[CVE-2026-64181][MODERATE REGULAR] mm: fix __vm_normal_page() to handle missing support for pmd_special()/pud_special()

[CVE-2026-64181][MODERATE REGULAR] mm: fix __vm_normal_page() to handle missing support for pmd_special()/pud_special()

[AL-KERNEL]

CVE: CVE-2026-64181
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mm: fix __vm_normal_page() to handle missing support for pmd_special()/pud_special()
Commit: 62153767e8fc3889bc6508e9ffe927aaf64c4334
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=62153767e8fc3889bc6508e9ffe927aaf64c4334
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64181
Analysis date: Sun, 19 Jul 2026 13:22:34 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended
Manual review required: YES

Summary:
A local user on affected 32-bit THP configurations may trigger incorrect huge zero page classification in __vm_normal_page(), leading to bad RSS/page state and possible kernel BUG based denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-64181 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64181

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.

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

CVE-2026-64181	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:L/I:L/A:H';CWE-754;CWE-703;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR '__vm_normal_page() could misclassify huge zero mappings as normal pages when pmd_special() or pud_special() support is missing for the relevant page table level. On affected systems this can trigger VM warnings, bad rss counter state, bad page state, and later kernel BUG conditions during reclaim or huge zero folio handling. For the CVSS the PR:L is used because a local user process can potentially exercise anonymous memory mappings and THP related paths, while reliable triggering still depends on a specific architecture and kernel configuration. The issue is not network reachable and does not come from packet input or a remote protocol. Impact is at least local denial of service via kernel instability or crash. For the paranoid score, C:L/I:L/A:H is used because the bug is in mm accounting and page classification, but the patch does not support treating it as a proven UAF or arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended (with actual score 4)  SKIP CVE-2026-64181 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: af38538801c6a97565b44700ee6695d7d60ad779 YES  NO NO unknown 	MAYBE	HARDWARE LINUS MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local user process can plausibly exercise anonymous memory, THP, zap, and reclaim paths without special privileges, although reliable triggering depends on architecture and configuration.

* Reliable kernel crash / strong DoS: +1
  The commit describes WARN followed by BUG: Bad rss-counter state and later BUG: Bad page state during reclaim.

* Availability impact realistic: +1
  The failure is not only a warning. It can progress to kernel BUG conditions and system instability on affected systems.

* Broad/default/common subsystem: +1
  The affected helper is in core mm page classification logic used by THP and page-table walking paths.

* Rare AND difficult-to-reach subsystem/configuration: -1
  The described reproducer depends on x86 32-bit with THP and missing PMD/PUD special support, which is not a broad modern default deployment profile.

Paranoid additional signal:

* Weak/indirect corruption candidate: +1
  The bug misclassifies huge zero mappings as normal pages and causes bad RSS/page state. This is mm state corruption and page classification failure, but the patch does not show UAF, OOB write, arbitrary write, controlled reclaim, or privilege escalation primitive.

Not applied:

* Remote reachable: +0
  No network path.

* Strong memory corruption primitive: +0
  No demonstrated UAF, double free, OOB write, type confusion, or arbitrary write.

* Privilege escalation plausible: +0
  No concrete LPE primitive is shown.

* Confidentiality / integrity impact: +0 for ActionableScore
  smaps/pagemap/numamaps misreporting is noted, but no sensitive disclosure or attacker-controlled modification primitive is demonstrated.

Call-site confidence: medium-high. The patch does not include all callers, but the commit gives concrete affected paths: zap_huge_pmd(), smaps, pagemap, numamaps, folio_walk_start(), and shrink_huge_zero_folio_scan().

## 3. Reachability analysis

A local unprivileged user is the realistic trigger model, because memory mapping and THP-related behavior can be influenced by ordinary process memory activity. Namespaces or containers do not materially change the primitive, although a containerized workload on an affected host could still exercise host mm paths if THP and the affected architecture/configuration are present.

The issue is not remotely reachable and is not packet-driven. The main limiting factor is deployment exposure: the described high-confidence failure depends on x86 32-bit with THP enabled and missing PMD/PUD special support. That makes the issue narrower than typical core-mm bugs on common 64-bit server kernels.

## 4. Severity interpretation

This behaves more like a borderline Moderate mm correctness bug with real DoS impact than an Important-class vulnerability. The realistic impact is local kernel instability or crash through bad page accounting/state. The theoretical concern is that page classification bugs in mm can sometimes hide deeper corruption, but here the patch context supports only weak/indirect corruption, not a demonstrated privilege escalation path.

The paranoid score is therefore raised to 4 for manual-review sensitivity, but not to Actionable Moderate or Important because there is no concrete memory corruption primitive beyond misclassification and bad accounting state.

## 5. One-sentence report phrase

A local user on affected 32-bit THP configurations may trigger incorrect huge zero page classification in __vm_normal_page(), leading to bad RSS/page state and possible kernel BUG based denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is core mm code and the failure reaches bad page state and kernel BUG conditions. However, it does not currently justify Important-class handling without additional evidence of UAF, arbitrary write, controlled object reuse, or privilege escalation.

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

Patch: mm: fix __vm_normal_page() to handle missing support for pmd_special()/pud_special()
Commit: 62153767e8fc3889bc6508e9ffe927aaf64c4334
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=62153767e8fc3889bc6508e9ffe927aaf64c4334

Commit description:

On x86 32-bit with THP enabled, zap_huge_pmd() is seen to generate a
"WARNING: mm/memory.c:735 at __vm_normal_page+0x6a/0x7d", from the
VM_WARN_ON_ONCE(is_zero_pfn(pfn) || is_huge_zero_pfn(pfn)); followed by
"BUG: Bad rss-counter state"s, then later "BUG: Bad page state"s when
reclaim gets to call shrink_huge_zero_folio_scan().

It's as if the _PAGE_SPECIAL bit never got set in the huge_zero pmd: and
indeed, whereas pte_special() and pte_mkspecial() are subject to a
dedicated CONFIG_ARCH_HAS_PTE_SPECIAL, pmd_special() and pmd_mkspecial()
are subject to CONFIG_ARCH_SUPPORTS_PMD_PFNMAP, which is never enabled on
any 32-bit architecture.

While the problem was exposed through commit d80a9cb
("mm/huge_memory: add and use normal_or_softleaf_folio_pmd()"), it was an
oversight in commit af38538 ("mm/memory: factor out common code from
vm_normal_page_*()") and would result in other problems:
* huge zero folio accounted in smaps, pagemap (PAGE_IS_FILE) and
  numamaps as file-backed THP
* folio_walk_start() returning the folio even without FW_ZEROPAGE set.
  Callers seem to tolerate that, though.

... and triggering the VM_WARN_ON_ONE(), although never reported so far.

To fix it, teach vm_normal_page_pmd()/vm_normal_page_pud() to consider
whether pmd_special/pud_special is actually implemented.

Link: https://lore.kernel.org/20260430-pmd_special-v1-1-dbcbcfd72c20@kernel.org
Fixes: af38538 ("mm/memory: factor out common code from vm_normal_page_*()")
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Reported-by: Hugh Dickins <hughd@google.com>
Closes: https://lore.kernel.org/r/74a75b59-2e13-3985-ee99-d5521f39df2a@google.com
Reported-by: Bibo Mao <maobibo@loongson.cn>
Closes: https://lore.kernel.org/r/20260430041121.2839350-1-maobibo@loongson.cn
Debugged-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Tested-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
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:
  mm/memory.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=62153767e8fc3889bc6508e9ffe927aaf64c4334

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

The original Linux kernel CVE announcement for CVE-2026-64181 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64181

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: 3
  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).