fix deadlock in cifs_ioctl_clone()

It really needs to check that src is non-directory *and* use
{un,}lock_two_nodirectories().  As it is, it's trivial to cause
double-lock (ioctl(fd, CIFS_IOC_COPYCHUNK_FILE, fd)) and if the
last argument is an fd of directory, we are asking for trouble
by violating the locking order - all directories go before all
non-directories.  If the last argument is an fd of parent
directory, it has 50% odds of locking child before parent,
which will cause AB-BA deadlock if we race with unlink().

Cc: stable@vger.kernel.org @ 3.13+
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/cifs/ioctl.c

@@ -86,21 +86,16 @@ static long cifs_ioctl_clone(unsigned int xid, struct file *dst_file,
 	}
 
 	src_inode = file_inode(src_file.file);
+	rc = -EINVAL;
+	if (S_ISDIR(src_inode->i_mode))
+		goto out_fput;
 
 	/*
 	 * Note: cifs case is easier than btrfs since server responsible for
 	 * checks for proper open modes and file type and if it wants
 	 * server could even support copy of range where source = target
 	 */
-
-	/* so we do not deadlock racing two ioctls on same files */
-	if (target_inode < src_inode) {
-		mutex_lock_nested(&target_inode->i_mutex, I_MUTEX_PARENT);
-		mutex_lock_nested(&src_inode->i_mutex, I_MUTEX_CHILD);
-	} else {
-		mutex_lock_nested(&src_inode->i_mutex, I_MUTEX_PARENT);
-		mutex_lock_nested(&target_inode->i_mutex, I_MUTEX_CHILD);
-	}
+	lock_two_nondirectories(target_inode, src_inode);
 
 	/* determine range to clone */
 	rc = -EINVAL;
@@ -124,13 +119,7 @@ static long cifs_ioctl_clone(unsigned int xid, struct file *dst_file,
 out_unlock:
 	/* although unlocking in the reverse order from locking is not
 	   strictly necessary here it is a little cleaner to be consistent */
-	if (target_inode < src_inode) {
-		mutex_unlock(&src_inode->i_mutex);
-		mutex_unlock(&target_inode->i_mutex);
-	} else {
-		mutex_unlock(&target_inode->i_mutex);
-		mutex_unlock(&src_inode->i_mutex);
-	}
+	unlock_two_nondirectories(src_inode, target_inode);
 out_fput:
 	fdput(src_file);
 out_drop_write: