[CVE-2026-90259][MODERATE REGULAR] btrfs: qgroup: fix a wrong length calculation in qgroup_free_reserved_data() [ Upstream

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

CVE: CVE-2026-90259
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: btrfs: qgroup: fix a wrong length calculation in qgroup_free_reserved_data() [ Upstream
Commit: e6edde29990af8064b9d12217ec03db231ccd55d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e6edde29990af8064b9d12217ec03db231ccd55d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90259
Analysis date: Thu, 17 Sep 2026 14:28:34 -0400
ActionableScore: 4
ActionableScore lower bound: 3
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-2026-90259 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90259

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-90259	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90259 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:L/A:H';CWE-682;CWE-697;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.3';DESCR 'A wrong length calculation in btrfs qgroup_free_reserved_data() can make the function treat the rounded end offset as the rounded length. This may cause qgroup reserved data to be freed outside the requested range and can weaken quota accounting for a Btrfs filesystem with qgroups enabled. For the CVSS the PR:L is used because a local user with write access to affected Btrfs files can potentially trigger qgroup reservation and release paths without full administrator privileges. The issue is not network reachable and does not show a memory corruption primitive. Impact is mainly integrity impact against quota policy and possible availability impact from inconsistent quota accounting rather than privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	OOB DISK UAF SKIP  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=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. No floor or cap applied.
* Raw paranoid score: 4
* Paranoid score: 4. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Positive conservative signals:

* +1 Local below-host-admin trigger. Once Btrfs qgroups are enabled and the user has write access to affected files or subvolumes, ordinary filesystem operations can reach qgroup reservation and release paths.
* +1 Important filesystem/storage path. The bug is in Btrfs qgroup accounting, which is a storage quota and resource-control mechanism.
* +1 Quota policy/accounting impact. The wrong aligned length can free reserved qgroup ranges outside the intended range, plausibly weakening quota enforcement or producing inconsistent accounting.

Additional paranoid signal:

* +1 Availability concern from sustained quota/accounting inconsistency. In a qgroup-enabled multi-tenant setup, incorrect reservation freeing may contribute to resource exhaustion or quota enforcement failure, but this is not shown as immediate host-wide crash.

Non-awarded signals:

* +0 Real UAF. No use-after-free, stale pointer, double release, refcount misuse, or lifetime corruption is established.
* +0 Memory corruption. The patch fixes arithmetic and range accounting, not an OOB write, arbitrary write, controlled reclaim, or type confusion.
* +0 Confidentiality. No disclosure path is shown.
* +0 Strong integrity primitive. The issue may affect quota accounting, but there is no demonstrated protected-file overwrite, COW violation, metadata corruption, or authorization bypass.
* +0 Reliable kernel crash. No panic, BUG, or filesystem shutdown is established.
* +0 Remote reachability. The trigger is local filesystem activity, not network traffic.
* +0 Race penalty. This is not a timing race.
* +0 Host-admin penalty. Host admin is needed to enable or mount the relevant Btrfs/qgroup environment, but runtime triggering by a user with write access is not shown to require host-admin-equivalent privilege.

3. Reachability analysis

The realistic attacker is a local user or workload with write access to a Btrfs filesystem where qgroups are enabled. The relevant operations are ordinary filesystem operations that can create and release qgroup reserved data ranges.

Host administrator action may be required as prior setup to mount Btrfs and enable qgroups, but the supplied patch does not show that the vulnerable runtime path itself requires host root. Therefore HostAdminRequired is NO for the trigger model, while prior setup remains deployment-dependent.

The issue is not network reachable. No namespace-scoped capability is central to the trigger. No special hardware is required. No race or narrow timing window is involved.

4. Primitive and severity interpretation

The primitive is an arithmetic/range-accounting error in `qgroup_free_reserved_data()`. The old code assigned `len = round_up(start + len, sectorsize)`, which stores the rounded end offset as if it were a length. Later use of `start + len` could therefore cover a much larger range than intended.

This is Class C: invalid filesystem quota/accounting behavior with potential policy and availability consequences. It is not Class A because there is no concrete authorization bypass, ownership/COW violation, controlled corruption, or LPE chain. It is not Class B because no kernel lifetime, UAF, RCU, refcount, or active stale-object primitive is established.

Demonstrated consequence is incorrect qgroup reserved-space freeing. Speculative consequence is weakened quota enforcement or resource exhaustion in qgroup-dependent environments. ImportantCandidate is NO because no Important-grade primitive is established. AutoDowngradeAllowed is YES because there is no unresolved memory-corruption, lifetime, authorization, or ownership primitive, though manual review is still useful for products relying on Btrfs qgroups for tenant isolation.

5. One-sentence report phrase

A wrong aligned length calculation in btrfs qgroup_free_reserved_data() can free qgroup reserved ranges outside the requested extent, weakening Btrfs qgroup quota accounting for local users with write access but without showing memory corruption or privilege escalation.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is not memory corruption and is unlikely to be Important by itself, but Btrfs qgroups may be used as a resource isolation mechanism, so environments relying on qgroup quota enforcement should review practical impact.

7. Call-site confidence

Call-site confidence: medium

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

Patch: btrfs: qgroup: fix a wrong length calculation in qgroup_free_reserved_data() [ Upstream
Commit: e6edde29990af8064b9d12217ec03db231ccd55d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e6edde29990af8064b9d12217ec03db231ccd55d

Changed files:
  fs/btrfs/qgroup.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=e6edde29990af8064b9d12217ec03db231ccd55d

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

The original Linux kernel CVE announcement for CVE-2026-90259 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90259

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:L/PR:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H
  The Best / paranoid CVSS score: 5.3

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: 3
  Paranoid ActionableScore: 4

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).