[CVE-2026-93251][MODERATE REGULAR] ACPI: bus: Introduce acpi_bus_get_primary_device() [ Upstream

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

CVE: CVE-2026-93251
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ACPI: bus: Introduce acpi_bus_get_primary_device() [ Upstream
Commit: 5657859851abb65105220a6cdb5804926249f714
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5657859851abb65105220a6cdb5804926249f714
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93251
Analysis date: Thu, 24 Sep 2026 21:24:27 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-93251 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93251

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: YES

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

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

CVE-2026-93251	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-93251 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:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in acpi_get_first_physical_node can return a stale struct device pointer after the physical_node_lock is released. Concurrent ACPI device unbinding can drop the final reference before a caller invokes get_device, resulting in a use-after-free condition. For the CVSS the PR:L value is used for the paranoid score because local users may be able to influence hotplug capable devices or delegated device management in some deployments. The issue is not directly network reachable. Impact is at least local denial of service through a kernel fault and in the worst case may allow confidentiality or integrity impact due to use-after-free memory corruption. Existing callers and follow up commits should be reviewed manually because this commit introduces a refcounted helper but does not itself migrate all callers.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	LOCK UAF HARDWARE  DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 4
* Raw paranoid score: 2
* Paranoid score: 4
* Applied conservative floor: 2 -> 4. A concrete post-lifetime use of an active struct device pointer is established and host-admin-equivalent privilege is not positively established.
* Applied paranoid floor: 2 -> 4.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Positive signals:

* +2 Real UAF or active lifetime violation. acpi_get_first_physical_node returns an unreferenced struct device pointer after releasing physical_node_lock. acpi_unbind_one may remove the association and drop the final device reference before the caller invokes get_device on the returned pointer.
* +1 Plausible kernel fault or local DoS. get_device dereferences and increments lifetime state in a potentially freed struct device. A stale or reused pointer can cause an Oops or other kernel fault.

Negative signals:

* -1 Hard timing window. The vulnerable interval is specifically between releasing physical_node_lock and the caller obtaining the device reference. The commit characterizes the scenario as mostly in theory, supporting a narrow and not demonstrated repeatable race penalty.

Important non-awarded signals:

* Controlled reclaim or attacker-selected replacement of the freed struct device is not demonstrated.
* Arbitrary kernel read or write is not demonstrated.
* Remote reachability is not established.
* Local unprivileged reachability is not established.
* Structured stale-object leverage is not awarded. struct device is an active structured object, but the supplied evidence shows only a stale get_device operation and does not establish attacker control of reused fields, callbacks, object replacement, or post-free contents.
* Concrete repeated release or refcount mutation is not awarded. The unbind path may drop the last valid reference, but the vulnerable caller performs get_device on an unreferenced returned pointer. The evidence does not establish that the exact same dangling reference is released or put a second time.
* Weak LPE concern is not awarded solely because the bug is a UAF.
* Confidentiality and integrity impact are not awarded. No disclosure, arbitrary write, controlled replacement, callback use, or security-boundary bypass is established.
* Extra Availability impact is not awarded because the potential kernel fault is already counted as the DoS signal.

3. Reachability analysis

The affected sequence requires concurrent ACPI physical-device unbinding and a caller of acpi_get_first_physical_node that subsequently invokes get_device on the returned pointer.

The supplied patch does not establish which actor can reliably initiate acpi_unbind_one, device removal, driver unbinding, or the affected caller path. It therefore does not prove that host-admin-equivalent privilege is required, but it also does not prove reachability by an ordinary local user.

No network input path is shown. ACPI hotplug, device removal, firmware events, driver lifecycle changes, or host device-management actions may participate, but the patch does not establish that an attacker can control the complete sequence through a namespace-scoped capability or a delegated interface.

NamespaceScopedPrivilege is UNKNOWN. There is no evidence that all required operations, including the relevant physical-device unbind and the stale-pointer caller path, can be completed inside an attacker-controlled user or device namespace.

The race is likely difficult to reproduce reliably because the relevant window begins after physical_node_lock is dropped and ends before the caller obtains a device reference. The commit itself describes it as mostly theoretical.

4. Primitive and severity interpretation

The exact primitive is a real lifetime violation. A pointer to an active structured kernel object, struct device, is returned without a reference held. Concurrent acpi_unbind_one can remove the physical-node relationship and drop the final device reference before the caller calls get_device.

This is not fixed stale DMA storage. It is a stale pointer to an active structured device object. However, the demonstrated stale operation is shallow: get_device attempts to acquire a reference on the returned object. The supplied material does not show later processing of attacker-influenced struct device fields, object replacement, callback dispatch, type confusion, controlled reclaim, arbitrary write, or disclosure.

There is no concrete repeated release or refcount pattern of the form put followed by a second put on the exact same dangling reference. The unbind path may legitimately release the final valid reference, while a separate caller later invokes get_device through an unprotected non-reference pointer. This is a UAF, but not the stronger repeated-release lifetime-mutation case.

The appropriate classification is CLASS_B because an actual stale post-lifetime object use is directly established. The demonstrated practical consequence is a possible kernel fault and local denial of service. Privilege escalation, confidentiality impact, integrity impact, and controlled kernel corruption remain speculative and are not established by this patch.

ImportantCandidate is NO because no meaningful exploitation leverage beyond the stale get_device operation and possible crash is demonstrated. AutoDowngradeAllowed is YES because the evidence currently supports a constrained UAF with no concrete reuse, replacement, write, callback, security-boundary, or repeated dangling-refcount primitive. Manual review remains recommended because later commits may migrate callers to the new refcounted helper and can reveal the practical caller exposure.

5. One-sentence report phrase

A race between acpi_get_first_physical_node and ACPI device unbinding can return an unreferenced stale struct device pointer, allowing a later get_device call to access freed memory and potentially cause a local kernel crash.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review all acpi_get_first_physical_node callers and the follow-up migration commits to determine whether a reachable caller can race with device unbinding and whether any caller performs additional operations on the stale device beyond get_device.

7. Call-site confidence

Call-site confidence: medium

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

Patch: ACPI: bus: Introduce acpi_bus_get_primary_device() [ Upstream
Commit: 5657859851abb65105220a6cdb5804926249f714
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5657859851abb65105220a6cdb5804926249f714

Changed files:
  drivers/acpi/bus.c
  include/acpi/acpi_bus.h

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=5657859851abb65105220a6cdb5804926249f714

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

The original Linux kernel CVE announcement for CVE-2026-93251 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93251

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:H/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 4

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).