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
