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

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Date:	Tue, 7 Oct 2014 21:32:57 +0000
From:	Serge Hallyn <serge.hallyn@...ntu.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.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

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

Oh, ok.  

Sorry, I'm getting the two thread confused anyway.  I'm going to bow out
here until I can pay proper attention.

> >> 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 :)
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