A flaw was found in the way Linux kernel KVM hypervisor before 4.18 emulated instructions such as sgdt/sidt/fxsave/fxrstor. It did not check current privilege(CPL) level while emulating unprivileged instructions. An unprivileged guest user/process could use this flaw to potentially escalate privileges inside guest. Andy Lutomirski and Mika Penttilä discovered that the KVM implementation in the Linux kernel did not properly check privilege levels when emulating some instructions. An unprivileged attacker in a guest VM could use this to escalate privileges within the guest.