CVE-2017-12855

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. 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. Xen 4.9, 4.8, 4.7, 4.6, and 4.5 are affected.