linux-kernel - Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the current root

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Message-ID: <87wq8bvbzg.fsf@x220.int.ebiederm.org>
Date:	Tue, 07 Oct 2014 13:52:35 -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):
>> Al Viro <viro@...IV.linux.org.uk> writes:
>> 
>> 2> On Tue, Oct 07, 2014 at 02:30:40PM +0100, Al Viro wrote:
>> >> On Tue, Oct 07, 2014 at 04:12:57PM +0400, Andrey Vagin wrote:
>> >> > Another problem is that rootfs can't be hidden from a container, because
>> >> > rootfs can't be moved or umounted.
>> >> 
>> >> ... which is a bug in mntns_install(), AFAICS.
>> >
>> > Ability to get to exposed rootfs, that is.
>> 
>> The container side of this argument is pretty bogus.  It only applies
>> if user namespaces are not used for the container.
>
> User namespaces are still far too restricted for many container use
> cases.  We can't say "we have user namespaces so now privileged
> containers can be ignored".  Yes you never should have handed the
> keys to a privileged container to an untrusted person, but we do
> still try to protect the host from accidental damage due to a
> privileged container.

What I meant is that it isn't about containers.   It is about something
root can do.  So this is not a "container" problem.

>> 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.

Eric
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