Re: [PATCH v6 6/9] KVM: x86: lapic: don't allow to change APIC ID unconditionally

Chao Gao
2022-03-15T14:57:27Z

On Sun, Mar 13, 2022 at 05:09:08PM +0200, Maxim Levitsky wrote:
>> > > This won't work with nested AVIC - we can't just inhibit a nested guest using its own AVIC,
>> > > because migration happens.
>> >
>> > I mean because host decided to change its apic id, which it can in theory do any time,
>> > even after the nested guest has started. Seriously, the only reason guest has to change apic id,
>> > is to try to exploit some security hole.
>>
>> Hi
>>
>> Thanks for the information.
>>
>> IIUC, you mean KVM applies APICv inhibition only to L1 VM, leaving APICv
>> enabled for L2 VM. Shouldn't KVM disable APICv for L2 VM in this case?
>> It looks like a generic issue in dynamically toggling APICv scheme,
>> e.g., qemu can set KVM_GUESTDBG_BLOCKIRQ after nested guest has started.
>>
>
>That is the problem - you can't disable it for L2, unless you are willing to emulate it in software.
>Or in other words, when nested guest uses a hardware feature, you can't at some point say to it:
>sorry buddy - hardware feature disappeared.

Hi Maxim,

I may miss something. When reading Sean's APICv inhibition cleanups, I
find AVIC is disabled for L1 when nested is enabled (SVM is advertised
to L1). Then, I think the new inhibition introduced for changed xAPIC ID
shouldn't be a problem for L2 VM. Or, you plan to remove
APICV_INHIBIT_REASON_NESTED and expose AVIC to L1?

svm_vcpu_after_set_cpuid:
/*
* Currently, AVIC does not work with nested virtualization.
* So, we disable AVIC when cpuid for SVM is set in the L1 guest.
*/
if (nested && guest_cpuid_has(vcpu, X86_FEATURE_SVM))
kvm_request_apicv_update(vcpu->kvm, false,
APICV_INHIBIT_REASON_NESTED);