[CVE-2026-90264][MODERATE REGULAR] btrfs: always wait for ordered extents to avoid OE races [ Upstream

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

CVE: CVE-2026-90264
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: btrfs: always wait for ordered extents to avoid OE races [ Upstream
Commit: 58ce50b1802a446d3b26df1b9c58be1ab8913d0e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=58ce50b1802a446d3b26df1b9c58be1ab8913d0e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90264
Analysis date: Fri, 18 Sep 2026 09:37:43 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90264 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90264

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: 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-2026-90264	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90264 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-362;CWE-667;BEST CVSS score: '4.7';DESCR 'A race in Btrfs ordered extent handling can occur when a short direct append write restores the previous file size and then falls back to buffered I/O. The buffered write path may skip waiting for an existing direct I/O ordered extent because the write starts beyond the restored i_size. A second ordered extent can then be inserted for the same file range while the completed first extent is still present in the tree. The overlap check triggers a kernel BUG in btrfs_alloc_ordered_extent and can cause an Oops or kernel panic leading to denial of service. For the CVSS the PR:L value is selected because an unprivileged local user may trigger the affected write path when allowed to write to a file on the vulnerable Btrfs filesystem. AC:H is selected because exploitation depends on a short direct append write buffered fallback and a race window before removal of the previous ordered extent. The issue is not directly network reachable although a network service writing attacker controlled data to Btrfs could expose an indirect path. No UAF OOB access arbitrary write or information disclosure primitive is demonstrated so the grounded impact is C:N I:N A:H.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)  SKIP CVE-2026-90264 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	WRITE OOB DISK INIT RACE LEAK SKIP KERNEL_PANIC_PLUS_UAF HARDWARE SYZBOT  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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

ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
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: **Borderline / manual review recommended**::

2. Signal breakdown

* +1 Local unprivileged trigger. A user with write access to a file on an already mounted Btrfs filesystem can reach the affected `pwritev2` and direct-I/O fallback path without host-admin privilege.
* +1 Reliable kernel crash / strong DoS. Inserting the conflicting ordered extent reaches an explicit kernel `BUG` in `btrfs_alloc_ordered_extent`.
* +1 Important filesystem/storage path. The defect affects Btrfs ordered-extents, direct-I/O fallback, buffered writeback, and extent-range serialization.
* 0 Race penalty. The issue is timing-dependent, but syzbot reproduced it and both relevant I/O operations can be repeatedly driven. The evidence does not establish a narrow, one-shot, externally uncontrollable race.
* 0 Filesystem metadata-corruption bonus. The conflicting in-memory ordered extent is detected by the `BUG`; persistent on-disk metadata corruption is not demonstrated.
* 0 Additional Availability point. The Oops or panic is the same consequence already counted as strong DoS.
* 0 UAF or corruption bonus. No freed object, stale post-lifetime reference, OOB access, arbitrary write, or controlled overwrite is established.
* 0 Confidentiality or integrity impact. No disclosure, cross-file write, COW violation, or unauthorized modification of another principal's data is shown.
* 0 paranoid escalation. Stronger consequences would require an additional memory-corruption or persistent-filesystem-corruption primitive not supported by the supplied evidence.

No concrete repeated release or refcount operation exists. No lifetime floor applies.

3. Reachability analysis

An ordinary local user can trigger the relevant system-call path when permitted to write to a file on a vulnerable Btrfs mount. Creating and mounting the filesystem requires prior administrator action, but that is ordinary system provisioning rather than a per-attack host-admin operation.

No capability check or namespace-scoped administrative operation is required for the file write itself, so `HostAdminRequired=NO`. `NamespaceScopedPrivilege=NO` because namespace capabilities are not part of the trigger. No special hardware, unusual boot mode, or uncommon Btrfs feature is identified.

The required sequence is a short direct append write, restoration of the original `i_size`, buffered-I/O fallback, and insertion before the completed direct-I/O ordered extent is removed. It is a race, but repeated attempts are feasible and the supplied syzbot report demonstrates practical occurrence. Direct network reachability is not established. A network service writing attacker-controlled data to Btrfs would only provide an indirect application-dependent path.

4. Primitive and severity interpretation

The exact primitive is an ordered-extent synchronization failure. Reverting `i_size` causes the buffered-write path to skip extent locking and waiting, allowing two ordered extents for the same file range to coexist temporarily. The overlap invariant check then triggers a kernel `BUG`.

This is not stale storage or an active-object UAF. There is no first lifetime-ending operation, surviving dangling reference, or second refcount mutation. The behavior is broader than passive bookkeeping because it reaches the active writeback path, but the demonstrated terminal consequence is an assertion-driven crash.

`PrimitiveClass=CLASS_C` because the established behavior is invalid filesystem state followed by Oops or panic, without an independently useful corruption primitive. Privilege escalation, arbitrary kernel access, persistent filesystem corruption, and cross-file ownership violation remain unsupported theoretical possibilities and are not scored.

`ImportantCandidate=NO` because no Class A or unresolved Class B primitive exists. `AutoDowngradeAllowed=YES` because triage may retain this as a crash-only local race after review without discarding a concrete memory-corruption or security-boundary primitive. Operational severity is ordinary-to-borderline Moderate.

5. One-sentence report phrase

A local user with write access to a vulnerable Btrfs filesystem can race short direct append I/O with buffered fallback, creating overlapping ordered extents and triggering a kernel BUG and denial of service.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm affected versions, practical reproducibility with unprivileged file access, and whether product-specific panic behavior or downstream Btrfs changes broaden the impact beyond the demonstrated crash.

7. Call-site confidence

Call-site confidence: high

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

Patch: btrfs: always wait for ordered extents to avoid OE races [ Upstream
Commit: 58ce50b1802a446d3b26df1b9c58be1ab8913d0e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=58ce50b1802a446d3b26df1b9c58be1ab8913d0e

Changed files:
  fs/btrfs/ordered-data.c
  fs/btrfs/file.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=58ce50b1802a446d3b26df1b9c58be1ab8913d0e

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

The original Linux kernel CVE announcement for CVE-2026-90264 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90264

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).