KVM: svm: unconditionally intercept #DB

This is needed to avoid the possibility that the guest triggers
an infinite stream of #DB exceptions (CVE-2015-8104).

VMX is not affected: because it does not save DR6 in the VMCS,
it already intercepts #DB unconditionally.

Reported-by: Jan Beulich <jbeulich@suse.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
static int interrupt_window_interception(struct vcpu_svm *svm)
{
	kvm_make_request(KVM_REQ_EVENT, &svm->vcpu);
	svm_clear_vintr(svm);
	svm->vmcb->control.int_ctl &= ~V_IRQ_MASK;
	mark_dirty(svm->vmcb, VMCB_INTR);
	++svm->vcpu.stat.irq_window_exits;
	return 1;
}

static int interrupt_window_interception(struct vcpu_svm *svm)
{
	kvm_make_request(KVM_REQ_EVENT, &svm->vcpu);
	svm_clear_vintr(svm);
	svm->vmcb->control.int_ctl &= ~V_IRQ_MASK;
	mark_dirty(svm->vmcb, VMCB_INTR);
	++svm->vcpu.stat.irq_window_exits;
	return 1;
}
