[CVE-2026-90095][LOW] fuse: Fix the condition to enable over-io-uring [ Upstream

[CVE-2026-90095][LOW] fuse: Fix the condition to enable over-io-uring [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90095
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: fuse: Fix the condition to enable over-io-uring [ Upstream
Commit: 8f9a725d89711ad027f6b7183586ef91528f106d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8f9a725d89711ad027f6b7183586ef91528f106d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90095
Analysis date: Thu, 17 Sep 2026 13:34:48 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90095 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90095

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-90095	LOW	https://www.kernelcve.org/list/?q=CVE-2026-90095 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:N/I:N/A:H';CWE-667;*CWE-833;CWE-841;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A FUSE connection could accept IORING_OP_URING_CMD even when the userspace FUSE server did not negotiate FUSE_OVER_IO_URING in its FUSE_INIT reply. In this state the synchronization barrier in fuse_uring_ready is not effective and ordinary FUSE I O may already be active while io_uring setup proceeds. Concurrent operations can enter conflicting lock paths and cause a deadlock, including a lock dependency between fch bg_lock and queue lock. For the CVSS the PR:L reflects that a local user or process must control the relevant FUSE server and submit commands through the local io_uring interface. AC:H reflects the need for concurrent operations to reach the unsafe lock ordering. The issue is not directly network reachable. The confirmed impact is local denial of service through blocked FUSE operations, with a worst case availability impact if blocked tasks accumulate or filesystem activity becomes unusable. No UAF, OOB access, information leak, or write primitive is supported by the supplied code, so confidentiality and integrity remain unaffected.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)  SKIP CVE-2026-90095 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	DEADLOCK DISK INIT LINUS IO_URING  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive signals:

* +1 Local unprivileged trigger. A userspace FUSE server controlling its connection can submit the local `IORING_OP_URING_CMD` operation without host-admin-equivalent privilege on a system exposing the feature.
* +1 Reliable DoS. Bypassing the negotiation-dependent barrier permits conflicting lock acquisition, including the documented `fch->bg_lock` and `queue->lock` deadlock.

Negative signals:

* No negative points. The timing is concurrency-dependent, but the evidence does not establish a narrow or difficult-to-repeat race.
* No rare-configuration penalty. FUSE over io_uring must be enabled, but the supplied evidence does not establish that the configuration is both genuinely unusual and difficult to reach in affected deployments.

Important non-awarded signals:

* No UAF, OOB access, stale post-lifetime object, overwrite, or disclosure is established.
* No strong or weak corruption bonus was awarded.
* No extra Availability point was awarded because blocked FUSE activity is the same DoS consequence already counted.
* No Confidentiality or Integrity point was awarded.
* No remote-reachability point was awarded.
* No authorization or security-boundary bypass was established.
* No active-state retirement bonus was awarded because the patch restores a synchronization and feature-negotiation gate, not an object retirement protocol.

3. Reachability analysis

The evaluated attacker is a local userspace process operating the relevant FUSE server and connection. The process can submit `IORING_OP_URING_CMD` after initialization without having returned `FUSE_OVER_IO_URING`, causing io_uring setup to overlap with ordinary FUSE I/O.

This is below-host-admin reachability on systems that permit the user to operate the FUSE server, so `HostAdminRequired=NO`. No namespace capability is needed for the described trigger, so `NamespaceScopedPrivilege=NO`. The host must support and expose FUSE over io_uring, potentially through a module parameter or equivalent administrator-controlled configuration, but that is prior feature enablement rather than host-admin privilege required for each trigger.

The issue is not directly network-triggerable. No special hardware is required. Concurrent operations are necessary, but the supplied evidence does not show an exceptionally narrow or one-shot timing window.

4. Primitive and severity interpretation

The exact primitive is a protocol-state and synchronization-gating error. The kernel accepts an io_uring command for a FUSE connection that did not negotiate the required feature. Consequently, the barrier in `fuse_uring_ready()` does not provide the expected ordering while ordinary I/O may already be active.

This is not a lifetime violation and no object is shown to be used after release. There is no concrete repeated release or refcount mutation. The demonstrated consequence is a lock-order deadlock and local denial of service, making this `CLASS_C`.

No useful exploitation primitive remains after excluding the deadlock. Privilege escalation, kernel memory corruption, disclosure, and integrity impact would require unsupported additional defects. Therefore `ImportantCandidate=NO` and `AutoDowngradeAllowed=YES`. The likely operational severity is Low to ordinary Moderate, depending on whether the deadlock affects only one FUSE connection or blocks broader filesystem activity.

5. One-sentence report phrase

A local userspace FUSE server can submit io_uring commands without negotiating FUSE_OVER_IO_URING, bypassing the required synchronization barrier and potentially deadlocking FUSE I/O through conflicting kernel lock acquisition.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a local synchronization-related DoS without memory corruption, authorization bypass, disclosure, repeated lifetime mutation, or unresolved exploitation leverage.

7. Call-site confidence

Call-site confidence: medium

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

Patch: fuse: Fix the condition to enable over-io-uring [ Upstream
Commit: 8f9a725d89711ad027f6b7183586ef91528f106d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8f9a725d89711ad027f6b7183586ef91528f106d

Changed files:
  fs/fuse/dev_uring.c
  fs/fuse/inode.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=8f9a725d89711ad027f6b7183586ef91528f106d

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

The original Linux kernel CVE announcement for CVE-2026-90095 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90095

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: unknown
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).