Xen through 4.8.x does not validate memory allocations during certain P2M operations, which allows guest OS users to obtain privileged host OS access, aka XSA-222.
https://xenbits.xen.org/xsa/advisory-222.html
https://security.gentoo.org/glsa/201710-17
https://security.gentoo.org/glsa/201708-03
http://www.securitytracker.com/id/1038732