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: Stefan Pietsch <stefan.pietsch@lsexperts.de>
Tested-by: Stefan Pietsch <stefan.pietsch@lsexperts.de>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index a7e18551c9689b5192d528ccae10ee9dbb20e84a..064d0be67ecc23734aa465541138d9b5be295277 100644 (file)
--- a/arch/x86/kvm/vmx.c
+++ b/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)