CVE-2026-31561

/// Technical description

In the Linux kernel, the following vulnerability has been resolved:

x86/cpu: Remove X86_CR4_FRED from the CR4 pinned bits mask

Commit in Fixes added the FRED CR4 bit to the CR4 pinned bits mask so

that whenever something else modifies CR4, that bit remains set. Which

in itself is a perfectly fine idea.

However, there's an issue when during boot FRED is initialized: first on

the BSP and later on the APs. Thus, there's a window in time when

exceptions cannot be handled.

This becomes particularly nasty when running as SEV-{ES,SNP} or TDX

guests which, when they manage to trigger exceptions during that short

window described above, triple fault due to FRED MSRs not being set up

yet.

See Link tag below for a much more detailed explanation of the

situation.

So, as a result, the commit in that Link URL tried to address this

shortcoming by temporarily disabling CR4 pinning when an AP is not

online yet.

However, that is a problem in itself because in this case, an attack on

the kernel ne

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References