[CVE-2026-64040][LOW] cachefiles: Fix error return when vfs_mkdir() fails [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-64040
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: cachefiles: Fix error return when vfs_mkdir() fails [ Upstream
Commit: 0940108d27c6995e02819ff832be11892f0b208b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0940108d27c6995e02819ff832be11892f0b208b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64040
Analysis date: Sun, 19 Jul 2026 12:22:23 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
cachefiles_get_directory() may return NULL instead of a proper ERR_PTR(-errno) when vfs_mkdir() fails, which can lead to a local configuration-dependent NULL pointer dereference and kernel crash.

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

The original Linux kernel CVE announcement for CVE-2026-64040 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64040

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.

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

CVE-2026-64040	LOW	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-252;**CWE-476;CWE-703;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'cachefiles_get_directory() can return ERR_PTR(0), effectively NULL, when vfs_mkdir() fails because the error pointer from vfs_mkdir() is not converted into a negative errno value. A caller that expects either a valid dentry or an ERR_PTR error may later dereference NULL and crash the kernel. For the CVSS the PR:L is used in the paranoid score because a local user may be able to trigger cache object creation through a cache enabled filesystem while the backing cache hits an error condition. The issue is not directly network reachable and requires local cachefiles usage plus a vfs_mkdir failure condition. Impact is denial of service via kernel crash. No UAF, OOB access, or write primitive is indicated, so confidentiality and integrity remain C:N/I:N. NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)  SKIP CVE-2026-64040 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 406fad7698f5bf21ab6b5ca195bf4b9e0b3990ed YES  NO NO unknown 	MAYBE	DISK SIMPLEFIX LINUS  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	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

Triggered signals:

* Local unprivileged trigger, paranoid only: +1. A local user may indirectly cause cache object creation through a cache enabled filesystem, but this also requires cachefiles to be active and a mkdir failure in the backing cache.
* Reliable kernel crash / strong DoS, paranoid only: +1. Returning ERR_PTR(0), effectively NULL, can cause a caller expecting either a valid dentry or an ERR_PTR error to dereference NULL.
* Requires admin/root/CAP in typical deployments, conservative: -2. cachefiles setup and backing cache configuration are normally administrator controlled.
* Rare or difficult-to-reach configuration: -1. The issue requires cachefiles/fscache usage plus a vfs_mkdir() failure condition.

Not triggered:

* No remote/network trigger.
* No UAF, double free, OOB write, type confusion, refcount takeover, or arbitrary write.
* No plausible privilege escalation primitive.
* No confidentiality or integrity impact supported by the patch.
* No Dirty-Pipe-like shared page or page-cache corruption primitive.

Call-site confidence: medium. The patch shows the wrong return value in cachefiles_get_directory(), but the exact downstream caller crash path is not included in the provided diff.

## 3. Reachability analysis

The bug is local and configuration dependent. It requires cachefiles to be enabled and used by a cached filesystem, and it requires vfs_mkdir() to fail while cachefiles_get_directory() is creating a cache directory. In normal deployments, cachefiles configuration is administrator controlled, but a local user may be able to trigger cache object creation indirectly through filesystem access once the cache is configured. Namespaces do not obviously turn this into a broad unprivileged primitive. The path is not network reachable by itself.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like kernel DoS issue rather than an Important vulnerability. The realistic impact is a NULL pointer dereference and kernel crash under specific error-path conditions. Theoretical memory corruption potential is not supported because the patch only fixes error-pointer conversion and does not indicate UAF, stale object reuse, overwrite, refcount abuse, or object confusion.

## 5. One-sentence report phrase

cachefiles_get_directory() may return NULL instead of a proper ERR_PTR(-errno) when vfs_mkdir() fails, which can lead to a local configuration-dependent NULL pointer dereference and kernel crash.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED.

This can be handled as auto-close or low-priority deferred triage unless additional evidence shows a concrete downstream memory corruption primitive or a reliable unprivileged trigger in the target product configuration.

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

Patch: cachefiles: Fix error return when vfs_mkdir() fails [ Upstream
Commit: 0940108d27c6995e02819ff832be11892f0b208b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0940108d27c6995e02819ff832be11892f0b208b

Changed files:
  fs/cachefiles/namei.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=0940108d27c6995e02819ff832be11892f0b208b

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

The original Linux kernel CVE announcement for CVE-2026-64040 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64040

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:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 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).