1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * nf_nat_pptp.c
4  *
5  * NAT support for PPTP (Point to Point Tunneling Protocol).
6  * PPTP is a protocol for creating virtual private networks.
7  * It is a specification defined by Microsoft and some vendors
8  * working with Microsoft.  PPTP is built on top of a modified
9  * version of the Internet Generic Routing Encapsulation Protocol.
10  * GRE is defined in RFC 1701 and RFC 1702.  Documentation of
11  * PPTP can be found in RFC 2637
12  *
13  * (C) 2000-2005 by Harald Welte <laforge@gnumonks.org>
14  *
15  * Development of this code funded by Astaro AG (http://www.astaro.com/)
16  *
17  * (C) 2006-2012 Patrick McHardy <kaber@trash.net>
18  *
19  * TODO: - NAT to a unique tuple, not to TCP source port
20  * 	   (needs netfilter tuple reservation)
21  */
22 
23 #include <linux/module.h>
24 #include <linux/tcp.h>
25 
26 #include <net/netfilter/nf_nat.h>
27 #include <net/netfilter/nf_nat_helper.h>
28 #include <net/netfilter/nf_conntrack_helper.h>
29 #include <net/netfilter/nf_conntrack_expect.h>
30 #include <net/netfilter/nf_conntrack_zones.h>
31 #include <linux/netfilter/nf_conntrack_proto_gre.h>
32 #include <linux/netfilter/nf_conntrack_pptp.h>
33 
34 #define NF_NAT_PPTP_VERSION "3.0"
35 
36 #define REQ_CID(req, off)		(*(__be16 *)((char *)(req) + (off)))
37 
38 MODULE_LICENSE("GPL");
39 MODULE_AUTHOR("Harald Welte <laforge@gnumonks.org>");
40 MODULE_DESCRIPTION("Netfilter NAT helper module for PPTP");
41 MODULE_ALIAS_NF_NAT_HELPER("pptp");
42 
pptp_nat_expected(struct nf_conn * ct,struct nf_conntrack_expect * exp)43 static void pptp_nat_expected(struct nf_conn *ct,
44 			      struct nf_conntrack_expect *exp)
45 {
46 	struct net *net = nf_ct_net(ct);
47 	const struct nf_conn *master = ct->master;
48 	struct nf_conntrack_expect *other_exp;
49 	struct nf_conntrack_tuple t = {};
50 	const struct nf_ct_pptp_master *ct_pptp_info;
51 	const struct nf_nat_pptp *nat_pptp_info;
52 	struct nf_nat_range2 range;
53 	struct nf_conn_nat *nat;
54 
55 	nat = nf_ct_nat_ext_add(ct);
56 	if (WARN_ON_ONCE(!nat))
57 		return;
58 
59 	nat_pptp_info = &nat->help.nat_pptp_info;
60 	ct_pptp_info = nfct_help_data(master);
61 
62 	/* And here goes the grand finale of corrosion... */
63 	if (exp->dir == IP_CT_DIR_ORIGINAL) {
64 		pr_debug("we are PNS->PAC\n");
65 		/* therefore, build tuple for PAC->PNS */
66 		t.src.l3num = AF_INET;
67 		t.src.u3.ip = master->tuplehash[!exp->dir].tuple.src.u3.ip;
68 		t.src.u.gre.key = ct_pptp_info->pac_call_id;
69 		t.dst.u3.ip = master->tuplehash[!exp->dir].tuple.dst.u3.ip;
70 		t.dst.u.gre.key = ct_pptp_info->pns_call_id;
71 		t.dst.protonum = IPPROTO_GRE;
72 	} else {
73 		pr_debug("we are PAC->PNS\n");
74 		/* build tuple for PNS->PAC */
75 		t.src.l3num = AF_INET;
76 		t.src.u3.ip = master->tuplehash[!exp->dir].tuple.src.u3.ip;
77 		t.src.u.gre.key = nat_pptp_info->pns_call_id;
