DNS: Separate out CIFS DNS Resolver code

Separate out the DNS resolver key type from the CIFS filesystem into its own
module so that it can be made available for general use, including the AFS
filesystem module.

This facility makes it possible for the kernel to upcall to userspace to have
it issue DNS requests, package up the replies and present them to the kernel
in a useful form.  The kernel is then able to cache the DNS replies as keys
can be retained in keyrings.

Resolver keys are of type "dns_resolver" and have a case-insensitive
description that is of the form "[<type>:]<domain_name>".  The optional <type>
indicates the particular DNS lookup and packaging that's required.  The
<domain_name> is the query to be made.

If <type> isn't given, a basic hostname to IP address lookup is made, and the
result is stored in the key in the form of a printable string consisting of a
comma-separated list of IPv4 and IPv6 addresses.

This key type is supported by userspace helpers driven from /sbin/request-key
and configured through /etc/request-key.conf.  The cifs.upcall utility is
invoked for UNC path server name to IP address resolution.

The CIFS functionality is encapsulated by the dns_resolve_unc_to_ip() function,
which is used to resolve a UNC path to an IP address for CIFS filesystem.  This
part remains in the CIFS module for now.

See the added Documentation/networking/dns_resolver.txt for more information.

Signed-off-by: Wang Lei <wang840925@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

1 files changed, 159 insertions, 0 deletions

diff --git a/net/dns_resolver/dns_query.c b/net/dns_resolver/dns_query.c
new file mode 100644
index 000000000000..6c0cf31ea00d
--- /dev/null
+++ b/net/dns_resolver/dns_query.c
@@ -0,0 +1,159 @@
+/* Upcall routine, designed to work as a key type and working through
+ * /sbin/request-key to contact userspace when handling DNS queries.
+ *
+ * See Documentation/networking/dns_resolver.txt
+ *
+ *   Copyright (c) 2007 Igor Mammedov
+ *   Author(s): Igor Mammedov (niallain@gmail.com)
+ *              Steve French (sfrench@us.ibm.com)
+ *              Wang Lei (wang840925@gmail.com)
+ *              David Howells (dhowells@redhat.com)
+ *
+ *   The upcall wrapper used to make an arbitrary DNS query.
+ *
+ *   This function requires the appropriate userspace tool dns.upcall to be
+ *   installed and something like the following lines should be added to the
+ *   /etc/request-key.conf file:
+ *
+ *      create dns_resolver * * /sbin/dns.upcall %k
+ *
+ *   For example to use this module to query AFSDB RR:
+ *
+ *      create dns_resolver afsdb:* * /sbin/dns.afsdb %k
+ *
+ *   This library is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU Lesser General Public License as published
+ *   by the Free Software Foundation; either version 2.1 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This library is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See
+ *   the GNU Lesser General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Lesser General Public License
+ *   along with this library; if not, write to the Free Software
+ *   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/dns_resolver.h>
+#include <keys/dns_resolver-type.h>
+#include <keys/user-type.h>
+
+#include "internal.h"
+
+/*
+ * dns_query - Query the DNS
+ * @type: Query type (or NULL for straight host->IP lookup)
+ * @name: Name to look up
+ * @namelen: Length of name
+ * @options: Request options (or NULL if no options)
+ * @_result: Where to place the returned data.
+ * @_expiry: Where to store the result expiry time (or NULL)
+ *
+ * The data will be returned in the pointer at *result, and the caller is
+ * responsible for freeing it.
+ *
+ * The description should be of the form "[<query_type>:]<domain_name>", and
+ * the options need to be appropriate for the query type requested.  If no
+ * query_type is given, then the query is a straight hostname to IP address
+ * lookup.
+ *
+ * The DNS resolution lookup is performed by upcalling to userspace by way of
+ * requesting a key of type dns_resolver.
+ *
+ * Returns the size of the result on success, -ve error code otherwise.
+ */
+int dns_query(const char *type, const char *name, size_t namelen,
+              const char *options, char **_result, time_t *_expiry)
+{
+        struct key *rkey;
+        struct user_key_payload *upayload;
+        const struct cred *saved_cred;
+        size_t typelen, desclen;
+        char *desc, *cp;
+        int ret, len;
+
+        kenter("%s,%*.*s,%zu,%s",
+               type, (int)namelen, (int)namelen, name, namelen, options);
+
+        if (!name || namelen == 0 || !_result)
+                return -EINVAL;
+
+        /* construct the query key description as "[<type>:]<name>" */
+        typelen = 0;
+        desclen = 0;
+        if (type) {
+                typelen = strlen(type);
+                if (typelen < 1)
+                        return -EINVAL;
+                desclen += typelen + 1;
+        }
+
+        if (!namelen)
+                namelen = strlen(name);
+        if (namelen < 3)
+                return -EINVAL;
+        desclen += namelen + 1;
+
+        desc = kmalloc(desclen, GFP_KERNEL);
+        if (!desc)
+                return -ENOMEM;
+
+        cp = desc;
+        if (type) {
+                memcpy(cp, type, typelen);
+                cp += typelen;
+                *cp++ = ':';
+        }
+        memcpy(cp, name, namelen);
+        cp += namelen;
+        *cp = '\0';
+
+        if (!options)
+                options = "";
+        kdebug("call request_key(,%s,%s)", desc, options);
+
+        /* make the upcall, using special credentials to prevent the use of
+         * add_key() to preinstall malicious redirections
+         */
+        saved_cred = override_creds(dns_resolver_cache);
+        rkey = request_key(&key_type_dns_resolver, desc, options);
+        revert_creds(saved_cred);
+        kfree(desc);
+        if (IS_ERR(rkey)) {
+                ret = PTR_ERR(rkey);
+                goto out;
+        }
+
+        down_read(&rkey->sem);
+        rkey->perm |= KEY_USR_VIEW;
+
+        ret = key_validate(rkey);
+        if (ret < 0)
+                goto put;
+
+        upayload = rcu_dereference_protected(rkey->payload.data,
+                                             lockdep_is_held(&rkey->sem));
+        len = upayload->datalen;
+
+        ret = -ENOMEM;
+        *_result = kmalloc(len + 1, GFP_KERNEL);
+        if (!*_result)
+                goto put;
+
+        memcpy(*_result, upayload->data, len + 1);
+        if (_expiry)
+                *_expiry = rkey->expiry;
+
+        ret = len;
+put:
+        up_read(&rkey->sem);
+        key_put(rkey);
+out:
+        kleave(" = %d", ret);
+        return ret;
+}
+EXPORT_SYMBOL(dns_query);