[CVE-2026-53167][MODERATE 7.0] fuse: limit FUSE_NOTIFY_RETRIEVE to uptodate folios

[CVE-2026-53167][MODERATE 7.0] fuse: limit FUSE_NOTIFY_RETRIEVE to uptodate folios

[AL-KERNEL]

CVE: CVE-2026-53167
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: fuse: limit FUSE_NOTIFY_RETRIEVE to uptodate folios
Commit: 56763afa013444a9d84ca1b74e4b7130942177ba
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=56763afa013444a9d84ca1b74e4b7130942177ba
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53167
Analysis date: Thu, 25 Jun 2026 05:26:30 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
FUSE_NOTIFY_RETRIEVE could return non uptodate page cache folios to userspace, exposing uninitialized memory to a local FUSE daemon on systems without automatic allocation zeroing.

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

The original Linux kernel CVE announcement for CVE-2026-53167 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53167

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
======================================================================

CVE-2026-53167	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:H/I:N/A:N';CWE-908;*CWE-200;CWE-226;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N';BEST CVSS score: '5.5';DESCR 'Commit SHA was provided only as short 4e3d1b2. This is an info leak class issue, not a DoS or memory corruption issue. Paranoid scoring raises exploitability via AC:L but stays within the same confidentiality only class.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	DISK SIMPLEFIX LINUS  KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local user or process able to operate a FUSE mount or FUSE daemon can potentially reach the FUSE_NOTIFY_RETRIEVE path.
* Confidentiality impact plausible: +1. Non uptodate folios may contain uninitialized memory and can be returned to userspace.
* Important filesystem/storage path: +1. The bug is in FUSE page cache retrieval semantics.
* Practical exploit path strongly implied: +1. The commit directly states that FUSE_NOTIFY_RETRIEVE can return !uptodate folios containing uninitialized data.
* Configuration and reachability limitation: -1. Security impact depends on systems without automatic page allocation zeroing and on FUSE_NOTIFY_RETRIEVE being reachable through a FUSE daemon.

Paranoid adjustment:

* Broad/common subsystem exposure: +1 instead of applying the configuration penalty. FUSE and page cache paths are common enough that this should not be auto-closed when uninitialized memory disclosure is plausible.

No memory corruption, UAF, OOB write, Dirty-Pipe-like page-cache corruption, privilege escalation, or reliable DoS primitive is shown by the patch.

## 3. Reachability analysis

The bug is local, not network reachable. The likely trigger is a local process controlling or interacting with a FUSE daemon that can issue FUSE_NOTIFY_RETRIEVE. On many systems unprivileged FUSE is available through /dev/fuse and fusermount policy, but containers may restrict it unless /dev/fuse is delegated. The path is not necessarily exercised by every FUSE filesystem, and the security impact is reduced or removed when CONFIG_INIT_ON_ALLOC_DEFAULT_ON or init_on_alloc=1 is enabled.

## 4. Severity interpretation

This behaves like an actionable Moderate information disclosure issue, not an Important memory corruption issue. The realistic impact is disclosure of uninitialized memory through a filesystem cache retrieval path. There is no evidence of integrity impact, availability impact, or privilege escalation, but the confidentiality impact can be meaningful enough to require manual review.

## 5. One-sentence report phrase

FUSE_NOTIFY_RETRIEVE could return non uptodate page cache folios to userspace, exposing uninitialized memory to a local FUSE daemon on systems without automatic allocation zeroing.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. This is not a crash-only or resource-only bug. It is a plausible local uninitialized memory disclosure with potentially high confidentiality impact, so it should not be auto-closed.

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

Patch: fuse: limit FUSE_NOTIFY_RETRIEVE to uptodate folios
Commit: 56763afa013444a9d84ca1b74e4b7130942177ba
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=56763afa013444a9d84ca1b74e4b7130942177ba

Commit description:

FUSE_NOTIFY_RETRIEVE must be limited to uptodate folios; !uptodate folios
can contain uninitialized data.
Since FUSE_NOTIFY_RETRIEVE is intended to only return data that is already
in the page cache and not wait for data from the FUSE daemon, treat
!uptodate folios as if they weren't present.

This only has security impact on systems that don't enable automatic
zero-initialization of all page allocations via
CONFIG_INIT_ON_ALLOC_DEFAULT_ON or init_on_alloc=1.

Cc: stable@kernel.org
Fixes: 2d45ba3 ("fuse: add retrieve request")
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://patch.msgid.link/20260519-fuse-retrieve-uptodate-v1-1-a7a1912a37f9@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=56763afa013444a9d84ca1b74e4b7130942177ba

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

The original Linux kernel CVE announcement for CVE-2026-53167 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53167

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:H/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
  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: 4
  Paranoid ActionableScore: 5

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).