Linux 6.9-rc6 To Fix Accidentally Disabling Mitigations By Default For Non-x86 CPUs

Written by Michael Larabel in Linux Security on 28 April 2024 at 09:38 AM EDT. 2 Comments
A commit made to the Linux kernel three weeks ago accidentally broke the default CPU security mitigations for non-x86 CPUs. With code sent in today via x86/urgent ahead of tonight's Linux 6.9-rc6 release, that accidental default breakage is being addressed.

A patch to be merged today to Linux Git will re-enable CPU security mitigations by default for non-x86 architectures like POWER and Arm.

Arm server CPU

Sean Christopherson of Google explained in the patch fixing the issue:
"Rename x86's to CPU_MITIGATIONS, define it in generic code, and force it on for all architectures exception x86. A recent commit to turn mitigations off by default if SPECULATION_MITIGATIONS=n kinda sorta missed that "cpu_mitigations" is completely generic, whereas SPECULATION_MITIGATIONS is x86 specific.

Rename x86's SPECULATIVE_MITIGATIONS instead of keeping both and have it select CPU_MITIGATIONS, as having two configs for the same thing is unnecessary and confusing. This will also allow x86 to use the knob to manage mitigations that aren't strictly related to speculative execution.

Use another Kconfig to communicate to common code that CPU_MITIGATIONS is already defined instead of having x86's menu depend on the common CPU_MITIGATIONS. This allows keeping a single point of contact for all of x86's mitigations, and it's not clear that other architectures *want* to allow disabling mitigations at compile-time."

That patch is part of today's x86 fixes pull request. There's another patch to also now ignore the "mitigations=" kernel boot parameter if the kernel is configured with CPU_MITIGATIONS=n to already have mitigations disabled.

Today's x86 fixes pull also carries the patch further extending the range of AMD Zen 5 processor models.
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