[CVE-2026-53296][MODERATE 7.0] mailbox: mailbox-test: free channels on probe error [ Upstream

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

CVE: CVE-2026-53296
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mailbox: mailbox-test: free channels on probe error [ Upstream
Commit: 0ad8c4a03a358de7811ba1ab8cbd1fe76ad0ff6b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0ad8c4a03a358de7811ba1ab8cbd1fe76ad0ff6b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53296
Analysis date: Fri, 26 Jun 2026 16:40:26 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A mailbox-test probe error can leave mailbox channels allocated after the devm managed client is freed, creating a stale lifetime relationship that may lead to use after free and kernel crash under local device-management conditions.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53296 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53296

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 7.0
Manual review required: YES

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

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-53296	MODERATE	CHECK	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-772;*CWE-401;CWE-703;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A probe error in mailbox-test can leave previously obtained mailbox channels allocated while the devm managed client structure is removed. This creates a stale channel to client relationship and can lead to use after free if the mailbox core or controller later touches the channel state. For the CVSS the PR:L is used in the paranoid score because a reduced capability root, service account, or delegated device management context may be enough in some deployments, although typical triggering is closer to administrator controlled probe and driver setup. The issue is local and is not network reachable. Impact is at least denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to the explicit UAF class, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	INIT NOMEMCHK LEAK UAF DEBUGFS LINUS TEST  INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Generic memory corruption: +2. The commit explicitly states that the leak can also create UAF scenarios because the devm managed client structure is removed while mailbox channels remain allocated.
* Real lifetime corruption: +1. This is a real object lifetime mismatch between mailbox channels and the devm allocated test client.
* Reliable kernel crash / strong DoS: +1. If a retained channel later dereferences the removed client, the most realistic outcome is a kernel crash.
* Privileged kernel/device-management memory corruption path: +1. The issue is in a driver probe error path involving mailbox channel ownership and cleanup.
* Requires admin/root/device-management privileges in typical deployments: -2. Triggering generally requires control over platform driver probing, device setup, or fault injection in probe.
* Rare and difficult-to-reach subsystem/configuration: -1. mailbox-test is a testing driver and not a broad default production attack surface.

Paranoid additional signals:

* Weak LPE concern: +1. The stale channel to client relationship may involve later mailbox core or controller access to a freed client object, but no attacker-controlled reclaim or overwrite primitive is shown.
* Confidentiality impact plausible: +1. Only in the paranoid interpretation because UAF can sometimes lead beyond crash.
* Integrity impact plausible: +1. Only in the paranoid interpretation because the patch context explicitly mentions UAF scenarios.

## 3. Reachability analysis

The realistic trigger is local and tied to the mailbox-test probe path. An attacker would need the mailbox-test driver present and a way to cause probe failure after mailbox channels were obtained, such as allocation failure or debugfs setup failure. This is usually administrator or device-management territory, not a normal unprivileged user interface.

Namespaces and containers do not normally make this broadly reachable unless a container or service has delegated control over device probing, driver binding, or a test platform environment. The path is not network reachable. The affected component is a test mailbox driver, so exposure is narrow and configuration dependent.

## 4. Severity interpretation

Conservatively, this behaves like an ordinary Moderate or Low-like issue because triggering is privileged, uncommon, and constrained to a probe error path. The demonstrated practical impact is mainly resource leak and possible kernel crash.

Paranoid triage should not auto-close it because the commit explicitly identifies UAF scenarios. There is no demonstrated reclaim, controlled overwrite, type confusion, callback hijack, or reliable LPE primitive, so it is not a strong Important candidate. It is best treated as Actionable Moderate at minimum for manual review.

## 5. One-sentence report phrase

A mailbox-test probe error can leave mailbox channels allocated after the devm managed client is freed, creating a stale lifetime relationship that may lead to use after free and kernel crash under local device-management conditions.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Explicit UAF is mentioned by the commit, even though realistic reachability is narrow and typical triggering requires privileged device-management control.

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

Patch: mailbox: mailbox-test: free channels on probe error [ Upstream
Commit: 0ad8c4a03a358de7811ba1ab8cbd1fe76ad0ff6b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0ad8c4a03a358de7811ba1ab8cbd1fe76ad0ff6b

Changed files:
  drivers/mailbox/mailbox-test.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=0ad8c4a03a358de7811ba1ab8cbd1fe76ad0ff6b

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

The original Linux kernel CVE announcement for CVE-2026-53296 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53296

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: 2
  Paranoid ActionableScore: 5

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 7.0

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).