[CVE-2026-89755][MODERATE 7.0] mm/migrate_device: clear stale mapping after freeing swapcache

[CVE-2026-89755][MODERATE 7.0] mm/migrate_device: clear stale mapping after freeing swapcache

[AL-KERNEL]

CVE: CVE-2026-89755
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mm/migrate_device: clear stale mapping after freeing swapcache
Commit: 8ffedc6573a665cdc31ebe47eae7b32b78d0df83
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8ffedc6573a665cdc31ebe47eae7b32b78d0df83
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89755
Analysis date: Sat, 12 Sep 2026 00:03:06 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A stale folio mapping after `folio_free_swap()` can make device-page migration operate on `swap_space.i_pages` with invalid folio reference accounting, causing a local kernel crash and requiring manual review for possible MM lifetime-corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-89755 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89755

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-89755	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89755 CHECK WITH IMPACT FROM ORIG NN MODERATE	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-664;*CWE-672;CWE-911;*CWE-416;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 stale folio mapping can be used during migrate_device after folio_free_swap removes the folio from the swap cache. The migration path may then treat the folio as still mapped through swap_space and operate on swap_space.i_pages with invalid reference accounting. This can trigger a folio reference count BUG and crash the kernel. For the CVSS the PR:L value is used because practical triggering requires local code execution with access to a device migration path such as a GPU or accelerator driver interface, but not necessarily full administrator privileges. The issue is not network reachable. Impact is at least local denial of service via kernel crash. In the paranoid interpretation, limited confidentiality or integrity impact is kept possible because MM reference accounting errors can be UAF adjacent, but no arbitrary write primitive is proven by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	INIT SIMPLEFIX KPANIC MEMORY 	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

## 1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

No floor or cap was applied.

## 2. Signal breakdown

Conservative signals:

* +1 weak / indirect corruption candidate: stale `mapping` points to `swap_space` after `folio_free_swap()` removed the folio from swap cache.
* +1 active semantic state mismatch: a folio no longer in swap cache can still be processed as if it were mapped through `swap_space`.
* +1 reliable kernel crash / strong DoS: commit states this can trigger a folio reference count BUG.
* +1 important MM lifetime / reference accounting path: the issue affects folio migration, swapcache state, and folio reference accounting.

Paranoid additional signal:

* +1 weak exploitation concern: MM reference-accounting corruption is more serious than a pure NULL dereference and may be UAF-adjacent, but no controlled reclaim or arbitrary write is established.

Non-awarded signals:

* RealUAF not awarded: stale `mapping` is a stale semantic pointer to `swap_space`, but supplied evidence does not show that `swap_space` itself is freed.
* Concrete repeated release / refcount not awarded: invalid reference accounting is described, but the patch does not show a clear first release, exact surviving dangling reference, and second lifetime mutation on that same reference.
* Strong LPE +2 not awarded: no controlled reclaim, object replacement, arbitrary write, credential manipulation, or demonstrated privilege escalation.
* Confidentiality / integrity direct bonuses not awarded: no disclosure path or unauthorized write primitive is proven.
* Remote reachability not awarded: this is not network-triggered.
* Race penalty not applied: this appears to be an ordered stale-state bug, not a narrow timing race.
* Hardware penalty not applied: device migration may involve GPU / accelerator paths, but rare physical-only hardware is not established.

## 3. Reachability analysis

The likely trigger is local and depends on reaching `migrate_device` behavior through HMM / GPU / accelerator / device-memory migration paths. The supplied evidence does not prove that host administrator privilege is required, but it also does not prove ordinary unprivileged reachability for the complete trigger chain.

`HostAdminRequired=UNKNOWN` is used because access to the full migration path is deployment and driver dependent. Some accelerator or GPU compute interfaces may be exposed below host admin, while other setups may require privileged device management.

`NamespaceScopedPrivilege=NO` because no namespace-scoped capability path is established or relevant from the supplied patch.

No direct network vector is present. Prior setup may require a kernel and driver configuration supporting device page migration and swapcache-backed folios. Special hardware or driver support may be required, but the evidence does not justify a rare-hardware penalty.

Race repeatability is not the main issue. The bug is caused by using stale saved state after `folio_free_swap()` changes the folio mapping state.

## 4. Primitive and severity interpretation

The exact primitive is stale semantic state in the MM migration path. `__migrate_device_pages()` previously saved a folio mapping, then `folio_free_swap()` could remove the folio from swap cache, leaving the saved mapping inconsistent with the current folio state. Passing that stale mapping to `folio_migrate_mapping()` can make migration operate on `swap_space.i_pages` with invalid folio reference accounting.

This is not proven to be a literal UAF from the supplied patch because the stale object is not shown to be freed. It is better classified as CLASS_B because it affects active MM lifetime / reference accounting and can corrupt kernel object accounting enough to trigger a refcount BUG.

Demonstrated consequence: local kernel crash / DoS.

Speculative consequence: limited exploitation leverage may exist because wrong folio reference accounting in core MM can be adjacent to lifetime bugs, but no arbitrary R/W, controlled reclaim, or privilege escalation is established.

ImportantCandidate=YES because the bug affects core MM folio reference accounting and the post-bug state is not merely a harmless warning or local bookkeeping issue.

AutoDowngradeAllowed=NO because automatic closure would risk losing a concrete MM lifetime/accounting corruption candidate that needs human review.

Likely operational severity: actionable Moderate, with Important-class review warranted if the affected device-migration path is reachable by unprivileged or delegated local users.

## 5. One-sentence report phrase

A stale folio mapping after `folio_free_swap()` can make device-page migration operate on `swap_space.i_pages` with invalid folio reference accounting, causing a local kernel crash and requiring manual review for possible MM lifetime-corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the demonstrated impact is a folio refcount BUG, but the affected area is core MM migration / swapcache reference accounting, which is sufficiently close to lifetime corruption to require human triage rather than automatic downgrade.

## 7. Call-site confidence

Call-site confidence: medium

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

Patch: mm/migrate_device: clear stale mapping after freeing swapcache
Commit: 8ffedc6573a665cdc31ebe47eae7b32b78d0df83
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8ffedc6573a665cdc31ebe47eae7b32b78d0df83

Commit description:

__migrate_device_pages() reads the folio mapping before calling
folio_free_swap().  When folio_free_swap() succeeds, the folio is removed
from the swap cache, but the saved mapping still points to swap_space.

Passing the stale mapping to folio_migrate_mapping() makes it use the
mapped-folio path for a folio that is no longer in swapcache.  It can then
operate on swap_space.i_pages with invalid reference accounting,
eventually triggering a folio reference count BUG.

After a successful split, nr still contains the number of pages in the
original large folio, although each resulting page is now a separate
order-0 folio.  Reset nr to 1 so each split folio is processed separately,
including its own swapcache removal and mapping lookup.

Refresh the saved mapping after folio_free_swap() so the current folio
state is used during migration.

Link: https://lore.kernel.org/20260728062832.1107127-1-arvind.yadav@intel.com
Fixes: df263d9 ("mm/migrate_device: try to handle swapcache pages")
Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Balbir Singh <balbirs@nvidia.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.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:
  mm/migrate_device.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=8ffedc6573a665cdc31ebe47eae7b32b78d0df83

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

The original Linux kernel CVE announcement for CVE-2026-89755 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89755

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).