Re: [PATCH v2 2/7] KVM: nVMX: Release enlightened VMCS on VMCLEAR

[Maxim Levitsky]

On Mon, 2021-05-17 at 15:50 +0200, Vitaly Kuznetsov wrote:
> Unlike VMREAD/VMWRITE/VMPTRLD, VMCLEAR is a valid instruction when
> enlightened VMCS is in use. TLFS has the following brief description:
> "The L1 hypervisor can execute a VMCLEAR instruction to transition an
> enlightened VMCS from the active to the non-active state". Normally,
> this change can be ignored as unmapping active eVMCS can be postponed
> until the next VMLAUNCH instruction but in case nested state is migrated
> with KVM_GET_NESTED_STATE/KVM_SET_NESTED_STATE, keeping eVMCS mapped
> may result in its synchronization with VMCS12 and this is incorrect:
> L1 hypervisor is free to reuse inactive eVMCS memory for something else.
> 
> Inactive eVMCS after VMCLEAR can just be unmapped.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  arch/x86/kvm/vmx/nested.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 3080e00c8f90..ea2869d8b823 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -5008,6 +5008,8 @@ static int handle_vmclear(struct kvm_vcpu *vcpu)
>  				     vmptr + offsetof(struct vmcs12,
>  						      launch_state),
>  				     &zero, sizeof(zero));
> +	} else if (vmx->nested.hv_evmcs && vmptr == vmx->nested.hv_evmcs_vmptr) {
> +		nested_release_evmcs(vcpu);
>  	}

Releasing the eVMCS is a good thing not only for migration 
but for instance for nested_vmx_fail which could write to 
current eVMCS even outside of nested mode 
(there is a bug there but you fixed it in the patch 4).


Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky


>  
>  	return nested_vmx_succeed(vcpu);