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SubjectRe: [RFC] KVM: SVM: do not drop VMCB CPL to 0 if SS is not present
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On 19/05/2017 18:14, Roman Penyaev wrote:
>
> 1. Simple one, KVM SVM side, which makes sure that CPL is not updated
> if segment is unusable:
>
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1549,7 +1549,7 @@ static void svm_set_segment(struct kvm_vcpu *vcpu,
> * forces SS.DPL to 3 on sysret, so we ignore that case; fixing it
> * would entail passing the CPL to userspace and back.
> */
> - if (seg == VCPU_SREG_SS)
> + if (seg == VCPU_SREG_SS && !var->unusable)
> svm->vmcb->save.cpl = (s->attrib >>
> SVM_SELECTOR_DPL_SHIFT) & 3;

Based on the discussion between you and Andy, my understanding is that
it would not be enough to ensure that the attributes are preserved
across a roundtrip through KVM_GET_SEGMENT and KVM_SET_SEGMENT. We need
a workaround in the hypervisor if we don't want to pass the CPL to
userspace and back.

Maybe if 1) in 64-bit mode 2) SS.P=0 3) SS selector != 0, then the CPL
can be taken from SS.RPL?

Thanks,

Paolo

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