[CVE-2026-98090][MODERATE 7.0] btrfs: restore active device pointers after failed sprout [ Upstream

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

CVE: CVE-2026-98090
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: btrfs: restore active device pointers after failed sprout [ Upstream
Commit: 2ee5c4bc11007c51f63f0c1be5d3f07f2b404ff7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2ee5c4bc11007c51f63f0c1be5d3f07f2b404ff7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98090
Analysis date: Fri, 25 Sep 2026 14:12:29 -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.

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

The original Linux kernel CVE announcement for CVE-2026-98090 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98090

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 7.0
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-2026-98090	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98090 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:L/I:L/A:H';*CWE-416;*CWE-672;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A use-after-free condition can occur after a failed Btrfs sprout operation because latest_dev and possibly s_bdev may continue to reference the new device after that device has been removed and released. A later call to btrfs_show_devname() can dereference the stale device pointer and cause a kernel crash. For the CVSS the PR:L value is used for the paranoid score because a reduced-capability service account or container root may have delegated access to Btrfs administration and the relevant block device without having unrestricted host root access. The attack is local and requires a failed sprout setup followed by access to the path that obtains the active device name. It is not directly network reachable. Impact is at least a local denial of service via kernel crash. Reuse of the freed allocation may permit limited confidentiality or integrity impact in a worst case interpretation, but the visible path does not establish an arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4) 	MAYBE	DISK INIT ERRORPATH UAF SKIP KERNEL_PANIC_PLUS_UAF HARDWARE  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

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

2. Signal breakdown

Positive signals:

* +2 Real UAF / lifetime violation. The failed sprout path releases the new `btrfs_device` while `latest_dev`, and possibly `s_bdev`, can continue referencing it.
* +1 Reliable kernel crash. A later `btrfs_show_devname()` call dereferences the stale device and can crash the kernel.
* +1 Important filesystem/storage path. The defect affects Btrfs device ownership and active-device selection during seed-to-sprout conversion.

Negative signals:

* None applied. The evidence does not positively establish host-admin-equivalent privilege, a hard race, rare hardware, or another qualifying reduction.

Material non-awarded signals:

* Remote reachability is not established.
* Controlled reclaim, attacker-shaped replacement data, arbitrary read/write, callback control, and privilege escalation are not demonstrated.
* Structured stale-object leverage is not awarded. Although `struct btrfs_device` is an active structured object, the demonstrated stale use is a shallow device-name dereference resulting in a crash.
* Confidentiality is not awarded merely because `btrfs_show_devname()` reads through a stale object. No concrete disclosure of reclaimed sensitive data is established.
* Integrity is not awarded merely from theoretical allocator reuse.
* Attacker-influenced lifetime manipulation is not awarded. The supplied evidence does not show that one below-host-admin actor can repeatedly control the failure, reclaim, and stale use in an exploitation-relevant way.
* Extra Availability is not awarded because the same kernel crash is already counted.
* The first release can end the new device lifetime, but no second release, put, free, or refcount mutation through the exact dangling reference is shown.
* Concrete repeated release/refcount +1 is not applicable.
* Paranoid repeated-refcount leverage +1 is not applicable.
* FLOOR 5 is not applicable because there is no concrete second lifetime mutation or independently established post-free exploitation leverage.

Arithmetic:

* Conservative: `2 + 1 + 1 = 4`
* Paranoid: `2 + 1 + 1 = 4`

3. Reachability analysis

The vulnerable sequence requires a Btrfs seed filesystem, an attempted sprout operation that installs a new writable device, failure during writable chunk creation or later sprout setup, and a subsequent call to `btrfs_show_devname()`.

Btrfs device-add and seed-to-sprout administration normally involve `CAP_SYS_ADMIN`, access to the mounted filesystem, and access to a suitable block device. However, the supplied patch does not show the complete ioctl permission checks, user-namespace relationship, mount ownership, or device-access boundary. Therefore, host-admin-equivalent privilege is not positively established and no `-2` penalty is applied.

The complete operation is not proven to work using capabilities obtained inside an attacker-created user namespace. `NamespaceScopedPrivilege` is consequently `UNKNOWN`, not `YES`.

Some privileged setup is inherently needed to have a mounted seed filesystem and an available target device, but the evidence does not establish whether the same attacker must have performed that setup. No special hardware, boot option, race, or timing window is required by the documented sequence. This is an ordered failure-path bug rather than a concurrency race.

4. Primitive and severity interpretation

The exact primitive is a stale post-lifetime pointer to a freed `struct btrfs_device`. The stale pointer escapes the failure cleanup through active filesystem fields and is later dereferenced by `btrfs_show_devname()`.

This is an active structured object rather than fixed asynchronous storage. Nevertheless, the demonstrated operation only consumes device-name-related state and produces a kernel fault. There is no demonstrated write through the dangling object, second refcount mutation, controlled reclaim, callback dispatch, or useful type confusion.

This is therefore `CLASS_B`, not `CLASS_A`. The demonstrated consequence is local kernel DoS. Information disclosure, filesystem corruption, and privilege escalation remain theoretical and are not scored.

`ImportantCandidate=NO` because the supplied evidence establishes a shallow crash-capable stale dereference without meaningful post-free exploitation leverage. `AutoDowngradeAllowed=YES` because no unresolved repeated lifetime mutation, controlled replacement, authorization bypass, or ownership violation is present. This does not make the defect harmless. It remains a real kernel UAF deserving review and correction.

The likely operational severity is Moderate, with practical exposure concentrated in systems that use Btrfs seed-device sprouting and encounter a failure during conversion.

5. One-sentence report phrase

A failed Btrfs seed-to-sprout conversion can leave active device pointers referencing a released device, allowing a later device-name query to trigger a kernel use-after-free and denial of service.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The UAF is concrete, but the visible stale use is shallow and crash-oriented. Review should confirm the exact privilege checks and whether any additional callers consume other fields through `latest_dev` or `s_bdev` before automatic downgrade.

7. Call-site confidence

Call-site confidence: medium

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

Patch: btrfs: restore active device pointers after failed sprout [ Upstream
Commit: 2ee5c4bc11007c51f63f0c1be5d3f07f2b404ff7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2ee5c4bc11007c51f63f0c1be5d3f07f2b404ff7

Changed files:
  fs/btrfs/volumes.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=2ee5c4bc11007c51f63f0c1be5d3f07f2b404ff7

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

The original Linux kernel CVE announcement for CVE-2026-98090 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98090

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:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).