[CVE-2026-89646][LOW] ceph: fix leaked inode reference on writeback abort at umount

[CVE-2026-89646][LOW] ceph: fix leaked inode reference on writeback abort at umount

[AL-KERNEL]

CVE: CVE-2026-89646
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: ceph: fix leaked inode reference on writeback abort at umount
Commit: ec32015a955c5d326c5e26610edfdcccd52f2314
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ec32015a955c5d326c5e26610edfdcccd52f2314
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89646
Analysis date: Sat, 12 Sep 2026 06:49:27 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A Ceph writeback abort during unmount can leak a wrbuffer claim and inode reference, preventing inode eviction and causing a kernel BUG during filesystem teardown, with impact limited to local denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-89646 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89646

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-2026-89646	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89646 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-772;CWE-404;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:R/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'A Ceph writeback abort during unmount can leak a wrbuffer claim and an inode reference because collected dirty folios are redirtied and unlocked before writepages_finish can release the claim. The leaked reference can keep i_wrbuffer_ref above zero and prevent inode eviction, causing a VFS busy inode condition and a kernel BUG during Ceph filesystem teardown. For the CVSS the PR:L is used for the paranoid score because a local user with write access to the mounted Ceph filesystem may prepare dirty folios, while the final crash is reached during unmount or teardown. The issue is not directly network reachable and requires a local Ceph client mount plus a writeback abort during unmount. Impact is denial of service via kernel crash. No confidentiality or integrity impact is assigned because the bug is a reference leak rather than a demonstrated UAF, OOB access, information leak, or write primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)  SKIP CVE-2026-89646 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	WRITE OOB DISK LEAK  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

## 1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap changed the score.

## 2. Signal breakdown

Positive signals:

* +1 Concrete persistent kernel resource/reference leak. The patch establishes that a wrbuffer claim, snap_context reference, and inode reference can be lost on the ceph_submit_write abort path.
* +1 Reliable kernel crash / strong DoS. The commit shows that the leaked inode reference can prevent inode eviction and lead to VFS Busy inodes after unmount followed by a kernel BUG.

Non-awarded signals:

* RealUAF not awarded. The bug keeps references alive too long. It does not use an object after its lifetime ended.
* Concrete repeated release/refcount not awarded. The patch fixes a missing release, not a second release through a dangling reference.
* Strong or weak LPE leverage not awarded. No controlled reclaim, arbitrary write, refcount takeover, callback control, or privilege escalation primitive is established.
* Confidentiality and integrity not awarded. The established effect is reference leakage and unmount-time crash, not data disclosure or unauthorized modification.
* Extra Availability not awarded. The kernel BUG is already counted as the DoS signal.
* Remote reachability not awarded. This is a local Ceph client filesystem path, not a network packet handling path directly reachable by an attacker.
* Important filesystem/storage corruption not awarded. The path is filesystem writeback, but the established consequence is a leaked reference and teardown BUG, not metadata corruption, COW ownership failure, or cross-file permission violation.

Repeated release/refcount checklist:

* First release can end lifetime: not applicable.
* Exact same reference survives to a second release: no.
* Second operation mutates lifetime/refcount: no.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.

## 3. Reachability analysis

A local user with write access to a mounted Ceph filesystem may be able to create dirty folios that participate in the affected writeback path. The fatal condition occurs when ceph_inc_osd_stopping_blocker fails during umount or filesystem teardown, causing the abort path to redirty and unlock pages without releasing the wrbuffer claim.

The complete trigger is not proven to be reachable by an ordinary unprivileged user alone because unmount or teardown is usually controlled by an administrator or system lifecycle event. HostAdminRequired is therefore UNKNOWN rather than YES or NO. There is no evidence that the complete operation is namespace-scoped. No special hardware is required. No race bonus or race penalty is applied because this is an ordered abort-path leak, not a timing-window UAF.

## 4. Primitive and severity interpretation

The exact primitive is a missing cleanup of a Ceph wrbuffer claim and associated inode/snap_context references on a writeback abort path. This is a persistent reference/resource leak, not a UAF, OOB access, double free, or active stale-object corruption.

The demonstrated consequence is denial of service through unmount failure and kernel BUG. The speculative consequence beyond crash is weak because the bug retains objects too long rather than accessing freed storage. There is no meaningful exploitation leverage beyond DoS established by the patch.

PrimitiveClass is CLASS_C because the behavior is primarily crash/DoS from invalid teardown state. ImportantCandidate is NO because no Class A primitive and no strong unresolved Class B lifetime corruption exist. AutoDowngradeAllowed is YES because this is suitable for ordinary Moderate handling and does not preserve an unresolved corruption or security-boundary primitive.

## 5. One-sentence report phrase

A Ceph writeback abort during unmount can leak a wrbuffer claim and inode reference, preventing inode eviction and causing a kernel BUG during filesystem teardown, with impact limited to local denial of service.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

The issue is a concrete reference leak with a demonstrated DoS consequence, but no UAF, repeated lifetime mutation, memory corruption, authorization bypass, or plausible privilege escalation primitive is established.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: ceph: fix leaked inode reference on writeback abort at umount
Commit: ec32015a955c5d326c5e26610edfdcccd52f2314
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ec32015a955c5d326c5e26610edfdcccd52f2314

Commit description:

ceph_dirty_folio() takes a wrbuffer claim on each newly dirtied folio: it
bumps i_wrbuffer_ref (taking an ihold() on the 0->1 transition) and
attaches the snap_context to folio->private.  That claim is released only
by ceph_put_wrbuffer_cap_refs(), which for a submitted write runs from
writepages_finish().

In ceph_submit_write(), if ceph_inc_osd_stopping_blocker() fails -- which
happens during umount -- the request is aborted before submission: the
already-collected folios are only redirtied and unlocked, so
writepages_finish() never runs and the claim is leaked.
redirty_page_for_writepage() -> folio_redirty_for_writepage() ->
filemap_dirty_folio() sets PG_dirty directly and does not go through
->dirty_folio, so ceph_dirty_folio() is not re-entered to rebalance it.
Because every subsequent writeback also fails the osd_stopping_blocker,
i_wrbuffer_ref never returns to 0, the ihold() is never dropped, and the
inode cannot be evicted:

  VFS: Busy inodes after unmount of ceph
  kernel BUG at fs/super.c:650!

Release the orphaned claim in the abort path before redirtying, via
ceph_undo_wrbuffer_claim(): detach the snap_context, drop the wrbuffer
reference (letting i_wrbuffer_ref reach 0 and iput() the inode), and drop
the snap_context reference -- i.e. do what writepages_finish() would have
done for these never-submitted folios.

Only the locked_pages entries are undone; folios still in the fbatch were
never dirty-cleared by this call (folio_clear_dirty_for_io() is the
ownership-transfer point, and a successful move NULLs the fbatch slot), so
they hold no claim this call owns.

Cc: stable@vger.kernel.org
Fixes: fd7449d ("ceph: fix generic/421 test failure")
Signed-off-by: Matthew Brown <matthew@bargrove.com>
Reviewed-by: Xiubo Li <xiubo.li@clyso.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/super.c
  fs/ceph/addr.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=ec32015a955c5d326c5e26610edfdcccd52f2314

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

The original Linux kernel CVE announcement for CVE-2026-89646 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89646

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:R/S:U/C:N/I:N/A:H
  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: unknown
  Paranoid ActionableScore: 2

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).