[CVE-2026-98083][MODERATE 7.0] btrfs: fix transaction use-after-free in raid stripe insertion [ Upstream

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

CVE: CVE-2026-98083
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: btrfs: fix transaction use-after-free in raid stripe insertion [ Upstream
Commit: b185bdf30313e213fa1c887fe22fc3d513c061d9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b185bdf30313e213fa1c887fe22fc3d513c061d9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98083
Analysis date: Fri, 25 Sep 2026 13:31:04 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
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-2026-98083 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98083

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-2026-98083	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98083 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:H/I:H/A:H';*CWE-416;*CWE-664;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A transaction use-after-free can occur in the Btrfs RAID stripe insertion error path when allocation of a stripe extent fails. `btrfs_insert_one_raid_extent()` prematurely ends the transaction while its caller still owns and later uses the transaction handle. The first transaction end can free the handle and drop the final transaction reference, after which the caller can access freed objects while aborting or ending the transaction again. The directly supported impact is a local kernel crash and denial of service. Because this is a real kernel use-after-free involving mutable transaction objects, memory corruption and local privilege escalation remain plausible worst case consequences and require manual review. For the CVSS the PR:L is selected because an unprivileged local user with write access to an affected Btrfs filesystem may be able to generate the required filesystem activity, although reliable triggering also depends on reaching the rare allocation failure path. The issue is not directly network reachable. AC:H reflects the need to trigger the RAID stripe insertion path while the relevant allocation fails and transaction cleanup releases the objects before their next use.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6) 	MAYBE	DISK UAF SKIP  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* No floor or cap changed either score. FLOOR 5 was checked and satisfied independently by the concrete repeated transaction-lifetime operation.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF. The premature `btrfs_end_transaction()` can free the transaction handle and drop the transaction reference while the caller still owns and uses both objects.
* +1 Concrete repeated release/refcount operation. The exact transaction handle survives the premature end and later reaches the caller's common `btrfs_end_transaction()` path again.
* +1 Reliable kernel crash or strong DoS. The caller performs abort and transaction-ending operations through objects whose lifetime may already have ended.
* +1 Important filesystem/storage path. The affected objects control Btrfs transaction abort, reference lifetime, and ordered-write completion.

Paranoid-only signal:

* +1 Repeated-refcount paranoid leverage. The later abort/end sequence performs write-like transaction-state and lifetime operations through storage that may already be freed. This is an unresolved post-lifetime corruption mechanism, not merely theoretical allocator reuse.

Repeated-release checks:

* The first `btrfs_end_transaction()` can end the handle lifetime and drop a transaction reference: yes.
* The exact same handle remains reachable by the caller: yes.
* A later lifetime-mutating operation is concrete: yes, the common exit path ends the transaction again.
* Concrete repeated release/refcount +1 awarded: yes.
* Paranoid repeated-refcount leverage +1 awarded: yes.
* FLOOR 5 applicable: yes, but raw conservative arithmetic already reached 5.

Not awarded:

* Controlled reclaim, refcount takeover, arbitrary write, disclosure, and callback control are not demonstrated.
* Structured stale-object leverage is not separately awarded because the relevant evidence is already counted as the concrete repeated lifetime mutation.
* Weak LPE is not separately awarded for the same repeated-release fact.
* Confidentiality and integrity points are not awarded merely because post-free mutation could theoretically become exploitable.
* Extra Availability is not awarded because it would double-count the same possible kernel crash.
* Local unprivileged trigger is not awarded because ordinary write access may reach the caller, but reliable control over the required `GFP_NOFS` allocation failure is not established.
* No timing penalty is applied. This is an ordered error-path lifetime sequence, not a demonstrated hard race.
* No rarity or host-admin penalty is applied because the supplied evidence does not establish those restrictions.

3. Reachability analysis

The production path is reached from Btrfs ordered-write completion on a filesystem using RAID stripe extents. A user with write access may generate the filesystem activity, but successful triggering additionally requires the stripe-extent allocation to fail. The input does not establish that an ordinary user can reliably force that allocation failure.

Creation, mounting, and feature configuration of the affected Btrfs filesystem may involve prior administration, but the supplied evidence does not prove that host-admin-equivalent privilege is required for the complete runtime trigger. Therefore `HostAdminRequired=UNKNOWN`.

No namespace-scoped capability path is established, so `NamespaceScopedPrivilege=UNKNOWN`. There is no direct network trigger and no special hardware requirement. The vulnerable sequence is deterministic after the relevant allocation failure and is not a race.

4. Primitive and severity interpretation

The primitive is an active structured-object lifetime violation involving `btrfs_trans_handle` and its referenced transaction. It is not fixed stale storage.

This is category B, concrete repeated lifetime corruption:

1. The callee prematurely ends the transaction.
2. That operation may free the handle and permit transaction cleanup.
3. The caller retains the same handle.
4. The caller aborts through the potentially dangling handle.
5. Its common exit path ends that transaction handle again.

The demonstrated consequence is kernel UAF with plausible crash or filesystem disruption. The patch does not establish controlled reclaim, arbitrary kernel read/write, metadata corruption, data loss, or privilege escalation. The second lifetime mutation nevertheless leaves meaningful exploitation leverage unresolved, making this CLASS_B rather than CLASS_C.

`ImportantCandidate=YES` because a concrete second lifetime operation occurs through a potentially dangling active transaction object. `AutoDowngradeAllowed=NO` because automatically treating this as ordinary crash-only cleanup would discard the unresolved post-lifetime transaction/refcount mutation. Operationally, it is Actionable Moderate at minimum, with Important classification requiring manual exploitation-leverage analysis.

5. One-sentence report phrase

A failed Btrfs RAID stripe allocation can prematurely release a transaction handle that remains owned by its caller, causing subsequent abort and transaction-end operations to access and mutate potentially freed transaction state.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The exact same transaction handle can undergo a second lifetime-mutating operation after the first end may have freed it, while the resulting post-free corruption leverage remains unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: btrfs: fix transaction use-after-free in raid stripe insertion [ Upstream
Commit: b185bdf30313e213fa1c887fe22fc3d513c061d9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b185bdf30313e213fa1c887fe22fc3d513c061d9

Changed files:
  fs/btrfs/raid-stripe-tree.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=b185bdf30313e213fa1c887fe22fc3d513c061d9

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

The original Linux kernel CVE announcement for CVE-2026-98083 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98083

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:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 5
  Paranoid ActionableScore: 6

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).