Google Research

SiliFuzz: Fuzzing CPUs by proxy

Google (2021)

Abstract

CPUs are getting more complex with every generation, on both the logical and the physical levels. Unsurprisingly, this leads to more bugs and defects in CPUs being overlooked during testing, which causes data corruption or other undesirable effects when these CPUs are used in production. Some defects may also be caused by aging.

If the RTL (“source code”) of a CPU is available, we could apply greybox fuzzing to the CPU model almost like any other software [Tri21]. However our targets are general purpose x86_64 CPUs produced by third parties, where we do not have the source, so in our case CPU implementations are opaque. Moreover, we are more interested in CPU defects (manufacturing problems that affect just one or several cores) as opposed to bugs (design problems that affect all cores of a given family of CPUs).

In this paper we present SiliFuzz, a work-in-progress system that finds CPU defects by fuzzing proxies, like CPU simulators or disassemblers, and then executing the accumulated test vectors (“corpus”) on actual CPUs on a large scale. The major difference between this work and traditional software fuzzing is that a software bug fixed once will be fixed for all installations of this software, while with CPU defects we have to test every individual core repeatedly over its lifetime due to wear and tear. We also analyze four groups of CPU defects that SiliFuzz has uncovered and the patterns shared by other findings.

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