ISSUE DESCRIPTION
=================

Xen maintains the _GTF_{read,writ}ing bits as appropriate, to inform the
guest that a grant is in use.  A guest is expected not to modify the
grant details while it is in use, whereas the guest is free to
modify/reuse the grant entry when it is not in use.

Under some circumstances, Xen will clear the status bits too early,
incorrectly informing the guest that the grant is no longer in use.

IMPACT
======

A guest may prematurely believe that a granted frame is safely private
again, and reuse it in a way which contains sensitive information, while
the domain on the far end of the grant is still using the grant.

VULNERABLE SYSTEMS
==================

All systems are vulnerable.

MITIGATION
==========

There are no mitigations.

External References:

http://xenbits.xen.org/xsa/advisory-230.html

Acknowledgements:

Name: the Xen project
Upstream: Jan Beulich (SUSE)