[CVE-2026-53294][MODERATE 7.0] mailbox: mailbox-test: don't free the reused channel [ Upstream

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

CVE: CVE-2026-53294
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mailbox: mailbox-test: don't free the reused channel [ Upstream
Commit: fc0089f82c3e36060c2c79156bc2018bfb16b56b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fc0089f82c3e36060c2c79156bc2018bfb16b56b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53294
Analysis date: Fri, 26 Jun 2026 16:47:33 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A double-free in mailbox-test can occur when RX and TX reuse the same mailbox channel and the driver frees both pointers during probe error handling or remove, leading primarily to local kernel crash risk with memory-corruption concerns in privileged device-management paths.

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

The original Linux kernel CVE announcement for CVE-2026-53294 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53294

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-2026-53294	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-415;*CWE-416;*CWE-664;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'mailbox-test can free the same mailbox channel twice when the RX channel is an alias of the TX channel. The double free can occur in the probe error path or during driver remove, because both tx_channel and rx_channel were released without checking whether they point to the same channel object. For the CVSS the PR:L is used in the paranoid score because some systems may delegate driver bind unbind or test device control to a service account or container root, although full administrative access is more typical. The issue is local only and is not network reachable. Impact is at least denial of service through kernel crash and in worst case may involve memory corruption from a double free, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	DANGER UAF 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=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Memory corruption, Strong corruption primitive: +2. The patch prevents freeing the same mailbox channel twice, which is a concrete double-free.
* Real lifetime corruption: +1. The bug is a lifetime management error in channel ownership and release.
* Reliable kernel crash / strong DoS: +1. A double-free in kernel object management can realistically crash the kernel.
* Privileged kernel/device-management memory corruption path: +1. The corruption occurs in a driver probe/remove path managing kernel mailbox channel objects.
* Requires admin/root/CAP-like control in typical deployments: -2. Triggering normally requires driver load, bind/unbind, platform device control, or remove/probe paths.
* Conservative total: 3.

Paranoid signals:

* Same positive signals as above: +5 total.
* No full admin penalty applied in the paranoid interpretation because some systems may delegate driver bind/unbind or test-device control to service accounts, container root, or reduced-capability management contexts.
* No privilege-escalation bonus is added because the patch does not show attacker-controlled reclaim, overwrite target control, callback dispatch, or object replacement.
* Paranoid total: 5.

## 3. Reachability analysis

The bug is local only and is not network reachable. Realistic triggering requires the mailbox-test driver to be present and a configuration where RX and TX channels alias the same channel object. In normal deployments, reaching probe error handling or driver remove requires administrative device-management control. Namespaces or containers may matter only if a reduced-privilege service or container root can control driver bind/unbind or a test platform device, which is not the default but is plausible in test or embedded environments.

## 4. Severity interpretation

This is not an ordinary validation bug. It is a concrete double-free memory lifetime issue, so it should not be auto-closed as low risk. Realistic impact is most likely local kernel crash or system instability because the freed object and trigger path are constrained. Theoretical privilege escalation cannot be fully dismissed due to the double-free class, but the patch does not demonstrate attacker-controlled reclaim, type confusion, function pointer control, or write primitive. This fits Actionable Moderate at minimum rather than a strong Important candidate.

## 5. One-sentence report phrase

A double-free in mailbox-test can occur when RX and TX reuse the same mailbox channel and the driver frees both pointers during probe error handling or remove, leading primarily to local kernel crash risk with memory-corruption concerns in privileged device-management paths.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: the bug class is a concrete double-free in kernel object lifetime handling. Even though the realistic trigger is local and usually privileged, memory corruption in a driver management path should receive manual review rather than automatic closure.

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

Patch: mailbox: mailbox-test: don't free the reused channel [ Upstream
Commit: fc0089f82c3e36060c2c79156bc2018bfb16b56b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fc0089f82c3e36060c2c79156bc2018bfb16b56b

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=fc0089f82c3e36060c2c79156bc2018bfb16b56b

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

The original Linux kernel CVE announcement for CVE-2026-53294 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53294

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: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: 3
  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).