[CVE-2026-89567][MODERATE REGULAR] jbd2: bound shrinker scans by examined checkpoint buffers

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

CVE: CVE-2026-89567
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: jbd2: bound shrinker scans by examined checkpoint buffers
Commit: edf5fcd0469b7467bd5b37a79502c8d9c3257dbb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=edf5fcd0469b7467bd5b37a79502c8d9c3257dbb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89567
Analysis date: Sat, 12 Sep 2026 02:59:06 -0400
ActionableScore: 3
ActionableScore lower bound: 2
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-2026-89567 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89567

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: NO

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

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

CVE-2026-89567	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89567 CHECK WITH IMPACT FROM ORIG NN 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-400;CWE-667;CWE-834;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'jbd2 checkpoint shrinking could hold journal j_list_lock for too long because busy checkpoint buffers did not consume the shrinker scan budget. If a checkpoint transaction contains many busy buffers, the shrinker can walk a very large checkpoint list while other CPUs spin on the same lock, causing soft lockups or RCU stalls. For the CVSS the PR:L is used because a local user or process with write access to an affected filesystem may be able to create the required IO pressure and checkpoint activity, while no administrative mount control is normally required for the workload itself. The issue is not network reachable and is triggered through local filesystem activity and memory reclaim paths. Impact is denial of service due to excessive lock hold time and system responsiveness loss, with no supported confidentiality or integrity impact.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 3)	YES	DEADLOCK DISK INIT UAF HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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

ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2. No floor or cap applied.
* Raw paranoid score: 3
* Paranoid score: 3. No floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Conservative signals:

* +1 Local unprivileged trigger. A local user or process with write access to an affected jbd2 backed filesystem can plausibly create filesystem IO and checkpoint activity that reaches the shrinker path.
* +1 Reliable kernel crash / strong DoS. The commit explicitly describes excessive lock hold time causing soft lockups or RCU stalls while other CPUs spin on journal->j_list_lock.

Paranoid additional signal:

* +1 Important filesystem/storage path. The affected code is in jbd2 checkpoint shrinking, a journaling path used by important filesystems such as ext4, and the failure can affect system responsiveness under memory reclaim and IO pressure.

Non-awarded signals:

* No memory corruption signal awarded. The patch bounds scanning work and lock hold time, not pointer lifetime, refcount, OOB, UAF, or overwrite behavior.
* No RealUAF. No stale post-lifetime object use is established.
* No controlled reclaim, arbitrary write, refcount mutation, or semantic reinterpretation is established.
* No confidentiality or integrity impact awarded.
* No remote reachability awarded. The trigger is local filesystem activity and reclaim behavior, not network input.
* No extra Availability point awarded because the soft lockup or RCU stall is the same DoS consequence already counted.
* No race penalty applied. This is mainly unbounded scan behavior under a lock, not a narrow timing race.

3. Reachability analysis

The practical trigger is local filesystem activity on a jbd2 backed filesystem plus memory reclaim or shrinker activity. A normal local user with write access to such a filesystem may contribute to large checkpoint lists and busy buffers, so host administrator privilege is not positively required for the workload itself. The complete behavior depends on filesystem usage, IO pressure, memory pressure, and checkpoint state, but no special hardware, boot mode, or network exposure is indicated.

Namespace scoped privilege is unknown because the supplied patch does not establish a namespace based complete trigger chain. Prior privileged setup may exist in the ordinary sense that the filesystem must be mounted, but that is normal system operation and not a separate attacker controlled prerequisite for scoring.

4. Primitive and severity interpretation

The exact issue is an unbounded or poorly budgeted shrinker scan of busy checkpoint buffers while holding journal->j_list_lock. This is a resource consumption and lock hold time problem, not a kernel memory corruption primitive. The demonstrated consequence is soft lockup or RCU stall, causing denial of service or degraded system responsiveness.

There is no active structured stale object, no fixed stale storage UAF, and no concrete repeated release or refcount operation. ImportantCandidate is NO because the patch does not establish exploitation leverage beyond DoS. AutoDowngradeAllowed is YES because the issue is DoS-only, has no memory corruption or boundary bypass primitive, and no unresolved Important-grade mechanism remains.

5. One-sentence report phrase

jbd2 checkpoint shrinking can hold journal->j_list_lock for too long because busy checkpoint buffers did not consume the shrinker scan budget, allowing local filesystem activity to cause soft lockups or RCU stalls as a denial of service.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a DoS-only unbounded scan and lock hold issue with no UAF, refcount corruption, arbitrary write, information disclosure, security boundary bypass, or privilege escalation evidence.

7. Call-site confidence

Call-site confidence: high

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

Patch: jbd2: bound shrinker scans by examined checkpoint buffers
Commit: edf5fcd0469b7467bd5b37a79502c8d9c3257dbb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=edf5fcd0469b7467bd5b37a79502c8d9c3257dbb

Commit description:

The jbd2 shrinker currently accounts only checkpoint buffers that it
successfully releases against nr_to_scan.  Busy buffers therefore do not
consume the scan budget.

If a checkpoint transaction contains mostly busy buffers, the shrinker
can scan its entire checkpoint list while holding journal->j_list_lock.
Large checkpoint lists can result in excessive lock hold times and leave
other CPUs spinning on j_list_lock, causing soft lockups or RCU stalls.

Pass nr_to_scan into journal_shrink_one_cp_list() and decrement it for
every buffer examined, including busy buffers.  Pass NULL from checkpoint
cleanup paths so their existing full-list behavior is preserved.

This restores the scan-budget semantics that existed before
journal_shrink_one_cp_list() was changed to always scan a complete
checkpoint list.

Fixes: b98dba2 ("jbd2: remove journal_clean_one_cp_list()")
Cc: stable@vger.kernel.org
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260713102229.1598812-3-max.kellermann@ionos.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/jbd2/checkpoint.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=edf5fcd0469b7467bd5b37a79502c8d9c3257dbb

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

The original Linux kernel CVE announcement for CVE-2026-89567 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89567

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: 2
  Paranoid ActionableScore: 3

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).