The PV pagetable code in arch/x86/mm.c in Xen 4.7.x and earlier allows local 32-bit PV guest OS administrators to gain host OS privileges by leveraging fast-paths for updating pagetable entries.
https://security.gentoo.org/glsa/201611-09
http://xenbits.xen.org/xsa/xsa182-unstable.patch
http://xenbits.xen.org/xsa/xsa182-4.6.patch
http://xenbits.xen.org/xsa/xsa182-4.5.patch
http://xenbits.xen.org/xsa/advisory-182.html
http://www.securitytracker.com/id/1036446
http://www.securityfocus.com/bid/92131
http://www.oracle.com/technetwork/topics/security/ovmbulletinjul2016-3090546.html