Deconstructing Xen

Hypervisors have quickly become essential but are vulnerable to attack. Unfortunately, efficiently hardening hypervisors is challenging because they lack a privileged security monitor and decomposition strategies. In this work we systematically analyze the 191 Xen hypervisor vulnerabilities from Xen Security Advisories, revealing that the majority (144) are in the core hypervisor not Dom0. We then use the analysis to provide a novel deconstruction of Xen, called Nexen, into a security monitor, a shared service domain, and per-VM Xen slices that are isolated by a least-privileged sandboxing framework. We implement Nexen using the Nested Kernel architecture, efficiently nesting itself within the Xen address space, and extend the Nested Kernel design by adding services for arbitrarily many protection domains along with dynamic allocators, data isolation, and cross-domain control-flow integrity. The effect is that Nexen confines VM-based hypervisor compromises to single Xen VM instances, thwarts 74% (107/144) of known Xen vulnerabilities, and enforces Xen code integrity (defending against all code injection compromises) while observing negligible overhead (1.2% on average). Overall, we believe that Nexen is uniquely positioned to provide a fundamental need for hypervisor hardening at minimal performance and implementation costs.