Xen through 4.8.x allows local x86 PV guest OS kernel administrators to cause a denial of service (host hang or crash) by modifying the instruction stream asynchronously while performing certain kernel operations.
https://support.citrix.com/article/CTX219378
https://security.gentoo.org/glsa/201612-56
http://xenbits.xen.org/xsa/advisory-202.html
http://www.securitytracker.com/id/1037517