Re: [PATCH v2 1/2] KVM: Inject #GP on invalid write to APIC_SELF_IPI register

[Sean Christopherson <seanjc@google.com>]

On Wed, May 25, 2022, Venkatesh Srinivas wrote:
> From: Marc Orr <marcorr@google.com>
> 
> From: Venkatesh Srinivas <venkateshs@chromium.org>

Only Marc should have an explicit From:.  git am processes only the first From:,
e.g. your From: line ends up in the changelog.

> The upper bytes of the x2APIC APIC_SELF_IPI register are reserved.

Uber nit, please be more precise than "upper bytes".  The comment about "Bits 7:0"
saved me from having to lookup up APIC_VECTOR_MASK, but it'd be nice to have that
in the changelog.

E.g.

  Inject a #GP if the guest attempts to set reserved bits in the x2APIC-only
  Self-IPI register.  Bits 7:0 hold the vector, all other bits are reserved.

> Inject a #GP into the guest if any of these reserved bits are set.
> 
> Signed-off-by: Marc Orr <marcorr@google.com>
> Signed-off-by: Venkatesh Srinivas <venkateshs@chromium.org>
> ---
>  arch/x86/kvm/lapic.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 21ab69db689b..6f8522e8c492 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2169,10 +2169,16 @@ static int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
>  		break;
>  
>  	case APIC_SELF_IPI:
> -		if (apic_x2apic_mode(apic))
> -			kvm_apic_send_ipi(apic, APIC_DEST_SELF | (val & APIC_VECTOR_MASK), 0);
> -		else
> +		/*
> +		 * Self-IPI exists only when x2APIC is enabled.  Bits 7:0 hold
> +		 * the vector, everything else is reserved.
> +		 */
> +		if (!apic_x2apic_mode(apic) || (val & ~APIC_VECTOR_MASK)) {
>  			ret = 1;
> +			break;
> +		}
> +		kvm_lapic_reg_write(apic, APIC_ICR,
> +				    APIC_DEST_SELF | (val & APIC_VECTOR_MASK));
>  		break;
>  	default:
>  		ret = 1;
> -- 
> 2.36.1.124.g0e6072fb45-goog
>