[CVE-2026-80685][LOW] mm/util: don't read __page_2 for order-1 folios in snapshot_page()

[CVE-2026-80685][LOW] mm/util: don't read __page_2 for order-1 folios in snapshot_page()

[AL-KERNEL]

CVE: CVE-2026-80685
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: mm/util: don't read __page_2 for order-1 folios in snapshot_page()
Commit: 9668ffe0e2a5e2399dce281620198a2e415871fc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9668ffe0e2a5e2399dce281620198a2e415871fc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80685
Analysis date: Fri, 28 Aug 2026 06:31:45 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-80685 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80685

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: LOW
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-2026-80685	LOW	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';CWE-125;CWE-754;**CWE-476;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.4';DESCR 'snapshot_page can read __page_2 for an order 1 folio even though the folio has only two pages. If that folio is placed at the end of a vmemmap section, the read can access an adjacent absent vmemmap section and trigger a kernel fault during memory isolation or memory remove. For the CVSS the PR:H is used because reliable triggering normally requires administrative control over DLPAR memory remove or memory hotplug operations. The issue is not network reachable and is tied to local memory management activity on affected systems. Impact is denial of service via kernel Oops or crash. No practical confidentiality or integrity primitive is visible from the patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 0)  SKIP CVE-2026-80685 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	READ SIMPLEFIX LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=0

1. ActionableScore
* Conservative score: 0
* Paranoid score: 0
* Final recommended bucket: **Ordinary Moderate / Low-like**::

2. Signal breakdown
* Reliable kernel crash / strong DoS: +1. The patch and report show a kernel fault or Oops when `snapshot_page()` reads `__page_2` for an order-1 folio at a vmemmap section boundary.
* Requires admin/root/CAP_* in typical deployments: -2. Reliable triggering normally requires DLPAR memory remove or memory hotplug control, which is administrative.
* Rare AND difficult-to-reach subsystem/configuration: -1. The observed path depends on memory hotplug/DLPAR, ppc64le LPAR-style environments, vmemmap holes, and a specific folio placement.
* Weak corruption / invalid read only: +0. This is an invalid kernel read from absent vmemmap causing a fault, not a demonstrated UAF, write primitive, object reuse, or info leak.

3. Reachability analysis
A normal local user is not expected to trigger this reliably. The realistic trigger is an administrator or privileged management component performing memory remove or isolation on affected large-memory/hotplug systems. Namespaces or containers do not materially reduce the privilege requirement because memory hotplug/DLPAR is host-level management. The path is not network reachable and is not a default packet or syscall data-plane exposure.

4. Severity interpretation
This behaves like a low-end Moderate or Low-like DoS issue. Theoretical memory-corruption impact is not supported by the patch context: the faulty access reads from an unmapped/absent vmemmap area and faults. There is no evidence of attacker-controlled overwrite, stale object reuse, UAF reclaim, page-cache corruption, privilege escalation, or confidentiality exposure. Practical impact is kernel Oops/crash during privileged memory management operations.

5. One-sentence report phrase
`snapshot_page() can fault during memory remove because it reads __page_2 for order 1 folios that only contain two pages, causing a privileged local DoS on affected memory hotplug or DLPAR systems.`

6. Manual review recommendation
NO MANUAL CHECK NEEDED. NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED.
Reason: admin-only trigger, rare configuration, DoS-only invalid read, and no supported LPE or data exposure primitive.

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

Patch: mm/util: don't read __page_2 for order-1 folios in snapshot_page()
Commit: 9668ffe0e2a5e2399dce281620198a2e415871fc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9668ffe0e2a5e2399dce281620198a2e415871fc

Commit description:

snapshot_page() currently reads __page_2 after checking nr_pages > 1, but
it should only do so when nr_pages > 2.

If an order-1 folio is allocated at the end of a vmemmap section,
__page_2 will not exist and reading it will cause a fault.

During DLPAR memory remove on a 22 TB ppc64le LPAR, snapshot_page() oopsed
on the page isolation path while reading an order-1 folio's __page_2 from
an adjacent absent section (unmapped vmemmap).

Fix this to avoid reading memmap that doesn't exist (e.g., a vmemmap
hole).

Link: https://lore.kernel.org/20260708201954.686111-1-aboorvad@linux.ibm.com
Fixes: 31a31da ("mm: move _pincount in folio to page[2] on 32bit")
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Luiz Capitulino <luizcap@redhat.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org> # v6.15+
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/util.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=9668ffe0e2a5e2399dce281620198a2e415871fc

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

The original Linux kernel CVE announcement for CVE-2026-80685 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80685

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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.4

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: unknown
  Paranoid ActionableScore: 0

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).