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 0af18077012e659fee211d37c5271791b59a7c36..0e2f2a405522ec226c41e2ab55baa5ee8477fe78 100644 (file)
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3259,15 +3259,22 @@ 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)