78 		t.dst.u3.ip = master->tuplehash[!exp->dir].tuple.dst.u3.ip;
79 		t.dst.u.gre.key = nat_pptp_info->pac_call_id;
80 		t.dst.protonum = IPPROTO_GRE;
81 	}
82 
83 	pr_debug("trying to unexpect other dir: ");
84 	nf_ct_dump_tuple_ip(&t);
85 	other_exp = nf_ct_expect_find_get(net, nf_ct_zone(ct), &t);
86 	if (other_exp) {
87 		nf_ct_unexpect_related(other_exp);
88 		nf_ct_expect_put(other_exp);
89 		pr_debug("success\n");
90 	} else {
91 		pr_debug("not found!\n");
92 	}
93 
94 	/* This must be a fresh one. */
95 	BUG_ON(ct->status & IPS_NAT_DONE_MASK);
96 
97 	/* Change src to where master sends to */
98 	range.flags = NF_NAT_RANGE_MAP_IPS;
99 	range.min_addr = range.max_addr
100 		= ct->master->tuplehash[!exp->dir].tuple.dst.u3;
101 	if (exp->dir == IP_CT_DIR_ORIGINAL) {
102 		range.flags |= NF_NAT_RANGE_PROTO_SPECIFIED;
103 		range.min_proto = range.max_proto = exp->saved_proto;
104 	}
105 	nf_nat_setup_info(ct, &range, NF_NAT_MANIP_SRC);
106 
107 	/* For DST manip, map port here to where it's expected. */
108 	range.flags = NF_NAT_RANGE_MAP_IPS;
109 	range.min_addr = range.max_addr
110 		= ct->master->tuplehash[!exp->dir].tuple.src.u3;
111 	if (exp->dir == IP_CT_DIR_REPLY) {
112 		range.flags |= NF_NAT_RANGE_PROTO_SPECIFIED;
113 		range.min_proto = range.max_proto = exp->saved_proto;
114 	}
115 	nf_nat_setup_info(ct, &range, NF_NAT_MANIP_DST);
116 }
117 
118 /* outbound packets == from PNS to PAC */
119 static int
pptp_outbound_pkt(struct sk_buff * skb,struct nf_conn * ct,enum ip_conntrack_info ctinfo,unsigned int protoff,struct PptpControlHeader * ctlh,union pptp_ctrl_union * pptpReq)120 pptp_outbound_pkt(struct sk_buff *skb,
121 		  struct nf_conn *ct,
122 		  enum ip_conntrack_info ctinfo,
123 		  unsigned int protoff,
124 		  struct PptpControlHeader *ctlh,
125 		  union pptp_ctrl_union *pptpReq)
126 
127 {
128 	struct nf_ct_pptp_master *ct_pptp_info;
129 	struct nf_conn_nat *nat = nfct_nat(ct);
130 	struct nf_nat_pptp *nat_pptp_info;
131 	u_int16_t msg;
132 	__be16 new_callid;
133 	unsigned int cid_off;
134 
135 	if (WARN_ON_ONCE(!nat))
136 		return NF_DROP;
137 
138 	nat_pptp_info = &nat->help.nat_pptp_info;
139 	ct_pptp_info = nfct_help_data(ct);
140 
141 	new_callid = ct_pptp_info->pns_call_id;
142 
143 	switch (msg = ntohs(ctlh->messageType)) {
144 	case PPTP_OUT_CALL_REQUEST:
145 		cid_off = offsetof(union pptp_ctrl_union, ocreq.callID);
146 		/* FIXME: ideally we would want to reserve a call ID
147 		 * here.  current netfilter NAT core is not able to do
148 		 * this :( For now we use TCP source port. This breaks
149 		 * multiple calls within one control session */
150 
151 		/* save original call ID in nat_info */
152 		nat_pptp_info->pns_call_id = ct_pptp_info->pns_call_id;
153 
154 		/* don't use tcph->source since we are at a DSTmanip
155 		 * hook (e.g. PREROUTING) and pkt is not mangled yet */
156 		new_callid = ct->tuplehash[IP_CT_DIR_REPLY].tuple.dst.u.tcp.port;
157 
158 		/* save new call ID in ct info */
159 		ct_pptp_info->pns_call_id = new_callid;
160 		break;
161 	case PPTP_IN_CALL_REPLY:
162 		cid_off = offsetof(union pptp_ctrl_union, icack.callID);
163 		break;
164 	case PPTP_CALL_CLEAR_REQUEST:
165 		cid_off = offsetof(union pptp_ctrl_union, clrreq.callID);
166 		break;
167 	default:
168 		pr_debug("unknown outbound packet 0x%04x:%s\n", msg,
169 			 pptp_msg_name(msg));
170 		fallthrough;
171 	case PPTP_SET_LINK_INFO:
172 		/* only need to NAT in case PAC is behind NAT box */
173 	case PPTP_START_SESSION_REQUEST:
174 	case PPTP_START_SESSION_REPLY:
175 	case PPTP_STOP_SESSION_REQUEST:
176 	case PPTP_STOP_SESSION_REPLY:
177 	case PPTP_ECHO_REQUEST:
178 	case PPTP_ECHO_REPLY:
179 		/* no need to alter packet */
180 		return NF_ACCEPT;
181 	}
182 
183 	/* only OUT_CALL_REQUEST, IN_CALL_REPLY, CALL_CLEAR_REQUEST pass
184 	 * down to here */
185 	pr_debug("altering call id from 0x%04x to 0x%04x\n",
186 		 ntohs(REQ_CID(pptpReq, cid_off)), ntohs(new_callid));
187 
188 	/* mangle packet */
189 	if (!nf_nat_mangle_tcp_packet(skb, ct, ctinfo, protoff,
190 				      cid_off + sizeof(struct pptp_pkt_hdr) +
191 				      sizeof(struct PptpControlHeader),
192 				      sizeof(new_callid), (char *)&new_callid,
193 				      sizeof(new_callid)))
194 		return NF_DROP;
195 	return NF_ACCEPT;
196 }
197 
198 static void
pptp_exp_gre(struct nf_conntrack_expect * expect_orig,struct nf_conntrack_expect * expect_reply)199 pptp_exp_gre(struct nf_conntrack_expect *expect_orig,
200 	     struct nf_conntrack_expect *expect_reply)
201 {
202 	const struct nf_conn *ct = expect_orig->master;
203 	struct nf_conn_nat *nat = nfct_nat(ct);
204 	struct nf_ct_pptp_master *ct_pptp_info;
205 	struct nf_nat_pptp *nat_pptp_info;
206 
207 	if (WARN_ON_ONCE(!nat))
208 		return;
209 
210 	nat_pptp_info = &nat->help.nat_pptp_info;
211 	ct_pptp_info = nfct_help_data(ct);
212 
213 	/* save original PAC call ID in nat_info */
214 	nat_pptp_info->pac_call_id = ct_pptp_info->pac_call_id;
215 
216 	/* alter expectation for PNS->PAC direction */
217 	expect_orig->saved_proto.gre.key = ct_pptp_info->pns_call_id;
218 	expect_orig->tuple.src.u.gre.key = nat_pptp_info->pns_call_id;
219 	expect_orig->tuple.dst.u.gre.key = ct_pptp_info->pac_call_id;
220 	expect_orig->dir = IP_CT_DIR_ORIGINAL;
221 
222 	/* alter expectation for PAC->PNS direction */
223 	expect_reply->saved_proto.gre.key = nat_pptp_info->pns_call_id;
224 	expect_reply->tuple.src.u.gre.key = nat_pptp_info->pac_call_id;
225 	expect_reply->tuple.dst.u.gre.key = ct_pptp_info->pns_call_id;
226 	expect_reply->dir = IP_CT_DIR_REPLY;
227 }
228 
229 /* inbound packets == from PAC to PNS */
230 static int
pptp_inbound_pkt(struct sk_buff * skb,struct nf_conn * ct,enum ip_conntrack_info ctinfo,unsigned int protoff,struct PptpControlHeader * ctlh,union pptp_ctrl_union * pptpReq)231 pptp_inbound_pkt(struct sk_buff *skb,
