The AMX Lockdown: Critical KVM Bug Allows Guest VMs to Crash Linux Hosts

On 25 December 2025, security coverage describes a flaw in Linux KVM that can trigger a kernel panic on the host when a guest VM uses Intel AMX. The issue arises from how KVM handles FPU and XSAVE state, with AMX inside a guest potentially provoking an unexpected #NM exception and crashing the host.

A series of fixes has been proposed, and Linux KVM maintainer Paolo Bonzini of Red Hat published patches detailing the root cause; the XFD value configured by the guest and stored in fpstate->xfd is reused during both guest execution and host-state saves via XSAVE. The patches aim to separate the guest XFD from the host value and add selftests to reproduce the desynchronisation between XFD and XSAVE state. At the time of writing, no public CVE appears to have been assigned, and the issue is described as a potential denial-of-service vulnerability.