[CVE-2026-45920][LOW] ext4: fix dirtyclusters double decrement on fs shutdown [ Upstream

[CVE-2026-45920][LOW] ext4: fix dirtyclusters double decrement on fs shutdown [ Upstream

[AL-KERNEL]

CVE: CVE-2026-45920
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: ext4: fix dirtyclusters double decrement on fs shutdown [ Upstream
Commit: 523d5a4df3c649fa305c89efb552ec62a1ce9d3d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=523d5a4df3c649fa305c89efb552ec62a1ce9d3d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45920
Analysis date: Wed, 27 May 2026 21:22:26 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
An ext4 shutdown error path can double decrement `s_dirtyclusters_counter`, causing negative in-memory accounting and a WARN during unmount, but no memory corruption or privilege escalation is indicated.

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

The original Linux kernel CVE announcement for CVE-2026-45920 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45920

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: LOW
Manual review required: NO

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

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

CVE-2026-45920	LOW	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:N/I:L/A:L';CWE-682;*CWE-664;CWE-754;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '3.6';DESCR 'An ext4 dirty cluster accounting bug can double decrement s_dirtyclusters_counter when allocation error handling races with filesystem shutdown or metadata dirtying failure. This can make the in memory dirty cluster counter negative and trigger a warning during ext4_put_super, but the patch does not indicate kernel memory corruption or an overwrite primitive. For the CVSS the PR:L is used only in the paranoid score because a local writer may participate in the allocation path if the filesystem is already entering the relevant shutdown or error state, while reliable standalone triggering usually needs privileged shutdown or error injection. The issue is not network reachable. Impact is mainly limited availability risk through warnings or accounting inconsistency, with only weak integrity concern for transient filesystem accounting state.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1) 	MAYBE	DISK WARNONLY LEAK ERRORPATH HARDWARE 	-	-	checked

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

ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

* Availability impact realistic: +1
  The demonstrated effect is a WARN in `ext4_put_super()` from a negative in-memory dirty cluster counter. This can matter if `panic_on_warn` is enabled, but the patch does not show ordinary filesystem shutdown or persistent outage by itself.

* Broad/default/common subsystem: +1 in paranoid scoring
  ext4 is widely deployed, but this specific path is an error/shutdown accounting path rather than normal metadata corruption.

* Local unprivileged participation: +1 in paranoid scoring only
  A local writer can participate in allocation paths, but reliable triggering requires the filesystem to already hit shutdown or metadata dirtying failures.

* Hard or special timing required: -1
  The reproduced case comes from shutdown injection plus fsstress. The vulnerable condition depends on a specific error path between `ext4_mb_mark_diskspace_used()` and `ext4_mb_new_blocks()`.

* Requires privileged shutdown/error injection in reliable scenarios: -2 conservative
  The clearest trigger requires forced shutdown or error injection, which is normally administrative or test-only.

* No generic memory corruption: +0
  This is a double decrement of an accounting counter, not UAF, OOB write, heap corruption, arbitrary write, or page-cache corruption.

* No strong integrity impact: +0
  The patch indicates transient in-memory accounting inconsistency, not persistent ext4 metadata corruption or overlapping allocation.

## 3. Reachability analysis

Reliable reproduction appears to require filesystem shutdown injection or metadata dirtying failure while ext4 allocation is in progress. A normal local user may create write allocation pressure, but cannot usually force the required shutdown or error state without administrative help. Namespaces and containers do not significantly lower this unless the environment gives tenants control over filesystem fault injection or shutdown behavior.

The issue is not network reachable. Realistic exploitation is mostly a warning or low-grade DoS in test or fault-injection scenarios.

## 4. Severity interpretation

This behaves like Low or ordinary Moderate. It is an ext4 accounting bug in an error path, not a memory safety issue and not clear persistent filesystem metadata corruption. Theoretical availability impact exists through WARN or `panic_on_warn`, but realistic exploitation evidence points to stress testing with shutdown injection rather than a practical unprivileged attack.

## 5. One-sentence report phrase

An ext4 shutdown error path can double decrement `s_dirtyclusters_counter`, causing negative in-memory accounting and a WARN during unmount, but no memory corruption or privilege escalation is indicated.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

The issue is a low-risk accounting inconsistency in a shutdown/error path, with no demonstrated memory corruption, persistent metadata corruption, network reachability, or privilege escalation primitive.

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

Patch: ext4: fix dirtyclusters double decrement on fs shutdown [ Upstream
Commit: 523d5a4df3c649fa305c89efb552ec62a1ce9d3d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=523d5a4df3c649fa305c89efb552ec62a1ce9d3d

Changed files:
  fs/ext4/super.c
  fs/ext4/mballoc.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=523d5a4df3c649fa305c89efb552ec62a1ce9d3d

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

The original Linux kernel CVE announcement for CVE-2026-45920 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45920

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS score: 3.6

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: 0
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).