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Date:	Tue, 7 Oct 2014 15:19:12 -0700
From:	Andy Lutomirski <>
To:	"Eric W. Biederman" <>
Cc:	Serge Hallyn <>,
	Al Viro <>,
	Andrey Vagin <>,
	Linux FS Devel <>,
	"" <>,
	Linux API <>,
	Andrey Vagin <>,
	Andrew Morton <>,
	Cyrill Gorcunov <>,
	Pavel Emelyanov <>,
	Serge Hallyn <>,
	Rob Landley <>
Subject: Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the
 current root

On Tue, Oct 7, 2014 at 2:42 PM, Eric W. Biederman <> wrote:
> 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.

Yes.  rootfs might not be empty, it might have totally insane
permissions, and it's globally shared, which makes it into a wonderful
channel to pass things around that shouldn't be passed around.

Can non-root do this?  You'd need to be in a userns with a "/" that
isn't MNT_LOCKED.  Can this happen on any normal setup?

FWIW, I think we should unconditionally MNT_LOCKED the root on userns
unshare, even if it's the only mount.

>>> >> 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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Andy Lutomirski
AMA Capital Management, LLC
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