULL)
185 ai->route(&init_net, (struct dst_entry **)&rt, &fl, false);
186 rcu_read_unlock();
187
188 if (rt != NULL) {
189 mtu = dst_mtu(&rt->dst);
190 dst_release(&rt->dst);
191 }
192 return mtu;
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 tcpmss_reverse_mtu(skb, 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;
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 tcpmss_reverse_mtu(skb, 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 ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
240 }
241 return XT_CONTINUE;
242}
243#endif
244
245
246static inline bool find_syn_match(const struct xt_entry_match *m)
247{
248 const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
249
250 if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
251 tcpinfo->flg_cmp & TCPHDR_SYN &&
252 !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
253 return true;
254
255 return false;
256}
257
258static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
259{
260 const struct xt_tcpmss_info *info = par->targinfo;
261 const struct ipt_entry *e = par->entryinfo;
262 const struct xt_entry_match *ematch;
263
264 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
265 (par->hook_mask & ~((1 << NF_INET_FORWARD) |
266 (1 << NF_INET_LOCAL_OUT) |
267 (1 << NF_INET_POST_ROUTING))) != 0) {
268 pr_info("path-MTU clamping only supported in "
269 "FORWARD, OUTPUT and POSTROUTING hooks\n");
270 return -EINVAL;
271 }
272 xt_ematch_foreach(ematch, e)
273 if (find_syn_match(ematch))
274 return 0;
275 pr_info("Only works on TCP SYN packets\n");
276 return -EINVAL;
277}
278
279#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
280static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
281{
282 const struct xt_tcpmss_info *info = par->targinfo;
283 const struct ip6t_entry *e = par->entryinfo;
284 const struct xt_entry_match *ematch;
285
286 if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
287 (par->hook_mask & ~((1 << NF_INET_FORWARD) |
288 (1 << NF_INET_LOCAL_OUT) |
289 (1 << NF_INET_POST_ROUTING))) != 0) {
290 pr_info("path-MTU clamping only supported in "
291 "FORWARD, OUTPUT and POSTROUTING hooks\n");
292 return -EINVAL;
293 }
294 xt_ematch_foreach(ematch, e)
295 if (find_syn_match(ematch))
296 return 0;
297 pr_info("Only works on TCP SYN packets\n");
298 return -EINVAL;
299}
300#endif
301
302static struct xt_target tcpmss_tg_reg[] __read_mostly = {
303 {
304 .family = NFPROTO_IPV4,
305 .name = "TCPMSS",
306 .checkentry = tcpmss_tg4_check,
307 .target = tcpmss_tg4,
308 .targetsize = sizeof(struct xt_tcpmss_info),
309 .proto = IPPROTO_TCP,
310 .me = THIS_MODULE,
311 },
312#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
313 {
314 .family = NFPROTO_IPV6,
315 .name = "TCPMSS",
316 .checkentry = tcpmss_tg6_check,
317 .target = tcpmss_tg6,
318 .targetsize = sizeof(struct xt_tcpmss_info),
319 .proto = IPPROTO_TCP,
320 .me = THIS_MODULE,
321 },
322#endif
323};
324
325static int __init tcpmss_tg_init(void)
326{
327 return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
328}
329
330static void __exit tcpmss_tg_exit(void)
331{
332 xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
333}
334
335module_init(tcpmss_tg_init);
336module_exit(tcpmss_tg_exit);
337
