Re: Linux guest kernel threat model for Confidential Computing

Christophe de Dinechin Dupont de Dinechin
2023-02-01T13:15:32Z

> On 1 Feb 2023, at 12:01, Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Wed, Feb 01, 2023 at 11:52:27AM +0100, Christophe de Dinechin Dupont de Dinechin wrote:
>>
>>
>>> 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.
>
> Not just that, also because it can and does emulate some devices.
> For example, are disk encryption systems protected against timing of
> disk accesses?
> This is why some people keep saying "forget about emulated devices, require
> passthrough, include devices in the trust zone".
>
>>>
>>>> </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).
>
> Host. Lots of existing systems do this. As an extreme boot a RO disk,
> limit which packages are allowed.

Is that provable to the guest?

Consider a cloud provider doing that: how do they prove to their guest:

a) What firmware, kernel and kvm they run

b) That what they booted cannot be maliciouly modified, e.g. by a rogue
device driver installed by a rogue sysadmin

My understanding is that SecureBoot is only intended to prevent non-verified
operating systems from booting. So the proof is given to the cloud provider,
and the proof is that the system boots successfully.

After that, I think all bets are off. SecureBoot does little AFAICT
to prevent malicious modifications of the running system by someone with
root access, including deliberately loading a malicious kvm-zilog.ko

It does not mean it cannot be done, just that I don’t think we
have the tools at the moment.

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