Silhouette: Efficient Intra-Address Space Isolation for Protected Shadow Stacks on Embedded Systems
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Embedded systems are increasingly deployed in devices that can have physical consequences if compromised by an attacker - including automobile control systems, smart locks, drones, and implantable medical devices. Due to resource and execution-time constraints, C is the primary programming language for writing both operating systems and bare-metal applications on these devices. Unfortunately, C is neither memory safe nor type safe. In this paper, we present an efficient intra-address space isolation technique for embedded ARM processors that leverages unprivileged store instructions. Using a compiler transformation, dubbed store promotion, which transforms regular stores into unprivileged equivalents, we can restrict a subset of the program's memory accesses to unprivileged memory while simultaneously allowing security instrumentation to access security-critical data structures (e.g., a shadow stack) - all without the need for expensive context switching. Using store promotion, we built Silhouette: a software defense that mitigates control-flow hijacking attacks using a combination of hardware configuration, runtime instrumentation, and code transformation. Silhouette enforces Control-Flow Integrity and provides an incorruptible shadow stack for return addresses. We implemented Silhouette on an ARMv7-M board and our evaluation shows that Silhouette incurs an arithmetic mean of 9.23 and 16.93 alternative implementation of Silhouette, which incurs just 2.54 overhead and 5.40
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