[CVE-2026-80719][MODERATE REGULAR] mm: mglru: fix stale batch updates after memcg reparenting

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

CVE: CVE-2026-80719
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mm: mglru: fix stale batch updates after memcg reparenting
Commit: fceb6b7f3ddec6ea9fc11577f4cf1b2da73a3101
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fceb6b7f3ddec6ea9fc11577f4cf1b2da73a3101
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80719
Analysis date: Fri, 28 Aug 2026 05:38:04 -0400
ActionableScore: 4
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
MGLRU batch accounting can race with memcg reparenting, causing pending generation deltas to be applied to a dying child lruvec and leading to stale nr_pages accounting, reclaim failure, and premature OOM based denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-80719 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80719

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.

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

CVE-2026-80719	MODERATE	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-667;CWE-682;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'MGLRU accounting can become stale when the page table walker batches generation size deltas while memcg reparenting moves folios from a dying child memcg to its parent. The pending deltas may be folded into the old child lruvec after reparenting, leaving lrugen nr_pages inconsistent and causing premature OOM because reclaim may be skipped when nr_pages appears to be zero. For the CVSS the PR:L value is used in the paranoid score for environments where cgroup or memcg control is delegated to a container root or service account rather than full host root. The issue is not network reachable and requires local control over memory pressure and memcg lifecycle conditions. Impact is denial of service through premature OOM or reclaim failure. There is no clear UAF, OOB access, or arbitrary write primitive visible from this patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3)  SKIP CVE-2026-80719 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	READ LOCK WARNONLY HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=4
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The commit describes user-visible premature OOMs caused by stale MGLRU accounting.
* Availability impact realistic: +1. Reclaim can be skipped incorrectly when nr_pages reaches zero while pages still exist, causing premature OOM.
* Cross-boundary isolation/policy bypass: +1. The bug involves memcg reparenting and can affect memory accounting across child and parent cgroups.
* Broad/default/common subsystem: +1. This is core MM reclaim and MGLRU code, not a narrow device-only path.
* Hard or unreliable race / special timing required: -1. Trigger requires concurrency between page-table walking and memcg offlining/reparenting.
* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2. Reliable triggering usually requires control over cgroup or memcg lifecycle and memory pressure.

Paranoid interpretation:

* Replace the typical admin assumption with delegated cgroup or container/service-account control: +1 local lower-privilege trigger.
* Do not add memory corruption, LPE, confidentiality, or integrity signals. The patch shows stale accounting and wrong lruvec target selection, not UAF, OOB write, arbitrary write, type confusion, or attacker-controlled reuse.

## 3. Reachability analysis

The bug is locally reachable through memory pressure plus memcg lifecycle activity, especially cgroup offlining/reparenting while MGLRU page-table walking is active. In typical host deployments, creating and destroying relevant cgroups or controlling memcg hierarchy is administrative or orchestrator-controlled, so the conservative score applies a privilege penalty. In containerized or delegated-cgroup environments, a reduced-privilege container root or service account may be able to create enough memcg churn and memory pressure, so the paranoid score treats PR-like reachability as lower than full host root. The path is not network reachable.

Call-site confidence: high. The patch includes the affected reset_batch_size callers in walk_mm and evict_folios, and the commit describes the exact race.

## 4. Severity interpretation

This behaves like a race-driven MM accounting DoS, not a demonstrated memory-corruption or privilege-escalation issue. The realistic impact is premature OOM or reclaim failure due to stale MGLRU nr_pages accounting after memcg reparenting. Theoretical concern comes from MM and RCU lifecycle complexity, but the supplied patch context does not support UAF, OOB access, attacker-controlled overwrite, or LPE. This is therefore not an Important-class candidate on current evidence, but it should not be blindly ignored because it affects core reclaim and cgroup accounting.

## 5. One-sentence report phrase

MGLRU batch accounting can race with memcg reparenting, causing pending generation deltas to be applied to a dying child lruvec and leading to stale nr_pages accounting, reclaim failure, and premature OOM based denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a core MM race involving memcg lifecycle and RCU-sensitive accounting, but current evidence supports DoS only and does not show a concrete memory corruption or privilege escalation primitive.

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

Patch: mm: mglru: fix stale batch updates after memcg reparenting
Commit: fceb6b7f3ddec6ea9fc11577f4cf1b2da73a3101
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fceb6b7f3ddec6ea9fc11577f4cf1b2da73a3101

Commit description:

The mglru page table walker batches per-generation size deltas in
walk->nr_pages while walking page tables without holding the lruvec lock.
The reset_batch_size() later folds those deltas into walk->lruvec under
the lruvec lock.

The page table walker can run concurrently with the memcg reparenting path
as follows:

CPU0                           CPU1
====                           ====

walk_mm
--> walk_page_range
    --> update_batch_size
        --> walk->nr_pages += delta

                              mem_cgroup_css_offline
                              --> memcg_reparent_objcgs
                                  --> lock lruvec
                                      lru_gen_reparent_memcg
                                      --> reparent child folios to parent
                                      unlock lruvec

    lock lruvec
    reset_batch_size
    --> child lrugen->nr_pages += delta

This will trigger the following warning in lru_gen_exit_memcg():

	VM_WARN_ON_ONCE(memchr_inv(lruvec->lrugen.nr_pages, 0,
				   sizeof(lruvec->lrugen.nr_pages)));

And the user-visible impact of underestimated nr_pages in MGLRU was
premature OOMs because MGLRU does not try to reclaim memory when nr_pages
reaches zero, but there are still more pages.

To fix it, make reset_batch_size() check CSS_DYING under RCU before
flushing the pending batch.  A non-dying memcg keeps the original lruvec
stable against RCU-delayed offlining; a dying memcg redirects the deltas
to the first non-dying ancestor.

Link: https://lore.kernel.org/20260710154318.75388-1-qi.zheng@linux.dev
Fixes: f304652 ("mm: vmscan: prepare for reparenting MGLRU folios")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Reported-by: Peiyang He <peiyang_he@smail.nju.edu.cn>
Closes: https://lore.kernel.org/all/5A9E929D82717101+12fcf643-efb8-4b9a-a53a-1e28cc894f0b@smail.nju.edu.cn
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
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:
  include/linux/memcontrol.h
  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=fceb6b7f3ddec6ea9fc11577f4cf1b2da73a3101

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

The original Linux kernel CVE announcement for CVE-2026-80719 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80719

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: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: 1
  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).