Re: [PATCH] KVM: VMX: mark unusable segment as nonpresent

Paolo Bonzini
2013-07-02T08:15:54Z

Il 28/06/2013 12:17, Gleb Natapov ha scritto:
> 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>
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 260a919..5402c94 100644
> --- 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)
> --
> Gleb.
>

Looks good, but for now I'm leaving it out of the 3.11 pull request.
Applied to queue.

Paolo