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Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE
 [ver #12]
To: David Howells <dhowells@redhat.com>, viro@zeniv.linux.org.uk
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From: Alan Jenkins <alan.christopher.jenkins@gmail.com>
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Date: Thu, 11 Oct 2018 13:23:35 +0100
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On 11/10/2018 13:14, David Howells wrote:
> David Howells <dhowells@redhat.com> 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