[CVE-2026-45858][IMPORTANT] ext4: don't zero the entire extent if EXT4_EXT_DATA_PARTIAL_VALID1 [ Upstream

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

CVE: CVE-2026-45858
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ext4: don't zero the entire extent if EXT4_EXT_DATA_PARTIAL_VALID1 [ Upstream
Commit: 58ddae5d77b1db3a27b891c75a8fa120239ac092
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=58ddae5d77b1db3a27b891c75a8fa120239ac092
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45858
Analysis date: Thu, 28 May 2026 00:30:56 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An ext4 unwritten extent split can incorrectly mark stale blocks as written after a failed partial split, allowing a local writer to potentially read old disk contents that should have remained unwritten or zeroed.

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

The original Linux kernel CVE announcement for CVE-2026-45858 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45858

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: IMPORTANT
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-2026-45858	IMPORTANT	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:N';*CWE-200;CWE-226;CWE-665;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:L/A:N';BEST CVSS score: '6.3';DESCR 'An ext4 unwritten extent split can leave stale disk data exposed when the first split fails due to temporary metadata space pressure and the code marks too much of the extent as written. A local user who can create or write files on the affected ext4 filesystem may trigger the condition with large unwritten extents and a split in the middle, potentially reading old block contents that should have remained unwritten or zeroed. For the CVSS the PR:L is used because the paranoid scenario requires only local filesystem access, not administrative privileges, although reliable triggering needs specific extent layout and metadata pressure. The issue is not network reachable. Impact is primarily confidentiality exposure of stale data, with possible file data integrity impact, but there is no supported kernel memory corruption or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) 	MAYBE	DISK IMPROVEONLY  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local user who can create or write files on an affected ext4 filesystem may reach the unwritten extent split path.
* Filesystem/storage metadata issue reachable by local user: +1. The bug is in ext4 extent state handling and can incorrectly convert part of an unwritten extent to written.
* Confidentiality impact plausible: +1. Stale disk data can become visible where zeros or valid new data were expected.
* Integrity impact plausible: +1. File contents and extent initialization state can become inconsistent with the write operation semantics.
* Important filesystem/storage path: +1. ext4 extent conversion and unwritten extent splitting are important filesystem data integrity paths.
* Broad/default/common subsystem: +1. ext4 is widely deployed.
* Hard/specific trigger conditions: -1. Reliable triggering requires a large unwritten extent, a middle split, temporary metadata allocation pressure, and then a successful second split.

Paranoid additional signal:

* Sensitive file or stale data exposure primitive: +2. In the worst defensible case, stale blocks exposed to a newly written file may contain data from previously allocated files, including sensitive data owned by another user or service.

Not counted:

* No generic kernel memory corruption.
* No UAF, OOB write, arbitrary write, or privilege escalation primitive.
* No remote/network trigger.
* No kernel crash or strong DoS shown by the patch.

## 3. Reachability analysis

The trigger is local filesystem activity. A user with write access to an affected ext4 filesystem can potentially create large unwritten extents through common file allocation and write patterns, but reliable exploitation depends on precise extent layout and temporary metadata space pressure.

Namespaces and containers matter if a container workload writes to a host-backed ext4 filesystem or shared volume. The attacker does not normally need CAP_SYS_ADMIN if the filesystem is already mounted and writable, but they do need write access to the affected filesystem.

The path is not network reachable by itself. ext4 is broadly deployed, but this specific stale data scenario requires nontrivial filesystem state and allocation timing.

## 4. Severity interpretation

This behaves like an actionable Moderate issue, with a paranoid Strong Important candidate interpretation for environments where stale block exposure could disclose sensitive data. The realistic impact is confidentiality exposure of stale filesystem contents and possible file data integrity violation.

It does not behave like a kernel memory corruption or LPE issue. The higher score is driven by filesystem data isolation risk, not by code execution or kernel object corruption.

## 5. One-sentence report phrase

An ext4 unwritten extent split can incorrectly mark stale blocks as written after a failed partial split, allowing a local writer to potentially read old disk contents that should have remained unwritten or zeroed.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because it affects ext4 stale data exposure in an important filesystem path, and the practical severity depends on whether local users or containers can trigger the extent pattern and whether exposed stale blocks may contain sensitive data.

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

Patch: ext4: don't zero the entire extent if EXT4_EXT_DATA_PARTIAL_VALID1 [ Upstream
Commit: 58ddae5d77b1db3a27b891c75a8fa120239ac092
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=58ddae5d77b1db3a27b891c75a8fa120239ac092

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=58ddae5d77b1db3a27b891c75a8fa120239ac092

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

The original Linux kernel CVE announcement for CVE-2026-45858 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45858

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:H/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N
  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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).