Medium [CVE-2026-64284] Ensure vendor's exit handler runs before fastpath userspace exits

In the Linux kernel, the following vulnerability has been resolved: KVM: x86: Ensure vendor's exit handler runs before fastpath userspace exits Move the handling of fastpath userspace exits into vendor code to ensure KVM runs vendor specific operations that need to run before userspace gains control of the vCPU.

E.g. for VMX (and soon to be for SVM as well), KVM needs to flush the PML buffer prior to exiting to userspace, otherwise any memory written by the final KVM_RUN might never be flagged as dirty. Note, waiting to snapshot CR0 and CR3 until svm_handle_exit() is flawed in general, as that risks consuming stale state in a fastpath handler.

That will be addressed in a future change. A flaw was found in the Linux kernel's Kernel-based Virtual Machine (KVM) for x86 systems.

This vulnerability occurs when KVM handles fastpath exits, where it may fail to execute vendor-specific cleanup operations before returning control to the virtual machine's userspace. Consequently, memory written by the virtual machine might not be correctly flagged as dirty, leading to the consumption of outdated data.

A local attacker with access to a virtual machine could potentially exploit this to cause data integrity issues within the virtualized environment. Red Hat severity: Moderate — CVSS 5.5 (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H).

Weakness: CWE-367.