CVE-2026-89958

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

Published Sep 16, 2026 CVSS n/a EPSS < 1% KEV · Not listed

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.

Affected Products

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	< 2fbe430fe40bc5adb7e439d0136d2c5fe34613ba
`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
`6.1.191`	unaffected	≤ 6.1.*
`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`	unaffected	≤ *

No data.

Vendor Product Versions

Linux

Linux Kernel

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

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	—

Remediation

No solution or workaround provided in the CVE record

Tracking

Advisories

Source	ID	Title
Debian DLA	DLA-4817-1	linux-6.12 security update
Debian DSA	DSA-6528-1	linux security update
Ubuntu USN	USN-9019-1	Linux kernel (Azure) vulnerabilities
Ubuntu USN	USN-9020-1	Linux kernel vulnerabilities
Ubuntu USN	USN-9022-1	Linux kernel vulnerabilities
Ubuntu USN	USN-9023-1	Linux kernel vulnerabilities
Ubuntu USN	USN-9024-1	Linux kernel vulnerabilities
Ubuntu USN	USN-9025-1	Linux kernel vulnerabilities
Ubuntu USN	USN-9026-1	Linux kernel (Azure) vulnerabilities
Ubuntu USN	USN-9039-1	Linux kernel (Azure CVM) vulnerabilities
Ubuntu USN	USN-9040-1	Linux kernel (Low Latency NVIDIA) vulnerabilities
Ubuntu USN	USN-9066-1	Linux kernel (HWE) vulnerabilities
Ubuntu USN	USN-9091-1	Linux kernel (NVIDIA Tegra) vulnerabilities

Metrics

No CVSS v4.0

No CVSS v3.1

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

Exploit prediction score: < 1% (0.215% probability within 30 days)

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/2fbe430fe40bc5adb7e439d0136d2c5fe34613ba
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

Oct 30, 2026 12:45 PM Added references View changes

Type	Values Removed	Values Added
References		https://git.kernel.org/stable/c/2fbe430fe40bc5adb7e439d0136d2c5fe34613ba

Sep 18, 2026 4:30 AM Weaknesses updated View changes

Type	Values Removed	Values Added
Weaknesses		CWE-476

Sep 16, 2026 10:45 AM Description updated. Title updated. First appearance in the feed. Affected versions updated. Affected vendors updated. Added references View changes

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