KVM: VMX: mark unusable segment as nonpresent

commit 03617c188f41eeeb4223c919ee7e66e5a114f2c6 upstream.

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.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 260a9193955538b4fea743045b2f964b2736b24e..5402c94ab76835aa6a00f0d503b8992a899b28b6 100644 (file)
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3399,15 +3399,22 @@ 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)