[CVE-2026-64265][IMPORTANT] fuse: clear intr_entry in fuse_resend and fuse_remove_pending_req

[CVE-2026-64265][IMPORTANT] fuse: clear intr_entry in fuse_resend and fuse_remove_pending_req

[AL-KERNEL]

CVE: CVE-2026-64265
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: fuse: clear intr_entry in fuse_resend and fuse_remove_pending_req
Commit: 1d8ecd0cd696a5df0b2f72046a4ccee5d2a8ec2c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1d8ecd0cd696a5df0b2f72046a4ccee5d2a8ec2c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64265
Analysis date: Sat, 25 Jul 2026 15:11:17 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale FUSE interrupt list entry can survive request resend and pending request removal, causing `fuse_read_interrupt()` to perform a UAF read and UAF write on a freed `fuse_req`, with local PR:L reachability on systems exposing unprivileged FUSE and potential DoS, information disclosure, and memory corruption impact.

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

The original Linux kernel CVE announcement for CVE-2026-64265 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64265

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: IMPORTANT
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-2026-64265	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN LOW	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-200;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A stale FUSE interrupt list entry can remain after fuse_resend moves an interrupted request back to the pending queue and a later fatal signal removes and frees that request. fuse_read_interrupt can then iterate the stale entry and access a freed fuse_req object, causing a UAF read of the request unique value and a UAF write through list_del_init on the freed slab object. For the CVSS the PR:L is used because a local user or process with access to FUSE functionality is needed, while full administrative privileges are not necessarily required on systems that allow unprivileged FUSE mounts. The issue is not directly network reachable and requires local interaction with FUSE request handling. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to memory corruption. For the paranoid score, the highest still defensible interpretation is used because the bug includes UAF read and UAF write on a freed slab object.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)  SKIP CVE-2026-64265 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 760eac73f9f69aa28fcb3050b4946c2dcc656d12 YES  NO NO unknown 	MAYBE	DISK INIT LEAK UAF DMAorINTERRUPT LINUS  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: **7**
* Paranoid score: **8**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: **+1**. FUSE is commonly reachable by local users on systems allowing unprivileged FUSE mounts or access to `/dev/fuse`.
* Privilege escalation plausible: **+1 conservative / +2 paranoid**. The bug provides UAF read and UAF write, but no proven reclaim or controlled object replacement is shown. Paranoid scoring treats writable UAF on freed slab as strong enough for Important review.
* Generic memory corruption: **+2**. The commit explicitly describes use-after-free on `struct fuse_req`.
* Real lifetime corruption: **+1**. `intr_entry` remains linked after the request can be freed, creating a stale list entry.
* Reliable kernel crash / strong DoS: **+1**. Iterating the stale interrupt list can access freed memory and plausibly crash the kernel.
* Confidentiality impact plausible: **+1**. The commit states `req->in.h.unique` can be read from freed memory and leaked to userspace.
* Integrity impact plausible: **+1**. `list_del_init(&req->intr_entry)` performs a write through a freed object.
* Hard or unreliable race / special timing required: **-1**. Triggering depends on resend, prior interruption, fatal signal timing, and later interrupt-list traversal.

## 3. Reachability analysis

The bug is local, not network reachable. A realistic attacker needs local ability to interact with FUSE request handling, typically via an unprivileged FUSE mount or a process with access to `/dev/fuse`. Namespaces and containers can matter: if `/dev/fuse` is exposed to a container or unprivileged FUSE mounts are allowed, this should be treated closer to PR:L rather than PR:H. The affected code is in a common filesystem subsystem, but the exact vulnerable path requires the newer resend request mechanism, interrupted requests, and a pending request removal window.

Call-site confidence: **high**, because the commit message identifies the concrete stale list path and the later `fuse_read_interrupt()` UAF read/write behavior.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is not just a WARN or NULL dereference. It is a lifetime bug with an explicit stale list entry, UAF read, and UAF write on a freed slab object. Realistic exploitation beyond crash is not demonstrated, so conservative scoring avoids assuming reliable LPE. However, the memory corruption primitive is concrete enough that the paranoid score should land in Strong Important candidate territory and should not be auto-closed.

## 5. One-sentence report phrase

A stale FUSE interrupt list entry can survive request resend and pending request removal, causing `fuse_read_interrupt()` to perform a UAF read and UAF write on a freed `fuse_req`, with local PR:L reachability on systems exposing unprivileged FUSE and potential DoS, information disclosure, and memory corruption impact.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Manual review is required because the patch fixes a concrete UAF with both read and write access to freed slab memory, and local privilege escalation cannot be ruled out even though reliable exploitation is timing-dependent and not demonstrated.

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

Patch: fuse: clear intr_entry in fuse_resend and fuse_remove_pending_req
Commit: 1d8ecd0cd696a5df0b2f72046a4ccee5d2a8ec2c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1d8ecd0cd696a5df0b2f72046a4ccee5d2a8ec2c

Commit description:

When fuse_resend() moves a request from fpq->processing back to
fiq->pending, it sets FR_PENDING and clears FR_SENT but does not
remove the requests intr_entry from fiq->interrupts.  If the
request had FR_INTERRUPTED set from a prior signal, intr_entry
remains dangling on fiq->interrupts.  When the requesting task
then receives a fatal signal, fuse_remove_pending_req() sees
FR_PENDING=1, removes the request from fiq->pending and frees it
via the refcount path, also without cleaning intr_entry.  The
stale intr_entry causes use-after-free when fuse_read_interrupt()
iterates fiq->interrupts:
  - list_del_init(&req->intr_entry) -> UAF write on freed slab
  - req->in.h.unique -> UAF read, data leaked to userspace

Remove intr_entry from fiq->interrupts in fuse_resend() for
interrupted requests before they are placed back on fiq->pending.

Add a WARN_ON if the intr_entry is not empty on request destruction.

Fixes: 760eac7 ("fuse: Introduce a new notification type for resend pending requests")
Cc: stable@vger.kernel.org # 6.9
Signed-off-by: Ji'an Zhou <eilaimemedsnaimel@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=1d8ecd0cd696a5df0b2f72046a4ccee5d2a8ec2c

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

The original Linux kernel CVE announcement for CVE-2026-64265 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64265

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: 7
  Paranoid ActionableScore: 8

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).