Re: [RFC 0/4] Adding Virtual Memory Fuses to Xen

[Demi Marie Obenour <demi@invisiblethingslab.com>]

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On Wed, Dec 21, 2022 at 04:53:46PM -0800, Stefano Stabellini wrote:
> On Tue, 20 Dec 2022, Demi Marie Obenour wrote:
> > On Tue, Dec 20, 2022 at 10:17:24PM +0000, Smith, Jackson wrote:
> > > > Hi Stefano,
> > > >
> > > > On 16/12/2022 01:46, Stefano Stabellini wrote:
> > > > > On Thu, 15 Dec 2022, Julien Grall wrote:
> > > > >>>> On 13/12/2022 19:48, Smith, Jackson wrote:
> > > > >>> Yes, we are familiar with the "secret-free hypervisor" work. As
> > > you
> > > > >>> point out, both our work and the secret-free hypervisor remove the
> > > > >>> directmap region to mitigate the risk of leaking sensitive guest
> > > > >>> secrets. However, our work is slightly different because it
> > > > >>> additionally prevents attackers from tricking Xen into remapping a
> > > > guest.
> > > > >>
> > > > >> I understand your goal, but I don't think this is achieved (see
> > > > >> above). You would need an entity to prevent write to TTBR0_EL2 in
> > > > >> order to fully protect it.
> > > > >
> > > > > Without a way to stop Xen from reading/writing TTBR0_EL2, we
> > > > cannot
> > > > > claim that the guest's secrets are 100% safe.
> > > > >
> > > > > But the attacker would have to follow the sequence you outlines
> > > > above
> > > > > to change Xen's pagetables and remap guest memory before
> > > > accessing it.
> > > > > It is an additional obstacle for attackers that want to steal other
> > > > guests'
> > > > > secrets. The size of the code that the attacker would need to inject
> > > > > in Xen would need to be bigger and more complex.
> > > >
> > > > Right, that's why I wrote with a bit more work. However, the nuance
> > > > you mention doesn't seem to be present in the cover letter:
> > > >
> > > > "This creates what we call "Software Enclaves", ensuring that an
> > > > adversary with arbitrary code execution in the hypervisor STILL cannot
> > > > read/write guest memory."
> > > >
> > > > So if the end goal if really to protect against *all* sort of
> > > arbitrary 
> > > > code,
> > > > then I think we should have a rough idea how this will look like in
> > > Xen.
> > > >
> > > >  From a brief look, it doesn't look like it would be possible to
> > > prevent
> > > > modification to TTBR0_EL2 (even from EL3). We would need to
> > > > investigate if there are other bits in the architecture to help us.
> > > >
> > > > >
> > > > > Every little helps :-)
> > > >
> > > > I can see how making the life of the attacker more difficult is 
> > > > appealing.
> > > > Yet, the goal needs to be clarified and the risk with the approach
> > > > acknowledged (see above).
> > > >
> > > 
> > > You're right, we should have mentioned this weakness in our first email.
> > > Sorry about the oversight! This is definitely still a limitation that we
> > > have not yet overcome. However, we do think that the increase in
> > > attacker workload that you and Stefano are discussing could still be
> > > valuable to security conscious Xen users.
> > > 
> > > It would nice to find additional architecture features that we can use
> > > to close this hole on arm, but there aren't any that stand out to me
> > > either.
> > > 
> > > With this limitation in mind, what are the next steps we should take to
> > > support this feature for the xen community? Is this increase in attacker
> > > workload meaningful enough to justify the inclusion of VMF in Xen?
> > 
> > Personally, I don’t think so.  The kinds of workloads VMF is usable
> > for (no hypercalls) are likely easily portable to other hypervisors,
> > including formally verified microkernels such as seL4 that provide... 
> 
> What other hypervisors might or might not do should not be a factor in
> this discussion and it would be best to leave it aside.

Indeed so, sorry.

> From an AMD/Xilinx point of view, most of our customers using Xen in
> productions today don't use any hypercalls in one or more of their VMs.
> Xen is great for these use-cases and it is rather common in embedded.
> It is certainly a different configuration from what most are come to
> expect from Xen on the server/desktop x86 side. There is no question
> that guests without hypercalls are important for Xen on ARM.

I was completely unaware of this.

> As a Xen community we have a long history and strong interest in making
> Xen more secure and also, more recently, safer (in the ISO 26262
> safety-certification sense). The VMF work is very well aligned with both
> of these efforts and any additional burder to attackers is certainly
> good for Xen.

That it is.

> Now the question is what changes are necessary and how to make them to
> the codebase. And if it turns out that some of the changes are not
> applicable or too complex to accept, the decision will be made purely
> from a code maintenance point of view and will have nothing to do with
> VMs making no hypercalls being unimportant (i.e. if we don't accept one
> or more patches is not going to have anything to do with the use-case
> being unimportant or what other hypervisors might or might not do).

-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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