CIFS: Fix a malicious redirect problem in the DNS lookup code

Fix the security problem in the CIFS filesystem DNS lookup code in which a
malicious redirect could be installed by a random user by simply adding a
result record into one of their keyrings with add_key() and then invoking a
CIFS CFS lookup [CVE-2010-2524].

This is done by creating an internal keyring specifically for the caching of
DNS lookups.  To enforce the use of this keyring, the module init routine
creates a set of override credentials with the keyring installed as the thread
keyring and instructs request_key() to only install lookup result keys in that
keyring.

The override is then applied around the call to request_key().

This has some additional benefits when a kernel service uses this module to
request a key:

 (1) The result keys are owned by root, not the user that caused the lookup.

 (2) The result keys don't pop up in the user's keyrings.

 (3) The result keys don't come out of the quota of the user that caused the
     lookup.

The keyring can be viewed as root by doing cat /proc/keys:

2a0ca6c3 I-----     1 perm 1f030000     0     0 keyring   .dns_resolver: 1/4

It can then be listed with 'keyctl list' by root.

	# keyctl list 0x2a0ca6c3
	1 key in keyring:
	726766307: --alswrv     0     0 dns_resolver: foo.bar.com

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-and-Tested-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Steve French <smfrench@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/cifs/cifsfs.c

@@ -923,7 +923,7 @@ init_cifs(void)
 		goto out_unregister_filesystem;
 #endif
 #ifdef CONFIG_CIFS_DFS_UPCALL
-	rc = register_key_type(&key_type_dns_resolver);
+	rc = cifs_init_dns_resolver();
 	if (rc)
 		goto out_unregister_key_type;
 #endif
@@ -935,7 +935,7 @@ init_cifs(void)
 
  out_unregister_resolver_key:
 #ifdef CONFIG_CIFS_DFS_UPCALL
-	unregister_key_type(&key_type_dns_resolver);
+	cifs_exit_dns_resolver();
  out_unregister_key_type:
 #endif
 #ifdef CONFIG_CIFS_UPCALL
@@ -961,7 +961,7 @@ exit_cifs(void)
 	cifs_proc_clean();
 #ifdef CONFIG_CIFS_DFS_UPCALL
 	cifs_dfs_release_automount_timer();
-	unregister_key_type(&key_type_dns_resolver);
+	cifs_exit_dns_resolver();
 #endif
 #ifdef CONFIG_CIFS_UPCALL
 	unregister_key_type(&cifs_spnego_key_type);

fs/cifs/dns_resolve.c

@@ -24,12 +24,16 @@
  */
 
 #include <linux/slab.h>
+#include <linux/keyctl.h>
+#include <linux/key-type.h>
 #include <keys/user-type.h>
 #include "dns_resolve.h"
 #include "cifsglob.h"
 #include "cifsproto.h"
 #include "cifs_debug.h"
 
+static const struct cred *dns_resolver_cache;
+
 /* Checks if supplied name is IP address
  * returns:
  * 		1 - name is IP
@@ -94,6 +98,7 @@ struct key_type key_type_dns_resolver = {
 int
 dns_resolve_server_name_to_ip(const char *unc, char **ip_addr)
 {
+	const struct cred *saved_cred;
 	int rc = -EAGAIN;
 	struct key *rkey = ERR_PTR(-EAGAIN);
 	char *name;
@@ -133,8 +138,15 @@ dns_resolve_server_name_to_ip(const char *unc, char **ip_addr)
 		goto skip_upcall;
 	}
 
+	saved_cred = override_creds(dns_resolver_cache);
 	rkey = request_key(&key_type_dns_resolver, name, "");
+	revert_creds(saved_cred);
 	if (!IS_ERR(rkey)) {
+		if (!(rkey->perm & KEY_USR_VIEW)) {
+			down_read(&rkey->sem);
+			rkey->perm |= KEY_USR_VIEW;
+			up_read(&rkey->sem);
+		}
 		len = rkey->type_data.x[0];
 		data = rkey->payload.data;
 	} else {
@@ -165,4 +177,61 @@ dns_resolve_server_name_to_ip(const char *unc, char **ip_addr)
 	return rc;
 }
 
+int __init cifs_init_dns_resolver(void)
+{
+	struct cred *cred;
+	struct key *keyring;
+	int ret;
+
+	printk(KERN_NOTICE "Registering the %s key type\n",
+	       key_type_dns_resolver.name);
+
+	/* create an override credential set with a special thread keyring in
+	 * which DNS requests are cached
+	 *
+	 * this is used to prevent malicious redirections from being installed
+	 * with add_key().
+	 */
+	cred = prepare_kernel_cred(NULL);
+	if (!cred)
+		return -ENOMEM;
+
+	keyring = key_alloc(&key_type_keyring, ".dns_resolver", 0, 0, cred,
+			    (KEY_POS_ALL & ~KEY_POS_SETATTR) |
+			    KEY_USR_VIEW | KEY_USR_READ,
+			    KEY_ALLOC_NOT_IN_QUOTA);
+	if (IS_ERR(keyring)) {
+		ret = PTR_ERR(keyring);
+		goto failed_put_cred;
+	}
+
+	ret = key_instantiate_and_link(keyring, NULL, 0, NULL, NULL);
+	if (ret < 0)
+		goto failed_put_key;
+
+	ret = register_key_type(&key_type_dns_resolver);
+	if (ret < 0)
+		goto failed_put_key;
+
+	/* instruct request_key() to use this special keyring as a cache for
+	 * the results it looks up */
+	cred->thread_keyring = keyring;
+	cred->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
+	dns_resolver_cache = cred;
+	return 0;
+
+failed_put_key:
+	key_put(keyring);
+failed_put_cred:
+	put_cred(cred);
+	return ret;
+}
 
+void __exit cifs_exit_dns_resolver(void)
+{
+	key_revoke(dns_resolver_cache->thread_keyring);
+	unregister_key_type(&key_type_dns_resolver);
+	put_cred(dns_resolver_cache);
+	printk(KERN_NOTICE "Unregistered %s key type\n",
+	       key_type_dns_resolver.name);
+}

fs/cifs/dns_resolve.h

@@ -24,8 +24,8 @@
 #define _DNS_RESOLVE_H
 
 #ifdef __KERNEL__
-#include <linux/key-type.h>
-extern struct key_type key_type_dns_resolver;
+extern int __init cifs_init_dns_resolver(void);
+extern void __exit cifs_exit_dns_resolver(void);
 extern int dns_resolve_server_name_to_ip(const char *unc, char **ip_addr);
 #endif /* KERNEL */