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