Sharing is leaking: blocking transient-execution attacks with core-gapped confidential VMs

Confidential VMs on platforms such as Intel TDX, AMD SEV and Arm CCA promise greater security for cloud users against even a hypervisor-level attacker, but this promise has been shattered by repeated transient-execution vulnerabilities and CPU bugs. At the root of this problem lies the need to multiplex CPU cores with all their complex microarchitectural state among distrusting entities, with an untrusted hypervisor in control of the multiplexing. We propose core-gapped confidential VMs, a set of software-only modifications that ensure that no distrusting code shares a core, thus removing all same-core side-channels and transient-execution vulnerabilities from the guest's TCB. We present an Arm-based prototype along with a performance evaluation showing that, not only does core-gapping offer performance competitive with non-confidential VMs, the greater locality achieved by avoiding shared cores can even improve performance for CPU-intensive workloads.