linux-kernel - Re: unprivileged mounts git tree

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Message-Id: <E1KXZpC-0008Ie-6w@pomaz-ex.szeredi.hu>
Date:	Mon, 25 Aug 2008 13:01:30 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	serue@...ibm.com
CC:	ebiederm@...ssion.com, miklos@...redi.hu,
	akpm@...ux-foundation.org, hch@...radead.org,
	viro@...IV.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: unprivileged mounts git tree

On Thu, 7 Aug 2008, Serge E. Hallyn wrote:
> Quoting Eric W. Biederman (ebiederm@...ssion.com):
> > "Serge E. Hallyn" <serue@...ibm.com> writes:
> > > so on the bright side I pulled this tree today and it compiled and
> > > passed ltp with no problems.
> > >
> > > But then I played around a bit and found I could do the following:
> > >
> > > (hmm, i'm trying to remember the exact order :)
> > >
> > > as root:
> > > 	mmount --bind -o user=500 /home/hallyn/etc/ /home/hallyn/etc/
> > > 	mount --bind /mnt /mnt
> > > 	mount --make-rshared /mnt
> > > 	mount --bind /dev /mnt/dev
> > >
> > > as hallyn:
> > > 	mmount --bind /mnt /home/hallyn/etc/mnt
> > > 	/usr/src/mmount-0.3/mmount --bind mnt/dev mnt/src
> > 
> > You are using relative directory names here which makes it confusing.
> > I'm assuming you in /home/hallyn/etc ?
> 
> Sorry, yeah.
> 
> > > Now /mnt/src contained /dev.
> > >
> > > Is this what we want?
> > 
> > I don't think so.
> > 
> > I think the simplest answer is to not allow mounting of shared
> > subtrees controlled by a different user.
> > 
> > Serge I think you are right downgrading the mount from shared to slave
> > looks like the sane thing to do if the mount owners match.
> 
> I assume you mean "if the mount owners don't match"?
> 
> Miklos, what do you think?

Sorry about the late reply: I was on a long summer vacation...

Serge, thanks for spotting this: it looks indeed a nasty hole!  I also
agree about the solution.

> The next question then becomes, how can we prove to ourselves that that
> closes the last security hole with unprivileged mounts?  So long as
> we treat each mount event as a piece of information and look at it as an
> information flow problem, maybe we can actually come up with a good
> description of the logic that is implemented and show that there is no
> way a user can "leak" info...  (where a leak is a mount event, a
> violation of intended DAC on open(file) or mkdir, etc)

"Information flow problem" doesn't mean much to me (I'm actually an
electric engineer, who ended up doing programming for living ;)

But yeah, we should think this over very carefully.  Especially
interaction with mount propagation, which has very complicated and
sometimes rather counter-intuitive semantics.

Thanks,
Miklos
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