Re: [PATCH v4 06/32] KVM: x86: Track xAPIC ID only on userspace SET, _after_ vAPIC is updated

[Maxim Levitsky]

On Sat, 2022-10-01 at 00:58 +0000, Sean Christopherson wrote:
> Track potential changes to a vCPU's xAPIC ID only for KVM_SET_LAPIC, i.e.
> not for KVM_GET_LAPIC, and process the update after the incoming state
> provided by userspace is copied to KVM's in-kernel vAPIC.  The latter bug
> is the most problematic issue, as processing the update before KVM's
> vAPIC is actually updated can result in false positives, e.g. due to the
> APIC holding an x2APIC ID (wrong format), and false negatives, e.g. due
> to KVM failing to detect an xAPIC ID "mismatch".
> 
> Processing an "update" in KVM_GET_LAPIC is likely a benign bug now that
> the update helper ignores mismatches, but prior to that fix, invoking
> KVM_GET_LAPIC while the APIC is disabled could effectively cause KVM to
> consume stale state, e.g. if the APIC were in x2APIC mode before being
> hardware disabled.
> 
> Fixes: 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base")
> Reported-by: Alejandro Jimenez <alejandro.j.jimenez@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/lapic.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 67260f7ce43a..251856ba0750 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2720,8 +2720,6 @@ static int kvm_apic_state_fixup(struct kvm_vcpu *vcpu,
>  			icr = __kvm_lapic_get_reg64(s->regs, APIC_ICR);
>  			__kvm_lapic_set_reg(s->regs, APIC_ICR2, icr >> 32);
>  		}
> -	} else {
> -		kvm_lapic_xapic_id_updated(vcpu->arch.apic);
>  	}
>  
>  	return 0;
> @@ -2757,6 +2755,9 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
>  	}
>  	memcpy(vcpu->arch.apic->regs, s->regs, sizeof(*s));
>  
> +	if (!apic_x2apic_mode(vcpu->arch.apic))
> +		kvm_lapic_xapic_id_updated(vcpu->arch.apic);
> +
>  	atomic_set_release(&apic->vcpu->kvm->arch.apic_map_dirty, DIRTY);
>  	kvm_recalculate_apic_map(vcpu->kvm);
>  	kvm_apic_set_version(vcpu);


Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky