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 */
69int dns_query(const char *type, const char *name, size_t namelen,
70	      const char *options, char **_result, time_t *_expiry)
71{
72	struct key *rkey;
73	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 = rkey->type_data.x[0];
141	if (ret)
142		goto put;
143
144	upayload = rcu_dereference_protected(rkey->payload.data,
145					     lockdep_is_held(&rkey->sem));
146	len = upayload->datalen;
147
148	ret = -ENOMEM;
149	*_result = kmalloc(len + 1, GFP_KERNEL);
150	if (!*_result)
151		goto put;
152
153	memcpy(*_result, upayload->data, len);
154	(*_result)[len] = '\0';
155
156	if (_expiry)
157		*_expiry = rkey->expiry;
158
159	ret = len;
160put:
161	up_read(&rkey->sem);
162	key_put(rkey);
163out:
164	kleave(" = %d", ret);
165	return ret;
166}
167EXPORT_SYMBOL(dns_query);
168