# CVE-2026-89958: s390/vfio-ap: Fix dereference matrix_mdev->kvm without checking for NULL

> A local attacker can cause a denial of service in the Linux kernel by triggering a NULL pointer dereference in the s390/vfio-ap driver.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-101511, BELL-CVE-2026-89958, DEBIAN-CVE-2026-89958, GHSA-hgjh-2rgq-2xcv, UBUNTU-CVE-2026-89958

## What is CVE-2026-89958?

CVE-2026-89958 is a vulnerability in Linux, published on 16 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

s390/vfio-ap: Fix dereference matrix_mdev->kvm without checking for NULL

The ap_driver structure has two fields which are function pointers to
callbacks:

* .on_config_changed: called at the start of the AP bus scan function to
                      notify the device driver that the host AP
                      configuration has changed and the associated AP
                      devices will be added or removed accordingly. This
                      gives the implementor a chance to evaluate the
                      configuration changes and respond to them before
                      the associated devices are added or removed.

* .on_scan_complete:  Called at the end of the AP bus scan function to
                      notify the device driver that the host AP
                      configuration has changed and the AP devices have
                      been added or removed accordingly. This gives the
                      implementor the opportunity to respond to the
                      changes after the associated devices are added or
                      removed.

These two callbacks are implemented in the vfio_ap device driver via the
vfio_ap_on_cfg_changed and vfio_ap_on_scan_complete functions respectively.

Within the call stack of these two callback functions the
matrix_mdev->kvm->lock mutex is taken without checking whether
matrix_mdev->kvm is NULL or not. If matrix_mdev->kvm has never been set,
trying to take the lock will trigger a NULL pointer dereference. This patch
adds checks for matrix_mdev->kvm == NULL before taking the
matrix_mdev->kvm->lock mutex.

Note that the matrix_mdev->kvm->lock mutex taken in the
vfio_ap_mdev_hot_plug_config function is moved to the calling function
along with the matrix_dev->mdevs_lock which is needed there to access
the fields of the matrix_mdev. It makes little sense to make the change
the check for matrix_mdev->kvm there before taking the kvm->lock
mutex only to have to move it out via another patch, so it is done in
this patch.

It is important to make note of the following:
1. The matrix_dev->guests_lock is acquired at the start of both callback
   functions. This ensures that matrix_mdev will not be removed via the
   vfio_ap_mdev_remove function because it too takes matrix_dev_guests_lock
   before removing the object; so, matrix_mdev will be available for the
   duration of the callback functions.

2. The matrix_dev->mdevs_lock mutex must be taken in order to access
   fields within the matrix_mdev structure

3. matrix_mdev->kvm->lock mutex must be taken before the
   matrix_dev->mdevs_lock to prevent a lockdep splat.

4: The kvm->lock must be held while plugging the guest's AP configuration
   into its SIE state description via the vfio_ap_mdev_update_guest_apcb
   function.

5. The vfio_ap_mdev_update_guest_apcb checks matrix_mdev->kvm to verify it
   is not NULL before doing the hot plug of the guest's AP configuration.

## Which versions are affected by CVE-2026-89958?


## How do I fix CVE-2026-89958?

Apply the vendor's patch.

## Is CVE-2026-89958 being exploited?

There is no known exploitation of CVE-2026-89958 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89958 and fix it if it is affected.

What it is: CVE-2026-89958 is a vulnerability in Linux, published on 16 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-89958 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-89958/ (Markdown: https://insights.sbomify.com/CVE-2026-89958/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89958
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89958
- OSV: https://osv.dev/vulnerability/CVE-2026-89958

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89958/
Summaries are generated and may contain mistakes; the official description above is the record.