Re: [PATCH 6/6] KVM: VMX: Intercept reads to invalid and write-only x2APIC registers

[Maxim Levitsky]

On Sat, 2023-01-07 at 01:10 +0000, Sean Christopherson wrote:
> Intercept reads to invalid (non-existent) and write-only x2APIC registers
> when configuring VMX's MSR bitmaps for x2APIC+APICv.  When APICv is fully
> enabled, Intel hardware doesn't validate the registers on RDMSR and
> instead blindly retrieves data from the vAPIC page, i.e. it's software's
> responsibility to intercept reads to non-existent and write-only MSRs.
> 
> Fixes: 8d14695f9542 ("x86, apicv: add virtual x2apic support")
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/vmx/vmx.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 82c61c16f8f5..1be2bc7185be 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4031,7 +4031,7 @@ static void vmx_update_msr_bitmap_x2apic(struct kvm_vcpu *vcpu)
>  	u64 *msr_bitmap = (u64 *)vmx->vmcs01.msr_bitmap;
>  	u8 mode;
>  
> -	if (!cpu_has_vmx_msr_bitmap())
> +	if (!cpu_has_vmx_msr_bitmap() || WARN_ON_ONCE(!lapic_in_kernel(vcpu)))
>  		return;
>  
>  	if (cpu_has_secondary_exec_ctrls() &&
> @@ -4053,11 +4053,11 @@ static void vmx_update_msr_bitmap_x2apic(struct kvm_vcpu *vcpu)
>  	 * Reset the bitmap for MSRs 0x800 - 0x83f.  Leave AMD's uber-extended
>  	 * registers (0x840 and above) intercepted, KVM doesn't support them.
>  	 * Intercept all writes by default and poke holes as needed.  Pass
> -	 * through all reads by default in x2APIC+APICv mode, as all registers
> -	 * except the current timer count are passed through for read.
> +	 * through reads for all valid registers by default in x2APIC+APICv
> +	 * mode, only the current timer count needs on-demand emulation by KVM.
>  	 */
>  	if (mode & MSR_BITMAP_MODE_X2APIC_APICV)
> -		msr_bitmap[read_idx] = 0;
> +		msr_bitmap[read_idx] = ~kvm_lapic_readable_reg_mask(vcpu->arch.apic);
>  	else
>  		msr_bitmap[read_idx] = ~0ull;
>  	msr_bitmap[write_idx] = ~0ull;

Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky