Dynamic Virtual Address Range Adjustment for Intra-Level Privilege Separation on ARM

Abstract

Privilege separation has long been considered as a fundamental principle in software design to mitigate the potential damage of a security attack. Much effort has been given to develop various privilege separation schemes where a monolithic OS or hypervisor is divided into two privilege domains where one domain is logically more privileged than the other even if both run at an identical processor privilege level. We say that privilege separation is intra-level if it is implemented for software of a certain privilege level without any involvement or assistance of more privileged software. In general, realizing intra-level privilege separation mandates developers to rely on certain security features of the underlying hardware. So far, such development efforts however have been much less focused on ARM architectures than on the Intel x86 family mainly because the architectural provision of ARM security features was relatively insufficient. Unlike on x86, as a result, there exists no full intra-level scheme that can be universally applied to any privilege level on ARM. However, as malware and attacks increase against virtually every level of privileged software including an OS, a hypervisor and even the highest privileged software armored by TrustZone, we have been motivated to develop a technique, named as Hilps, to realize true intra-level privilege separation in all these levels of privileged software on ARM. Pivotal to the success of Hilps is the support from a new hardware feature of ARM s latest 64-bit architecture, called TxSZ, which we manipulate to elastically adjust the accessible virtual address range for a program. In our experiments, we have applied Hilps to retrofit the core software mechanisms for privilege separation into existing system software and evaluated the performance of the resulting system. According to the experimental results, the system incurs on average just less than 1 % overhead; hence, we conclude that Hilps is quite promising for practical use in real deployments.