[CVE-2026-89985][MODERATE 7.0] memcg: keep folio's objcg same as its node

[CVE-2026-89985][MODERATE 7.0] memcg: keep folio's objcg same as its node

[AL-KERNEL]

CVE: CVE-2026-89985
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: memcg: keep folio's objcg same as its node
Commit: 6165478eaa3094eaee1d5e33b12520064faf043d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6165478eaa3094eaee1d5e33b12520064faf043d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89985
Analysis date: Wed, 16 Sep 2026 15:03:18 -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.

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

The original Linux kernel CVE announcement for CVE-2026-89985 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89985

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-2026-89985	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89985 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:H/I:H/A:H';CWE-362;CWE-667;*CWE-416;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in memcg folio migration and objcg reparenting can break the invariant that an LRU folio uses the objcg for its own NUMA node. After migration across nodes, reparenting of the objcg and reparenting of the folio LRU list can happen in different lock sections, so an LRU operation may resolve the wrong lruvec and take the wrong lru_lock for the list that actually contains the folio. This can strand folios on the LRU list of a dying memcg and creates a kernel memory management corruption candidate with at least local DoS risk. For the CVSS the PR:L is used for the paranoid score because delegated cgroup control, container like environments, or service accounts may allow a local actor to influence memcg lifetime, memory pressure, and migration paths without full host root. The issue is not network reachable. Impact is at least a local denial of service and in worst case may allow confidentiality or integrity impact if the stale LRU or dead memcg state becomes exploitable as a UAF style primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)  SKIP CVE-2026-89985 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	LOCK UAF LINUS KPANIC MEMORY  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	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: 4
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Applied conservative floor: 4 -> 5 because this is an active memory-management lifetime or ownership corruption candidate involving stale LRU or dead memcg state, wrong lru_lock selection, and nontrivial later LRU processing.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2: Real active lifetime or ownership violation. A folio can carry an objcg from a different NUMA node, breaking the required folio node to objcg node invariant during memcg reparenting.
* +1: Reliable kernel DoS or strong availability concern. Wrong lru_lock selection and folio stranding on a dead memcg LRU list can plausibly cause memory-management corruption, lock/list inconsistency, warnings, stalls, or crashes.
* +1: Secondary list/index staleness. The LRU list membership and objcg/lruvec resolution become inconsistent, and later LRU walkers can act on the obsolete association.
* Floor 5 applied: stale active memory-management state is consumed beyond a simple cleanup fault.

Paranoid additional signals:

* +1: Attacker-influenced lifetime and state manipulation is plausible in delegated cgroup or container-like environments where a local actor can influence memcg lifetime, memory pressure, folio migration, and replacement paths.
* +1: Weak exploitation concern. The stale state affects active LRU locking, list placement, and dead memcg ownership state rather than only causing an immediate NULL dereference.

Non-awarded signals:

* +0: Controlled reclaim is not demonstrated.
* +0: Arbitrary or directed kernel write is not demonstrated.
* +0: Confidentiality and integrity are not awarded as direct signals. The patch supports memory-management corruption concern, but not a concrete disclosure or write primitive.
* +0: Remote reachability is not established.
* +0: Strong LPE leverage is not awarded. There is no shown callback control, credential overwrite, controlled replacement object, or refcount takeover.
* +0: Extra availability is not awarded because the same memory-management failure already accounts for DoS.
* No concrete repeated release/refcount signal is awarded. The patch changes objcg selection and migration ownership, not a visible first put followed by a second put on the exact same dangling reference.

Repeated-release/refcount checklist:

* First release can end lifetime: not established as the main primitive.
* Exact same reference survives to a second lifetime mutation: not established.
* Second operation mutates lifetime/refcount: not established.
* +1 Concrete repeated release/refcount: not awarded.
* Paranoid repeated-refcount +1: not awarded.
* FLOOR 5: applied for active stale LRU or dead memcg processing, not for repeated release/refcount.

3. Reachability analysis

The issue is local, not network reachable. A trigger requires interaction between folio migration across NUMA nodes, memcg reparenting of a dying cgroup, and an LRU operation that observes the mismatched folio objcg state.

