The load_segment_descriptor implementation in arch/x86/kvm/emulate.c in the Linux kernel before 4.9.5 improperly emulates a "MOV SS, NULL selector" instruction, which allows guest OS users to cause a denial of service (guest OS crash) or gain guest OS privileges via a crafted application.
https://usn.ubuntu.com/3754-1/
https://github.com/torvalds/linux/commit/33ab91103b3415e12457e3104f0e4517ce12d0f3
https://bugzilla.redhat.com/show_bug.cgi?id=1414735
https://access.redhat.com/errata/RHSA-2017:1616
https://access.redhat.com/errata/RHSA-2017:1615
http://www.securityfocus.com/bid/95673
http://www.openwall.com/lists/oss-security/2017/01/19/2
http://www.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.9.5