[CVE-2026-64295][LOW] mm: page_ext: add count limit to page_ext_iter_next to prevent invalid PFN access

[CVE-2026-64295][LOW] mm: page_ext: add count limit to page_ext_iter_next to prevent invalid PFN access

[AL-KERNEL]

CVE: CVE-2026-64295
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: mm: page_ext: add count limit to page_ext_iter_next to prevent invalid PFN access
Commit: 8dcaa0f87a88d720d13106f3a306c6b61d189d86
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8dcaa0f87a88d720d13106f3a306c6b61d189d86
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64295
Analysis date: Sat, 25 Jul 2026 08:53:22 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A page_ext iterator boundary bug can access one PFN past the requested range during memory hotplug online processing, causing a NULL pointer dereference and local privileged kernel crash.

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

The original Linux kernel CVE announcement for CVE-2026-64295 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64295

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: NO

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

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

CVE-2026-64295	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:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-129;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'The page_ext iterator can advance one PFN past the requested range during memory hotplug online processing. page_ext_iter_next() may call page_ext_lookup(start_pfn plus count) after the loop body has processed the last valid page, and that PFN may belong to a section that is not initialized yet. This can cause a NULL pointer dereference in lookup_page_ext and crash the kernel. For the CVSS the PR:L in the paranoid score is used only for environments where a reduced capability local administrator, service account, or delegated memory management path can online memory blocks. The base case remains PR:H because normal memory hotplug and memory block online operations require administrative privileges. The issue is not network reachable and the impact is denial of service only, with no supported UAF, OOB write, information leak, or privilege escalation primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)  SKIP CVE-2026-64295 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	DANGER NULLPTR HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=1
ActionableScoreLower=0

## 1. ActionableScore

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

## 2. Signal breakdown

Triggered signals:

* Reliable kernel crash / strong DoS: +1
  The commit shows a concrete NULL pointer dereference in `lookup_page_ext()` during memory hotplug online processing.

Paranoid-only signal:

* Availability impact realistic: +1
  If triggered, the NULL dereference can crash the kernel. This is counted only in the paranoid score because the DoS is tied to a privileged memory hotplug path rather than an ordinary unprivileged interface.

Subtracting signals:

* Requires admin/root/CAP_SYS_ADMIN-like privileges in typical deployments: -2
  Online memory block operations and dynamic memory hotplug are normally host-administrator controlled.

* Rare AND difficult-to-reach subsystem/configuration: -1
  The issue depends on memory hotplug plus `CONFIG_PAGE_EXTENSION`/page extension users such as page owner paths, and is not a common user data-plane interface.

Not triggered:

* No remote/network reachability.
* No local unprivileged trigger in typical deployments.
* No UAF, double-free, OOB write, type confusion, refcount takeover, or arbitrary write.
* No confidentiality or integrity primitive supported by the patch.
* No privilege escalation plausibility beyond a crash.

## 3. Reachability analysis

The realistic trigger is local and privileged: a host administrator or privileged memory-management agent must add or online memory. Normal containers and namespaces do not provide access to host memory hotplug controls unless those controls are explicitly exposed, which would already be a high-trust configuration. The affected path is not network reachable and is not triggered by ordinary application syscalls. The configuration is also conditional on page extension support and a hotplug boundary case.

Call-site confidence: high. The commit includes the relevant iterator macro, the changed helper functions, and the crash stack showing the memory hotplug call path.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like kernel DoS issue, not an Important-class vulnerability. The theoretical bug class is an invalid PFN lookup leading to NULL pointer dereference, not memory corruption. Realistic exploitation evidence supports a privileged local kernel crash only. There is no supported path to privilege escalation, information disclosure, persistent corruption, or cross-boundary policy bypass.

## 5. One-sentence report phrase

A page_ext iterator boundary bug can access one PFN past the requested range during memory hotplug online processing, causing a NULL pointer dereference and local privileged kernel crash.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a privileged local NULL pointer dereference DoS in a memory hotplug path, with no evidence of UAF, OOB write, information leak, privilege escalation, or network exposure.

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

Patch: mm: page_ext: add count limit to page_ext_iter_next to prevent invalid PFN access
Commit: 8dcaa0f87a88d720d13106f3a306c6b61d189d86
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8dcaa0f87a88d720d13106f3a306c6b61d189d86

Commit description:

The page_ext iteration API does not validate if the PFN still belongs to a
valid section while advancing the iterator.  When dynamically adding
memory in the hotplug path, it can lead to a NULL pointer dereference
during page_ext_lookup at the boundary of the last valid section when
iterator count equals __pgcount.

The for_each_page_ext() macro calls page_ext_iter_next() as its loop
increment.  for_each_page_ext() does a "__page_ext =
page_ext_iter_next(&__iter)" at the end.  This causes page_ext_iter_next()
to increment iter->index past __pgcount and call page_ext_lookup(start_pfn
+ __pgcount).  During memory hotplug (online), the PFN at start_pfn +
__pgcount may belong to a section that has not yet been initialized,
causing page_ext_lookup() to trigger a NULL pointer dereference.

[   14.555124][  T846] Call trace:
[   14.555125][  T846]  lookup_page_ext+0x6c/0x108 (P)
[   14.555127][  T846]  page_ext_lookup+0x30/0x3c
[   14.555129][  T846]  __reset_page_owner+0x11c/0x260
[   14.571201][  T846]  __free_pages_ok+0x5e8/0x8e0
[   14.571204][  T846]  __free_pages_core+0x78/0xf0
[   14.571206][  T846]  generic_online_page+0x14/0x24
[   14.597782][  T846]  online_pages+0x178/0x30c
[   14.597784][  T846]  memory_block_change_state+0x284/0x32c
[   14.597787][  T846]  memory_subsys_online+0x4c/0x64
[   14.597789][  T846]  device_online+0x88/0xb0
[   14.597791][  T846]  online_memory_block+0x30/0x40
[   14.597793][  T846]  walk_memory_blocks+0xac/0xe8
[   14.597794][  T846]  add_memory_resource+0x280/0x298
[   14.656161][  T846]  add_memory+0x60/0x98

Move the iteration boundary enforcement inside the iterator functions, so
callers cannot inadvertently access beyond the requested range.

Link: https://lore.kernel.org/20260623-page_ext-v3-1-a89799a5367c@oss.qualcomm.com
Fixes: 9039b90 ("mm: page_ext: add an iteration API for page extensions")
Signed-off-by: Ketan Kishore <ketan.kishore@oss.qualcomm.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Luiz Capitulino <luizcap@redhat.com>
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:
  include/linux/page_ext.h

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=8dcaa0f87a88d720d13106f3a306c6b61d189d86

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

The original Linux kernel CVE announcement for CVE-2026-64295 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64295

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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 0
  Paranoid ActionableScore: 1

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).