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, write to the Free Software
  36  *   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
  37  */
  38
  39 #include <linux/module.h>
  40 #include <linux/slab.h>
  41 #include <linux/dns_resolver.h>
  42 #include <linux/err.h>
  43 #include <keys/dns_resolver-type.h>
  44 #include <keys/user-type.h>
  45
  46 #include "internal.h"
  47
  48 /**
  49  * dns_query - Query the DNS
  50  * @type: Query type (or NULL for straight host->IP lookup)
  51  * @name: Name to look up
  52  * @namelen: Length of name
  53  * @options: Request options (or NULL if no options)
  54  * @_result: Where to place the returned data.
  55  * @_expiry: Where to store the result expiry time (or NULL)
  56  *
  57  * The data will be returned in the pointer at *result, and the caller is
  58  * responsible for freeing it.
  59  *
  60  * The description should be of the form "[<query_type>:]<domain_name>", and
  61  * the options need to be appropriate for the query type requested.  If no
  62  * query_type is given, then the query is a straight hostname to IP address
  63  * lookup.
  64  *
  65  * The DNS resolution lookup is performed by upcalling to userspace by way of
  66  * requesting a key of type dns_resolver.
  67  *
  68  * Returns the size of the result on success, -ve error code otherwise.
  69  */
  70 int dns_query(const char *type, const char *name, size_t namelen,
  71               const char *options, char **_result, time_t *_expiry)
  72 {
  73         struct key *rkey;
  74         struct user_key_payload *upayload;
  75         const struct cred *saved_cred;
  76         size_t typelen, desclen;
  77         char *desc, *cp;
  78         int ret, len;
  79
  80         kenter("%s,%*.*s,%zu,%s",
  81                type, (int)namelen, (int)namelen, name, namelen, options);
  82
  83         if (!name || namelen == 0 || !_result)
  84                 return -EINVAL;
  85
  86         /* construct the query key description as "[<type>:]<name>" */
  87         typelen = 0;
  88         desclen = 0;
  89         if (type) {
  90                 typelen = strlen(type);
  91                 if (typelen < 1)
  92                         return -EINVAL;
  93                 desclen += typelen + 1;
  94         }
  95
  96         if (!namelen)
  97                 namelen = strlen(name);
  98         if (namelen < 3)
  99                 return -EINVAL;
 100         desclen += namelen + 1;
 101
 102         desc = kmalloc(desclen, GFP_KERNEL);
 103         if (!desc)
 104                 return -ENOMEM;
 105
 106         cp = desc;
 107         if (type) {
 108                 memcpy(cp, type, typelen);
 109                 cp += typelen;
 110                 *cp++ = ':';
 111         }
 112         memcpy(cp, name, namelen);
 113         cp += namelen;
 114         *cp = '\0';
 115
 116         if (!options)
 117                 options = "";
 118         kdebug("call request_key(,%s,%s)", desc, options);
 119
 120         /* make the upcall, using special credentials to prevent the use of
 121          * add_key() to preinstall malicious redirections
 122          */
 123         saved_cred = override_creds(dns_resolver_cache);
 124         rkey = request_key(&key_type_dns_resolver, desc, options);
 125         revert_creds(saved_cred);
 126         kfree(desc);
 127         if (IS_ERR(rkey)) {
 128                 ret = PTR_ERR(rkey);
 129                 goto out;
 130         }
 131
 132         down_read(&rkey->sem);
 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 + 1);
 154         if (_expiry)
 155                 *_expiry = rkey->expiry;
 156
 157         ret = len;
 158 put:
 159         up_read(&rkey->sem);
 160         key_put(rkey);
 161 out:
 162         kleave(" = %d", ret);
 163         return ret;
 164 }
 165 EXPORT_SYMBOL(dns_query);