KVM: x86: work around infinite loop in microcode when #AC is delivered

It was found that a guest can DoS a host by triggering an infinite
stream of "alignment check" (#AC) exceptions.  This causes the
microcode to enter an infinite loop where the core never receives
another interrupt.  The host kernel panics pretty quickly due to the
effects (CVE-2015-5307).

Signed-off-by: Eric Northup <digitaleric@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
static void vmx_hwapic_isr_update(struct kvm *kvm, int isr)
{
	u16 status;
	u8 old;

	if (isr == -1)
		isr = 0;

	status = vmcs_read16(GUEST_INTR_STATUS);
	old = status >> 8;
	if (isr != old) {
		status &= 0xff;
		status |= isr << 8;
		vmcs_write16(GUEST_INTR_STATUS, status);
	}
}

static void vmx_hwapic_isr_update(struct kvm *kvm, int isr)
{
	u16 status;
	u8 old;

	if (isr == -1)
		isr = 0;

	status = vmcs_read16(GUEST_INTR_STATUS);
	old = status >> 8;
	if (isr != old) {
		status &= 0xff;
		status |= isr << 8;
		vmcs_write16(GUEST_INTR_STATUS, status);
	}
}
