Re: [PATCH 0/7] Initial support for user namespace owned mounts

Dave Chinner
2015-07-17T00:12:51Z

On Wed, Jul 15, 2015 at 11:47:08PM -0500, Eric W. Biederman wrote:
> Casey Schaufler <casey@schaufler-ca.com> writes:
> > On 7/15/2015 6:08 PM, Andy Lutomirski wrote:
> >> If I mount an unprivileged filesystem, then either the contents were
> >> put there *by me*, in which case letting me access them are fine, or
> >> (with Seth's patches and then some) I control the backing store, in
> >> which case I can do whatever I want regardless of what LSM thinks.
> >>
> >> So I don't see the problem. Why would Smack or any other LSM care at
> >> all, unless it wants to prevent me from mounting the fs in the first
> >> place?
> >
> > First off, I don't cotton to the notion that you should be able
> > to mount filesystems without privilege. But it seems I'm being
> > outvoted on that. I suspect that there are cases where it might
> > be safe, but I can't think of one off the top of my head.
>
> There are two fundamental issues mounting filesystems without privielge,
> by which I actually mean mounting filesystems as the root user in a user
> namespace.
>
> - Are the semantics safe.
> - Is the extra attack surface a problem.

I think the attack surface this exposes is the biggest problem
facing this proposal.

> Figuring out how to make semantics safe is what we are talking about.
>
> Once we sort out the semantics we can look at the handful of filesystems
> like fuse where the extra attack surface is not a concern.
>
> With that said desktop environments have for a long time been
> automatically mounting whichever filesystem you place in your computer,
> so in practice what this is really about is trying to align the kernel
> with how people use filesystems.

The key difference is that desktops only do this when you physically
plug in a device. With unprivileged mounts, a hostile attacker
doesn't need physical access to the machine to exploit lurking
kernel filesystem bugs. i.e. they can just use loopback mounts, and
they can keep mounting corrupted images until they find something
that works.

User namespaces are supposed to provide trust separation. The
kernel filesystems simply aren't hardened against unprivileged
attacks from below - there is a trust relationship between root and
the filesystem in that they are the only things that can write to
the disk. Mounts from within a userns destroys this relationship as
the userns root, by definition, is not a trusted actor.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com