CVE-2026-89958 - Vulnerability Details

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

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.

Published: 2026-09-16

Score: n/a

EPSS: < 1% Very Low

KEV: No

Impact:

Action:

Analysis

Impact

The vulnerability arises when the vfio-ap driver accesses the kvm pointer through matrix_mdev->kvm without checking for NULL. If matrix_mdev->kvm has never been set, taking the kvm->lock mutex triggers a NULL pointer dereference, causing the kernel to crash. Attackers can leverage this to force a system reboot or a reboot loop, denying service to all users on the affected system, especially on s390 machines using the VFIO AP driver.

Affected Systems

The flaw affects the Linux kernel’s s390 VFIO AP subsystem. Any kernel version that includes the vfio_ap driver on the s390 architecture and lacks the null check is vulnerable. The specific product is the Linux Linux kernel; affected releases are those before the patch commit that introduces the NULL check for matrix_mdev->kvm.

Risk and Exploitability

The EPSS score is less than 1 %, indicating a very low likelihood of exploitation in the wild. CISA KEV lists it as not listed, so no known active exploits. The attack would require local privilege or an environment where the system can load a guest that leaves matrix_mdev->kvm unset. Because the crash is locally exploitable, the severity is high, but the practical risk remains low due to the niche target and small attack surface.

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions

Linux

unaffected

Version	Status	Constraints
`eeb386aeb5b7c8d2dae6a3ba49255d8a97803182`	affected	< 5e932abfcce6189d067fffb0eb8214443bc04374
`eeb386aeb5b7c8d2dae6a3ba49255d8a97803182`	affected	< 334b435b951353f934d5110c4630e0cb58d2f9a7
`eeb386aeb5b7c8d2dae6a3ba49255d8a97803182`	affected	< 178ea7a1c2c3c0c01d14b8f60aa35f1e0cdc6b78
`eeb386aeb5b7c8d2dae6a3ba49255d8a97803182`	affected	< 4f7e869d0db24ae07b4c888fd3bc21554eef775d
`eeb386aeb5b7c8d2dae6a3ba49255d8a97803182`	affected	< d50346801b4f144e42b49cd4f1496010498ab114

Linux

affected

Version	Status	Constraints
`6.0`	affected	—
`0`	unaffected	< 6.0
`6.6.157`	unaffected	≤ 6.6.*
`6.12.110`	unaffected	≤ 6.12.*
`6.18.51`	unaffected	≤ 6.18.*
`7.2.5`	unaffected	≤ 7.2.*
`7.3-rc1`	unaffected	≤ *

No data.

Vendor Product Confidence Versions

Linux

Linux Kernel

99%

Version	Status	Scheme	Platform
`[eeb386aeb5b7c8d2dae6a3ba49255d8a97803182,5e932abfcce6189d067fffb0eb8214443bc04374)`	affected	code_commit	—
`[eeb386aeb5b7c8d2dae6a3ba49255d8a97803182,334b435b951353f934d5110c4630e0cb58d2f9a7)`	affected	code_commit	—
`[eeb386aeb5b7c8d2dae6a3ba49255d8a97803182,4f7e869d0db24ae07b4c888fd3bc21554eef775d)`	affected	code_commit	—
`[eeb386aeb5b7c8d2dae6a3ba49255d8a97803182,d50346801b4f144e42b49cd4f1496010498ab114)`	affected	code_commit	—
`[eeb386aeb5b7c8d2dae6a3ba49255d8a97803182,178ea7a1c2c3c0c01d14b8f60aa35f1e0cdc6b78)`	affected	code_commit	—

Linux

Linux Kernel

99%

Version	Status	Scheme	Platform
`6.0`	affected	semver	—
`[0,6.0)`	unaffected	semver	—
`[6.6.157,6.7.0)`	unaffected	semver	—
`[6.12.110,6.13.0)`	unaffected	semver	—
`[6.18.51,6.19.0)`	unaffected	semver	—
`[7.2.5,7.3.0)`	unaffected	semver	—
`[7.3-rc1,*]`	unaffected	semver	—

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Remediation

No solution or workaround provided in the CVE record.

OpenCVE Recommended Actions

Apply the kernel patch that adds the NULL check before acquiring matrix_mdev->kvm->lock (commits such as 178ea7a1… are available in the Linux kernel source).
Disable the vfio_ap module (e.g., rmmod vfio_ap) until the patched kernel is in use to avoid accidental crashes.
Reboot the system after updating the kernel to ensure the new patch is loaded and the driver functions correctly.

Generated by OpenCVE AI on September 18, 2026 at 04:14 UTC.

Tracking

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Advisories

Source	ID	Title
Debian DLA	DLA-4817-1	linux-6.12 security update
Debian DSA	DSA-6528-1	linux security update

No CVSS v4.0

No CVSS v3.1

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

The EPSS score is 0.00209.

Key SSVC decision points have not yet been added.

References

Link	Providers
https://git.kernel.org/stable/c/178ea7a1c2c3c0c01d14b8f60aa35f1e0cdc6b78
https://git.kernel.org/stable/c/334b435b951353f934d5110c4630e0cb58d2f9a7
https://git.kernel.org/stable/c/4f7e869d0db24ae07b4c888fd3bc21554eef775d
https://git.kernel.org/stable/c/5e932abfcce6189d067fffb0eb8214443bc04374
https://git.kernel.org/stable/c/d50346801b4f144e42b49cd4f1496010498ab114

History

Fri, 18 Sep 2026 04:30:00 +0000

Type	Values Removed	Values Added
Weaknesses		CWE-476

Wed, 16 Sep 2026 10:45:00 +0000

Type	Values Removed	Values Added
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.
Title		s390/vfio-ap: Fix dereference matrix_mdev->kvm without checking for NULL
First Time appeared		Linux Linux linux Kernel
CPEs		cpe:2.3:o:linux:linux_kernel::::::::
Vendors & Products		Linux Linux linux Kernel
References		https://git.kernel.org/stable/c/178ea7a1c2c3c0c01d14b8f60aa35f1e0cdc6b78 https://git.kernel.org/stable/c/334b435b951353f934d5110c4630e0cb58d2f9a7 https://git.kernel.org/stable/c/4f7e869d0db24ae07b4c888fd3bc21554eef775d https://git.kernel.org/stable/c/5e932abfcce6189d067fffb0eb8214443bc04374 https://git.kernel.org/stable/c/d50346801b4f144e42b49cd4f1496010498ab114

Subscriptions

Linux Linux Kernel

MITRE

Status: PUBLISHED

Assigner: Linux

Published: 2026-09-16T10:32:41.239Z

Updated: 2026-09-16T10:32:41.239Z

Reserved: 2026-09-11T19:38:34.778Z

Link: CVE-2026-89958

Vulnrichment

No data.

NVD

Status : Received

Published: 2026-09-16T11:17:06.177

Modified: 2026-09-16T11:17:06.177

Link: CVE-2026-89958

Redhat

No data.

OpenCVE Enrichment

Updated: 2026-09-18T04:15:02Z

Weaknesses

CWE-476
NULL Pointer Dereference

Impact

Affected Systems

Risk and Exploitability

Tracking

Subscriptions

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