[CVE-2026-89961][MODERATE 7.0] powerpc/mm: fix wrong addr_pfn tracking in compound vmemmap population

[CVE-2026-89961][MODERATE 7.0] powerpc/mm: fix wrong addr_pfn tracking in compound vmemmap population

[AL-KERNEL]

CVE: CVE-2026-89961
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: powerpc/mm: fix wrong addr_pfn tracking in compound vmemmap population
Commit: b96be860673f9fbbf12cdadb0b25fc4d6d4d207f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b96be860673f9fbbf12cdadb0b25fc4d6d4d207f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89961
Analysis date: Wed, 16 Sep 2026 13:20:42 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89961 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89961

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-2026-89961	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89961 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:H/I:H/A:H';CWE-682;CWE-665;CWE-119;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'Wrong addr_pfn tracking in powerpc compound vmemmap population can make the code compute pfn_offset from the wrong PFN and choose the wrong head or tail struct page mapping. This affects low level memory metadata for compound pages on powerpc Book3S64 radix systems with vmemmap optimization. The realistic trigger is local and configuration dependent, usually through privileged memory hotplug or device memory setup rather than a normal unprivileged syscall. For the CVSS the PR:L in the paranoid score reflects deployments where limited host management roles or service accounts can trigger memory add or device memory setup without full root equivalence. The issue is not network reachable. Impact is at least denial of service via kernel crash, and in the worst defensible case may affect confidentiality and integrity because incorrect struct page metadata mapping is a memory corruption candidate.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 4)	YES	READ OOB SIMPLEFIX HARDWARE MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=3
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3. No floor or cap applied.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 CLASS_B active memory metadata corruption candidate. The bug can compute pfn_offset from the wrong PFN and choose the wrong head or tail struct page mapping during compound vmemmap population.
* +1 Memory layout invariant restoration. The patch fixes inconsistent tracking between vmemmap address progression and data PFN progression.
* +1 Reliable kernel crash / strong DoS plausible. Incorrect vmemmap metadata for compound pages can plausibly lead to kernel faults or memory-management failure when the affected metadata is later used.
* -1 Specialized architecture and configuration. The affected path is powerpc Book3S64 radix vmemmap optimization, not a generic syscall path.

Paranoid additional signal:

* +1 Weak LPE concern. The wrong head/tail struct page mapping is a meaningful invalid structured-object interpretation in low-level page metadata, so limited integrity/confidentiality concern is defensible for manual triage, although not proven.

Non-awarded signals:

* RealUAF not awarded. The patch does not establish a freed object being used after lifetime end.
* Concrete repeated release/refcount not awarded. No first put/free followed by a second lifetime mutation on the same dangling reference is shown.
* Strong exploitation leverage not awarded. Controlled reclaim, arbitrary write, callback control, refcount takeover, or demonstrated LPE is not established.
* Remote reachability not awarded. This is not packet or network triggered.
* Local unprivileged trigger not awarded. Ordinary user reachability is not established.
* C/I as proven impact not awarded. The paranoid score reflects unresolved memory metadata corruption concern, not demonstrated disclosure or unauthorized write.

3. Reachability analysis

The vulnerable function is a low-level meminit path for powerpc Book3S64 radix vmemmap optimization. The realistic trigger appears tied to memory hotplug, device memory setup, or similar kernel memory-management initialization paths. Those operations are normally privileged, but the supplied input does not fully prove that host-admin-equivalent privilege is required for every possible affected deployment, so HostAdminRequired is UNKNOWN rather than YES.

No evidence shows that the complete trigger chain is namespace-scoped. NamespaceScopedPrivilege is NO. No network trigger is present. No ordinary syscall path for an unprivileged local user is established. No race timing is required by the patch itself. The main prerequisite is affected architecture and vmemmap optimization support.

4. Primitive and severity interpretation

The primitive is not a UAF. It is an active low-level memory metadata consistency violation: vmemmap_populate_compound_pages() may select the wrong head or tail struct page mapping because addr_pfn does not track the current vmemmap address correctly.

This is CLASS_B because it affects active struct page metadata and compound-page representation, but the patch does not establish a CLASS_A primitive such as controlled overwrite, controlled reclaim, arbitrary read/write, refcount takeover, or demonstrated privilege escalation.

Demonstrated consequence from the patch is incorrect vmemmap population logic. Practical consequence is likely crash or memory-management malfunction on affected systems. Speculative consequence is broader page metadata corruption that could affect integrity or confidentiality, but this remains unresolved and requires manual review.

ImportantCandidate is YES because wrong struct page metadata mapping is a meaningful kernel memory-management corruption candidate. AutoDowngradeAllowed is NO because automatically closing it as ordinary Moderate could miss unresolved exploitation leverage in core memory metadata.

5. One-sentence report phrase

Wrong addr_pfn tracking in powerpc compound vmemmap population can cause incorrect head or tail struct page metadata mappings, creating a kernel memory metadata corruption candidate with likely DoS impact and unresolved stronger security implications.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch affects low-level struct page metadata mapping for compound pages, and the worst defensible interpretation is broader memory metadata corruption even though UAF, arbitrary write, and LPE are not demonstrated.

7. Call-site confidence

Call-site confidence: medium

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

Patch: powerpc/mm: fix wrong addr_pfn tracking in compound vmemmap population
Commit: b96be860673f9fbbf12cdadb0b25fc4d6d4d207f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b96be860673f9fbbf12cdadb0b25fc4d6d4d207f

Commit description:

vmemmap_populate_compound_pages() uses addr_pfn to determine the PFN
offset within a compound page and to decide whether the current vmemmap
slot should be populated as a head page mapping or should reuse a tail
page mapping.

However, addr_pfn is advanced manually in parallel with addr.  The loop
itself progresses in vmemmap address space, so each PAGE_SIZE step in addr
covers PAGE_SIZE / sizeof(struct page) struct page slots.  Since addr_pfn
is compared against nr_pages in data-PFN units, it should advance by the
same number of PFNs.  The existing manual increments do not match that and
therefore do not reliably track the PFN corresponding to the current addr.

As a result, pfn_offset can be computed from the wrong PFN and the code
can make the head/tail decision for the wrong compound-page position.

Fix this by deriving addr_pfn directly from the current vmemmap address
instead of carrying it as loop state.

Link: https://lore.kernel.org/20260612035903.2468601-4-songmuchun@bytedance.com
Fixes: f2b79c0 ("powerpc/book3s64/radix: add support for vmemmap optimization for radix")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Frank van der Linden <fvdl@google.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador (SUSE) <osalvador@kernel.org>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Vlastimil Babka <vbabka@kernel.org>
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:
  arch/powerpc/mm/book3s64/radix_pgtable.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=b96be860673f9fbbf12cdadb0b25fc4d6d4d207f

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

The original Linux kernel CVE announcement for CVE-2026-89961 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89961

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: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: 3
  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: NO
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).