232 		 struct nf_conn *ct,
233 		 enum ip_conntrack_info ctinfo,
234 		 unsigned int protoff,
235 		 struct PptpControlHeader *ctlh,
236 		 union pptp_ctrl_union *pptpReq)
237 {
238 	const struct nf_nat_pptp *nat_pptp_info;
239 	struct nf_conn_nat *nat = nfct_nat(ct);
240 	u_int16_t msg;
241 	__be16 new_pcid;
242 	unsigned int pcid_off;
243 
244 	if (WARN_ON_ONCE(!nat))
245 		return NF_DROP;
246 
247 	nat_pptp_info = &nat->help.nat_pptp_info;
248 	new_pcid = nat_pptp_info->pns_call_id;
249 
250 	switch (msg = ntohs(ctlh->messageType)) {
251 	case PPTP_OUT_CALL_REPLY:
252 		pcid_off = offsetof(union pptp_ctrl_union, ocack.peersCallID);
253 		break;
254 	case PPTP_IN_CALL_CONNECT:
255 		pcid_off = offsetof(union pptp_ctrl_union, iccon.peersCallID);
256 		break;
257 	case PPTP_IN_CALL_REQUEST:
258 		/* only need to nat in case PAC is behind NAT box */
259 		return NF_ACCEPT;
260 	case PPTP_WAN_ERROR_NOTIFY:
261 		pcid_off = offsetof(union pptp_ctrl_union, wanerr.peersCallID);
262 		break;
263 	case PPTP_CALL_DISCONNECT_NOTIFY:
264 		pcid_off = offsetof(union pptp_ctrl_union, disc.callID);
265 		break;
266 	case PPTP_SET_LINK_INFO:
267 		pcid_off = offsetof(union pptp_ctrl_union, setlink.peersCallID);
268 		break;
269 	default:
270 		pr_debug("unknown inbound packet %s\n", pptp_msg_name(msg));
271 		fallthrough;
272 	case PPTP_START_SESSION_REQUEST:
273 	case PPTP_START_SESSION_REPLY:
274 	case PPTP_STOP_SESSION_REQUEST:
275 	case PPTP_STOP_SESSION_REPLY:
276 	case PPTP_ECHO_REQUEST:
277 	case PPTP_ECHO_REPLY:
278 		/* no need to alter packet */
279 		return NF_ACCEPT;
280 	}
281 
282 	/* only OUT_CALL_REPLY, IN_CALL_CONNECT, IN_CALL_REQUEST,
283 	 * WAN_ERROR_NOTIFY, CALL_DISCONNECT_NOTIFY pass down here */
284 
285 	/* mangle packet */
286 	pr_debug("altering peer call id from 0x%04x to 0x%04x\n",
287 		 ntohs(REQ_CID(pptpReq, pcid_off)), ntohs(new_pcid));
288 
289 	if (!nf_nat_mangle_tcp_packet(skb, ct, ctinfo, protoff,
290 				      pcid_off + sizeof(struct pptp_pkt_hdr) +
291 				      sizeof(struct PptpControlHeader),
292 				      sizeof(new_pcid), (char *)&new_pcid,
293 				      sizeof(new_pcid)))
294 		return NF_DROP;
295 	return NF_ACCEPT;
296 }
297 
298 static const struct nf_nat_pptp_hook pptp_hooks = {
299 	.outbound = pptp_outbound_pkt,
300 	.inbound = pptp_inbound_pkt,
301 	.exp_gre = pptp_exp_gre,
302 	.expectfn = pptp_nat_expected,
303 };
304 
nf_nat_helper_pptp_init(void)305 static int __init nf_nat_helper_pptp_init(void)
306 {
307 	WARN_ON(nf_nat_pptp_hook != NULL);
308 	RCU_INIT_POINTER(nf_nat_pptp_hook, &pptp_hooks);
309 
310 	return 0;
311 }
312 
nf_nat_helper_pptp_fini(void)313 static void __exit nf_nat_helper_pptp_fini(void)
314 {
315 	RCU_INIT_POINTER(nf_nat_pptp_hook, NULL);
316 	synchronize_rcu();
317 }
318 
319 module_init(nf_nat_helper_pptp_init);
320 module_exit(nf_nat_helper_pptp_fini);
321