[CVE-2026-89645][LOW] btrfs: drop recovered reloc root refs on recovery failure

[CVE-2026-89645][LOW] btrfs: drop recovered reloc root refs on recovery failure

[AL-KERNEL]

CVE: CVE-2026-89645
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: btrfs: drop recovered reloc root refs on recovery failure
Commit: 2256d6dc5b88841a8a8b4fd9a9f04730705fb6fd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2256d6dc5b88841a8a8b4fd9a9f04730705fb6fd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89645
Analysis date: Sat, 12 Sep 2026 00:40:11 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89645 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89645

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-89645	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89645 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-401;*CWE-772;CWE-404;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'Btrfs relocation recovery can leave fs_root reloc_root references behind when recovery fails before the normal merge and cleanup paths run. A local actor who can trigger mounting or recovery of a filesystem image with pending relocation may cause references to remain held after the mount failure, especially when an error such as ENOMEM or a failed transaction commit interrupts recovery. For the CVSS the PR:L is used for the paranoid case because some deployments may delegate mount like capabilities to a container root, service account, or restricted storage management role rather than full host root. The issue is not network reachable and requires local control over the mount or recovery path. Impact is resource exhaustion and denial of service only. The patch context supports a reference leak, not a UAF, arbitrary write, information disclosure, or privilege escalation primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	DANGER DISK INIT SKIP 	YES	NO	checked

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

ActionableScore=1
ActionableScoreLower=0
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 0
* Conservative score: 0
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap changed the score.

2. Signal breakdown

Conservative signals:

* +1 Concrete persistent kernel resource/reference leak. The patch directly shows missing cleanup of `fs_root->reloc_root` references on relocation recovery failure.
* +1 Important filesystem/storage path. The affected code is in Btrfs relocation recovery during mount.
* -2 Host-admin-equivalent operation. Triggering the affected recovery path requires mounting or recovering a Btrfs filesystem image, which is normally a host `CAP_SYS_ADMIN` operation.
* Total: 0.

Paranoid signals:

* +1 Concrete persistent kernel resource/reference leak. Same as conservative.
* +1 Important filesystem/storage path. Same as conservative.
* +1 Potential accumulated availability impact. Repeated failed recovery attempts could leave references behind and contribute to resource pressure.
* -2 Host-admin-equivalent operation. Same as conservative.
* Total: 1.

Important non-awarded signals:

* Real UAF not awarded. The bug leaves references behind but does not establish stale post-free use.
* Concrete repeated release/refcount not awarded. The issue is missing `btrfs_put_root()`, not a second put through a dangling reference.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity impact not awarded.
* Reliable kernel crash not awarded. The demonstrated effect is leaked references on mount failure, not an immediate panic.
* Remote reachability not established.
* Namespace-scoped privilege not established.
* ImportantCandidate not set because this is a resource/reference leak without memory corruption, ownership bypass, or privilege-escalation primitive.

3. Reachability analysis

The vulnerable path is reached during `btrfs_recover_relocation()` while mounting or recovering a Btrfs filesystem with pending relocation state. A realistic attacker would need control over a filesystem image or storage object and the ability to cause the kernel to mount or recover it. In normal deployments this requires host-admin-equivalent mount privilege.

The complete trigger chain is not shown to be namespace-scoped. The patch does not establish that an ordinary user can mount such a Btrfs image inside an attacker-created namespace. No network trigger is involved. No special hardware is required, but the failure condition is an error path during relocation recovery, such as `-ENOMEM` or first transaction commit failure. This is not a race.

4. Primitive and severity interpretation

The exact primitive is a persistent reference/resource leak: `fs_root->reloc_root` remains set and its reference is not dropped when relocation recovery fails before the normal merge and cleanup paths. This is not active structured UAF, not fixed stale storage UAF, and not a concrete repeated release/refcount mutation.

The demonstrated consequence is leaked Btrfs root references after failed mount recovery. The speculative consequence is resource exhaustion if the condition can be triggered repeatedly. There is no evidence of arbitrary read/write, controlled reclaim, privilege escalation, metadata corruption usable across ownership boundaries, or information disclosure.

PrimitiveClass is `NONE` because no meaningful corruption, authorization bypass, or post-lifetime object use is established. ImportantCandidate is `NO` because the issue is a local privileged reference leak with no unresolved Important-grade exploitation leverage. AutoDowngradeAllowed is `YES` because automatic downgrade would not discard a concrete memory-corruption or security-boundary primitive.

5. One-sentence report phrase

Btrfs relocation recovery can leak relocation root references on mount recovery failure, causing a local host-admin-triggered resource leak with potential DoS only under repeated failed recovery attempts.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be handled as an ordinary local privileged resource/reference leak because the patch does not establish UAF, repeated lifetime mutation, memory corruption, authorization bypass, or privilege escalation.

7. Call-site confidence

Call-site confidence: high

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

Patch: btrfs: drop recovered reloc root refs on recovery failure
Commit: 2256d6dc5b88841a8a8b4fd9a9f04730705fb6fd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2256d6dc5b88841a8a8b4fd9a9f04730705fb6fd

Commit description:

During relocation recovery, each fs root gets a reference to its relocation
root. If loading or adding a later root fails, or if the first transaction
commit fails, btrfs_recover_relocation() jumps to out_unset before
merge_reloc_roots() and clean_dirty_subvols().

put_reloc_control() drops the list-owned relocation root references, but it
does not clear fs_root->reloc_root or drop the references owned by those
pointers. Mount cleanup only drops them when BTRFS_FS_ERROR is set, so an
error such as -ENOMEM while processing a later root can leave references
behind.

Keep temporary references to the fs roots associated during recovery. On
failure, clear their reloc_root pointers and drop the corresponding
references. Once the first transaction commit succeeds, drop only the
temporary fs root references and let the normal merge and cleanup paths
handle the relocation roots.

Fault injection on a pending-relocation image confirmed the cleanup gap.
With an injected first-commit failure, 25 fs roots had reloc_root set with
fs_error=0. With this fix, the same failure path drops that count to 0
before mount fails.

Fixes: f44deb7 ("btrfs: hold a ref on the root->reloc_root")
CC: stable@vger.kernel.org
Signed-off-by: Guanghui Yang <3497809730@qq.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/btrfs/relocation.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=2256d6dc5b88841a8a8b4fd9a9f04730705fb6fd

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

The original Linux kernel CVE announcement for CVE-2026-89645 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89645

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:L
  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: 0
  Paranoid ActionableScore: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).