UNDOCUMENTED X86 INSTRUCTIONS TO CONTROL THE CPU AT THE MICROARCHITECTURE LEVEL IN MODERN INTEL PROCESSORS Mark Ermolov Dmitry Sklyarov Positive Technologies Positive Technologies
[email protected] [email protected] Maxim Goryachy independent researcher
[email protected] July 7, 2021 ABSTRACT At the beginning of 2020, we discovered the Red Unlock technique that allows extracting microcode (ucode) and targeting Intel Atom CPUs. Using the technique we were able to research the internal structure of the microcode and then x86 instructions implementation. We found two undocumented x86 instructions which are intendent to control the microarhitecture for debug purposes. In this paper we are going to introduce these instructions and explain the conditions under which they can be used on public-available platforms. We believe, this is a unique opportunity for third-party researchers to better understand the x86 architecture. Disclamer. All information is provided for educational purposes only. Follow these instructions at your own risk. Neither the authors nor their employer are responsible for any direct or consequential damage or loss arising from any person or organization acting or failing to act on the basis of information contained in this paper. Keywords Intel · Microcode · Undocumented · x86 1 Introduction The existence of undocumented mechanisms in the internals of modern CPUs has always been a concern for information security researchers and ordinary users. Assuming that such mechanisms do exist, the main worry is that