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

J. Bruce Fields
2015-07-22T14:09:27Z

On Wed, Jul 22, 2015 at 05:56:40PM +1000, Dave Chinner wrote:
> On Tue, Jul 21, 2015 at 01:37:21PM -0400, J. Bruce Fields wrote:
> > On Fri, Jul 17, 2015 at 12:47:35PM +1000, Dave Chinner wrote:
> > > On Thu, Jul 16, 2015 at 07:42:03PM -0500, Eric W. Biederman wrote:
> > > > Dave Chinner <david@fromorbit.com> writes:
> > > > > 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.
> > > >
> > > > Yep. That magnifies the problem quite a bit.
> > > >
> > > > > 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.
> > > >
> > > > I talked to Ted Tso a while back and ext4 is at least in principle
> > > > already hardened against that kind of attack. I am not certain I
> > > > believe it, but if it is true I think it is fantastic.
> > >
> > > No, it's not. No filesystem is, because to harden against such
> > > attacks requires complete verification of all metadata when it is
> > > read from disk, before it is used, or some method or ensuring the
> > > block was not tampered with. CRCs are not sufficient, because they
> > > can be tampered with, too.
> > >
> > > The only way a filesystem would be able to trust what it reads from
> > > disk has not been tampered with in a system with untrusted mounts is
> > > if it has some kind of cryptographically secure signature in the
> > > metadata and the attacker is unable to access the key for that
> > > signature.
> >
> > Preventing tampering is a little different from protecting the kernel
> > from attack, isn't it? I thought the latter was what people were asking
> > about.
>
> People might be asking for the latter, but the only attack vector
> that can be made against filesystems from below is via tampering
> with the on-disk structure.
>
> An untrusted user in an untrusted container can construct arbitrary
> untrusted filesystem structures and get them parsed by a context
> running as $DIETY that assumes the structure is from a trusted
> source. What can possibly go wrong?
>
> IOWs, To protect the kernel against attack from untrusted filesystem
> images, we either have to be able to guarantee the image can not be
> modified by untrusted parties (i.e. needs to be created with
> signed tools, contain only signed filesystem metadata and
> signed/encrypted data),

I don't think that works--who exactly would be the "trusted party"? It
can't be this kernel or this hardware--users expect to be able to mount
filesystems created by older kernels, on other machines, running other
distributions (even other operating systems). It can't be the
user--then any user could compromise the kernel by signing a bad
filesystem.

Authenticating the creator of the filesystem might be useful for other
reasons, but it sounds to me like at best only very weak protection
against corrupted filesystems.

As a similar example, browser makers are stuck both implementing SSL and
hardening their code against malicious content. Those address separate
problems.

> or we have to sandbox the filesystem parsing
> code completely (i.e. fuse).
>
> > So, for example, a screwed up on-disk directory structure shouldn't
> > result in creating a cycle in the dcache and then deadlocking.
>
> Therein lies the problem: how do you detect such structural defects
> without doing a full structure validation?

You can prevent cycles in a graph if you can prevent adding an edge
which would be part of a cycle.

For the dcache, it's d_splice_alias that does that (using d_ancestor).

(And I believe the main motivation for that was NFS, where you don't
need a filesystem cycle, just a server-side race that can briefly make
it look like there's one--an example of the changing filesystem problem
that you point out below.)

> e.g. cyclic links may
> only manifest when completely unrelated pieces of metadata are linked
> together in a specific way.
>
> Further, the problem is not restricted to validation at mount time -
> if the user can write to the filesystem image file, then they can
> modify it after it has been mounted, too. That means the attacker
> may be someone who has broken into a container, not necessarily the
> user you trusted with unprivileged mounts. That means every cold
> metadata read needs to be treated with suspicion, not just at mount
> time.

Yes. Agreed that this is difficult. (I can't actually give an example
of an existing problem of this sort, but I'd be surprised if they don't
exist.)

--b.