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Linux/net/netfilter/xt_TCPMSS.c

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  1 /*
  2  * This is a module which is used for setting the MSS option in TCP packets.
  3  *
  4  * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
  5  * Copyright (C) 2007 Patrick McHardy <kaber@trash.net>
  6  *
  7  * This program is free software; you can redistribute it and/or modify
  8  * it under the terms of the GNU General Public License version 2 as
  9  * published by the Free Software Foundation.
 10  */
 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 
 30 MODULE_LICENSE("GPL");
 31 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
 32 MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
 33 MODULE_ALIAS("ipt_TCPMSS");
 34 MODULE_ALIAS("ip6t_TCPMSS");
 35 
 36 static inline unsigned int
 37 optlen(const u_int8_t *opt, unsigned int offset)
 38 {
 39         /* Beware zero-length options: make finite progress */
 40         if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
 41                 return 1;
 42         else
 43                 return opt[offset+1];
 44 }
 45 
 46 static u_int32_t tcpmss_reverse_mtu(struct net *net,
 47                                     const struct sk_buff *skb,
 48                                     unsigned int family)
 49 {
 50         struct flowi fl;
 51         struct rtable *rt = NULL;
 52         u_int32_t mtu     = ~0U;
 53 
 54         if (family == PF_INET) {
 55                 struct flowi4 *fl4 = &fl.u.ip4;
 56                 memset(fl4, 0, sizeof(*fl4));
 57                 fl4->daddr = ip_hdr(skb)->saddr;
 58         } else {
 59                 struct flowi6 *fl6 = &fl.u.ip6;
 60 
 61                 memset(fl6, 0, sizeof(*fl6));
 62                 fl6->daddr = ipv6_hdr(skb)->saddr;
 63         }
 64 
 65         nf_route(net, (struct dst_entry **)&rt, &fl, false, family);
 66         if (rt != NULL) {
 67                 mtu = dst_mtu(&rt->dst);
 68                 dst_release(&rt->dst);
 69         }
 70         return mtu;
 71 }
 72 
 73 static int
 74 tcpmss_mangle_packet(struct sk_buff *skb,
 75                      const struct xt_action_param *par,
 76                      unsigned int family,
 77                      unsigned int tcphoff,
 78                      unsigned int minlen)
 79 {
 80         const struct xt_tcpmss_info *info = par->targinfo;
 81         struct tcphdr *tcph;
 82         int len, tcp_hdrlen;
 83         unsigned int i;
 84         __be16 oldval;
 85         u16 newmss;
 86         u8 *opt;
 87 
 88         /* This is a fragment, no TCP header is available */
 89         if (par->fragoff != 0)
 90                 return 0;
 91 
 92         if (!skb_make_writable(skb, skb->len))
 93                 return -1;
 94 
 95         len = skb->len - tcphoff;
 96         if (len < (int)sizeof(struct tcphdr))
 97                 return -1;
 98 
 99         tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
100         tcp_hdrlen = tcph->doff * 4;
101 
102         if (len < tcp_hdrlen || tcp_hdrlen < sizeof(struct tcphdr))
103                 return -1;
104 
105         if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
106                 struct net *net = xt_net(par);
107                 unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family);
108                 unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu);
109 
110                 if (min_mtu <= minlen) {
111                         net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
112                                             min_mtu);
113                         return -1;
114                 }
115                 newmss = min_mtu - minlen;
116         } else
117                 newmss = info->mss;
118 
119         opt = (u_int8_t *)tcph;
120         for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
121                 if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
122                         u_int16_t oldmss;
123 
124                         oldmss = (opt[i+2] << 8) | opt[i+3];
125 
126                         /* Never increase MSS, even when setting it, as
127                          * doing so results in problems for hosts that rely
128                          * on MSS being set correctly.
129                          */
130                         if (oldmss <= newmss)
131                                 return 0;
132 
133                         opt[i+2] = (newmss & 0xff00) >> 8;
134                         opt[i+3] = newmss & 0x00ff;
135 
136                         inet_proto_csum_replace2(&tcph->check, skb,
137                                                  htons(oldmss), htons(newmss),
138                                                  false);
139                         return 0;
140                 }
141         }
142 
143         /* There is data after the header so the option can't be added
144          * without moving it, and doing so may make the SYN packet
145          * itself too large. Accept the packet unmodified instead.
146          */
147         if (len > tcp_hdrlen)
148                 return 0;
149 
150         /* tcph->doff has 4 bits, do not wrap it to 0 */
151         if (tcp_hdrlen >= 15 * 4)
152                 return 0;
153 
154         /*
155          * MSS Option not found ?! add it..
156          */
157         if (skb_tailroom(skb) < TCPOLEN_MSS) {
158                 if (pskb_expand_head(skb, 0,
159                                      TCPOLEN_MSS - skb_tailroom(skb),
160                                      GFP_ATOMIC))
161                         return -1;
162                 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
163         }
164 
165         skb_put(skb, TCPOLEN_MSS);
166 
167         /*
168          * IPv4: RFC 1122 states "If an MSS option is not received at
169          * connection setup, TCP MUST assume a default send MSS of 536".
