linux/net/dns_resolver/dns_query.c
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   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 */
  70int 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;
 158put:
 159        up_read(&rkey->sem);
 160        key_put(rkey);
 161out:
 162        kleave(" = %d", ret);
 163        return ret;
 164}
 165EXPORT_SYMBOL(dns_query);
 166
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