[CVE-2026-89757][MODERATE REGULAR] mm/mglru: fix and remove redundant unevictable folio handling [ Upstream

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

CVE: CVE-2026-89757
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mm/mglru: fix and remove redundant unevictable folio handling [ Upstream
Commit: 54a58d6656dd403b686b247f0bad8507cbfb45df
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=54a58d6656dd403b686b247f0bad8507cbfb45df
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89757
Analysis date: Sat, 12 Sep 2026 03:22:27 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
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-2026-89757 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89757

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: YES

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

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

CVE-2026-89757	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89757 CHECK WITH IMPACT FROM ORIG NN LOW	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-362;CWE-665;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A bug in the MGLRU unevictable folio handling path can leave a folio with an invalid mlock_count because folio lru storage may contain list poison after lru_gen_del_folio. This can make munlock fail to clear PG_mlocked and can keep the folio unevictable until it is freed, inflating Mlocked accounting and degrading reclaim behavior. The same shortcut also changes LRU state in the wrong order, so concurrent isolation paths such as compaction or folio_isolate_lru may observe an inconsistent folio state. For the CVSS the PR:L is used because a local user can influence mlock and munlock behavior and memory pressure, although reliable triggering depends on MGLRU state and timing. The issue is not network reachable. Impact is mainly local denial of service through memory accounting and reclaim disruption rather than a demonstrated privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap was applied.

2. Signal breakdown

Conservative positive signals:

* +1 Local unprivileged trigger. A local user can influence mlock, munlock, memory pressure, and reclaim behavior within normal resource limits.
* +1 Weak / indirect corruption candidate. The bug causes invalid structured interpretation of folio->lru storage as mlock_count after list poison may have been written.
* +1 Availability / resource impact. The demonstrated result is inflated Mlocked accounting and folios remaining unevictable until freed, which can degrade reclaim behavior.

Paranoid additional signal:

* +1 Active LRU state consistency concern. The shortcut changes LRU state in the wrong order while PG_lru is still set, allowing concurrent isolation paths to observe an inconsistent folio state.

Important non-awarded signals:

* Real UAF not awarded. The patch does not establish that a folio is freed and then dereferenced through a stale reference.
* Concrete repeated release/refcount not awarded. There is no first put/free followed by a second lifetime-mutating operation on the same dangling reference.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity impact not awarded.
* Reliable kernel crash not awarded because the commit describes user-visible accounting/reclaim effects and possible unexpected behavior, not a concrete panic path.
* Remote reachability not established.
* Extra availability not awarded beyond the resource/reclaim disruption signal.

Repeated release/refcount checklist:

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

3. Reachability analysis

The plausible attacker is a local user process that can allocate memory, use mlock and munlock within RLIMIT_MEMLOCK, and create memory pressure that causes reclaim paths to process folios under MGLRU. Host administrator privileges are not positively required for those basic operations, so HostAdminRequired=NO.

The issue is not namespace-specific. NamespaceScopedPrivilege=NO because the relevant trigger is ordinary local memory-management behavior, not a namespaced capability-gated operation.

A prior host setup may be needed in practice if MGLRU is disabled or unavailable in the evaluated kernel, but the supplied patch is for the normal MGLRU implementation and does not establish a rare hardware or physical-only prerequisite. No network trigger is present. The concurrent isolation concern may depend on timing with compaction or folio isolation, but the main mlock_count accounting problem is an ordered state bug rather than a hard race-only exploit path.

4. Primitive and severity interpretation

The exact primitive is incorrect handling of unevictable folios in MGLRU. The code can move a folio toward the unevictable path without properly initializing mlock_count, while folio->lru storage may already contain LIST_POISON data from list_del. This results in bogus mlock_count values and prevents munlock from clearing PG_mlocked.

This is active kernel memory-management state corruption in the bookkeeping sense, but not a proven post-lifetime UAF, not a double free, and not a directed write primitive. The affected object is an active structured folio, but the patch does not show attacker-controlled replacement, controlled reclaim, callback use, credential manipulation, or useful stale-field consumption after free.

PrimitiveClass=CLASS_C because the demonstrated behavior is invalid state, accounting corruption, and reclaim disruption. The practical impact is local denial of service or memory-management degradation. Privilege escalation is speculative and not supported by the patch evidence.

ImportantCandidate=NO because no Class A primitive and no strong unresolved Class B lifetime/corruption primitive is established. ManualReview=RECOMMENDED because this is core mm code with active LRU state ordering and concurrent isolation implications. AutoDowngradeAllowed=YES because the evidence supports a borderline Moderate issue rather than an Important-preserving corruption primitive.

5. One-sentence report phrase

A bug in MGLRU unevictable folio handling can corrupt mlock_count accounting and expose inconsistent LRU state to concurrent isolation paths, causing local reclaim disruption or denial of service without a demonstrated UAF, arbitrary write, or privilege escalation primitive.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Core memory-management state and LRU ordering are involved, and the commit explicitly mentions concurrent isolation unexpected behavior, but the established evidence remains below an Important-class memory corruption or privilege-escalation primitive.

7. Call-site confidence

Call-site confidence: medium

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

Patch: mm/mglru: fix and remove redundant unevictable folio handling [ Upstream
Commit: 54a58d6656dd403b686b247f0bad8507cbfb45df
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=54a58d6656dd403b686b247f0bad8507cbfb45df

Changed files:
  mm/vmscan.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=54a58d6656dd403b686b247f0bad8507cbfb45df

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

The original Linux kernel CVE announcement for CVE-2026-89757 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89757

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: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: 3
  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: 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).