170          * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
171          * length IPv6 header of 60, ergo the default MSS value is 1220
172          * Since no MSS was provided, we must use the default values
173          */
174         if (xt_family(par) == NFPROTO_IPV4)
175                 newmss = min(newmss, (u16)536);
176         else
177                 newmss = min(newmss, (u16)1220);
178 
179         opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
180         memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));
181 
182         inet_proto_csum_replace2(&tcph->check, skb,
183                                  htons(len), htons(len + TCPOLEN_MSS), true);
184         opt[0] = TCPOPT_MSS;
185         opt[1] = TCPOLEN_MSS;
186         opt[2] = (newmss & 0xff00) >> 8;
187         opt[3] = newmss & 0x00ff;
188 
189         inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), false);
190 
191         oldval = ((__be16 *)tcph)[6];
192         tcph->doff += TCPOLEN_MSS/4;
193         inet_proto_csum_replace2(&tcph->check, skb,
194                                  oldval, ((__be16 *)tcph)[6], false);
195         return TCPOLEN_MSS;
196 }
197 
198 static unsigned int
199 tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
200 {
201         struct iphdr *iph = ip_hdr(skb);
202         __be16 newlen;
203         int ret;
204 
205         ret = tcpmss_mangle_packet(skb, par,
206                                    PF_INET,
207                                    iph->ihl * 4,
208                                    sizeof(*iph) + sizeof(struct tcphdr));
209         if (ret < 0)
210                 return NF_DROP;
211         if (ret > 0) {
212                 iph = ip_hdr(skb);
213                 newlen = htons(ntohs(iph->tot_len) + ret);
214                 csum_replace2(&iph->check, iph->tot_len, newlen);
215                 iph->tot_len = newlen;
216         }
217         return XT_CONTINUE;
218 }
219 
220 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
221 static unsigned int
222 tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
223 {
224         struct ipv6hdr *ipv6h = ipv6_hdr(skb);
225         u8 nexthdr;
226         __be16 frag_off, oldlen, newlen;
227         int tcphoff;
228         int ret;
229 
230         nexthdr = ipv6h->nexthdr;
231         tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
232         if (tcphoff < 0)
233                 return NF_DROP;
234         ret = tcpmss_mangle_packet(skb, par,
235                                    PF_INET6,
236                                    tcphoff,
237                                    sizeof(*ipv6h) + sizeof(struct tcphdr));
238         if (ret < 0)
239                 return NF_DROP;
240         if (ret > 0) {
241                 ipv6h = ipv6_hdr(skb);
242                 oldlen = ipv6h->payload_len;
243                 newlen = htons(ntohs(oldlen) + ret);
244                 if (skb->ip_summed == CHECKSUM_COMPLETE)
245                         skb->csum = csum_add(csum_sub(skb->csum, oldlen),
246                                              newlen);
247                 ipv6h->payload_len = newlen;
248         }
249         return XT_CONTINUE;
250 }
251 #endif
252 
253 /* Must specify -p tcp --syn */
254 static inline bool find_syn_match(const struct xt_entry_match *m)
255 {
256         const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
257 
258         if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
259             tcpinfo->flg_cmp & TCPHDR_SYN &&
260             !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
261                 return true;
262 
263         return false;
264 }
265 
266 static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
267 {
268         const struct xt_tcpmss_info *info = par->targinfo;
269         const struct ipt_entry *e = par->entryinfo;
270         const struct xt_entry_match *ematch;
271 
272         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
273             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
274                            (1 << NF_INET_LOCAL_OUT) |
275                            (1 << NF_INET_POST_ROUTING))) != 0) {
276                 pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
277                 return -EINVAL;
278         }
279         if (par->nft_compat)
280                 return 0;
281 
282         xt_ematch_foreach(ematch, e)
283                 if (find_syn_match(ematch))
284                         return 0;
285         pr_info_ratelimited("Only works on TCP SYN packets\n");
286         return -EINVAL;
287 }
288 
289 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
290 static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
291 {
292         const struct xt_tcpmss_info *info = par->targinfo;
293         const struct ip6t_entry *e = par->entryinfo;
294         const struct xt_entry_match *ematch;
295 
296         if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
297             (par->hook_mask & ~((1 << NF_INET_FORWARD) |
298                            (1 << NF_INET_LOCAL_OUT) |
299                            (1 << NF_INET_POST_ROUTING))) != 0) {
300                 pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
301                 return -EINVAL;
302         }
303         if (par->nft_compat)
304                 return 0;
305 
306         xt_ematch_foreach(ematch, e)
307                 if (find_syn_match(ematch))
308                         return 0;
309         pr_info_ratelimited("Only works on TCP SYN packets\n");
310         return -EINVAL;
311 }
312 #endif
313 
314 static struct xt_target tcpmss_tg_reg[] __read_mostly = {
315         {
316                 .family         = NFPROTO_IPV4,
317                 .name           = "TCPMSS",
318                 .checkentry     = tcpmss_tg4_check,
319                 .target         = tcpmss_tg4,
320                 .targetsize     = sizeof(struct xt_tcpmss_info),
321                 .proto          = IPPROTO_TCP,
322                 .me             = THIS_MODULE,
323         },
324 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
325         {
326                 .family         = NFPROTO_IPV6,
327                 .name           = "TCPMSS",
328                 .checkentry     = tcpmss_tg6_check,
329                 .target         = tcpmss_tg6,
330                 .targetsize     = sizeof(struct xt_tcpmss_info),
331                 .proto          = IPPROTO_TCP,
332                 .me             = THIS_MODULE,
333         },
334 #endif
335 };
336 
337 static int __init tcpmss_tg_init(void)
338 {
339         return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
340 }
341 
342 static void __exit tcpmss_tg_exit(void)
343 {
344         xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
345 }
346 
347 module_init(tcpmss_tg_init);
348 module_exit(tcpmss_tg_exit);
349 

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