Hello, fellow hackers and pen testers. Welcome to Programming Thursdays! Today, we’re going to talk about a topic that is often overlooked but is an essential part of the software stack: microcode.

Microcode is the lowest software layer in a computer system, below the operating system, drivers, and even the firmware. It controls the behavior of the CPU and allows us to run code on our machines.

In this article, we’ll high-level look into microcode, exploring its history, how it works, and its importance for pen testers and red teamers. So grab a cup of coffee and let’s get started!

The History of Microcode

Microcode has been around for quite some time, with its roots tracing back to the early days of computing. IBM first used Microcode in the mid-1950s in its first commercial computer, the IBM 704.

The idea behind microcode was to simplify the CPU’s design by breaking down complex instructions into simpler ones that the CPU could execute. This allowed for more efficient processing and reduced the CPU’s overall complexity.

Over the years, microcode has evolved alongside the CPU, with each new generation of processors incorporating more advanced microcode. Today, microcode is an integral part of every modern CPU, and it allows us to run the software we rely on daily.

How Microcode Works

So, how does microcode work? At a high level, a microcode is a set of instructions that controls the behavior of the CPU. These instructions are stored in a small amount of memory within the CPU, called the microcode ROM.

When the CPU receives an instruction, it looks up the corresponding microcode instruction in the microcode ROM and executes it. This allows the CPU to break down complex instructions into simpler ones to perform more efficiently.

For example, let’s say we want to add two numbers together. The high-level instruction might look something like this:

ADD R1, R2

To execute this instruction, the CPU would need to perform several steps, including fetching the values of R1 and R2, adding them together, and storing the result back in R1. Instead of performing all of these steps directly, the CPU would break down the ADD instruction into a series of more straightforward microcode instructions like this:

LOAD R1
LOAD R2
ADD
STORE R1

Each of these microcode instructions performs a simple operation that the CPU can execute quickly and efficiently. By breaking down the ADD instruction into these more straightforward microcode instructions, the CPU can operate much more rapidly and with less complexity.

The Importance of Microcode for Pen Testers and Red Teamers

So, why is microcode necessary for pen testers and red teamers? There are a few key reasons:

  • Low-level access to the CPU: Because microcode is the lowest layer of software in the system, it provides direct access to the CPU itself. This allows pen testers and red teamers to perform low-level attacks that would be impossible using higher-level software.
  • Debugging and reverse engineering: Microcode is often used for debugging and reverse engineering purposes, allowing pen testers and red teamers to analyze the CPU’s behavior and understand how it executes code.
  • Exploiting vulnerabilities: Because microcode is such a low-level component of the system, software developers often overlook it when it comes to security. This means there may be vulnerabilities in the microcode that pen testers and red teamers can exploit to gain access to the system.

Examples of Microcode Attacks

Now that we understand the importance of microcode for pen testers and red teamers, let’s examine some examples of microcode attacks.

Meltdown and Spectre

In 2018, researchers discovered two significant vulnerabilities in modern CPUs’ microcode, Meltdown and Spectre. These vulnerabilities allowed attackers to read sensitive data from the memory of other processes running on the same system.

Meltdown and Spectre were caused by a flaw in how modern CPUs execute out-of-order instructions. By exploiting this flaw, attackers could force the CPU to execute instructions that should have been protected, allowing them to read sensitive data like passwords and encryption keys.

Meltdown and Spectre were industry wake-up calls, highlighting the importance of microcode security and the need for better process isolation.

Rowhammer

Another example of a microcode attack is Rowhammer, first discovered in 2014. Rowhammer is a technique that exploits a weakness in the design of modern DRAM memory.

By repeatedly accessing specific rows of memory, an attacker can cause nearby rows to flip, corrupting data or even allowing them to execute arbitrary code. This technique can bypass security protections and gain access to sensitive data.

Rowhammer is a powerful technique that highlights the importance of microcode security. It reminds us that even seemingly small system vulnerabilities can have serious consequences.

ROP

Return-oriented programming (ROP) is a technique that attackers use to bypass security protections by constructing a chain of small, “gadget” instructions that already exist in the system’s microcode.

By carefully selecting and chaining these gadgets, attackers can construct a sequence of instructions that performs the desired operation, even if it’s not a valid instruction in the system’s high-level language.

ROP is a powerful technique that allows attackers to bypass even the most sophisticated security protections. It highlights the importance of securing microcode against these types of attacks.

Resources

For those interested in exploring microcode further, there are a few resources that can be helpful:

  1. Intel Microcode Update Guidance: This page from Intel provides information on the latest microcode updates for Intel CPUs and guidance on how to install them.
  2. AMD CPU Microcode Updates: This page from AMD provides information on the latest microcode updates for AMD CPUs and guidance on how to install them.
  3. Microcorruption: Microcorruption is a free online game that teaches players how to exploit vulnerabilities in microcode. It’s a fun and engaging way to learn more about microcode and its importance for security.
  4. Ghidra: Ghidra is a free and open-source reverse engineering tool that supports analyzing microcode. It’s a powerful tool for analyzing CPU behavior and understanding how they execute code.
  5. Microcode Research: There are several research papers and articles on microcode that can be helpful for those interested in exploring the topic further. Some notable examples include “Meltdown: Reading Kernel Memory from User Space” and “Rowhammer.js: A Remote Software-Induced Fault Attack in JavaScript.”

Conclusion

In conclusion, microcode is an essential part of the software stack, providing low-level control over the CPU’s behavior. While software developers often overlook it, microcode is a critical component of modern security, and it’s essential for pen testers and red teamers to understand its importance.

By understanding how microcode works and how it can be attacked, pen testers and red teamers can identify vulnerabilities and weaknesses in the system, allowing them to perform more effective attacks and improve overall security.

So, the next time you’re working on a security assessment or pen testing engagement, remember the importance of microcode. It may be the lowest layer of the software stack, but it can also be the most powerful tool in your arsenal. Happy hacking!