[CVE-2026-45899][MODERATE 7.0] ext4: drop extent cache when splitting extent fails [ Upstream

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

CVE: CVE-2026-45899
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ext4: drop extent cache when splitting extent fails [ Upstream
Commit: 6e54f8dfee359bbd58086c883ea8cffd5312999d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e54f8dfee359bbd58086c883ea8cffd5312999d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45899
Analysis date: Wed, 27 May 2026 22:27:24 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
An ext4 extent split failure can leave stale extent status cache entries after an error path, allowing local filesystem workloads to create block mapping inconsistency with integrity and availability impact.

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

The original Linux kernel CVE announcement for CVE-2026-45899 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45899

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.

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

CVE-2026-45899	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 failure can leave stale entries in the extent status tree when the function returns an error before dropping the affected cached range. A local workload that triggers extent split errors, for example through ENOSPC, EDQUOT, ENOMEM, or similar failure paths, may leave later filesystem operations with an incorrect in memory view of block mappings. For the CVSS the PR:L is used because the trigger can be reached through local filesystem operations by a user with write access to the affected ext4 filesystem, while full administrator privileges are not inherently required 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 NOMEMCHK SIMPLEFIX ERRORPATH  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 an affected mounted ext4 filesystem may exercise extent splitting paths.

* Filesystem/storage metadata or mapping consistency risk: +1
  The bug leaves stale entries in the ext4 extent status tree, which is an in-memory cache used for block mapping decisions.

* Integrity impact plausible: +1
  Stale extent status entries can make later filesystem operations rely on an incorrect view of extents or block mappings.

* Availability impact realistic: +1
  Incorrect extent cache 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
  ext4 extent splitting and extent status cache handling are core filesystem mapping paths.

* Hard or special failure condition required: -1
  The issue requires extent split failure, such as ENOSPC, EDQUOT, ENOMEM, or a similar error path.

* No generic kernel memory corruption: +0
  The patch does not show UAF, OOB write, arbitrary write, heap corruption, or LPE.

## 3. Reachability analysis

The issue is locally reachable through filesystem operations that cause ext4 extent splitting and then hit an error path. Full administrator privileges are not inherently required once the filesystem is mounted and writable, but reliably forcing the exact failure path may require resource pressure, quota limits, ENOSPC conditions, or fault-like conditions.

Namespaces and containers may matter if untrusted workloads can write to host-backed ext4 filesystems or consume quotas/storage. The issue is not network reachable.

## 4. Severity interpretation

This behaves like an actionable Moderate filesystem integrity issue. It is not kernel memory corruption and does not show a privilege escalation primitive. However, stale extent status cache entries in ext4 can affect block mapping correctness and filesystem availability, so it should not be treated like a simple WARN-only or cosmetic bug.

## 5. One-sentence report phrase

An ext4 extent split failure can leave stale extent status cache entries after an error path, allowing local filesystem workloads to create block mapping inconsistency with integrity and availability impact.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this affects ext4 extent mapping consistency in a core filesystem path, even though no kernel memory corruption or privilege escalation is demonstrated.

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

Patch: ext4: drop extent cache when splitting extent fails [ Upstream
Commit: 6e54f8dfee359bbd58086c883ea8cffd5312999d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e54f8dfee359bbd58086c883ea8cffd5312999d

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=6e54f8dfee359bbd58086c883ea8cffd5312999d

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

The original Linux kernel CVE announcement for CVE-2026-45899 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45899

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).