Linux Security Summit Europe 2025

Microarchitectural attacks such as Branch History Injection (BHI) can expose kernel data when instructions at a mispredicted indirect call target are executed speculatively with malicious data crafted by the attacker.

FineIBT (Fine-grain Control-flow Enforcement with Indirect Branch Tracking) is a hardening technique adopted by the Linux kernel (first merged in 6.2) that performs a check at each indirect call target to ensure that the target’s type (e.g., void (*)(int)) matches the type of the function pointer that was used to make the call. Although FineIBT can provide substantial defense-in-depth against architectural attacks such as Call-Oriented Programming (COP), its current implementation does not address microarchitectural attacks.

This talk introduces a new enhancement to FineIBT that hardens the Linux kernel against a plethora of microarchitectural attacks—including BHI—by poisoning the contents of live registers whenever the FineIBT check fails, thus preventing an attacker from using those registers to pass malicious data to a mis-predicted call target. This enhancement has been merged into Linux 6.15.