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.

A flaw was discovered in the Kernel Virtual Machine's (KVM) emulation of
the SYSTENTER instruction when the guest OS does not initialize the
SYSENTER MSRs. A guest OS user could exploit this flaw to cause a denial of
service of the guest OS (crash) or potentially gain privileges on the guest
OS.