[CVE-2026-46160][MODERATE 7.0] btrfs: fix missing last_unlink_trans update when removing a directory [ Upstream

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

CVE: CVE-2026-46160
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: btrfs: fix missing last_unlink_trans update when removing a directory [ Upstream
Commit: cc3c0a0f965754ce230d93ba44ee5b34fbe6138a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cc3c0a0f965754ce230d93ba44ee5b34fbe6138a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46160
Analysis date: Thu, 28 May 2026 11:45:26 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
Btrfs failed to update `last_unlink_trans` during `rmdir`, allowing fsync plus crash recovery to replay inconsistent directory metadata and potentially make the filesystem fail to mount.

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

The original Linux kernel CVE announcement for CVE-2026-46160 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46160

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
======================================================================

CVE-2026-46160	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-754;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H';BEST CVSS score: '6.3';DESCR 'Btrfs rmdir did not update last_unlink_trans for the removed directory, so fsync on an open file descriptor to that removed directory could miss the required transaction commit fallback. After a crash or power failure, log replay can restore inconsistent metadata such as an invalid directory link count and fail the next mount with filesystem corruption errors. For the CVSS the PR:L is used because a local user with write access to the mounted Btrfs filesystem can create, move, remove, and fsync directories without full administrative privileges. The issue is not network reachable and requires a crash or power loss timing condition. Impact is persistent filesystem integrity and availability loss, not kernel memory corruption or privilege escalation.';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
======================================================================

ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local user with write access to the mounted Btrfs filesystem can create, move, remove, and fsync directories.
* Filesystem/storage metadata corruption reachable by local user: +1. The bug affects Btrfs directory metadata and tree-log replay consistency.
* Important filesystem/storage path: +1. Btrfs fsync, rmdir, unlink tracking, and log replay are critical filesystem integrity paths.
* Integrity impact plausible: +1. Log replay can restore inconsistent metadata such as invalid directory link counts.
* Availability impact realistic: +1. The filesystem may fail to mount after crash recovery with `-EIO`.

Paranoid additional signal:

* Persistent corruption severity: +1. The corruption can survive reboot and prevent mount, so the availability and metadata integrity impact is operationally significant.

Limiting factor:

* Crash or power loss timing required: -1. The bad state requires a specific fsync/rmdir sequence followed by crash or power failure.

Not counted:

* No kernel memory corruption.
* No UAF, OOB write, arbitrary write, or information disclosure.
* No privilege escalation.
* No network reachability.

## 3. Reachability analysis

The trigger is local filesystem activity on a mounted Btrfs filesystem. A user with write permissions can perform the directory operations and fsync, but the failure requires a crash or power loss before the relevant state is safely committed.

Namespaces and containers matter if untrusted workloads write to Btrfs-backed storage and can fsync directories. The issue is not network reachable by itself.

## 4. Severity interpretation

This is an actionable Moderate filesystem-integrity issue. It is not an Important memory-corruption vulnerability, but it can cause persistent filesystem metadata corruption and mount failure after recovery.

The theoretical impact is limited to filesystem integrity and availability. There is no supported LPE or kernel compromise path.

## 5. One-sentence report phrase

Btrfs failed to update `last_unlink_trans` during `rmdir`, allowing fsync plus crash recovery to replay inconsistent directory metadata and potentially make the filesystem fail to mount.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Persistent Btrfs metadata corruption after local filesystem operations and crash recovery should not be auto-closed without checking exposure of writable Btrfs filesystems to untrusted users or containers.

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

Patch: btrfs: fix missing last_unlink_trans update when removing a directory [ Upstream
Commit: cc3c0a0f965754ce230d93ba44ee5b34fbe6138a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cc3c0a0f965754ce230d93ba44ee5b34fbe6138a

Changed files:
  fs/btrfs/disk-io.c
  fs/btrfs/inode.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=cc3c0a0f965754ce230d93ba44ee5b34fbe6138a

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

The original Linux kernel CVE announcement for CVE-2026-46160 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46160

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).