eCryptfs: Validate global auth tok keys

When searching through the global authentication tokens for a given key
signature, verify that a matching key has not been revoked and has not
expired.  This allows the `keyctl revoke` command to be properly used on
keys in use by eCryptfs.

Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: ecryptfs-devel@lists.launchpad.net
Cc: stable <stable@kernel.org>
Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>

fs/ecryptfs/keystore.c

@@ -416,7 +416,9 @@ ecryptfs_find_global_auth_tok_for_sig(
 			    &mount_crypt_stat->global_auth_tok_list,
 			    mount_crypt_stat_list) {
 		if (memcmp(walker->sig, sig, ECRYPTFS_SIG_SIZE_HEX) == 0) {
-			(*global_auth_tok) = walker;
+			rc = key_validate(walker->global_auth_tok_key);
+			if (!rc)
+				(*global_auth_tok) = walker;
 			goto out;
 		}
 	}