xref: /openbmc/linux/net/dns_resolver/dns_query.c (revision 6a613ac6)
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.txt
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/dns_resolver.h>
41 #include <linux/err.h>
42 #include <keys/dns_resolver-type.h>
43 #include <keys/user-type.h>
44 
45 #include "internal.h"
46 
47 /**
48  * dns_query - Query the DNS
49  * @type: Query type (or NULL for straight host->IP lookup)
50  * @name: Name to look up
51  * @namelen: Length of name
52  * @options: Request options (or NULL if no options)
53  * @_result: Where to place the returned data.
54  * @_expiry: Where to store the result expiry time (or NULL)
55  *
56  * The data will be returned in the pointer at *result, and the caller is
57  * responsible for freeing it.
58  *
59  * The description should be of the form "[<query_type>:]<domain_name>", and
60  * the options need to be appropriate for the query type requested.  If no
61  * query_type is given, then the query is a straight hostname to IP address
62  * lookup.
63  *
64  * The DNS resolution lookup is performed by upcalling to userspace by way of
65  * requesting a key of type dns_resolver.
66  *
67  * Returns the size of the result on success, -ve error code otherwise.
68  */
69 int dns_query(const char *type, const char *name, size_t namelen,
70 	      const char *options, char **_result, time64_t *_expiry)
71 {
72 	struct key *rkey;
73 	const struct user_key_payload *upayload;
74 	const struct cred *saved_cred;
75 	size_t typelen, desclen;
76 	char *desc, *cp;
77 	int ret, len;
78 
79 	kenter("%s,%*.*s,%zu,%s",
80 	       type, (int)namelen, (int)namelen, name, namelen, options);
81 
82 	if (!name || namelen == 0 || !_result)
83 		return -EINVAL;
84 
85 	/* construct the query key description as "[<type>:]<name>" */
86 	typelen = 0;
87 	desclen = 0;
88 	if (type) {
89 		typelen = strlen(type);
90 		if (typelen < 1)
91 			return -EINVAL;
92 		desclen += typelen + 1;
93 	}
94 
95 	if (!namelen)
96 		namelen = strnlen(name, 256);
97 	if (namelen < 3 || namelen > 255)
98 		return -EINVAL;
99 	desclen += namelen + 1;
100 
101 	desc = kmalloc(desclen, GFP_KERNEL);
102 	if (!desc)
103 		return -ENOMEM;
104 
105 	cp = desc;
106 	if (type) {
107 		memcpy(cp, type, typelen);
108 		cp += typelen;
109 		*cp++ = ':';
110 	}
111 	memcpy(cp, name, namelen);
112 	cp += namelen;
113 	*cp = '\0';
114 
115 	if (!options)
116 		options = "";
117 	kdebug("call request_key(,%s,%s)", desc, options);
118 
119 	/* make the upcall, using special credentials to prevent the use of
120 	 * add_key() to preinstall malicious redirections
121 	 */
122 	saved_cred = override_creds(dns_resolver_cache);
123 	rkey = request_key(&key_type_dns_resolver, desc, options);
124 	revert_creds(saved_cred);
125 	kfree(desc);
126 	if (IS_ERR(rkey)) {
127 		ret = PTR_ERR(rkey);
128 		goto out;
129 	}
130 
131 	down_read(&rkey->sem);
132 	set_bit(KEY_FLAG_ROOT_CAN_INVAL, &rkey->flags);
133 	rkey->perm |= KEY_USR_VIEW;
134 
135 	ret = key_validate(rkey);
136 	if (ret < 0)
137 		goto put;
138 
139 	/* If the DNS server gave an error, return that to the caller */
140 	ret = PTR_ERR(rkey->payload.data[dns_key_error]);
141 	if (ret)
142 		goto put;
143 
144 	upayload = user_key_payload(rkey);
145 	len = upayload->datalen;
146 
147 	ret = -ENOMEM;
148 	*_result = kmalloc(len + 1, GFP_KERNEL);
149 	if (!*_result)
150 		goto put;
151 
152 	memcpy(*_result, upayload->data, len);
153 	(*_result)[len] = '\0';
154 
155 	if (_expiry)
156 		*_expiry = rkey->expiry;
157 
158 	ret = len;
159 put:
160 	up_read(&rkey->sem);
161 	key_put(rkey);
162 out:
163 	kleave(" = %d", ret);
164 	return ret;
165 }
166 EXPORT_SYMBOL(dns_query);
167