arch/x86/mm.c in Xen allows local PV guest OS users to gain host OS privileges via vectors related to map_grant_ref.
www.debian.org/security/2017/dsa-3969
www.openwall.com/lists/oss-security/2017/08/15/2
www.securityfocus.com/bid/100342
www.securitytracker.com/id/1039174
xenbits.xen.org/xsa/advisory-227.html
bugzilla.redhat.com/show_bug.cgi?id=1477657
security.gentoo.org/glsa/201801-14
support.citrix.com/article/CTX225941