[CVE-2026-74738][MODERATE REGULAR] regmap: sdw-mbq: don't call an unset readable_reg callback [ Upstream

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

CVE: CVE-2026-74738
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: regmap: sdw-mbq: don't call an unset readable_reg callback [ Upstream
Commit: 6cf600b276a55a6481462830607a611f32e737c0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6cf600b276a55a6481462830607a611f32e737c0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74738
Analysis date: Wed, 26 Aug 2026 13:00:23 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline Moderate, manual review recommended
Manual review required: YES

Summary:
A NULL function pointer dereference in the SoundWire MBQ regmap polling path can crash the kernel when an affected device returns COMMAND_IGNORED and the optional readable_reg callback is unset.

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

The original Linux kernel CVE announcement for CVE-2026-74738 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74738

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-2026-74738	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-754;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL function pointer dereference can occur in the SoundWire MBQ regmap path because regmap_sdw_mbq_poll_busy calls ctx readable_reg even though the callback is optional. Drivers such as es9356 and tac5xx2_sdw can create an MBQ regmap without this callback, and a COMMAND_IGNORED response can make the poll path execute the missing callback and crash the kernel. For the CVSS the PR:L is used for the paranoid score because a local user or local process may be able to trigger SoundWire or audio control operations on affected hardware without full administrator privileges. The issue is not network reachable and requires a vulnerable local device and driver path. Impact is denial of service via kernel crash. No practical confidentiality or integrity impact is indicated because the primitive is a NULL callback call rather than UAF, OOB access, or an attacker controlled function pointer.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Moderate, manual review recommended (with actual score 3)	YES	SIMPLEFIX HARDWARE  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline Moderate, manual review recommended**::

## 2. Signal breakdown

Paranoid scoring signals:

* Firmware-mediated external influence: +1
  The crash depends on an affected SoundWire device returning COMMAND_IGNORED / -ENODATA, so the trigger is partly device or firmware behavior mediated.

* Local unprivileged trigger: +1
  In the paranoid interpretation, a local user or local process may be able to exercise exposed audio or SoundWire control paths on affected hardware without full administrator privileges.

* Reliable kernel crash / strong DoS: +1
  The vulnerable code calls ctx->readable_reg directly when it is NULL, producing a NULL function pointer dereference and likely kernel crash.

* Availability impact realistic: +1
  On affected systems, the failure mode is a host kernel crash, and repeated triggering may be practical if the relevant control path is exposed.

* Rare or device-specific reachability: -1
  The issue requires SoundWire MBQ regmap users without readable_reg, such as the mentioned es9356 or tac5xx2_sdw style configurations, and is not a broad default kernel path.

Not awarded:

* Generic memory corruption: +0
  This is a NULL function pointer dereference, not UAF, OOB write, double free, type confusion, or attacker-controlled callback corruption.

* Privilege escalation plausible: +0
  No reclaim, overwrite, object confusion, refcount abuse, or controlled function pointer primitive is supported.

* Confidentiality / integrity impact: +0
  The demonstrated primitive is crash-only.

## 3. Reachability analysis

The bug is local and hardware-dependent. Affected systems need a SoundWire MBQ regmap device whose driver omits the optional readable_reg callback, and the device or bus transaction must produce COMMAND_IGNORED / -ENODATA so that regmap_sdw_mbq_poll_busy enters the polling path.

Namespaces and containers do not meaningfully create a network-style boundary here. If audio controls or related device nodes are exposed into a container or to an unprivileged desktop user, PR:L is a reasonable paranoid interpretation. In more locked-down systems, triggering may require privileged device access or specific kernel driver activity.

The path is not network reachable and is not a common networking or filesystem path. It is device-specific, but the crash primitive is direct and credible on affected hardware.

Call-site confidence: high, because the patch and commit message show the direct NULL callback call and the exact condition fixed.

## 4. Severity interpretation

This behaves like a Moderate local DoS, not an Important-class memory corruption issue. The theoretical impact is limited because the function pointer is NULL rather than attacker-controlled, and there is no evidence of UAF, stale object reuse, arbitrary write, or privilege escalation.

The paranoid score is raised only to keep the issue from being auto-dismissed where local users can reach the affected audio control path on vulnerable hardware. It should not be treated as a strong LPE candidate based on the supplied patch.

## 5. One-sentence report phrase

A NULL function pointer dereference in the SoundWire MBQ regmap polling path can crash the kernel when an affected device returns COMMAND_IGNORED and the optional readable_reg callback is unset.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended to confirm whether affected SoundWire devices and user-reachable audio controls exist in the target product. If the hardware or drivers are absent, this can likely be deprioritized.

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

Patch: regmap: sdw-mbq: don't call an unset readable_reg callback [ Upstream
Commit: 6cf600b276a55a6481462830607a611f32e737c0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6cf600b276a55a6481462830607a611f32e737c0

Changed files:
  drivers/base/regmap/regmap-sdw-mbq.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=6cf600b276a55a6481462830607a611f32e737c0

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

The original Linux kernel CVE announcement for CVE-2026-74738 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74738

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:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2
  Paranoid ActionableScore: 3

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).