1/* Upcall routine, designed to work as a key type and working through
2 * /sbin/request-key to contact userspace when handling DNS queries.
3 *
4 * See Documentation/networking/dns_resolver.rst
5 *
6 *   Copyright (c) 2007 Igor Mammedov
7 *   Author(s): Igor Mammedov (niallain@gmail.com)
8 *              Steve French (sfrench@us.ibm.com)
9 *              Wang Lei (wang840925@gmail.com)
10 *		David Howells (dhowells@redhat.com)
11 *
12 *   The upcall wrapper used to make an arbitrary DNS query.
13 *
14 *   This function requires the appropriate userspace tool dns.upcall to be
15 *   installed and something like the following lines should be added to the
16 *   /etc/request-key.conf file:
17 *
18 *	create dns_resolver * * /sbin/dns.upcall %k
19 *
20 *   For example to use this module to query AFSDB RR:
21 *
22 *	create dns_resolver afsdb:* * /sbin/dns.afsdb %k
23 *
24 *   This library is free software; you can redistribute it and/or modify
25 *   it under the terms of the GNU Lesser General Public License as published
26 *   by the Free Software Foundation; either version 2.1 of the License, or
27 *   (at your option) any later version.
28 *
29 *   This library is distributed in the hope that it will be useful,
30 *   but WITHOUT ANY WARRANTY; without even the implied warranty of
31 *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See
32 *   the GNU Lesser General Public License for more details.
33 *
34 *   You should have received a copy of the GNU Lesser General Public License
35 *   along with this library; if not, see <http://www.gnu.org/licenses/>.
36 */
37
38#include <linux/module.h>
39#include <linux/slab.h>
40#include <linux/cred.h>
41#include <linux/dns_resolver.h>
42#include <linux/err.h>
43#include <net/net_namespace.h>
44
45#include <keys/dns_resolver-type.h>
46#include <keys/user-type.h>
47
48#include "internal.h"
49
50/**
51 * dns_query - Query the DNS
52 * @net: The network namespace to operate in.
53 * @type: Query type (or NULL for straight host->IP lookup)
54 * @name: Name to look up
55 * @namelen: Length of name
56 * @options: Request options (or NULL if no options)
57 * @_result: Where to place the returned data (or NULL)
58 * @_expiry: Where to store the result expiry time (or NULL)
59 * @invalidate: Always invalidate the key after use
60 *
61 * The data will be returned in the pointer at *result, if provided, and the
62 * caller is responsible for freeing it.
63 *
64 * The description should be of the form "[<query_type>:]<domain_name>", and
65 * the options need to be appropriate for the query type requested.  If no
66 * query_type is given, then the query is a straight hostname to IP address
67 * lookup.
68 *
69 * The DNS resolution lookup is performed by upcalling to userspace by way of
70 * requesting a key of type dns_resolver.
71 *
72 * Returns the size of the result on success, -ve error code otherwise.
73 */
74int dns_query(struct net *net,
75	      const char *type, const char *name, size_t namelen,
76	      const char *options, char **_result, time64_t *_expiry,
77	      bool invalidate)
78{
79	struct key *rkey;
80	struct user_key_payload *upayload;
81	const struct cred *saved_cred;
82	size_t typelen, desclen;
83	char *desc, *cp;
84	int ret, len;
85
86	kenter("%s,%*.*s,%zu,%s",
87	       type, (int)namelen, (int)namelen, name, namelen, options);
88
89	if (!name || namelen == 0)
90		return -EINVAL;
91
92	/* construct the query key description as "[<type>:]<name>" */
93	typelen = 0;
94	desclen = 0;
95	if (type) {
96		typelen = strlen(type);
97		if (typelen < 1)
98			return -EINVAL;
99		desclen += typelen + 1;
100	}
101
102	if (namelen < 3 || namelen > 255)
103		return -EINVAL;
104	desclen += namelen + 1;
105
106	desc = kmalloc(desclen, GFP_KERNEL);
107	if (!desc)
108		return -ENOMEM;
109
110	cp = desc;
111	if (type) {
112		memcpy(cp, type, typelen);
113		cp += typelen;
114		*cp++ = ':';
115	}
116	memcpy(cp, name, namelen);
117	cp += namelen;
118	*cp = '\0';
119
120	if (!options)
121		options = "";
122	kdebug("call request_key(,%s,%s)", desc, options);
123
124	/* make the upcall, using special credentials to prevent the use of
125	 * add_key() to preinstall malicious redirections
126	 */
127	saved_cred = override_creds(dns_resolver_cache);
128	rkey = request_key_net(&key_type_dns_resolver, desc, net, options);
129	revert_creds(saved_cred);
130	kfree(desc);
131	if (IS_ERR(rkey)) {
132		ret = PTR_ERR(rkey);
133		goto out;
134	}
135
136	down_read(&rkey->sem);
137	set_bit(KEY_FLAG_ROOT_CAN_INVAL, &rkey->flags);
138	rkey->perm |= KEY_USR_VIEW;
139
140	ret = key_validate(rkey);
141	if (ret < 0)
142		goto put;
143
144	/* If the DNS server gave an error, return that to the caller */
145	ret = PTR_ERR(rkey->payload.data[dns_key_error]);
146	if (ret)
147		goto put;
148
149	upayload = user_key_payload_locked(rkey);
150	len = upayload->datalen;
151
152	if (_result) {
153		ret = -ENOMEM;
154		*_result = kmemdup_nul(upayload->data, len, GFP_KERNEL);
155		if (!*_result)
156			goto put;
157	}
158
159	if (_expiry)
160		*_expiry = rkey->expiry;
161
162	ret = len;
163put:
164	up_read(&rkey->sem);
165	if (invalidate)
166		key_invalidate(rkey);
167	key_put(rkey);
168out:
169	kleave(" = %d", ret);
170	return ret;
171}
172EXPORT_SYMBOL(dns_query);
173