[CVE-2026-45942][MODERATE 7.0] ext4: fix e4b bitmap inconsistency reports [ Upstream

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

CVE: CVE-2026-45942
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ext4: fix e4b bitmap inconsistency reports [ Upstream
Commit: 57e83bfbe1e412ac42daced2086f3c6f9a17bba0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=57e83bfbe1e412ac42daced2086f3c6f9a17bba0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45942
Analysis date: Wed, 27 May 2026 20:14:53 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
A timing-dependent ext4 mballoc race between folio migration and buddy bitmap updates can let local filesystem allocation workloads cause bitmap or group accounting inconsistency, leading to filesystem integrity errors and availability impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-45942 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45942

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
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CVE-2026-45942	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-362;CWE-667;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H';BEST CVSS score: '6.3';DESCR 'An ext4 allocation bitmap race can occur when the buddy bitmap fast path uses an uptodate folio by reference only while folio migration is in progress. A local workload that performs ext4 allocations under memory migration or huge page pressure can hit a narrow timing window and leave bitmap or group accounting state inconsistent. For the CVSS the PR:L is used because a local user with write access to the affected filesystem may trigger allocation activity, while full administrator privileges are not inherently required after the filesystem is mounted. The issue is not network reachable. Impact is mainly filesystem availability and integrity risk, including ext4 errors, possible filesystem shutdown, or inconsistent allocation metadata. Kernel memory corruption or privilege escalation is not directly indicated by this patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 6) 	MAYBE	LOCK DANGER DISK INIT RACE IMPROVEONLY KPANIC 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* 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 can create allocation activity that may enter the vulnerable mballoc path.

* Filesystem/storage metadata corruption reachable by an unprivileged local user: +1
  The race affects ext4 buddy bitmap and group accounting consistency, which is filesystem allocation metadata.

* Integrity impact plausible: +1
  Bitmap inconsistency can affect allocation metadata correctness and may cause ext4 corruption reports or inconsistent free-space accounting.

* Availability impact realistic: +1
  ext4 metadata inconsistency can lead to ext4 errors, failed allocations, filesystem shutdown behavior, or operational DoS for the filesystem.

* Broad/default/common subsystem: +1
  ext4 is a broadly deployed default Linux filesystem.

* Important filesystem/storage path: +1
  The bug is in ext4 mballoc bitmap handling, a core filesystem allocation path.

* Hard or unreliable race / special timing required: -1
  The commit explicitly describes an extremely narrow race between folio migration and bitmap modification, although it was observed in stress testing.

* No generic kernel memory corruption: +0
  The patch supports metadata inconsistency, not UAF, double-free, OOB write, arbitrary write, or controlled heap corruption.

* No practical privilege escalation primitive: +0
  There is no demonstrated path to LPE, credential overwrite, page-cache corruption of privileged files, or attacker-controlled kernel overwrite.

## 3. Reachability analysis

The bug is locally reachable through ext4 allocation activity, assuming the attacker can write to files on an affected mounted ext4 filesystem. Triggering also requires concurrent folio migration or comparable memory-management pressure, making reliable exploitation timing-dependent. Namespaces or containers may matter if untrusted workloads can write heavily to host-backed ext4 filesystems, but the bug does not require CAP_SYS_ADMIN once the filesystem is mounted and writable.

The path is not network reachable. It is not a packet-processing or remote protocol parser issue. It is likely relevant in common deployments because ext4 is widely used, but the specific race requires stress-like conditions involving migration and bitmap updates.

## 4. Severity interpretation

This behaves like an actionable Moderate rather than an ordinary Low/Moderate DoS because it affects ext4 allocation metadata consistency in a widely deployed filesystem. The realistic impact is filesystem integrity and availability degradation, not kernel code execution. Theoretical escalation is not supported by the patch because the primitive is not kernel memory corruption. The paranoid score is higher only because persistent filesystem metadata inconsistency is more operationally serious than a simple warning or transient assertion.

## 5. One-sentence report phrase

A timing-dependent ext4 mballoc race between folio migration and buddy bitmap updates can let local filesystem allocation workloads cause bitmap or group accounting inconsistency, leading to filesystem integrity errors and availability impact.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this affects ext4 allocation metadata in a broadly deployed filesystem and can produce integrity-impacting inconsistency, even though the patch does not indicate kernel memory corruption or privilege escalation.

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

Patch: ext4: fix e4b bitmap inconsistency reports [ Upstream
Commit: 57e83bfbe1e412ac42daced2086f3c6f9a17bba0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=57e83bfbe1e412ac42daced2086f3c6f9a17bba0

Changed files:
  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=57e83bfbe1e412ac42daced2086f3c6f9a17bba0

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

The original Linux kernel CVE announcement for CVE-2026-45942 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45942

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:H
  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: 5
  Paranoid ActionableScore: 6

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).