[CVE-2026-89955][LOW] s390/vfio-ap: Fix NULL deref in status_show() during queue probe

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-89955
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: s390/vfio-ap: Fix NULL deref in status_show() during queue probe
Commit: 31fa0a8a3c337ed2d200166d6926ab23c14f8b2e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=31fa0a8a3c337ed2d200166d6926ab23c14f8b2e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89955
Analysis date: Wed, 16 Sep 2026 11:11:41 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89955 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89955

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89955	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89955 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;**CWE-476;CWE-667;BEST CVSS score: '4.7';DESCR 'A NULL pointer dereference in the s390 vfio ap sysfs status path can occur because the status attribute may become visible before queue driver data is installed during vfio_ap_mdev_probe_queue. A local user or process that reads the status attribute in this narrow race window can make status_show receive NULL from dev_get_drvdata and later dereference it through vfio_ap_mdev_for_queue, causing a kernel crash. For the CVSS the PR:L is used because the trigger is a local sysfs read and does not require full administrative privileges once the attribute is accessible. The issue is not network reachable and depends on vfio ap queue probing on s390 systems. Impact is denial of service only because the patch context indicates a NULL dereference race and does not show a UAF, OOB write, arbitrary write, or information leak primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	READ NULLPTR HARDWARE LINUS  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap changed the score.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive signals:

* +1 Reliable kernel crash / strong DoS: the patch and commit directly establish a NULL pointer dereference in `status_show()` via `vfio_ap_mdev_for_queue(q)` when `q == NULL`.
* +1 Local below-host-admin trigger: the triggering operation is a local sysfs `status` attribute read once the attribute is visible. The supplied context supports PR:L style reachability rather than host-admin-equivalent privilege for the read side.

Negative or limiting signals:

* No -1 hard race penalty was applied, although the commit describes a very small window. The race is real, but the scoring rules say not to subtract merely because a race exists unless the timing is materially difficult and externally uncontrollable. Here the window is narrow, but the local read path is straightforward once the attribute is exposed.
* No -1 rare subsystem penalty was applied. `s390/vfio-ap` is platform and device specific, but niche subsystem alone is not enough for a negative point.
* No C/I points were awarded: the primitive is NULL dereference, not information disclosure or unauthorized modification.
* No UAF points were awarded: the object is not freed and then reused. The issue is missing initialization / publication ordering of driver data.
* No structured stale-object leverage was awarded: there is no stale reclaimed object, controlled replacement, callback, function pointer, or post-free structured consumption.
* No concrete repeated release/refcount signal exists: there is no first lifetime-ending put/free followed by a second put/free on the same dangling reference.
* No extra availability point was awarded because the kernel crash / DoS is already counted once.
* No remote/network reachability was awarded.

3. Reachability analysis

The reachable trigger is a local sysfs read of the `status` attribute while `vfio_ap_mdev_probe_queue()` has created the sysfs group but has not yet installed queue driver data with `dev_set_drvdata()`. The supplied context indicates that a local user or local process can perform the read once the sysfs attribute is visible, so `HostAdminRequired=NO` is used for the vulnerable read operation.

The complete environmental setup still depends on an s390 system with vfio-ap/AP queue devices and queue probing activity. That affects deployment exposure, but it does not prove host-admin privilege is required for the actual vulnerable read. This is not namespace-scoped. There is no evidence that user or network namespaces are involved. There is no remote packet or network-triggered path.

Race repeatability is limited by the probe window, but the bug class is still a local race against sysfs attribute publication. The attacker does not appear to control object allocation, replacement, or lifetime reuse in a way useful beyond causing the crash.

4. Primitive and severity interpretation

The exact primitive is a NULL pointer dereference caused by a race between queue probe initialization and sysfs `status` reads. `sysfs_create_group()` exposes the attribute before the queue object is stored in device driver data. A concurrent `status_show()` can receive NULL from `dev_get_drvdata()` and pass it into `vfio_ap_mdev_for_queue()`, where `q->apqn` is dereferenced.

This is not a real UAF. It is not a fixed stale storage case. It is not an active structured post-lifetime object reuse case. It is a Class C invalid-state / NULL-deref crash primitive. The demonstrated consequence is local denial of service via kernel crash. Stronger consequences such as privilege escalation, arbitrary write, controlled reclaim, information disclosure, or security-boundary bypass are not supported by the patch or supplied context.

ImportantCandidate=NO because the evidence does not establish memory corruption or unresolved exploitation leverage beyond crash. AutoDowngradeAllowed=YES because this is a constrained NULL-deref race with DoS-only impact and no concrete corruption, repeated lifetime mutation, authorization bypass, or ownership violation.

5. One-sentence report phrase

A local sysfs status read can race with s390 vfio-ap queue probing before driver data is installed, causing `status_show()` to dereference NULL and crash the kernel.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a kernel race in a sysfs path and should be sanity-checked for permissions and device exposure, but the supplied evidence supports DoS-only NULL dereference and does not require Important-class handling.

7. Call-site confidence

Call-site confidence: high

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: s390/vfio-ap: Fix NULL deref in status_show() during queue probe
Commit: 31fa0a8a3c337ed2d200166d6926ab23c14f8b2e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=31fa0a8a3c337ed2d200166d6926ab23c14f8b2e

Commit description:

When vfio_ap_mdev_probe_queue() creates the sysfs attribute group,
the queue's driver data has not yet been set. A concurrent read of
the 'status' attribute can therefore call dev_get_drvdata() and
get NULL, which is then passed directly to
vfio_ap_mdev_for_queue() where q->apqn is unconditionally
dereferenced, causing a NULL pointer dereference.

Fix this by acquiring the update locks before calling
sysfs_create_group(). The status_show() function acquires
guests_lock before reading the driver data, so any concurrent
read will block until after dev_set_drvdata() has been called
and the update locks are released.

As a bonus, the APQN no longer needs to be read from the queue
struct after allocation — it can be read directly from apdev
before allocation and stored in a local variable, which is then
assigned to q->apqn once the allocation succeeds.

Fixes: 260f3ea ("s390/vfio-ap: move probe and remove callbacks to vfio_ap_ops.c")
Cc: stable@vger.kernel.org
Signed-off-by: Anthony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/s390/crypto/vfio_ap_ops.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=31fa0a8a3c337ed2d200166d6926ab23c14f8b2e

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89955 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89955

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: Not available
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: unknown
  Paranoid ActionableScore: 2

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).