CVE-2026-89957

CVE-2026-89957 is a logic flaw in the Linux kernel's s390/vfio-ap driver, which passes IBM Z Adjunct Processor (AP) crypto hardware into KVM guests via mediated devices. The hot-unplug handler vfio_ap_mdev_hot_unplug_cfg() relied on the return value of bitmap_andnot(), which returns false when the resulting bitmap is empty; therefore, when the only adapter, domain, or control domain assigned to an mdev is removed from the host AP configuration, do_hotplug stays 0 and vfio_ap_mdev_update_guest_apcb() is never called. As a result, a running KVM guest retains stale hardware access to AP crypto devices that were unplugged or de-assigned from its authorized configuration, breaching the host/guest isolation boundary around cryptographic hardware and potentially key material. Only IBM Z / LinuxONE systems running Linux KVM with vfio-ap crypto passthrough are affected; the CVSS vector (AV:L/PR:L/S:C) indicates a low-privileged local attacker inside a guest, not remote compromise. No public proof-of-concept is known, the issue is not listed in CISA KEV, and no exploitation has been reported.

What to do: Apply the distribution kernel update containing the s390/vfio-ap fix (which replaces the bitmap_andnot() return-value check with bitmap_intersects() to force the guest APCB update) when available. Until patched, avoid hot-removing the last assigned AP adapter, domain, or control domain while vfio-ap guests are running—perform such reconfiguration with guests shut down or restart guests afterward to clear stale queue access. Confirm exposure by checking for vfio-ap mediated devices / CONFIG_VFIO_AP on s390x KVM hosts; x86 and other architectures are not affected.