> 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. 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
