[CVE-2026-45892][MODERATE 7.0] ext4: drop extent cache after doing PARTIAL_VALID1 zeroout [ Upstream

[CVE-2026-45892][MODERATE 7.0] ext4: drop extent cache after doing PARTIAL_VALID1 zeroout [ Upstream

[AL-KERNEL]

CVE: CVE-2026-45892
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ext4: drop extent cache after doing PARTIAL_VALID1 zeroout [ Upstream
Commit: f0931a5c17005a0c4fc35bd1a001245effc3354b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f0931a5c17005a0c4fc35bd1a001245effc3354b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45892
Analysis date: Wed, 27 May 2026 22:51:20 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
An ext4 extent split zeroout path can leave stale unwritten extent status entries after conversion to written extents, creating local filesystem mapping integrity and availability risk.

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

The original Linux kernel CVE announcement for CVE-2026-45892 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45892

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: NO

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

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

CVE-2026-45892	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:N/I:H/A:H';CWE-459;*CWE-664;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.3';DESCR 'An ext4 extent split and zeroout error path can leave stale unwritten entries in the extent status tree after the on disk extent has been converted to written. A local workload with write access to the affected filesystem may trigger this through buffered writes into unwritten extents combined with a split failure and PARTIAL_VALID1 zeroout handling. For the CVSS the PR:L is used because the trigger is local filesystem write activity and does not inherently require full administrator privileges once the filesystem is mounted. The issue is not network reachable. Impact is primarily filesystem integrity and availability risk from stale extent cache state. No supported evidence indicates kernel memory corruption, information disclosure, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	DISK  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1
  A local user with write access to the mounted ext4 filesystem can exercise buffered writes into unwritten extents.

* Filesystem/storage metadata or mapping consistency risk: +1
  The bug leaves stale unwritten entries in the extent status tree while the on-disk extent has become written.

* Integrity impact plausible: +1
  Later filesystem operations may rely on an incorrect in-memory extent state, affecting block mapping or data consistency decisions.

* Availability impact realistic: +1
  Stale extent state can lead to filesystem errors, failed operations, or recovery paths.

* Broad/default/common subsystem: +1
  ext4 is widely deployed and commonly used as a default filesystem.

* Important filesystem/storage path: +1
  Extent splitting and extent status cache are core ext4 mapping mechanisms.

* Hard or special failure condition required: -1
  The trigger requires a specific unwritten extent split plus PARTIAL_VALID1 zeroout path after a split failure, such as temporary space or allocation failure.

* No kernel memory corruption: +0
  The issue is stale filesystem mapping state, not UAF, OOB write, arbitrary write, or heap corruption.

## 3. Reachability analysis

A local writer may trigger the path through buffered writes into unwritten extents, especially under allocation pressure or transient split failures. No full administrator privileges are inherently required once the filesystem is mounted and writable, but reliable reproduction likely needs carefully shaped extents and failure conditions.

Containers matter if untrusted workloads can write to host-backed ext4 filesystems. The issue is not network reachable.

## 4. Severity interpretation

This is an actionable Moderate filesystem integrity issue. It is not a proven kernel memory corruption or privilege escalation bug, but stale extent status entries in ext4 are important because they can affect mapping correctness. It should not be auto-closed as a simple DoS or warning-only issue.

## 5. One-sentence report phrase

An ext4 extent split zeroout path can leave stale unwritten extent status entries after conversion to written extents, creating local filesystem mapping integrity and availability risk.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the issue affects ext4 extent mapping consistency in a core filesystem path, although no kernel memory corruption or LPE primitive is shown.

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

Patch: ext4: drop extent cache after doing PARTIAL_VALID1 zeroout [ Upstream
Commit: f0931a5c17005a0c4fc35bd1a001245effc3354b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f0931a5c17005a0c4fc35bd1a001245effc3354b

Changed files:
  fs/ext4/extents.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=f0931a5c17005a0c4fc35bd1a001245effc3354b

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

The original Linux kernel CVE announcement for CVE-2026-45892 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45892

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:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H
  The Best / paranoid CVSS score: 6.3

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: 4
  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: YES
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).