[CVE-2026-98279][MODERATE REGULAR] btrfs: handle lack of space when cleaning up verity items [ Upstream

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

CVE: CVE-2026-98279
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: btrfs: handle lack of space when cleaning up verity items [ Upstream
Commit: 4b7ecabd87dc40998fe7870d7975279649229a3b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4b7ecabd87dc40998fe7870d7975279649229a3b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98279
Analysis date: Sat, 10 Oct 2026 17:34:57 -0400
ActionableScore: 4
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-2026-98279 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98279

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-2026-98279	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98279 CHECK WITH IMPACT FROM ORIG NN NONE | AEGIS IMPACT MODERATE AEGIS_AS=3 LEGACY=MODERATE RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-703;CWE-755;CWE-459;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A Btrfs fsverity failure can leave verity or orphan items that cannot be removed because rollback and cleanup attempt to obtain a normal metadata reservation after the qgroup limit or filesystem space has already been exhausted. The failed cleanup can force the entire filesystem into read-only mode, leave a subvolume unreachable after EDQUOT, or prevent a later read-write mount after ENOSPC. For the CVSS the PR:L value is used because a local user with permission to modify an eligible file may be able to invoke fsverity and consume an assigned quota without full administrative privileges. AC ranges from H to L because exploitation depends on fsverity support and a specific low-space or qgroup-limit condition, but a user may be able to arrange that condition through controlled storage consumption. The issue is not network reachable by itself. Impact is availability loss only, with no supported memory corruption, information disclosure, or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	MAYBE	DISK SIMPLEFIX SKIP  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=4
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 4
* No floor or cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Positive signals:

* +1 Local unprivileged trigger. A user with write access to an eligible regular file can invoke fsverity without host-administrator powers and can reach an assigned qgroup limit through storage consumption.
* +1 Reliable strong DoS. Failure to reserve metadata for rollback can force the entire Btrfs filesystem read-only.
* +1 Distinct persistent availability impact. Leftover verity or orphan items can leave a subvolume unreachable or prevent a later read-write mount, extending the impact beyond the immediate failed operation.
* +1 Important filesystem/storage path. The defect affects Btrfs transaction, orphan-cleanup, and filesystem recovery behavior.

Negative signals:

* No negative points. The qgroup or ENOSPC prerequisite affects trigger conditions but does not establish a rare, inaccessible, or host-admin-only path.
* No race penalty. This is an ordered transaction-reservation failure, not a timing race.

Important non-awarded signals:

* No memory corruption, UAF, OOB access, repeated release, or attacker-directed write is established.
* No filesystem metadata-corruption point was awarded because the patch establishes persistent unremoved items and mount failure, not unauthorized metadata modification.
* No second generic availability point was added for the same read-only transition. The additional point is specifically for persistent cleanup and subsequent mount failure.
* No confidentiality, integrity, authorization-bypass, privilege-escalation, or remote-reachability signal applies.
* B5-R is not established because there is no heap OOB write, retry-accumulated offset, or later consumption of corrupted descriptors.
* B3-PENDING is not established because no shared-backing write occurs.
* B4-PENDING is not established because no authorization decision changes from DENY to ALLOW.

3. Reachability analysis

The relevant attacker is a local user with permission to modify an eligible file on an affected Btrfs filesystem. `FS_IOC_ENABLE_VERITY` does not inherently require host UID 0 or host-admin-equivalent capabilities when the caller has the required file access.

The trigger requires Btrfs fsverity support and either an applicable qgroup limit or filesystem-wide ENOSPC. Qgroup configuration is normally established by an administrator, but the user can consume an already assigned quota and invoke verity on an accessible file. The global ENOSPC path may also occur without qgroups.

The attacker does not need to create or mount the filesystem and does not need namespace-scoped capabilities. Prior provisioning of the writable Btrfs filesystem is required, while qgroup provisioning is required only for the EDQUOT variant. No special hardware, network access, race, or timing window is involved.

4. Primitive and severity interpretation

The exact defect is improper error recovery under exhausted metadata reservations. Cleanup operations that only delete items incorrectly request an ordinary transaction reservation, which can be rejected by the same qgroup or space condition that caused `enable_verity` to fail.

There is no stale storage, active structured UAF, concrete repeated release, OOB operation, shared-page ownership violation, or authorization primitive. The appropriate classification is CLASS_C because the demonstrated security consequence is availability loss through filesystem shutdown and failed recovery.

The practical consequence can be significant because a per-subvolume quota failure may force the whole filesystem read-only, and cleanup failure can persist across remount attempts. Nevertheless, no useful corruption primitive remains after excluding the availability impact. ImportantCandidate is therefore NO, and AutoDowngradeAllowed is YES. Operational severity is Borderline Moderate and deployment-dependent.

5. One-sentence report phrase

A local user able to enable fsverity on an eligible Btrfs file may trigger a qgroup or ENOSPC rollback failure that forces the filesystem read-only and can prevent subsequent cleanup or read-write mounting.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm whether untrusted users can invoke fsverity on the affected filesystem, whether qgroups are enabled, and whether a whole-filesystem read-only transition would affect security-critical workloads.

7. Call-site confidence

Call-site confidence: high

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

Patch: btrfs: handle lack of space when cleaning up verity items [ Upstream
Commit: 4b7ecabd87dc40998fe7870d7975279649229a3b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4b7ecabd87dc40998fe7870d7975279649229a3b

Changed files:
  fs/btrfs/inode.c
  fs/btrfs/verity.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=4b7ecabd87dc40998fe7870d7975279649229a3b

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

The original Linux kernel CVE announcement for CVE-2026-98279 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98279

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:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 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).