linux-kernel - Re: Does MNT_LOCKED work as expected?

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Date:	Mon, 29 Sep 2014 16:32:22 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrey Wagin <avagin@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	linux-fsdevel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: Does MNT_LOCKED work as expected?

Andrey Wagin <avagin@...il.com> writes:

> Hi All,
>
> I think I found a case, when MNT_LOCKED (5ff9d8a65ce8 "vfs: Lock in
> place mounts from more privileged users") doesn't help to hide
> overmounted parts from unprivileged users.

> The problem exists for mounts, which are not overmounted entirely. In
> such cases we can open a directory from a target mount, which is not
> overmounted. Then we do pivot_root to move all mounts in a temporary
> directory. At the final step we deatch all mounts from the temporary
> directory. After that all children mounts are umounted from the target
> mount and we can use our file descriptor to open files, which have
> been overmount before.
>
> Here is a example https://github.com/avagin/userns_vs_mntns.

I am looking at this now to make certain I am understand what you are
seeing but so far it looks like lazy unmounts allow you to bypass
MNT_LOCKED :(

Grumble.

Eric
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