[CVE-2026-90261][LOW] btrfs: zoned: flush active metadata block group at btree_writepages() start [ Upstream

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

CVE: CVE-2026-90261
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: btrfs: zoned: flush active metadata block group at btree_writepages() start [ Upstream
Commit: e2de2989adb3e58c2320e57c255f52e065667b5e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e2de2989adb3e58c2320e57c255f52e065667b5e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90261
Analysis date: Thu, 17 Sep 2026 19:35:59 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90261 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90261

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: LOW
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-2026-90261	LOW	https://www.kernelcve.org/list/?q=CVE-2026-90261 CHECK WITH IMPACT FROM ORIG NN NONE	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:N/A:H';CWE-703;CWE-755;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'A zoned Btrfs filesystem can abort a transaction and become read only when metadata writeback encounters an active metadata or system block group above another block group containing dirty metadata. The ascending writeback walk may need to finish and replace the active block group, but unsent IO prevents that transition and waiting under zoned_meta_io_lock could deadlock. btrfs_check_meta_write_pointer can therefore return EAGAIN, which transaction commit treats as fatal. For the CVSS the PR:L value is used by the paranoid score because a local user may generate substantial metadata churn while relocation or another privileged maintenance operation is already active. Reliable direct reproduction normally requires administrative control over Btrfs relocation, and the timing dependent state makes AC:H appropriate. The issue is not network reachable. Impact is denial of service through loss of write availability on the affected filesystem. There is no memory corruption primitive supporting confidentiality impact, integrity impact, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)  SKIP CVE-2026-90261 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	OOB DEADLOCK DISK INIT SKIP 	-	-	checked

======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable strong DoS. The transaction is aborted and the affected Btrfs filesystem is forced read-only.
* +1 Important filesystem and storage path. The defect affects zoned Btrfs metadata writeback and transaction commit.
* +0 Additional availability. The read-only filesystem state is already represented by the strong DoS point and is not counted twice.
* +0 Memory corruption. No UAF, OOB access, stale pointer, invalid free, or write primitive is established.
* +0 Filesystem integrity impact. The patch prevents an unnecessary transaction abort but does not establish damaged metadata, cross-file corruption, or a COW ownership violation.
* +0 Local unprivileged trigger. A local user might contribute metadata churn, but the complete trigger below host-admin privilege is not established.
* +0 Race penalty. The failure depends on block-group ordering and pending IO, but the evidence does not establish a narrow attacker-driven race.
* +0 Rare-configuration penalty. Zoned Btrfs is specialized, but subsystem specialization alone is insufficient for a negative point.
* +0 Privilege penalty. Administrative relocation is the documented reproducer, but the evidence does not prove that every complete trigger requires host-admin-equivalent privilege.

The conservative and paranoid scores are identical because no technically supported corruption, disclosure, authorization bypass, or privilege-escalation primitive exists.

3. Reachability analysis

The demonstrated scenario occurs on an already mounted zoned Btrfs filesystem during metadata-heavy relocation, such as the btrfs/187 test. Direct control of relocation normally involves filesystem administration, while an ordinary writer may only contribute metadata churn during an operation initiated elsewhere. The supplied evidence does not establish that an ordinary user can independently perform the complete trigger, but it also does not prove that every occurrence requires attacker-controlled host administration. HostAdminRequired is therefore UNKNOWN.

No network trigger, namespace-scoped capability chain, special device input, or remote exposure is established. A zoned Btrfs deployment and an active metadata relocation or equivalent writeback state are prerequisites. The failure is intermittent, but it is an ordering and progress failure rather than a demonstrated narrow security race.

4. Primitive and severity interpretation

The exact defect is a writeback progress and error-handling failure. Dirty metadata in an active higher-address block group remains unsubmitted when the ascending walk reaches a lower-address block group. The required pivot cannot finish the active group, and waiting under zoned_meta_io_lock is deliberately avoided to prevent deadlock. The resulting EAGAIN is treated as fatal, aborting the transaction and forcing the filesystem read-only.

There is no stale object, post-lifetime access, repeated release, memory overwrite, semantic reinterpretation, authorization bypass, or ownership violation. PrimitiveClass is CLASS_C because the demonstrated behavior is availability failure without an independently useful corruption primitive.

The practical consequence is loss of write availability for the affected filesystem until administrative recovery. Privilege escalation and confidentiality or integrity compromise are not supported. ImportantCandidate is NO, and automatic downgrade is allowed because no unresolved security primitive would be discarded.

5. One-sentence report phrase

A writeback ordering failure on zoned Btrfs can abort a transaction and force the affected filesystem read-only during metadata-heavy relocation, with no demonstrated memory-corruption or privilege-escalation primitive.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The issue is a configuration-specific filesystem availability failure with an explicit read-only outcome and no established corruption, ownership, authorization, or cross-boundary primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: btrfs: zoned: flush active metadata block group at btree_writepages() start [ Upstream
Commit: e2de2989adb3e58c2320e57c255f52e065667b5e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e2de2989adb3e58c2320e57c255f52e065667b5e

Changed files:
  fs/btrfs/extent_io.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=e2de2989adb3e58c2320e57c255f52e065667b5e

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

The original Linux kernel CVE announcement for CVE-2026-90261 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90261

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: unknown
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).