[CVE-2026-90265][MODERATE REGULAR] btrfs: defrag: fix deadlock between defrag and delalloc space reservation [ Upstream

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

CVE: CVE-2026-90265
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: btrfs: defrag: fix deadlock between defrag and delalloc space reservation [ Upstream
Commit: a9701dd06e82c345fe70debdf8835b7eb5000c59
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a9701dd06e82c345fe70debdf8835b7eb5000c59
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90265
Analysis date: Thu, 17 Sep 2026 14:47:36 -0400
ActionableScore: 3
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-2026-90265 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90265

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 REGULAR
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-2026-90265	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90265 CHECK WITH IMPACT FROM ORIG NN MODERATE	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-833;CWE-667;BEST CVSS score: '4.7';DESCR 'A Btrfs deadlock can occur because defrag may reserve delalloc space while it already holds locked and dirty folios together with an extent range lock. Under metadata pressure the reservation can wait for asynchronous reclaim, while reclaim waits for a transaction commit or delalloc writeback and writeback waits for a folio or extent lock held by defrag. This circular dependency can leave filesystem workers and user operations blocked in uninterruptible sleep and can make the affected Btrfs filesystem unavailable. For the CVSS the PR:L value is used because a local user with write access to the filesystem may generate the file activity and metadata pressure needed to reach the vulnerable path without administrative control over the kernel. The issue is not directly network reachable, although file activity delivered through a separate network service could indirectly contribute to the required workload. AC:H is used because successful triggering depends on a specific resource pressure and timing combination. Impact is denial of service only. The analyzed code provides no UAF, OOB access, information disclosure, or write primitive that would support confidentiality impact, integrity impact, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 3)  SKIP CVE-2026-90265 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	LOCK DEADLOCK DANGER DISK INIT SKIP MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive signals:

* +1 Local unprivileged trigger. After the filesystem is mounted with autodefrag, a user with write access can generate dirty folios and metadata pressure that drive the affected kernel paths.
* +1 Reliable strong DoS. The supplied trace and dependency chain establish a persistent circular wait involving defrag, metadata reclaim, transaction commit, and writeback.
* +1 Important filesystem path. The deadlock affects Btrfs writeback and transaction processing and can make the affected filesystem unavailable.

Negative signals:

* No negative points. The bug requires several runtime conditions, but it is not shown to depend on a narrow or externally uncontrollable race.
* No rare-subsystem penalty. Btrfs autodefrag is optional, but the evidence does not establish that it is both genuinely unusual and difficult to exercise once enabled.
* No host-admin penalty. Administrative access may be needed to configure the mount, but it is not required for each trigger after an affected filesystem is available to a writable user.

Important non-awarded signals:

* No UAF, OOB access, stale post-lifetime reference, or other memory-corruption primitive is established.
* No repeated release or refcount mutation exists.
* No filesystem metadata corruption or cross-file ownership violation is demonstrated.
* No confidentiality, integrity, privilege-escalation, or authorization-bypass signal is established.
* No remote-reachability point is awarded. A network file service could relay workload, but that would be an indirect deployment-specific path.
* No extra Availability point is awarded because filesystem unavailability is the same consequence already counted as strong DoS.
* No paranoid point is added because stronger impact would require an unsupported corruption or security-boundary primitive.

3. Reachability analysis

The vulnerable sequence requires a Btrfs filesystem using autodefrag, dirty folios retained across defrag subranges, metadata pressure, and reclaim or transaction activity. The demonstrated flushoncommit sequence includes snapshot creation, but the commit explicitly identifies another path where asynchronous reclaim flushes delalloc without requiring flushoncommit.

Once autodefrag is enabled by prior host configuration, an ordinary local user with write access can contribute the file activity and metadata pressure needed to reach the vulnerable sequence. Host administration is therefore not required for the runtime trigger, although prior administrative mounting and configuration of the Btrfs filesystem are prerequisites.

No capability or namespace-scoped privileged operation is needed for the assessed write-workload trigger. The issue does not depend on special hardware. The deadlock is workload-dependent rather than a narrow one-shot race, and it was observed under fsstress.

4. Primitive and severity interpretation

The exact primitive is a circular lock and resource dependency. Defrag holds dirty, locked folios and an extent-range lock while waiting for delalloc space. Reclaim then depends on transaction commit or writeback, while writeback waits for state held by defrag.

This is neither fixed stale storage nor an active structured-object UAF. There is no cleanup-time stale dereference and no concrete repeated release or refcount operation. The demonstrated consequence is blocked kernel workers, tasks in uninterruptible sleep, and loss of availability for the affected filesystem.

PrimitiveClass is CLASS_C because the established behavior is DoS through deadlock without an independently useful corruption, ownership, lifetime, or authorization primitive. Stronger consequences such as kernel memory corruption, data disclosure, protected-data modification, or privilege escalation are not supported by the patch or surrounding explanation.

ImportantCandidate is NO because no Important-preserving primitive is present. AutoDowngradeAllowed is YES because the issue is an established availability defect with no unresolved corruption or security-boundary leverage. Its likely operational severity is ordinary Moderate.

5. One-sentence report phrase

A local user with write access to an autodefrag-enabled Btrfs filesystem may help trigger a circular wait between defrag, metadata reclaim, transaction commit, and writeback, making the affected filesystem unavailable without establishing memory corruption or privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The complete patch-supported consequence is a filesystem deadlock, and no memory-safety, ownership, authorization, confidentiality, or integrity primitive remains unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: btrfs: defrag: fix deadlock between defrag and delalloc space reservation [ Upstream
Commit: a9701dd06e82c345fe70debdf8835b7eb5000c59
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a9701dd06e82c345fe70debdf8835b7eb5000c59

Changed files:
  fs/btrfs/defrag.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=a9701dd06e82c345fe70debdf8835b7eb5000c59

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

The original Linux kernel CVE announcement for CVE-2026-90265 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90265

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: Not available
  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: 3

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).