Host-admin-equivalent privilege is not positively established from the supplied patch. However, full below-host-admin reachability is also not fully established, because practical triggering may depend on cgroup delegation, memory pressure control, NUMA placement or migration behavior, and the ability to create or destroy relevant cgroups. Therefore HostAdminRequired is UNKNOWN.

NamespaceScopedPrivilege is UNKNOWN. Cgroup and container delegation can make parts of the trigger reachable below host admin, but the supplied evidence does not prove that the complete vulnerable sequence is attacker-created namespace scoped.

Prior host setup may exist in real deployments if cgroup delegation, NUMA policy, or container memory-management controls are required. No special hardware beyond a NUMA-capable environment is implied. The race window is real but not scored with a timing penalty, because memcg_reparent_objcgs drops locks between per-node iterations and the bug is an invariant break across active state rather than a purely one-shot tiny interrupt race.

4. Primitive and severity interpretation

The primitive is an active memory-management lifetime/ownership consistency violation. The folio remains an active structured object on LRU lists, but its objcg can correspond to the wrong node after migration. During memcg reparenting, objcg redirection and LRU list splicing are no longer atomic for that folio. A later LRU operation can resolve the wrong lruvec and take the wrong lru_lock for the list that actually contains the folio.

This is stronger than passive bookkeeping corruption because it affects active LRU locking and list membership involving a dying or dead memcg. It is not a demonstrated CLASS_A primitive because controlled reclaim, arbitrary read/write, callback control, credential manipulation, or proven LPE is not shown.

PrimitiveClass is CLASS_B. RealUAF is YES in the broader stale post-lifetime sense because the patch-supported behavior can leave folios associated with dead memcg/LRU state and later active LRU operations can act through that stale association. Demonstrated consequence is at least local DoS or memory-management corruption. Speculative consequence is possible confidentiality or integrity impact if the stale LRU/dead memcg state can be shaped into a useful UAF-style primitive, but that is not proven.

ImportantCandidate is YES because this is unresolved active kernel memory-management corruption rather than a simple warning, NULL dereference, or resource leak. AutoDowngradeAllowed is NO because automatically closing it as ordinary Moderate would discard a meaningful unresolved lifetime/ownership corruption primitive in core mm/memcg code.

5. One-sentence report phrase

A memcg folio migration race can break the folio node to objcg invariant, causing later LRU operations to take the wrong lru_lock and operate on stale or dead memcg LRU state, with at least local DoS impact and unresolved memory-corruption exploitation risk.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The bug affects active mm/memcg LRU lifetime and locking invariants, and the patch context supports a real stale-state corruption candidate even though controlled exploitation is not demonstrated.

7. Call-site confidence

Call-site confidence: medium

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

Patch: memcg: keep folio's objcg same as its node
Commit: 6165478eaa3094eaee1d5e33b12520064faf043d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6165478eaa3094eaee1d5e33b12520064faf043d

Commit description:

memcg_reparent_objcgs() has an inherent assumption that a folio's objcg is
the objcg of the folio's node.  Folio migration across nodes breaks that
assumption: the new folio simply inherits the old folio's objcg while
living on a different node.

Once the assumption is broken, the reparenting of the folio's objcg and
the reparenting of the folio's LRU list are no longer atomic.
memcg_reparent_objcgs() handles one node per iteration and drops all the
locks in between, so the objcg gets reparented in the iteration for the
objcg's node while the LRU list gets spliced in the iteration for the
folio's node.  Any LRU operation on that folio in between resolves its
lruvec through the objcg, and thus takes the lru_lock of the wrong memcg,
not the lru_lock of the list the folio is actually on.

Fix this by selecting the objcg by folio_nid() at charge time, and by
re-deriving it for the destination node in mem_cgroup_migrate() and
mem_cgroup_replace_folio().

Link: https://lore.kernel.org/20260807142406.443516-1-shakeel.butt@linux.dev
Fixes: f1cf8d2 ("mm: memcontrol: eliminate the problem of dying memory cgroup for LRU folios")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reported-by: Karl Erik Hofseth <karl.e.hofseth@opoint.com>
Closes: https://lore.kernel.org/all/anMmd1ADrDVwMO6v@work/
Co-developed-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Muchun Song <muchun.song@linux.dev>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/memcontrol.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=6165478eaa3094eaee1d5e33b12520064faf043d

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

The original Linux kernel CVE announcement for CVE-2026-89985 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89985

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).