[CVE-2026-64417][LOW] mm: shrinker: fix NULL pointer dereference in debugfs [ Upstream

[CVE-2026-64417][LOW] mm: shrinker: fix NULL pointer dereference in debugfs [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64417
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: mm: shrinker: fix NULL pointer dereference in debugfs [ Upstream
Commit: ebb45c2648b1f60715fd283700f651e05e431231
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ebb45c2648b1f60715fd283700f651e05e431231
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64417
Analysis date: Sat, 25 Jul 2026 07:15:24 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A local actor with access to the affected shrinker debugfs file can trigger a NULL callback dereference and crash the kernel, but normal deployments restrict this path to administrator-controlled debugfs access and no privilege escalation primitive is evident.

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

The original Linux kernel CVE announcement for CVE-2026-64417 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64417

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-2026-64417	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-754;CWE-703;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference in the shrinker debugfs path occurs because shrinker_debugfs_add creates count and scan files even when the corresponding shrinker callback is missing. A local actor with write access to the affected debugfs scan file can cause shrinker_debugfs_scan_write to call a NULL scan_objects function pointer, leading to a kernel panic and denial of service. For the CVSS the PR:L is used for the paranoid score because a local user or service with delegated debugfs write access could trigger the file operation, while the normal deployment assumption is PR:H because debugfs access is administrator controlled. The issue is not network reachable and does not provide an evident UAF, OOB access, information leak, or arbitrary write primitive. Impact is denial of service only via kernel crash.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)  SKIP CVE-2026-64417 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	DANGER NULLPTR DEBUGFS LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The commit shows a direct NULL function pointer call from `shrinker_debugfs_scan_write()` leading to kernel panic.
* Requires admin/root/CAP_* in typical deployments: -2. Triggering requires access to debugfs shrinker files, and debugfs is normally administrator-controlled.
* Total: -1, clamped to 0.

Paranoid signals:

* Local unprivileged or reduced-privilege trigger: +1. If debugfs write access is delegated to a service account, test harness, container root, or reduced-capability local role, the trigger may be below full host root.
* Reliable kernel crash / strong DoS: +1. Writing the affected `scan` file can reliably call a NULL `scan_objects` callback.
* No memory corruption bonus. This is a NULL function pointer dereference to address 0, not UAF, OOB write, type confusion, reclaim, callback hijack, or arbitrary write.
* No confidentiality or integrity impact. No leak or write primitive is shown.
* Total: 2.

Call-site confidence: high. The patch and stack trace show the affected debugfs file operation path directly.

## 3. Reachability analysis

The bug is locally reachable through shrinker debugfs files created by `shrinker_debugfs_add()`. In normal deployments this requires root or administrator-controlled debugfs access, because debugfs is not intended as an unprivileged user interface. Namespace or container behavior only matters if the product exposes or delegates debugfs write access to a reduced-privilege context. The path is not network reachable. Realistic exploitation is a local kernel crash only, and depends on a shrinker that lacks the relevant callback, such as the cited xen-backend shrinker missing `scan_objects()`.

## 4. Severity interpretation

This behaves like ordinary Moderate or Low-like local DoS, not Important. The theoretical bug class is a kernel NULL function pointer dereference, but the realistic primitive is only a panic through a privileged debugfs control path. There is no evidence of attacker-controlled function pointer redirection, UAF reclaim, object replacement, arbitrary write, information disclosure, or privilege escalation.

## 5. One-sentence report phrase

A local actor with access to the affected shrinker debugfs file can trigger a NULL callback dereference and crash the kernel, but normal deployments restrict this path to administrator-controlled debugfs access and no privilege escalation primitive is evident.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed or handled as low-priority Moderate in most triage flows because it is a local debugfs-only DoS with no demonstrated memory corruption primitive beyond NULL dereference and no network reachability.

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

Patch: mm: shrinker: fix NULL pointer dereference in debugfs [ Upstream
Commit: ebb45c2648b1f60715fd283700f651e05e431231
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ebb45c2648b1f60715fd283700f651e05e431231

Changed files:
  mm/shrinker_debug.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=ebb45c2648b1f60715fd283700f651e05e431231

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

The original Linux kernel CVE announcement for CVE-2026-64417 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64417

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: 2

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).