KVM: VMX: mark unusable segment as nonpresent

Some userspaces do not preserve unusable property. Since usable
segment has to be present according to VMX spec we can use present
property to amend userspace bug by making unusable segment always
nonpresent. vmx_segment_access_rights() already marks nonpresent segment
as unusable.

Cc: stable@vger.kernel.org # 3.9+
Reported-by: NStefan Pietsch <stefan.pietsch@lsexperts.de>
Tested-by: NStefan Pietsch <stefan.pietsch@lsexperts.de>
Signed-off-by: NGleb Natapov <gleb@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -3404,15 +3404,22 @@ static void vmx_get_segment(struct kvm_vcpu *vcpu,
 	var->limit = vmx_read_guest_seg_limit(vmx, seg);
 	var->selector = vmx_read_guest_seg_selector(vmx, seg);
 	ar = vmx_read_guest_seg_ar(vmx, seg);
+	var->unusable = (ar >> 16) & 1;
 	var->type = ar & 15;
 	var->s = (ar >> 4) & 1;
 	var->dpl = (ar >> 5) & 3;
-	var->present = (ar >> 7) & 1;
+	/*
+	 * Some userspaces do not preserve unusable property. Since usable
+	 * segment has to be present according to VMX spec we can use present
+	 * property to amend userspace bug by making unusable segment always
+	 * nonpresent. vmx_segment_access_rights() already marks nonpresent
+	 * segment as unusable.
+	 */
+	var->present = !var->unusable;
 	var->avl = (ar >> 12) & 1;
 	var->l = (ar >> 13) & 1;
 	var->db = (ar >> 14) & 1;
 	var->g = (ar >> 15) & 1;
-	var->unusable = (ar >> 16) & 1;
 }
 
 static u64 vmx_get_segment_base(struct kvm_vcpu *vcpu, int seg)