Custom Processing Unit: Tracing and Patching Intel Atom Microcode

Sandro Schärfer

Discover the secrets of Intel Atom microcode updates, including tracing, patching, and decryption, to unlock custom CPU behavior and control flow redirection.

Key takeaways

The microcode update is done using an undocumented instruction that returns random data.
The CR bus is a critical component that allows CPU internal components to interact with each other.
Intel’s CPUs have a secret stored in the CR bus that can be accessed through the L dot component.
The microcode sequencer can be accessed through the L dot and allows modification of microcode.
The microcode can be decrypted and the decrypted content can be modified.
The matching patch mechanism allows the CPU to redirect control flow to microcode ROM.
By tracing the microcode update routine, the CPU’s internal state can be reconstructed.
The microcode updates can be patched to implement custom CPU behavior.
The CPU’s secret keys can be used to decrypt the microcode updates.
The CR bus is used to communicate between CPU internal components.
The CPU’s microcode can be analyzed to understand how it works.
By hooking the microcode update routine, the CPU’s internal state can be observed.
The microcode update can be used to implement custom CPU functionality.
The CPU’s microcode can be patched to implement custom CPU behavior.

Custom Processing Unit: Tracing and Patching Intel Atom Microcode

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