* [CVE-2026-90140][MODERATE REGULAR] cuse: wait for pending RCU callbacks on module exit [ Upstream
@ 2026-09-18 5:15 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-18 5:15 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90140
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: cuse: wait for pending RCU callbacks on module exit [ Upstream
Commit: 7fe415e1cd8fa875be263670c0ab47109818abb6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7fe415e1cd8fa875be263670c0ab47109818abb6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90140
Analysis date: Fri, 18 Sep 2026 01:15:50 -0400
ActionableScore: 2
ActionableScore lower bound: 1
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-2026-90140 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90140
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 REGULAR
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-2026-90140 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90140 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:H/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-672;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.4';DESCR 'A race between CUSE connection release and module unloading can leave a pending RCU callback that later calls cuse_fc_release after the cuse module text has been freed. The usual result is an invalid instruction fetch from unmapped module memory followed by a kernel Oops or panic. If the same virtual address is reused before the callback runs, the stale indirect call may transfer control to unrelated code, creating a plausible control flow corruption risk beyond denial of service. For the CVSS the PR:H reflects that reliable triggering normally requires permission to unload a kernel module through CAP_SYS_MODULE or equivalent administrative control. AC:H reflects the timing dependency between queuing the RCU callback, unloading the module, and executing the callback. The issue is local and is not network reachable. Impact is at least a local denial of service via kernel crash and in the worst case may affect confidentiality and integrity through execution at a reused kernel virtual address.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) SKIP CVE-2026-90140 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE OOB DISK UAF DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 2
* Paranoid score: 2
* No score floor or cap was applied. The real-UAF floors do not apply because host-admin-equivalent privilege is positively required for module removal.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
Conservative signals:
* +2 Real UAF and lifetime violation. A queued RCU callback survives CUSE module teardown and later calls a function located in freed module text.
* +1 Reliable kernel crash. Calling the unmapped address produces a page-fault Oops and may panic the kernel.
* -2 Host-admin-equivalent privilege. Completing the vulnerable sequence requires unloading the host-global CUSE module, which requires host CAP_SYS_MODULE or equivalent authority.
* Raw conservative score: 2 + 1 - 2 = 1.
Paranoid-only signal:
* +1 Weak indirect control-flow concern. The commit explicitly identifies a mechanism where reuse of the former module virtual address could cause the stale callback to execute unrelated code. This is weaker than controlled callback hijacking because useful address reuse or attacker control over the replacement code is not demonstrated.
* Raw paranoid score: 2 + 1 + 1 - 2 = 2.
Not awarded:
* No timing penalty. The bug requires unloading the module before the queued callback runs, but the evidence does not establish that this ordering is materially unreliable or difficult for the privileged actor to reproduce.
* No strong exploitation-leverage bonus. Controlled virtual-address reuse, controlled callback target selection, and attacker-shaped replacement code are not demonstrated.
* No local-unprivileged-trigger bonus. Closing `/dev/cuse` can queue the callback, but an ordinary user cannot complete the security-relevant module-unload stage.
* No concrete repeated-release or refcount bonus. There is one deferred release callback into unloaded code, not two lifetime mutations through the same dangling object.
* No structured stale-object leverage bonus. The stale target is a fixed module-text address and the demonstrated path is module-exit cleanup.
* No additional availability point. The Oops or panic is already represented by the reliable-crash signal.
* No confidentiality or integrity bonus. Execution at a reused address is explicitly possible, but no useful controlled execution, disclosure, or unauthorized modification is established.
* No remote-reachability signal. The path is local and depends on host module management.
* No RCU referenced-object mismatch bonus. The problem is an RCU callback outliving executable module text, not a demonstrated A-to-B referenced-object lifetime invariant of the separately defined form.
* FLOOR 5 was not applied.
3. Reachability analysis
The complete trigger consists of closing the CUSE channel so that `fuse_conn_put()` queues `delayed_release`, followed by unloading CUSE before the callback executes. An ordinary process may influence the close stage, but `rmmod cuse` requires host CAP_SYS_MODULE or equivalent host-administrator control.
Kernel module unloading is host-global and is not enabled merely by obtaining capabilities in an attacker-created user or network namespace. Therefore, host-admin privilege is positively established and the complete operation is not namespace-scoped.
The CUSE module must be loaded and a CUSE channel must exist. No special hardware, network access, sysctl, or boot mode is required. Although asynchronous ordering is necessary, the supplied evidence does not show a genuinely hard or low-probability race, so no timing penalty is applied.
4. Primitive and severity interpretation
The exact primitive is a deferred indirect call to `fc->release`, whose fixed target is `cuse_fc_release()` in module text that has already been unmapped. This is a real post-lifetime use and therefore CLASS_B rather than a mere invalid-state CLASS_C issue.
The stale resource is fixed executable module storage rather than an attacker-reclaimable heap object. The use occurs during module-unload cleanup. Its demonstrated consequence is an instruction-fetch fault, Oops, or panic.
The commit explicitly identifies possible execution of unrelated code if the virtual address is reused. That supports a paranoid weak control-flow concern, but not CLASS_A: controlled address reuse, attacker-selected replacement instructions, arbitrary read or write, and a privilege-escalation chain are not established.
There is no repeated release or refcount mutation. This is ordinary asynchronous stale callback execution, not a second lifetime mutation through an already dangling object.
ImportantCandidate is NO because the complete trigger requires authority that already permits loading and unloading kernel modules, and the only confirmed consequence is a host crash. AutoDowngradeAllowed is YES because this is a constrained, host-admin-gated cleanup UAF without controlled reuse, a repeated lifetime mutation, or an unresolved lower-privilege security boundary.
5. One-sentence report phrase
A pending CUSE RCU release callback can execute after the module text containing its fixed callback target has been unloaded, causing a host kernel Oops or panic and requiring host module-unload privileges to complete the trigger.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
The UAF and stale control-flow transfer are concrete, but the path is host-admin-gated and no controlled reuse or lower-privilege exploitation primitive is established.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: cuse: wait for pending RCU callbacks on module exit [ Upstream
Commit: 7fe415e1cd8fa875be263670c0ab47109818abb6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7fe415e1cd8fa875be263670c0ab47109818abb6
Changed files:
fs/fuse/cuse.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=7fe415e1cd8fa875be263670c0ab47109818abb6
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90140 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90140
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:H/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 6.4
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: 1
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: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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