[CVE-2026-46017][MODERATE 7.0] mm: fix deferred split queue races during migration

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

CVE: CVE-2026-46017
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mm: fix deferred split queue races during migration
Commit: cbf75cf212ee6e499abc1757fb4b5ae6d70ed0aa
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cbf75cf212ee6e499abc1757fb4b5ae6d70ed0aa
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46017
Analysis date: Wed, 27 May 2026 16:36:01 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-46017 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46017

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: NO

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

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

CVE-2026-46017	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;CWE-667;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A race in folio migration can make the destination folio visible before its deferred split queue state is restored, while concurrent rmap removal or shrinker activity can observe inconsistent split state. This may trigger WARNs in deferred_split_folio() or cause a folio to be lost from the split_queue when folio_trylock() fails during migration. For the CVSS the PR:L is used because a local user can create THP and memory pressure workloads that exercise migration and reclaim paths, although reliable triggering requires a narrow timing window. The issue is not network reachable. Impact is mainly local denial of service or memory management state degradation. Limited confidentiality and integrity concern is kept only for the paranoid score because the race affects core mm metadata, but no UAF, arbitrary write, or privilege escalation primitive is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	RACE SYZBOT KPANIC MEMORY 	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

1. ActionableScore
* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

2. Signal breakdown
* Local unprivileged trigger: +1. A local user can create THP, memory pressure, migration, reclaim, or rmap activity that exercises the affected paths.
* Weak or indirect corruption candidate: +1. The bug affects deferred split queue state and folio visibility ordering, but does not show UAF, OOB write, or arbitrary write.
* Reliable DoS or strong warning condition: +1. The race can trigger WARNs in `deferred_split_folio()` and can degrade memory management state.
* Availability impact realistic: +1. Losing folios from the split queue or tripping WARN paths can cause local DoS or memory-management instability.
* Broad core subsystem exposure: +1. This is core MM, THP, migration, rmap, and shrinker logic.
* Hard race / special timing required: -1. Reliable triggering requires a narrow concurrency window between migration, rmap removal, and deferred split scanning.
* No strong memory corruption primitive. No UAF reclaim, arbitrary write, controlled overwrite, page-cache corruption, or privilege escalation path is demonstrated.

3. Reachability analysis
The issue is locally reachable through workloads that create THP activity, memory pressure, reclaim, folio migration, and rmap changes. It is not network reachable. Containers may influence memory pressure and THP behavior if allowed by host policy, but they do not provide a direct special interface to the bug. Realistic exploitation is timing-sensitive and likely requires sustained stress rather than a simple deterministic trigger.

4. Severity interpretation
This behaves like an actionable Moderate MM race rather than a clear Important vulnerability. The realistic impact is local DoS, WARN, or memory-management state degradation. The theoretical concern is that core MM metadata is temporarily inconsistent, but the patch does not support a strong LPE primitive or direct memory corruption such as UAF or arbitrary write.

5. One-sentence report phrase
A race between folio migration, deferred split queue handling, and rmap or shrinker activity can expose inconsistent THP split state, causing WARNs or local memory-management DoS without a proven privilege-escalation primitive.

6. Manual review recommendation
MANUAL CHECK RECOMMENDED. This is a core MM race with state corruption potential, but current evidence supports DoS and metadata inconsistency rather than a demonstrated UAF, arbitrary write, or LPE path.

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

Patch: mm: fix deferred split queue races during migration
Commit: cbf75cf212ee6e499abc1757fb4b5ae6d70ed0aa
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cbf75cf212ee6e499abc1757fb4b5ae6d70ed0aa

Commit description:

migrate_folio_move() records the deferred split queue state from src and
replays it on dst.  Replaying it after remove_migration_ptes(src, dst, 0)
makes dst visible before it is requeued, so a concurrent rmap-removal path
can mark dst partially mapped and trip the WARN in deferred_split_folio().

Move the requeue before remove_migration_ptes() so dst is back on the
deferred split queue before it becomes visible again.

Because migration still holds dst locked at that point, teach
deferred_split_scan() to requeue a folio when folio_trylock() fails.
Otherwise a fully mapped underused folio can be dequeued by the shrinker
and silently lost from split_queue.

[ziy@nvidia.com: move the comment]
  Link: https://lkml.kernel.org/r/FB71A764-0F10-4E5A-B4A0-BA4C7F138408@nvidia.com
Link: https://syzkaller.appspot.com/bug?extid=a7067a757858ac8eb085
Link: https://lkml.kernel.org/r/20260401131032.13011-1-lance.yang@linux.dev
Fixes: 8a8ca142a488 ("mm: migrate: requeue destination folio on deferred split queue")
Signed-off-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: Zi Yan <ziy@nvidia.com>
Reported-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-mm/69ccb65b.050a0220.183828.003a.GAE@google.com/
Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Byungchul Park <byungchul@sk.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Deepanshu Kartikey <kartikey406@gmail.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Usama Arif <usama.arif@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/69ccb65b.050a0220.183828.003a.GAE
  mm/huge_memory.c
  mm/migrate.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=cbf75cf212ee6e499abc1757fb4b5ae6d70ed0aa

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

The original Linux kernel CVE announcement for CVE-2026-46017 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46017

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:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 4
  Paranoid ActionableScore: 5

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).