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Exploiting the Analog Properties of Digital Circuits for Malicious Hardware


chip

While the move to smaller transistors has been a boon for performance it has dramatically increased the cost to fabricate chips using those smaller transistors. This forces the vast majority of chip design companies to trust a third party—often overseas—to fabricate their design. To guard against shipping chips with errors (intentional or otherwise) chip design companies rely on post-fabrication testing. Unfortunately, this type of testing leaves the door open to malicious modifications since attackers can craft attack triggers requiring a sequence of unlikely events, which will never be encountered by even the most diligent tester. In this paper, we show how a fabrication-time attacker can leverage analog circuits to create a hardware attack that is small (i.e., requires as little as one gate) and stealthy (i.e., requires an unlikely trigger sequence before affecting a chip's functionality). In the open spaces of an already placed and routed design, we construct a circuit that uses capacitors to siphon charge from nearby wires as they transit between digital values. When the capacitors are fully charged, they deploy an attack that forces a victim flip-flop to a desired value. We weaponize this attack into a remotely controllable privilege escalation by attaching the capacitor to a controllable wire and by selecting a victim flip-flop that holds the privilege bit for our processor. We implement this attack in an OR1200 processor and fabricate a chip. Experimental results show that the purposed attack works. It eludes activation by a diverse set of benchmarks and evades known defenses.

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1. Introduction

The trend toward smaller transistors in integrated circuits, while beneficial for higher performance and lower power, has made fabricating a chip expensive. For example, it costs 15% more to set up the fabrication line for each successive process node and by 2020 it is expected that setting up a fabrication line for the smallest transistor size will require a $20 billion upfront investment.18 To amortize the cost of fabrication development, most hardware companies outsource fabrication.

Outsourcing of chip fabrication opens up hardware to attack. These hardware attacks can evade software checks because software must trust hardware to faithfully implement the instructions.6, 12 Even worse, if there is an attack in hardware, it can contaminate all layers of a system that depend on the hardware and violates high-level security policies correctly implemented by software.


 

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