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Date:	Tue, 07 Oct 2014 14:42:18 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Serge Hallyn <serge.hallyn@...ntu.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Andrey Vagin <avagin@...nvz.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org, Andrey Vagin <avagin@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Rob Landley <rob@...dley.net>
Subject: Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the current root

Serge Hallyn <serge.hallyn@...ntu.com> writes:

> Quoting Eric W. Biederman (ebiederm@...ssion.com):

>> What I meant is that it isn't about containers.   It is about something
>> root can do.  So this is not a "container" problem.
>
> Oh, ok.  
>
> Sorry, I'm getting the two thread confused anyway.  I'm going to bow out
> here until I can pay proper attention.

I think there is half a point here that may be legit, when you are using
mount namespaces to jail applications, there may be a problem
with umounting / and making it to the underlying rootfs filesystem.

I am squinting and looking this way and that but while I can imagine
someone more clever than I can think up some unique property of rootfs
that makes it a little more exploitable than just mounting a ramfs,
but since you have to be root to exploit those properties I think the
game is pretty much lost.

>> >> So it is only root (and not root in a container) who can get to the
>> >> exposed rootfs.
>> >> 
>> >> I have a vague memory someone actually had a real use in miminal systems
>> >> for being able to get back to the rootfs and being able to use rootfs as
>> >> the rootfs.  There was even a patch at that time that Andrew Morton was
>> >> carrying for a time to allow unmounting root and get at rootfs, and to
>> >> prevent the oops on rootfs unmount in some way.
>> >> 
>> >> So not only do I not think it is a bug to get back too rootfs, I think
>> >> it is a feature that some people have expressed at least half-way sane
>> >> uses for.
>> >
>> > They can still do that if they want, using chroot :)
>> 
>> It would take fchdir or fchroot and a directory file descriptor open on
>> rootfs.  Frequently there is no appropriate directory file descriptor.
>
> ?  you can always escape if you're simply chrooted.  waterbuffalo :)

filesystem type rootfs.

Eric

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