Re: [Xen-devel] Xen Security Advisory 99 - unexpected pitfall in xenaccess API

[Andres Lagar Cavilla <andres@xxxxxxxxxxxxxxxx>]

On Tue, Jun 17, 2014 at 6:36 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:

On Tue, 2014-06-17 at 06:13 -0700, Andres Lagar Cavilla wrote:

> But fundamentally, how is this a vulnerability? Since the dawn of time
> guests can poke at the qemu and PV frontend rings. So self DoS, check.
> But, privilege escalation?

PV frontend rings have an endpoint in the guest, but by contrast the
xenaccess ring is supposed to have its endpoint in Xen, having a guest
able to poke at it therefore requires additional consideration and
thought.

> Is this predicated on the potential (lack of) software quality of the
> xenaccess backends? That's a fair argument, but a different story.

An attacker who can poke at this particular ring can cause the xenaccess
backend's view of the world to become different from the actual state of
things within Xen, by virtue of injecting events for which Xen has not
made the appropriate state change.

Correct. So (1) Xen's handling of events won't change (2) the dom0 helper's view will.

So the bottomline question is how can a guest inject an event for a dom0 helper which will cause privilege escalation.

Such a helper would be (1) terribly designed (2) unduly powerful.

For instance imagine a xenaccess who was trying to enforce W^X but was
being fed false information by the guest about writing/executing pages
which did not correspond to actual changes being made in the p2m.

W^X is enforced by Xen and it won't be swayed by guest ring manipulation.

The helper would have been thrown off balance, and failed to audit something at worst. Maybe this means a security problem down the line for that helper toolchain, but outside the purview of the hypervisor.

One path that is not obvious is how would Xen react if the guest corrupts the ring in a way that makes it look full. The intended behavior is for Xen to put the guest vcpu in a wait/queue (or kill the guest). So that the damage at most might be self-DoS.

I see how helpers may be thrown totally off balance. I see self-DoS, but still do not see privilege escalation happening.

For qemu there is no Xen side state, so all a guest can do with ring
access here is to perform emulated I/O which it could otherwise have
achieved by doing the I/O.

Correct, my bad.

Thanks

AndresÂ

Ian.

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