[CVE-2026-46251][MODERATE 7.0] btrfs: fix block_group_tree dirty_list corruption [ Upstream

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

CVE: CVE-2026-46251
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: btrfs: fix block_group_tree dirty_list corruption [ Upstream
Commit: 6e10283b5519d987d880d71bec90cdc7f2ec62b3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e10283b5519d987d880d71bec90cdc7f2ec62b3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46251
Analysis date: Wed, 03 Jun 2026 15:20:48 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
Btrfs with EXTENT_TREE_V2 can corrupt the block_group_root dirty_list during transaction commit, allowing a local writer on the affected filesystem to trigger list corruption, transaction abort, filesystem corrupted state, and forced readonly DoS.

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

The original Linux kernel CVE announcement for CVE-2026-46251 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46251

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: 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-2026-46251	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:L/I:L/A:H';*CWE-664;*CWE-787;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H';BEST CVSS score: '5.8';DESCR 'Btrfs with EXTENT_TREE_V2 can corrupt an internal kernel list because block_group_root dirty_list is added to switch_commits even though the same list node may already be linked by normal root dirty tracking. A local user who can write to the affected Btrfs filesystem can dirty block groups and trigger transaction commit or sync, which can lead to list corruption, transaction abort, filesystem corrupted state, and forced readonly remount. For the CVSS the PR:L is used for the paranoid score because reliable triggering does not normally require full administrator privileges once an affected filesystem is mounted and writable by the user. The issue is not network reachable and is triggered through local filesystem activity. Impact is at least local denial of service and filesystem integrity degradation. In worst case the kernel list corruption may allow limited confidentiality or integrity impact and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 6) 	MAYBE	DANGER DISK SKIP HARDWARE  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	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 the affected mounted Btrfs filesystem can plausibly dirty block groups and trigger transaction commit or sync.

* Memory corruption, weak or indirect corruption candidate: +1
  The bug corrupts kernel list pointers in `block_group_root->dirty_list`, but the overwritten values are list metadata and kernel-derived rather than an attacker-selected payload.

* Weak LPE concern: +1
  Kernel list corruption is a real corruption primitive, but the patch does not show UAF reclaim, arbitrary write, controlled overwrite, or reliable object confusion. Treat as weak LPE concern, not strong LPE.

* Reliable kernel crash or strong DoS: +1
  The report shows `list_del corruption`, transaction abort, filesystem corrupted state, and forced readonly behavior.

* Filesystem/storage metadata corruption reachable by local user: +1
  The corruption affects Btrfs transaction and root-list handling, causing transaction abort and filesystem integrity failure.

* Important filesystem/storage path: +1
  This is in Btrfs transaction commit handling, a critical filesystem metadata path.

* Integrity impact plausible: +1
  The observed result includes mixed transaction lists, failed root update, transaction abort, and `Filesystem corrupted`.

* Availability impact realistic: +1
  Availability impact goes beyond a transient warning because the filesystem can be forced readonly and the transaction aborted.

* Rare AND configuration-dependent subsystem/configuration: -1
  Trigger requires Btrfs with `EXTENT_TREE_V2`, which is not a universal default deployment condition.

* Corruption primitive highly constrained or metadata-only: -1
  The corruption is list bookkeeping corruption without demonstrated attacker-controlled target or payload.

## 3. Reachability analysis

The issue is locally reachable through filesystem activity on an affected Btrfs filesystem with `EXTENT_TREE_V2` enabled. A non-root user may be able to trigger it if they can write to the filesystem, cause allocations, and induce transaction commit or sync indirectly or directly. Full administrator privileges are not necessarily required once the filesystem is mounted and writable.

Namespaces and containers can matter if a containerized or delegated workload has write access to the affected Btrfs mount. The issue is not network reachable by itself. It is not a packet parsing or remote protocol bug.

The path is not broadly default for all Linux systems because it depends on Btrfs and the `EXTENT_TREE_V2` incompat feature. However, Btrfs transaction commit is an important storage metadata path, and the demonstrated outcome includes real list corruption and persistent filesystem impact.

## 4. Severity interpretation

This is stronger than an ordinary Moderate DoS because it is not just a warning or assertion. The patch fixes concrete kernel list corruption that can mix Btrfs transaction lists and lead to filesystem corruption and forced readonly state.

Realistic exploitation evidence supports local DoS and filesystem integrity impact. Theoretical privilege escalation is not demonstrated, because there is no clear attacker-controlled overwrite target, UAF reclaim primitive, arbitrary write, or controlled object confusion. The paranoid score is still elevated because kernel list corruption in a filesystem transaction path deserves manual review.

## 5. One-sentence report phrase

Btrfs with EXTENT_TREE_V2 can corrupt the block_group_root dirty_list during transaction commit, allowing a local writer on the affected filesystem to trigger list corruption, transaction abort, filesystem corrupted state, and forced readonly DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is confirmed kernel list corruption in an important filesystem metadata path with persistent integrity and availability impact. It should not be auto-closed as a simple warning or low-risk DoS, even though a strong privilege-escalation primitive is not demonstrated.

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

Patch: btrfs: fix block_group_tree dirty_list corruption [ Upstream
Commit: 6e10283b5519d987d880d71bec90cdc7f2ec62b3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e10283b5519d987d880d71bec90cdc7f2ec62b3

Changed files:
  lib/list_debug.c
  fs/btrfs/root-tree.c
  fs/btrfs/transaction.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=6e10283b5519d987d880d71bec90cdc7f2ec62b3

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

The original Linux kernel CVE announcement for CVE-2026-46251 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46251

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:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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).