Linux Kernel CVE-2026-89959

Requires an IBM Z / s390 host running the Linux kernel with KVM and the vfio-ap module actively passing AP (Adjunct Processor) crypto adapters and control domains through to a guest via a matrix mdev. The flawed path is reached only when a host administrator explicitly removes/unplugs a control domain from that mdev; the failure is that the removal does not propagate to the guest. LIMITING FACTORS: exploitation is confined to systems using s390 crypto passthrough (a rare deployment mode), the trigger is a privileged host reconfiguration action rather than remote input, and the resulting exposure is limited to retained crypto control-domain access rather than arbitrary code execution.

The vendor CVSS of 8.8 (AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H) substantially overstates real-world risk for what is fundamentally a control-flow bug in host reconfiguration logic. The defect: vfio_ap_config_remove used bitmap_andnot instead of bitmap_and when clearing the matrix.adm bitmap, so control domains an administrator explicitly unplugs are NOT actually withdrawn from the running KVM guest - the guest retains stale access to AP crypto control domains it should no longer possess. This is an isolation/access-control failure, not a memory-corruption RCE, so the C:H/I:H/A:H triad is inflated: the practical impact is bounded (a guest can continue issuing administrative/control commands to crypto domains that were meant to be revoked). It is triggered by a legitimate host administrative action (removing a control domain from an mdev), so PR:L understates the privilege actually needed to reach the buggy path (host-side vfio-ap configuration = admin). There is NO KEV listing, NO public POC, and no EPSS signal in the input - consistent with a low-real-risk, niche s390 issue. No availability impact is evident from the description despite A:H being asserted. Net: this is a correctness/isolation fix for IBM Z crypto passthrough, not an urgent priority for most fleets.

On an IBM Z / s390 host running KVM with vfio-ap crypto passthrough, an administrator removes (unplugs) a control domain from a guest's mdev, expecting the guest to lose the ability to issue administrative crypto commands to that domain. Because of the bitmap_andnot bug, the domain is never actually cleared from the guest's adm bitmap, so the guest silently retains control-domain access and can continue performing privileged crypto operations (e.g., domain management) it was supposed to be denied. No public exploit code is identified at the time of analysis.