Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12]

[Alan Jenkins]

On 11/10/2018 13:14, David Howells wrote:
> David Howells <dhowells@xxxxxxxxxx> wrote:
>
> > The reason that you can do this with open_tree()/move_mount() is that it
> > allows you to create a mount tree (OPEN_TREE_CLONE) that has no namespace
> > assignment, pass it through the namespace switch and then attach it inside the
> > child namespace.  The cross-namespace checks in do_move_mount() are bypassed
> > because the root of the newly-cloned mount tree doesn't have one.
> It's worse than that.  The apparently disconnected tree given you by
> open_tree(OPEN_TREE_CLONE) is still subject to modification by outside
> forces.  All it takes is one shared object within that tree.
>
> So I do wonder if it's possible to form a ring, even in an upstream kernel, by
> using the propagation mechanism to push through an nsfs mount into itself,
> possibly with a layer of indirection (ie. having two mutually-referential
> namespaces).
>
> David

Upstream does cover the mount propagation case, by simply never 
propagating mounts of mount NS files. See commit 4ce5d2b1a8fd "vfs: 
Don't copy mount bind mounts of /proc/<pid>/ns/mnt between namespaces" / 
https://unix.stackexchange.com/questions/473717/what-code-prevents-mount-namespace-loops-in-a-more-complex-case-involving-mount-propagation