Re: Linux guest kernel threat model for Confidential Computing

Dr. David Alan Gilbert
2023-02-06T18:58:52Z

* Christophe de Dinechin (dinechin@redhat.com) wrote:
>
> On 2023-02-01 at 11:02 -05, "Michael S. Tsirkin" <mst@redhat.com> wrote...
> > On Wed, Feb 01, 2023 at 02:15:10PM +0100, Christophe de Dinechin Dupont de Dinechin wrote:
> >>
> >>
> >> > 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.
> >
> > I think I should have said measured boot not secure boot.
>
> The problem again is how you prove to the guest that you are not lying?
>
> We know how to do that from a guest [1], but you will note that in the
> normal process, a trusted hardware component (e.g. the PSP for AMD SEV)
> proves the validity of the measurements of the TCB by encrypting it with an
> attestation signing key derived from some chip-unique secret. For AMD, this
> is called the VCEK, and TDX has something similar. In the case of SEV, this
> goes through firmware, and you have to tell the firmware each time you
> insert data in the original TCB (using SNP_LAUNCH_UPDATE). This is all tied
> to a VM execution context. I do not believe there is any provision to do the
> same thing to measure host data. And again, it would be somewhat pointless
> if there isn't also a mechanism to ensure the host data is not changed after
> the measurement.
>
> Now, I don't think it would be super-difficult to add a firmware service
> that would let the host do some kind of equivalent to PVALIDATE, setting
> some physical pages aside that then get measured and become inaccessible to
> the host. The PSP or similar could then integrate these measurements as part
> of the TCB, and the fact that the pages were "transferred" to this special
> invariant block would ensure the guests that the code will not change after
> being measured.
>
> I am not aware that such a mechanism exists on any of the existing CC
> platforms. Please feel free to enlighten me if I'm wrong.
>
> [1] https://www.redhat.com/en/blog/understanding-confidential-containers-attestation-flow
> >
> >>
> >> 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
> >
> > So disable module loading then or don't allow root access?
>
> Who would do that?
>
> The problem is that we have a host and a tenant, and the tenant does not
> trust the host in principle. So it is not sufficient for the host to disable
> module loading or carefully control root access. It is also necessary to
> prove to the tenant(s) that this was done.
>
> >
> >>
> >> It does not mean it cannot be done, just that I don’t think we
> >> have the tools at the moment.
> >
> > Phones, chromebooks do this all the time ...
>
> Indeed, but there, this is to prove to the phone's real owner (which,
> surprise, is not the naive person who thought they'd get some kind of
> ownership by buying the phone) that the software running on the phone has
> not been replaced by some horribly jailbreaked goo.
>
> In other words, the user of the phone gets no proof whatsoever of anything,
> except that the phone appears to work. This is somewhat the situation in the
> cloud today: the owners of the hardware get all sorts of useful checks, from
> SecureBoot to error-correction for memory or I/O devices. However, someone
> running in a VM on the cloud gets none of that, just like the user of your
> phone.

Assuming you do a measured boot, the host OS and firmware is measured into the host TPM;
people have thought in the past about triggering attestations of the
host from the guest; then you could have something external attest the
host and only release keys to the guests disks if the attestation is
correct; or a key for the guests disks held in the hosts TPM.

Dave

> --
> Cheers,
> Christophe de Dinechin (https://c3d.github.io)
> Theory of Incomplete Measurements (https://c3d.github.io/TIM)
>
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK