The security of embedded systems can be dramatically improved through the use of formally verified isolation mechanisms such as separation kernels, hypervisors, or microkernels. For trustworthiness, particularly for system level behaviour, the verifications need precise models of the underlying hardware. Such models are hard to attain, highly complex, and proofs of their security properties may not easily apply to similar but different platforms. This may render verification economically infeasible. To address these issues, we propose a compositional top-down approach to embedded system specification and verification, where the system-on-chip is modeled as a network of distributed automata communicating via paired synchronous message passing. Using abstract specifications for each component allows to delay the development of detailed models for cores, devices, etc., while still being able to verify high level security properties like integrity and confidentiality, and soundly refine the result for different instantiations of the abstract components at a later stage. As a case study, we apply this methodology to the verification of information flow security for an industry scale security-oriented hypervisor on the ARMv8-A platform. The hypervisor statically assigns (multiple) cores to each guest system and implements a rudimentary, but usable, inter guest communication discipline. We have completed a pen-and-paper security proof for the hypervisor down to state transition level and report on a partially completed verification of guest mode security in the HOL4 theorem prover.

For the verification of system software, information flow properties of the instruction set architecture (ISA) are essential.They show how information propagates through the processor, including sometimes opaque control registers.Thus, they can be used to guarantee that user processes cannot infer the state of privileged system components, such as secure partitions.Formal ISA models - for example for the HOL4 theorem prover - have been available for a number of years. However, little work has been published on the formal analysis of these models.In this paper, we present a general framework for proving information flow properties of a number of ISAs automatically, for example for ARM.The analysis is represented in HOL4 using a direct semantical embedding of noninterference, and does not use an explicit type system, in order to (i) minimize the trusted computingbase, and to (ii) support a large degree of context-sensitivity, which is needed for the analysis.The framework determines automatically which system components are accessible at a given privilege level, guaranteeing both soundness and accuracy.

Caches pose a significant challenge to formal proofs of security for code executing on application processors, as the cache access pattern of security-critical services may leak secret information. This paper reveals a novel attack vector, exposing a low-noise cache storage channel that can be exploited by adapting well-known timing channel analysis techniques. The vector can also be used to attack various types of security-critical software such as hypervisors and application security monitors. The attack vector uses virtual aliases with mismatched memory attributes and self-modifying code to misconfigure the memory system, allowing an attacker to place incoherent copies of the same physical address into the caches and observe which addresses are stored in different levels of cache. We design and implement three different attacks using the new vector on trusted services and report on the discovery of an 128-bit key from an AES encryption service running in TrustZone on Raspberry Pi 2. Moreover, we subvert the integrity properties of an ARMv7 hypervisor that was formally verified against a cache-less model. We evaluate well-known countermeasures against the new attack vector and propose a verification methodology that allows to formally prove the effectiveness of defence mechanisms on the binary code of the trusted software.

Security monitor inlining is a technique for security policy enforcement whereby monitor functionality is injected into application code in the style of aspect-oriented programming. The intention is that the injected code enforces compliance with the policy (security), and otherwise interferes with the application as little as possible (conservativity and transparency). Such inliners are said to be correct. For sequential Java-like languages, inlining is well understood, and several provably correct inliners have been proposed. For multithreaded Java one difficulty is the need to maintain a shared monitor state. We show that this problem introduces fundamental limitations in the type of security policies that can be correctly enforced by inlining. A class of race-free policies is identified that precisely characterizes the inlineable policies by showing that inlining of a policy outside this class is either not secure or not transparent, and by exhibiting a concrete inliner for policies inside the class which is secure, conservative and transparent. The inliner is implemented for Java and applied to a number of practical application security policies. Finally, we discuss how certification in the style of proof-carrying code could be supported for inlined programs by using annotations to reduce a potentially complex verification problem for multithreaded Java bytecode to sequential verification of just the inlined code snippets.

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.

We present MProsper, a trustworthy system to prevent code injection in Linux on embedded devices. MProsper is a formally verified run-time monitor, which forces an untrusted Linux to obey the executable space protection policy; a memory area can be either executable or writable, but cannot be both. The executable space protection allows the MProsper’s monitor to intercept every change to the executable code performed by a user application or by the Linux kernel. On top of this infrastructure, we use standard code signing to prevent code injection. MProsper is deployed on top of the Prosper hypervisor and is implemented as an isolated guest. Thus MProsper inherits the security property verified for the hypervisor: (i) Its code and data cannot be tampered by the untrusted Linux guest and (ii) all changes to the memory layout is intercepted, thus enabling MProsper to completely mediate every operation that can violate the desired security property. The verification of the monitor has been performed using the HOL4 theorem prover and by extending the existing formal model of the hypervisor with the formal specification of the high level model of the monitor.

In order to host a general purpose operating system, hypervisors need to virtualize the CPU memory subsystem. This entails dynamically changing MMU resources, in particular the page tables, to allow a hosted OS to reconfigure its own memory. In this paper we present the verification of the isolation properties of a hypervisor design that uses direct paging. This virtualization approach allows to host commodity Oss without requiring either shadow data structures or specialized hardware support. Our verification targets a system consisting of a commodity CPU for embedded devices (ARMv7), a hypervisor and an untrusted guest running Linux.The verification involves three steps: (i) Formalization of an ARMv7 CPU that includes the MMU, (ii) Formalization of a system behavior that includes the hypervisor and the untrusted guest (iii) Verification of the isolation properties. Formalization and proof are done in the HOL4 theorem prover, thus allowing to re-use the existing HOL4 ARMv7 model developed in Cambridge.

Low level code is challenging: It lacks structure, it uses jumps and symbolic addresses, the control ow is often highly optimized, and registers and memory locations may be reused in ways that make typing extremely challenging. Information ow properties create additional complications: They are hyperproperties relating multiple executions, and the possibility of interrupts and concurrency, and use of devices and features like memory-mapped I/O requires a departure from the usual initial-state nal-state account of noninterference. In this work we propose a novel approach to relational verication for machine code. Verication goals are expressed as equivalence of traces decorated with observation points. Relational verication conditions are propagated between observation points using symbolic execution, and discharged using rst-order reasoning. We have implemented an automated tool that integrates with SMT solvers to automate the verication task. The tool transforms ARMv7 binaries into an intermediate, architecture-independent format using the BAP toolset by means of a veried translator. We demonstrate the capabilities of the tool on a separation kernel system call handler, which mixes hand-written assembly with gcc-optimized output, a UART device driver and a crypto service modular exponentiation routine.