Re: Linux guest kernel threat model for Confidential Computing

Christophe de Dinechin Dupont de Dinechin
2023-02-01T10:52:55Z

> On 31 Jan 2023, at 18:39, Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Tue, Jan 31, 2023 at 04:14:29PM +0100, Christophe de Dinechin wrote:
>> Finally, security considerations that apply irrespective of whether the
>> platform is confidential or not are also outside of the scope of this
>> document. This includes topics ranging from timing attacks to social
>> engineering.
>
> Why are timing attacks by hypervisor on the guest out of scope?

Good point.

I was thinking that mitigation against timing attacks is the same
irrespective of the source of the attack. However, because the HV
controls CPU time allocation, there are presumably attacks that
are made much easier through the HV. Those should be listed.

>
>> </doc>
>>
>> Feel free to comment and reword at will ;-)
>>
>>
>> 3/ PCI-as-a-threat: where does that come from
>>
>> Isn't there a fundamental difference, from a threat model perspective,
>> between a bad actor, say a rogue sysadmin dumping the guest memory (which CC
>> should defeat) and compromised software feeding us bad data? I think there
>> is: at leats inside the TCB, we can detect bad software using measurements,
>> and prevent it from running using attestation. In other words, we first
>> check what we will run, then we run it. The security there is that we know
>> what we are running. The trust we have in the software is from testing,
>> reviewing or using it.
>>
>> This relies on a key aspect provided by TDX and SEV, which is that the
>> software being measured is largely tamper-resistant thanks to memory
>> encryption. In other words, after you have measured your guest software
>> stack, the host or hypervisor cannot willy-nilly change it.
>>
>> So this brings me to the next question: is there any way we could offer the
>> same kind of service for KVM and qemu? The measurement part seems relatively
>> easy. Thetamper-resistant part, on the other hand, seems quite difficult to
>> me. But maybe someone else will have a brilliant idea?
>>
>> So I'm asking the question, because if you could somehow prove to the guest
>> not only that it's running the right guest stack (as we can do today) but
>> also a known host/KVM/hypervisor stack, we would also switch the potential
>> issues with PCI, MSRs and the like from "malicious" to merely "bogus", and
>> this is something which is evidently easier to deal with.
>
> Agree absolutely that's much easier.
>
>> I briefly discussed this with James, and he pointed out two interesting
>> aspects of that question:
>>
>> 1/ In the CC world, we don't really care about *virtual* PCI devices. We
>> care about either virtio devices, or physical ones being passed through
>> to the guest. Let's assume physical ones can be trusted, see above.
>> That leaves virtio devices. How much damage can a malicious virtio device
>> do to the guest kernel, and can this lead to secrets being leaked?
>>
>> 2/ He was not as negative as I anticipated on the possibility of somehow
>> being able to prevent tampering of the guest. One example he mentioned is
>> a research paper [1] about running the hypervisor itself inside an
>> "outer" TCB, using VMPLs on AMD. Maybe something similar can be achieved
>> with TDX using secure enclaves or some other mechanism?
>
> Or even just secureboot based root of trust?

You mean host secureboot? Or guest?

If it’s host, then the problem is detecting malicious tampering with
host code (whether it’s kernel or hypervisor).

If it’s guest, at the moment at least, the measurements do not extend
beyond the TCB.

>
> --
> MST
>