[CVE-2026-64267][MODERATE 7.0] fuse: avoid 32-bit prune notification count wrap

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

CVE: CVE-2026-64267
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: fuse: avoid 32-bit prune notification count wrap
Commit: 6e2d84fdeac05bfd858e84a76353fdb84f23a43e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e2d84fdeac05bfd858e84a76353fdb84f23a43e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64267
Analysis date: Sat, 25 Jul 2026 09:31:43 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A 32 bit integer wrap in FUSE_NOTIFY_PRUNE payload validation allows a malicious local FUSE daemon to submit a malformed prune notification that reaches a kernel BUG path and can crash the system.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64267 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64267

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-2026-64267	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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-190;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.6';DESCR 'A 32 bit integer wraparound in FUSE_NOTIFY_PRUNE payload validation can allow a malicious FUSE daemon to submit a prune notification where the count field passes the length check even though the nodeid payload is missing. The kernel then enters the prune copy loop and the FUSE device copy path may try to consume nodeids that are not present in the userspace write buffer, reaching a BUG_ON path and causing a kernel crash. For the CVSS the PR:L value is used because reliable triggering requires local code execution as a FUSE daemon or equivalent control over a FUSE connection, not remote network access. The issue is not network reachable. Impact is at least local denial of service via kernel crash. For the paranoid score, limited confidentiality or integrity impact is kept only as a review sensitivity signal because the bug class is integer wraparound with invalid copy state, but the patch does not show a strong UAF or write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-64267 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 3f29d59e92a96d843c2ff10ebfed92ac26878658 YES  NO NO unknown 	MAYBE	DISK SIMPLEFIX HARDWARE LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	checked

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ACTIONABLESCORE ANALYSIS
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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 attacker who can operate as a FUSE daemon or control a FUSE connection can provide the malformed prune notification.
* Reliable kernel crash / strong DoS: +1. The commit describes a path reaching `fuse_copy_fill()` `BUG_ON(!err)` in QEMU.
* Availability impact realistic: +1. The expected result is a host kernel crash on affected 32 bit kernels.
* Broad/default/common subsystem exposure: +1. FUSE is a common local filesystem interface, although this specific prune notification path is narrower.
* Legacy / non-default execution mode: -1. The integer wrap depends on 32 bit kernels where `size_t` is 32 bits.

Paranoid additional signal:

* Weak/indirect corruption candidate: +1. The wrapped count makes the kernel copy path consume nodeids not present in the userspace write buffer. This is not a demonstrated arbitrary read/write primitive, but it is more than a pure validation typo because it reaches a kernel copy/BUG path.

Not counted:

* No remote/network trigger.
* No strong LPE plausibility.
* No strong memory corruption primitive such as UAF, double free, OOB write, arbitrary write, or controlled object replacement.
* No Dirty Pipe style shared page or page cache corruption primitive.

## 3. Reachability analysis

The likely trigger is a local process acting as a malicious FUSE daemon or otherwise controlling a FUSE connection. This is PR:L in practical terms, because it requires local code execution but not full host root in common FUSE deployments.

The issue is not network reachable. Containers or user namespaces may matter if they allow untrusted users to create or interact with FUSE mounts, but the bug still remains a local attack surface.

The main limiting condition is architecture. The described wrap requires a 32 bit kernel, so exposure is narrower than typical 64 bit Linux deployments.

Call-site confidence: medium. The patch gives the affected validation and the commit describes the downstream `fuse_copy_fill()` BUG path, but the full surrounding copy loop and notification call chain are not included.

## 4. Severity interpretation

This behaves like an actionable Moderate rather than a clear Important vulnerability. The demonstrated impact is a reliable local kernel crash on affected 32 bit systems.

Theoretical concern exists because the bug is an integer wrap in a kernel copy path and causes the kernel to consume missing payload data, but the patch does not show a practical privilege escalation primitive, attacker-controlled overwrite, UAF reclaim, type confusion, or information disclosure path.

## 5. One-sentence report phrase

A 32 bit integer wrap in FUSE_NOTIFY_PRUNE payload validation allows a malicious local FUSE daemon to submit a malformed prune notification that reaches a kernel BUG path and can crash the system.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a local kernel parser/copy-path integer overflow with confirmed crash impact and architecture-specific exposure. It is not a proven LPE, but the copy-path behavior and 32 bit wrap make it unsuitable for automatic closure without human review.

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

Patch: fuse: avoid 32-bit prune notification count wrap
Commit: 6e2d84fdeac05bfd858e84a76353fdb84f23a43e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e2d84fdeac05bfd858e84a76353fdb84f23a43e

Commit description:

FUSE_NOTIFY_PRUNE validates the nodeid payload length with:

    size - sizeof(outarg) != outarg.count * sizeof(u64)

On 32-bit kernels, size_t is also 32 bits, so the daemon-controlled
count multiplication can wrap.  A prune notification with count
0x20000000 and no nodeid payload passes the check, enters the copy
loop, and asks the device copy path to read nodeids that are not
present in the userspace write buffer.  In QEMU this reaches the
fuse_copy_fill() BUG_ON(!err) path.

Validate the payload length with array_size() instead.  That accepts
exactly the same valid messages, but avoids wrapping arithmetic before
the copy loop consumes the count.

Assisted-by: Codex:gpt-5.5-cyber-preview
Fixes: 3f29d59 ("fuse: add prune notification")
Cc: stable@vger.kernel.org
Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
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=6e2d84fdeac05bfd858e84a76353fdb84f23a43e

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

The original Linux kernel CVE announcement for CVE-2026-64267 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64267

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: 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).