MxU: Towards Predictable, Flexible, and Efficient Memory Access Control for the Secure IoT

The advanced functionality requirements of modern embedded and Internet of Things (IoT) devices - from autonomous vehicles, to city and power-grid management - are driving an ever-increasing software complexity. At the same time, the pervasive internet connections of these systems necessitate the fundamental design of security into these devices. The isolation of complex features from those that are critical through protection domains is an effective means to constrain the scope of faults and security breaches. Common hardware-provided memory facilities to enforce protection domains through memory access control - including Memory Management Units (MMUs) usually found in microprocessors, and Memory Protection Units (MPUs) usually found in microcontrollers - must meet the goals of enabling flexible, efficient and dynamic management of memory, and must enable tight bounds on the worst-case execution of critical code. Unfortunately, current system memory management facilities are ill-prepared to handle this challenge: MMUs that use extensive caches to achieve strong average-case performance suffer from debilitating worst-case and even average-case behavior under hefty interference, while MPUs struggle to provide flexible memory management. This paper details MxU, a memory protection and allocation abstraction that integrates temporal specifications into the memory management subsystem, to enable portable code to achieve both predictable, tightly-bounded execution and dynamic management across both MMU- and MPU-based systems. We implement MxU in the Composite microkernel, and evaluate its flexibility and predictability over two different architectures: a MPU-based Cortex-M7 microcontroller and a MMU-based Cortex-A9 microprocessor using a suite of modern applications including neural network-based inference, SQLite, and a javascript runtime. For MMU-based systems, MxU reduces application TLB stall by up to 68.0%. For MPU-based systems, MxU enables flexible dynamic memory management often with application overheads of 1%, increasing to 6.1% under significant interference.