The em_sysenter function in arch/x86/kvm/emulate.c in the Linux kernel before 3.18.5, when the guest OS lacks SYSENTER MSR initialization, allows guest OS users to gain guest OS privileges or cause a denial of service (guest OS crash) by triggering use of a 16-bit code segment for emulation of a SYSENTER instruction.
https://github.com/torvalds/linux/commit/f3747379accba8e95d70cec0eae0582c8c182050
https://bugzilla.redhat.com/show_bug.cgi?id=1186448
http://www.ubuntu.com/usn/USN-2518-1
http://www.ubuntu.com/usn/USN-2517-1
http://www.ubuntu.com/usn/USN-2516-1
http://www.ubuntu.com/usn/USN-2515-1
http://www.ubuntu.com/usn/USN-2514-1
http://www.ubuntu.com/usn/USN-2513-1
http://www.securityfocus.com/bid/72842
http://www.oracle.com/technetwork/topics/security/linuxbulletinoct2015-2719645.html
http://www.oracle.com/technetwork/topics/security/linuxbulletinjan2016-2867209.html
http://www.mandriva.com/security/advisories?name=MDVSA-2015:058
http://www.kernel.org/pub/linux/kernel/v3.x/ChangeLog-3.18.5
http://www.debian.org/security/2015/dsa-3170