The __switch_to function in arch/x86/kernel/process_64.c in the Linux kernel does not properly context-switch IOPL on 64-bit PV Xen guests, which allows local guest OS users to gain privileges, cause a denial of service (guest OS crash), or obtain sensitive information by leveraging I/O port access.
http://www.debian.org/security/2016/dsa-3607
http://www.oracle.com/technetwork/topics/security/linuxbulletinapr2016-2952096.html
http://www.securitytracker.com/id/1035308
http://www.ubuntu.com/usn/USN-2968-1
http://www.ubuntu.com/usn/USN-2968-2
http://www.ubuntu.com/usn/USN-2969-1
http://www.ubuntu.com/usn/USN-2970-1
http://www.ubuntu.com/usn/USN-2971-1
http://www.ubuntu.com/usn/USN-2971-2
http://www.ubuntu.com/usn/USN-2971-3
http://www.ubuntu.com/usn/USN-2996-1