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: [email protected] # 3.9+
Reported-by: Stefan Pietsch <[email protected]>
Tested-by: Stefan Pietsch <[email protected]>
Signed-off-by: Gleb Natapov <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

1 files changed, 9 insertions, 2 deletions

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index a7e18551c968..064d0be67ecc 100644
--- 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)