SMT Vulnerabilities on Linux: MMIO_Stale_Data

While checking CPU vulnerabilities in Linux, if the result shows mmio_stale_data: Mitigation: Clear CPU buffers; SMT vulnerability, it indicates that your CPU has a hardware flaw known as MMIO Stale Data.

( For a full guide on checking CPU vulnerabilities in Linux, refer to this -> blog.)

Reason for This Vulnerability

Intel’s MMIO Stale Data flaw affects all processors up to the 11th generation. These CPUs require microcode and OS updates to reduce the risk, but the flaw remains in the hardware. Starting with the 12th generation (Alder Lake), Intel redesigned the chips, fixing the issue in hardware.

Linux mitigates the risk by clearing CPU buffers, but if Hyper-Threading (SMT) is enabled, a small chance of data leakage remains.

For normal use, this is generally safe, but the only way to completely block it is to disable Hyper-Threading in the BIOS, which may reduce system performance.

What is Hyper-Threading (SMT)?

The image shows the Linux kernel clears CPU buffers during key moments, like context switches, to reduce MMIO Stale Data risks.  

But if Hyper-Threading (SMT) is enabled, it poses a small risk of data leakage; that is why the output is marked as vulnerable.

Intel calls it Hyper-Threading, while Linux refers to it as SMT (Simultaneous Multithreading). It allows a single physical CPU core to run two threads simultaneously, sharing internal resources like execution units, caches, and buffers.

This improves performance, especially for multitasking, but it also means one thread can potentially access leftover data from the other thread.

Why Does It Cause Vulnerabilities Like MMIO Stale Data?

Many CPU vulnerabilities are side-channel leaks. When two threads run on the same physical core with SMT enabled, one thread can potentially infer data from the other by probing shared resources.

Even though Linux clears CPU buffers, sharing the same core leaves a theoretical leak path, which is why Linux reports “SMT vulnerable.”

How to Check if Hyper-Threading (SMT) Is Enabled on Linux

Run the following command:

lscpu | grep -E 'Thread|Core|Socket'

The output will look similar to this:

If the thread count per core is 2, Hyper-Threading is enabled.

If it shows 1, Hyper-Threading is disabled.

Conclusion

MMIO Stale Data is a known vulnerability in Intel CPUs, where attackers could potentially read residual data from CPU buffers.

For a more detailed explanation of the MMIO Stale Data vulnerability, refer to the blog below.