net/dns_resolver/dns_query.c

   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, 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;
 160 put:
 161         up_read(&rkey->sem);
 162         key_put(rkey);
 163 out:
 164         kleave(" = %d", ret);
 165         return ret;
 166 }
 167 EXPORT_SYMBOL(dns_query);