The x86 segment base write emulation functionality in Xen 4.4.x through 4.7.x allows local x86 PV guest OS administrators to cause a denial of service (host crash) by leveraging lack of canonical address checks.
https://support.citrix.com/article/CTX218775
https://security.gentoo.org/glsa/201612-56
http://xenbits.xen.org/xsa/advisory-193.html