Xen 4.4.x, when running on an ARM system and "handling an unknown system register access from 64-bit userspace," returns to an instruction of the trap handler for kernel space faults instead of an instruction that is associated with faults in 64-bit userspace, which allows local guest users to cause a denial of service (crash) and possibly gain privileges via a crafted process.
https://exchange.xforce.ibmcloud.com/vulnerabilities/95233
http://xenbits.xenproject.org/xsa/advisory-103.html
http://www.securitytracker.com/id/1030725