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