Trustworthy Memory Isolation of Linux on Embedded Devices

The isolation of security critical components from an untrusted OS allows to both protect applications and to harden the OS itself, for instance by run-time monitoring. Virtualization of the memory subsystem is a key component to provide such isolation. We present the design, implementation and verification of a virtualization platform for the ARMv7-A processor family. Our design is based on direct paging, an MMU virtualization mechanism previously introduced by Xen for the x86 architecture, and used later with minor variants by the Secure Virtual Architecture, SVA. We show that the direct paging mechanism can be implemented using a compact design, suitable for formal verification down to a low level of abstraction, without penalizing system performance. The verification is performed using the HOL4 theorem prover and uses a detailed model of the ARMv7-A ISA, including the MMU. We prove memory isolation of the hosted components along with information flow security for an abstract top level model of the virtualization mechanism. The abstract model is refined down to a HOL4 transition system closely resembling a C implementation. The virtualization mechanism is demonstrated on real hardware via a hypervisor capable of hosting Linux as an untrusted guest.