EPTI: Efficient Defence against Meltdown Attack for Unpatched VMs

The Meltdown vulnerability, which exploits the inherent out-of-order execution in common processors like x86, ARM and PowerPC, has shown to break the fundamental isolation boundary between user and kernel space. This has stimulated a non-trivial patch to modern OS to separate page tables for user space and kernel space, namely, KPTI (kernel page table isolation). While this patch stops kernel memory leakages from rouge user processes, it mandates users to patch their kernels (usually requiring a reboot), and is currently only available on the latest versions of OS kernels. Further, it also introduces non-trivial performance overhead due to page table switching during user/kernel crossings. In this paper, we present EPTI, an alternative approach to defending against the Meltdown attack for unpatched VMs (virtual machines) in cloud, yet with better performance than KPTI. Specifically, instead of using two guest page tables, we use two EPTs (extended page tables) to isolate user space and kernel space, and unmap all the kernel space in user’s EPT to achieve the same effort of KPTI. The switching of EPTs is done through a hardware-support feature called EPT switching within guest VMs without hypervisor involvement. Meanwhile, EPT switching does not flush TLB since each EPT has its own TLB, which further reduces the overhead. We have implemented our design and evaluated it on Intel Kaby Lake CPU with different versions of Linux kernel. The results show that EPTI only introduces up to 13% overhead, which is around 45% less than KPTI.