  Programmable logic controllers (PLCs) are crucial devices for implementing
automated control in various industrial control systems (ICS), such as smart
power grids, water treatment systems, manufacturing, and transportation
systems. Owing to their importance, PLCs are often the target of cyber
attackers that are aiming at disrupting the operation of ICS, including the
nation's critical infrastructure, by compromising the integrity of control
logic execution. While a wide range of cybersecurity solutions for ICS have
been proposed, they cannot counter strong adversaries with a foothold on the
PLC devices, which could manipulate memory, I/O interface, or PLC logic itself.
These days, many ICS devices in the market, including PLCs, run on ARM-based
processors, and there is a promising security technology called ARM TrustZone,
to offer a Trusted Execution Environment (TEE) on embedded devices. Envisioning
that such a hardware-assisted security feature becomes available for ICS
devices in the near future, this paper investigates the application of the ARM
TrustZone TEE technology for enhancing the security of PLC. Our aim is to
evaluate the feasibility and practicality of the TEE-based PLCs through the
proof-of-concept design and implementation using open-source software such as
OP-TEE and OpenPLC. Our evaluation assesses the performance and resource
consumption in real-world ICS configurations, and based on the results, we
discuss bottlenecks in the OP-TEE secure OS towards a large-scale ICS and
desired changes for its application on ICS devices. Our implementation is made
available to public for further study and research.
