[CVE-2026-64266][MODERATE 7.0] fuse: re-lock request before returning from fuse_ref_folio()

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

CVE: CVE-2026-64266
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: fuse: re-lock request before returning from fuse_ref_folio()
Commit: 1f9156714592356b4fda57beac7eab9c2a462dd3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1f9156714592356b4fda57beac7eab9c2a462dd3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64266
Analysis date: Sat, 25 Jul 2026 14:08:45 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in the FUSE request copy path can leave request args freed by an async abort callback while later copy-chain logic still accesses them, causing a local kernel use-after-free with reliable DoS impact and possible but unproven privilege-escalation concern.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64266 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64266

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-2026-64266	MODERATE	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:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in the FUSE device path because fuse_ref_folio unlocks the request and previously returned without re-locking it. During that window fuse_chan_abort can end the request and an async end callback such as fuse_writepage_free can free the request args while later copy chain logic still accesses them. For the CVSS the PR:L is used because a local user or process that can operate a FUSE mount or FUSE daemon path is needed to trigger the vulnerable request lifetime race. The issue is not directly network reachable. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel use-after-free. For the paranoid score, the highest still defensible interpretation is used because the bug class is kernel UAF and the patch context supports manual-review sensitivity over autoclosed false negatives.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	LOCK DANGER DISK UAF HARDWARE LINUS KPANIC 	NO	NO	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: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

* Local unprivileged trigger: +1. FUSE paths are commonly reachable by local users or services that can operate a FUSE mount or FUSE daemon path.
* Memory corruption, weak primitive: +1. The commit explicitly describes a use-after-free, but the patch does not show attacker-controlled reclaim, overwrite, type confusion, or callback control.
* Real lifetime corruption: +1. The bug is a request lifetime race where `fuse_chan_abort()` and async completion can free request args while later copy-chain code still accesses them.
* Reliable kernel crash / strong DoS: +1. UAF in this path can realistically crash the kernel.
* Broad/default/common subsystem exposure: +1. FUSE is a common Linux filesystem interface, though exact reachability depends on `/dev/fuse` permissions and distribution policy.
* Hard or unreliable race / special timing required: -1. Triggering requires a race window between request unlock, abort, async end callback, and later copy-chain access.

Conservative total: 4.
Paranoid total: 5, because kernel UAF in a local filesystem request path should not be auto-closed even without a demonstrated reclaim primitive.

Call-site confidence: medium. The commit names the relevant competing paths and callback, but the pasted patch does not include the full caller chain after `fuse_ref_folio()`.

## 3. Reachability analysis

A local user or local service that can use FUSE can plausibly reach the affected request path. This is not network-reachable by itself. Containers and namespaces may matter because FUSE access is often delegated to non-root users or containerized workloads, but some deployments restrict `/dev/fuse` or require specific permissions. The vulnerable path is not a rare hardware or legacy mode, but practical exploitation requires winning a race around request abort and asynchronous completion.

## 4. Severity interpretation

This is more than an ordinary Moderate because the patch fixes an explicit kernel use-after-free, not just a warning or resource leak. Realistic impact is most clearly local DoS through kernel crash. Privilege escalation is not demonstrated by the patch, and the race-UAF downgrade rule applies because there is no shown attacker-controlled reclaim, write-after-free primitive, type confusion, or callback dispatch control. Still, the paranoid score should remain Actionable Moderate because local kernel UAF lifetime bugs are historically worth manual review.

## 5. One-sentence report phrase

A race in the FUSE request copy path can leave request args freed by an async abort callback while later copy-chain logic still accesses them, causing a local kernel use-after-free with reliable DoS impact and possible but unproven privilege-escalation concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: explicit race-based kernel UAF in a common local filesystem interface. YES REQUIRES MANUAL CHECK.

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

Patch: fuse: re-lock request before returning from fuse_ref_folio()
Commit: 1f9156714592356b4fda57beac7eab9c2a462dd3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1f9156714592356b4fda57beac7eab9c2a462dd3

Commit description:

fuse_ref_folio() unlocks the request but does not re-lock it before
returning. fuse_chan_abort() can end the request and the async end
callback (eg fuse_writepage_free()) can free the args while the
subsequent copy chain logic after fuse_ref_folio() accesses them,
leading to use-after-free issues.

Fix this by locking the request in fuse_ref_folio() before returning.

Fixes: c302162 ("fuse: support splice() reading from fuse device")
Cc: stable@vger.kernel.org
Signed-off-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=1f9156714592356b4fda57beac7eab9c2a462dd3

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

The original Linux kernel CVE announcement for CVE-2026-64266 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64266

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 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).