[CVE-2026-45912][MODERATE 7.0] ext4: don't cache extent during splitting extent

[CVE-2026-45912][MODERATE 7.0] ext4: don't cache extent during splitting extent

[AL-KERNEL]

CVE: CVE-2026-45912
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ext4: don't cache extent during splitting extent
Commit: 8302b5b4aacdbb378f7b1216bb2ee782b5142415
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8302b5b4aacdbb378f7b1216bb2ee782b5142415
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45912
Analysis date: Wed, 27 May 2026 09:40:31 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-45912 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45912

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-2026-45912	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-664;CWE-682;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.3';DESCR 'ext4 can leave a stale hole in the extent status tree while splitting an unwritten extent, because extents being modified may be cached during the split. A local user with write access to an ext4 file may be able to create the pattern with fallocate, Direct IO to part of the unwritten extent, and later buffered delayed writes, causing inconsistent space accounting or filesystem metadata state. For the CVSS the PR:L is used because ordinary file write access can influence the affected extent conversion path without administrator privileges. The issue is not network reachable and does not show kernel memory corruption or privilege escalation. Impact is primarily filesystem integrity and availability, not confidentiality.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	DISK WARNONLY IMPROVEONLY  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	NO	NO	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 ext4 file can plausibly influence unwritten extent splitting through fallocate, DIO, and buffered writes.
* Filesystem/storage metadata corruption reachable by unprivileged local user: +1. The bug can leave stale extent-status entries and cause incorrect space accounting.
* Important filesystem/storage path: +1. ext4 extent splitting, unwritten extent conversion, and delayed allocation accounting are core filesystem paths.
* Integrity impact plausible: +1. Incorrect extent status and space accounting can affect filesystem metadata/accounting correctness.
* Availability impact realistic: +1. Space accounting errors or later consistency checks can cause operation failure, warnings, or filesystem disruption.
* Hard/specific sequence required: -1. Triggering requires a particular unwritten extent split pattern, DIO write to part of the extent, and later buffered delayed write.
* No kernel memory corruption: +0. This is filesystem state/accounting inconsistency, not UAF, OOB write, arbitrary write, or LPE.
* No confidentiality impact: +0. No read disclosure or cross-file data exposure is shown.

3. Reachability analysis
The affected path can be reached by local file operations on ext4, especially a sequence involving unwritten extents, partial Direct IO writes, and later buffered delayed writes. Administrative privileges are not inherently required if the user can create and write files on an ext4 filesystem. Namespaces or containers may still reach the bug if they write to an ext4-backed filesystem and can issue the needed I/O pattern. The issue is not network reachable except indirectly through services that expose local file writes.

4. Severity interpretation
This is an actionable Moderate filesystem integrity issue. It is not an Important-class memory corruption bug, but it affects ext4 metadata/accounting correctness in a path potentially reachable by unprivileged local file writes. The realistic impact is filesystem accounting inconsistency and possible availability degradation. Theoretical impact should not be upgraded to privilege escalation without stronger evidence.

5. One-sentence report phrase
ext4 may cache stale extent status during unwritten extent splitting, allowing local file write patterns to create incorrect space accounting and filesystem metadata state.

6. Manual review recommendation
MANUAL CHECK RECOMMENDED
The issue affects a core ext4 metadata/accounting path and may be reachable by unprivileged local file operations, but it does not show kernel memory corruption or privilege escalation.

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

Patch: ext4: don't cache extent during splitting extent
Commit: 8302b5b4aacdbb378f7b1216bb2ee782b5142415
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8302b5b4aacdbb378f7b1216bb2ee782b5142415

Commit description:

Caching extents during the splitting process is risky, as it may result
in stale extents remaining in the status tree. Moreover, in most cases,
the corresponding extent block entries are likely already cached before
the split happens, making caching here not particularly useful.

Assume we have an unwritten extent, and then DIO writes the first half.

  [UUUUUUUUUUUUUUUU] on-disk extent        U: unwritten extent
  [UUUUUUUUUUUUUUUU] extent status tree
  |<-   ->| ----> dio write this range

First, when ext4_split_extent_at() splits this extent, it truncates the
existing extent and then inserts a new one. During this process, this
extent status entry may be shrunk, and calls to ext4_find_extent() and
ext4_cache_extents() may occur, which could potentially insert the
truncated range as a hole into the extent status tree. After the split
is completed, this hole is not replaced with the correct status.

  [UUUUUUU|UUUUUUUU] on-disk extent        U: unwritten extent
  [UUUUUUU|HHHHHHHH] extent status tree    H: hole

Then, the outer calling functions will not correct this remaining hole
extent either. Finally, if we perform a delayed buffer write on this
latter part, it will re-insert the delayed extent and cause an error in
space accounting.

In adition, if the unwritten extent cache is not shrunk during the
splitting, ext4_cache_extents() also conflicts with existing extents
when caching extents. In the future, we will add checks when caching
extents, which will trigger a warning. Therefore, Do not cache extents
that are being split.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Baokun Li <libaokun1@huawei.com>
Cc: stable@kernel.org
Message-ID: <20251129103247.686136-6-yi.zhang@huaweicloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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=8302b5b4aacdbb378f7b1216bb2ee782b5142415

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

The original Linux kernel CVE announcement for CVE-2026-45912 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45912

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).