CVE-2024-26620

What it actually does

Incomplete AP matrix filtering in vfio-ap allows APQNs referencing unbound queue devices to remain in a guest's configuration when domains or adapters are reassigned after unbinding operations.

Why it matters to the business

Guest virtual machines on s390 systems may gain unauthorized access to adjunct processor queues, violating isolation requirements and potentially exposing cryptographic or other hardware resources.

How an attack unfolds

An attacker with control over mdev matrix assignments unassigns a domain, unbinds a queue device, then reassigns the domain; because only the newly assigned domain's APQNs are inspected, previously unbound queues remain passed through to the guest.

How to fix it

Upgrade Linux kernel to a fixed version such as 6.1.7-1 on Debian 12/13 or apply commits including 21e5eca0ac9046da9918a919bc92b7b5a78d27e7 and d6b8d034b576f406af920a7bee81606c027b24c6; rebuild and reboot affected systems.

How to confirm the fix worked

Verify running kernel contains the fix by checking version or presence of the updated vfio_ap_mdev_filter_matrix logic; confirm all assigned APQNs reference driver-bound queues after domain/adapter changes.

Typical attack path

1. Obtain ability to manage vfio-ap mdev matrix assignments for a guest
2. Unassign a domain from the mdev while a queue in that domain is bound
3. Unbind the target queue device from vfio_ap driver
4. Reassign the domain to the mdev
5. Observe that the unbound queue is still presented to the guest

How to detect it

Review kernel logs and vfio-ap driver messages around mdev assignment events for unexpected queue pass-through; correlate with guest AP configuration changes and driver bind/unbind activity.

What your security team should do

• Apply the kernel update to all s390 systems using vfio-ap within the next maintenance window given the 947-day exposure window
• Audit current mdev matrix configurations for any unbound queues being passed through
• Restrict mdev management privileges to trusted administrators until patches are deployed