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.securityfocus.com/bid/84594
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
Source: MITRE
Published: 2016-04-12
Updated: 2016-12-03
Type: CWE-264
Base Score: 7.2
Vector: AV:L/AC:L/Au:N/C:C/I:C/A:C
Impact Score: 10
Exploitability Score: 3.9
Severity: HIGH
Base Score: 7.8
Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Impact Score: 5.9
Exploitability Score: 1.8
Severity: HIGH