namespace: restrict OPEN_TREE_NAMESPACE/FSMOUNT_NAMESPACE to directories

namespace: restrict OPEN_TREE_NAMESPACE/FSMOUNT_NAMESPACE to directories

open_tree(..., OPEN_TREE_NAMESPACE) and
fsmount(..., FSMOUNT_NAMESPACE, ...) currently work on non-directories,
like regular files. That's bad for two reasons:

 - It ends up mounting a regular file over the inherited namespace root,
   which is a directory; mounting a non-directory over a directory is
   normally explicitly forbidden, see for example do_move_mount()

 - It causes setns() on the new namespace to set the cwd to a regular
   file, which the rest of VFS does not expect

Fix it by restricting create_new_namespace() (which is used by both of
these flags) to directories.

Leave the behavior for OPEN_TREE_CLONE as-is, that seems unproblematic.

Fixes: 9b8a0ba68246 ("mount: add OPEN_TREE_NAMESPACE")
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: stable@kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>