[CVE-2026-53129][MODERATE REGULAR] fs/mbcache: cancel shrink work before destroying the cache [ Upstream

[CVE-2026-53129][MODERATE REGULAR] fs/mbcache: cancel shrink work before destroying the cache [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53129
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fs/mbcache: cancel shrink work before destroying the cache [ Upstream
Commit: a88d39a74a208e197c03bffaa2df34de732af19f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a88d39a74a208e197c03bffaa2df34de732af19f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53129
Analysis date: Wed, 24 Jun 2026 13:57:57 -0400
ActionableScore: 4
ActionableScore lower bound: 2
Actionable bucket: Borderline manual review required. Not auto close because the patch fixes an explicit UAF lifetime race
Manual review required: YES

Summary:
A race in fs/mbcache can leave c_shrink_work pending while mb_cache_destroy frees the cache, allowing a privileged local user with filesystem teardown control to trigger a use-after-free and likely kernel crash.

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

The original Linux kernel CVE announcement for CVE-2026-53129 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53129

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-2026-53129	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-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in fs mbcache because mb_cache_destroy can free the cache while cache c_shrink_work is still pending or running. The worker may later access freed mb_cache memory after a mounted ext2/ext4/ocfs2 filesystem is torn down, causing a kernel crash and potentially giving a memory corruption primitive. For the CVSS the PR:L is used in the paranoid score because delegated CAP_SYS_ADMIN, container root, or a service account with filesystem teardown control may be less than full host administrator access in practical risk terms. The issue is local only and is not network reachable. Attack complexity is high because reliable triggering depends on a race between scheduled work and cache destruction. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to use-after-free memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review required. Not auto close because the patch fixes an explicit UAF lifetime race (with actual score 3)	YES	DISK UAF LINUS  DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7 DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review required. Not auto close because the patch fixes an explicit UAF lifetime race**::

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak primitive: +1
  The patch fixes a use-after-free where `mb_cache_shrink_worker()` may access `cache` after `mb_cache_destroy()` frees it. This is a race-UAF, but no attacker-controlled reclaim, overwrite, callback control, type confusion, or refcount takeover is shown.

* Real lifetime corruption: +1
  The bug is an async workqueue lifetime mismatch: pending `c_shrink_work` can outlive the cache object.

* Reliable kernel crash / strong DoS: +1
  A worker accessing freed kernel memory can realistically cause KASAN splats, Oops, or panic.

* Important filesystem/storage path: +1
  The affected cache is used by ext2/ext4/ocfs2 related filesystem paths, though the vulnerable operation is teardown rather than normal metadata writeback.

* Hard or unreliable race / special timing: -1
  Triggering depends on pending or running work racing with cache destruction.

* Requires root or CAP_SYS_ADMIN in typical deployments: -2
  The commit states this is reachable only by a privileged user who can trigger the last put of a mounted ext2/ext4/ocfs2 filesystem.

Paranoid adjustment:

* Reduced privilege interpretation avoids the full -2 penalty
  In delegated environments, container root, service accounts, or limited CAP_SYS_ADMIN holders may be less than full host root in practical risk terms. This raises the practical score, but not enough to classify as Important without a stronger exploitation primitive.

## 3. Reachability analysis

The bug is local only and is not network reachable. A typical trigger requires root or CAP_SYS_ADMIN because the attacker must control mounted filesystem teardown enough to cause the last put of an ext2/ext4/ocfs2 mbcache instance. Namespaces and containers matter only if CAP_SYS_ADMIN or mount control is delegated in a way that does not already imply full host compromise. The affected subsystem is common enough to deserve review, especially because ext4 is broadly deployed, but the vulnerable path is a privileged teardown race rather than a default unprivileged data path.

## 4. Severity interpretation

This behaves more like a privileged local UAF DoS with manual-review value than a clear Important-class LPE. Theoretical memory corruption exists because a workqueue can dereference freed `mb_cache` memory. Realistic privilege escalation is not demonstrated by the patch because there is no shown attacker-controlled reclaim, stale object replacement, write-after-free, callback dispatch, or arbitrary write primitive. The conservative result is ordinary Moderate / Low-like due to CAP_SYS_ADMIN reachability, while the paranoid result is borderline manual-review because explicit UAF lifetime bugs are historically easy to underestimate.

## 5. One-sentence report phrase

A race in fs/mbcache can leave c_shrink_work pending while mb_cache_destroy frees the cache, allowing a privileged local user with filesystem teardown control to trigger a use-after-free and likely kernel crash.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch fixes an explicit use-after-free caused by async workqueue lifetime mismatch. Even though the realistic trigger is privileged and race-dependent, UAF lifetime bugs should not be auto-closed without checking whether delegated CAP_SYS_ADMIN, container mount control, or reclaim timing can make exploitation more practical.

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

Patch: fs/mbcache: cancel shrink work before destroying the cache [ Upstream
Commit: a88d39a74a208e197c03bffaa2df34de732af19f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a88d39a74a208e197c03bffaa2df34de732af19f

Changed files:
  fs/mbcache.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=a88d39a74a208e197c03bffaa2df34de732af19f

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

The original Linux kernel CVE announcement for CVE-2026-53129 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53129

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: 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).