Xen 4.0.x through 4.7.x mishandle x86 task switches to VM86 mode, which allows local 32-bit x86 HVM guest OS users to gain privileges or cause a denial of service (guest OS crash) by leveraging a guest operating system that uses hardware task switching and allows a new task to start in VM86 mode.
https://support.citrix.com/article/CTX218775
https://security.gentoo.org/glsa/201612-56
http://xenbits.xen.org/xsa/advisory-192.html