[CVE-2026-93238][MODERATE REGULAR] s390/vfio-ap: fix potential use of uninitialized apm_filtered bitmap

[CVE-2026-93238][MODERATE REGULAR] s390/vfio-ap: fix potential use of uninitialized apm_filtered bitmap

[AL-KERNEL]

CVE: CVE-2026-93238
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: s390/vfio-ap: fix potential use of uninitialized apm_filtered bitmap
Commit: 4ba8a08f5f26ed59f356a6318bca3aa7cc0af3e2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4ba8a08f5f26ed59f356a6318bca3aa7cc0af3e2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93238
Analysis date: Thu, 24 Sep 2026 12:32:29 -0400
ActionableScore: 0
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-2026-93238 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93238

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: MODERATE REGULAR
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-2026-93238	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-93238 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:C/C:N/I:N/A:H';*CWE-457;CWE-908;CWE-665;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6';DESCR 'An uninitialized apm_filtered stack bitmap in the s390 VFIO AP hot plug path can be passed to reset_queues_for_apids when a configuration change adds only control domains. Stack garbage bits can then be interpreted as APIDs and cause unintended cryptographic queue resets. This may disrupt a guest VFIO AP device or other affected queue users and can result in denial of service. For the CVSS the PR:H rating reflects that configuring VFIO AP mediated devices and their hot plug configuration normally requires privileged host administration. The issue is not directly network reachable. No confidentiality or integrity impact is established because the patch shows unintended hardware queue reset behavior rather than a memory corruption primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)	YES	DANGER INIT SIMPLEFIX HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

ActionableScore=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: -3
* Conservative score: 0
* Raw paranoid score: -3
* Paranoid score: 0
* Applied score range clamp: -3 -> 0
* Final recommended bucket: **Ordinary Moderate**::

The score is low because the demonstrated behavior is an unintended hardware queue reset or crash caused by uninitialized stack bits. No memory corruption, UAF, authorization bypass, information disclosure, or privilege escalation primitive is established.

2. Signal breakdown

Positive signals:

* +1 Reliable kernel crash or strong DoS. The added source comment states that reset_queues_for_apids can crash when it accesses the uninitialized bitmap state.

Negative signals:

* -2 Host-admin-equivalent privilege. Configuring an s390 VFIO AP mediated device and changing its AP matrix or hot plug configuration is a host-side privileged management operation.
* -2 Rare hardware-only requirement. The vulnerable path requires IBM s390 systems with AP cryptographic hardware, VFIO AP support, and mediated-device configuration.

Important non-awarded signals:

* +0 Real UAF. No object is freed and later dereferenced.
* +0 Controlled reclaim or object replacement. Not demonstrated.
* +0 Arbitrary or directed kernel write. Not demonstrated.
* +0 Confidentiality impact. Stack contents are consumed internally as APID selection bits and are not copied to an attacker-visible output.
* +0 Integrity impact. The patch establishes unintended queue resets, not unauthorized modification of protected data or security state.
* +0 Cross-guest isolation bypass. The supplied patch does not establish that arbitrary bitmap bits can reset queues assigned to another guest or security domain.
* +0 Remote reachability. The path is not directly reachable from network traffic.
* +0 Strong semantic reinterpretation. The bitmap is incorrectly interpreted as APID selection data, but the attacker does not control the stack contents and no security-relevant semantic boundary bypass is established.
* +0 Additional availability impact. The possible crash and queue disruption are part of the same demonstrated availability consequence and are not counted twice.

3. Reachability analysis

The evaluated attacker is a host administrator or an equivalently privileged host-side management process. The relevant operation is the configuration or hot plug update of an s390 VFIO AP mediated device.

The vulnerable case requires a configuration update that adds control domains while adding neither adapters nor regular domains. This bypasses vfio_ap_mdev_filter_matrix and leaves apm_filtered uninitialized before reset_queues_for_apids consumes it.

The complete path is not namespace-scoped. VFIO AP mediated device lifecycle and AP matrix management concern host-owned s390 cryptographic hardware resources. A network namespace or container capability does not establish access to this operation.

Prior host setup is required. The system must have compatible s390 AP hardware, the VFIO AP driver, mediated-device support, and an existing configured VFIO AP device. No remote trigger and no ordinary local-user trigger are established. This is not a timing race.

4. Primitive and severity interpretation

The exact primitive is use of an uninitialized stack bitmap as an APID selection mask. Arbitrary residual stack bits can be treated as identifiers of queues to reset.

This is not a UAF, double free, refcount error, out-of-bounds access, controlled overwrite, or active stale-object lifetime violation. The bitmap is transient stack storage rather than a freed active structured object. No repeated release or refcount mutation exists.

The demonstrated consequence is unintended queue reset behavior and a possible kernel crash in reset_queues_for_apids. The speculative consequence is disruption of additional queue users if the bitmap selects unexpected APIDs, but the supplied evidence does not establish a cross-guest reset, host-wide integrity effect, or security-boundary bypass.

PrimitiveClass is CLASS_C because the evidence supports invalid-state behavior and DoS rather than a useful kernel corruption primitive. ImportantCandidate is NO because no meaningful exploitation leverage beyond queue disruption or crash is shown. AutoDowngradeAllowed is YES because the issue is constrained to privileged management of specialized hardware and has no unresolved corruption or authorization primitive.

Likely operational severity is low for general-purpose deployments but may matter operationally on affected s390 virtualization hosts because an administrative configuration change can disrupt assigned cryptographic queues.

5. One-sentence report phrase

An uninitialized APID bitmap in the s390 VFIO AP hot plug path can cause unintended hardware queue resets and possible kernel crash during privileged mediated-device configuration.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review is recommended to confirm whether reset_queues_for_apids can affect queues belonging to other guests or host workloads, since that would change the availability-boundary assessment. The supplied patch alone does not establish such cross-domain impact.

7. Call-site confidence

Call-site confidence: low

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

Patch: s390/vfio-ap: fix potential use of uninitialized apm_filtered bitmap
Commit: 4ba8a08f5f26ed59f356a6318bca3aa7cc0af3e2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4ba8a08f5f26ed59f356a6318bca3aa7cc0af3e2

Commit description:

The DECLARE_BITMAP(apm_filtered, AP_DEVICES) macro allocates the bitmap
on the stack without zero-initializing it.

In vfio_ap_mdev_hot_plug_cfg(), the vfio_ap_mdev_filter_matrix() function
is only called to initialize and populate apm_filtered if either
filter_adapters or filter_domains is true. If the hot plug configuration
change only adds control domains (meaning filter_cdoms is true, but
filter_adapters and filter_domains are both false),
vfio_ap_mdev_filter_matrix() is bypassed.

Consequently, apm_filtered is passed to reset_queues_for_apids() with
uninitialized stack garbage. This can cause reset_queues_for_apids() to
interpret arbitrary stack garbage bits as valid APIDs to reset, potentially
performing unintended guest hardware queue resets.

Fix this by zero-initializing the apm_filtered bitmap at the beginning of
vfio_ap_mdev_hot_plug_cfg() using bitmap_zero().

Fixes: eeb386a ("s390/vfio-ap: handle config changed and scan complete notification")
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=4ba8a08f5f26ed59f356a6318bca3aa7cc0af3e2

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

The original Linux kernel CVE announcement for CVE-2026-93238 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93238

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: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:H/UI:N/S:C/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6

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: 0

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: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).