[CVE-2026-53168][MODERATE 7.0] fuse: reject fuse_notify() pagecache ops on directories

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

CVE: CVE-2026-53168
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: fuse: reject fuse_notify() pagecache ops on directories
Commit: 15487f98863dc7156ed43c5be26d478beb82ba35
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15487f98863dc7156ed43c5be26d478beb82ba35
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53168
Analysis date: Thu, 25 Jun 2026 05:34:45 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local FUSE daemon could use FUSE_NOTIFY_STORE or FUSE_NOTIFY_RETRIEVE on directory inodes to access or corrupt kernel-internal readdir pagecache state, causing WARN_ON based DoS and possible limited cache confidentiality or integrity impact.

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

The original Linux kernel CVE announcement for CVE-2026-53168 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53168

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-53168	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';*CWE-20;CWE-284;CWE-668;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.6';DESCR 'FUSE_NOTIFY_STORE and FUSE_NOTIFY_RETRIEVE could be accepted for directory inodes even though directory pagecache with FOPEN_CACHE_DIR is used as kernel internal readdir cache storage. A local FUSE daemon could write bogus data into that cache or try to retrieve cache contents through notify operations that are intended for regular file pagecache. This can make fuse_parse_cache process attacker controlled directory cache data and hit a WARN_ON, which may become a kernel denial of service on systems where warnings are fatal or where the warning path disrupts the mount. For the CVSS the PR:L is used because triggering requires local code execution as the FUSE daemon or access to the affected FUSE connection, but it does not require full host root in common unprivileged FUSE configurations. The issue is not network reachable by itself and depends on local FUSE mount behavior. Impact is at least local denial of service. In the paranoid interpretation, limited confidentiality or integrity impact is kept because the bug gives userspace direct access to a cache format that is meant to be kernel internal, but the patch does not show a concrete UAF, OOB write, or arbitrary kernel write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	DISK WARNONLY SIMPLEFIX LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local FUSE daemon or process controlling the FUSE connection can issue FUSE_NOTIFY_STORE or FUSE_NOTIFY_RETRIEVE. This does not normally require full host root when unprivileged FUSE is enabled.
* Page-cache / kernel-internal cache corruption concern: +2. The bug allows notify pagecache operations on directory inodes where the pagecache is used as kernel-internal readdir cache storage, not as normal userspace-visible regular file cache.
* WARN_ON / DoS concern: +1. Bogus directory cache contents can make fuse_parse_cache() hit WARN_ON(). This is at least a local DoS concern, especially on systems using panic_on_warn or where the warning disrupts the mount.

Paranoid additional signals:

* Confidentiality impact plausible: +1. FUSE_NOTIFY_RETRIEVE may expose directory cache contents that userspace was not intended to access directly.
* Integrity impact plausible: +1. FUSE_NOTIFY_STORE may let userspace inject malformed data into a kernel-internal directory cache format.
* No arbitrary write / UAF / OOB primitive is shown. Therefore this should not be treated as a strong kernel memory-corruption or LPE primitive.

## 3. Reachability analysis

The issue is local, not network reachable by itself. The practical attacker is a local user or service able to run or control a FUSE daemon and issue notify operations on the affected FUSE connection. Namespace and distribution policy matter because unprivileged FUSE is common on many systems, while some environments restrict /dev/fuse or FUSE mounts. The affected path is not a packet-processing or remote protocol path. Realistic exploitation is most credible as local denial of service or internal cache integrity abuse, not direct privilege escalation.

## 4. Severity interpretation

This behaves stronger than an ordinary low-risk validation bug because the rejected operations directly access directory pagecache that is meant to be kernel-internal readdir cache storage. However, the patch does not show UAF, double-free, OOB write, arbitrary write, callback corruption, or attacker-controlled object reuse. Conservative handling is borderline/manual-review Moderate. Paranoid handling reaches Actionable Moderate because local unprivileged FUSE plus pagecache/internal-cache modification is a historically sensitive class and should not be auto-closed without review.

## 5. One-sentence report phrase

A local FUSE daemon could use FUSE_NOTIFY_STORE or FUSE_NOTIFY_RETRIEVE on directory inodes to access or corrupt kernel-internal readdir pagecache state, causing WARN_ON based DoS and possible limited cache confidentiality or integrity impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is not just a harmless -EINVAL validation cleanup. It involves userspace access to kernel-internal directory pagecache state through FUSE notify operations, with demonstrated WARN_ON behavior and plausible limited C/I impact, even though no strong LPE primitive is shown.

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

Patch: fuse: reject fuse_notify() pagecache ops on directories
Commit: 15487f98863dc7156ed43c5be26d478beb82ba35
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15487f98863dc7156ed43c5be26d478beb82ba35

Commit description:

The operations FUSE_NOTIFY_STORE and FUSE_NOTIFY_RETRIEVE allow the
FUSE daemon to actively write/read pagecache contents.

For directories with FOPEN_CACHE_DIR, the pagecache is used as
kernel-internal cache storage, and userspace is not supposed to have
direct access to this cache - in particular, fuse_parse_cache() will hit
WARN_ON() if the cache contains bogus data.

Reject FUSE_NOTIFY_STORE and FUSE_NOTIFY_RETRIEVE on anything other than
regular files with -EINVAL.

Fixes: 5d7bc7e ("fuse: allow using readdir cache")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://patch.msgid.link/20260519-fuse-dir-pagecache-v2-1-5428fa48e175@google.com
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/fuse/dev.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=15487f98863dc7156ed43c5be26d478beb82ba35

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

The original Linux kernel CVE announcement for CVE-2026-53168 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53168

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:L/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:L/I:L/A:H
  The Best / paranoid CVSS score: 6.6

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: 4
  Paranoid ActionableScore: 6

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: YES
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).