[CVE-2026-90189][MODERATE 7.0] null_blk: register configfs subsystem after creating default devices [ Upstream

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

CVE: CVE-2026-90189
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: null_blk: register configfs subsystem after creating default devices [ Upstream
Commit: 2679b11e9256bd70a5ef41783bab6ad6c34a3db1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2679b11e9256bd70a5ef41783bab6ad6c34a3db1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90189
Analysis date: Thu, 17 Sep 2026 15:50:54 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90189 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90189

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.

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

CVE-2026-90189	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90189 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-362;*CWE-416;CWE-667;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'null_blk registered the configfs subsystem before the block major and default devices were fully initialized. A local race with configfs mkdir and poweron during module load can reach null_add_dev while null_major is still zero, causing a WARN and failed disk registration. The more serious path is the error cleanup path, where a user created null_blk device can be freed while its configfs item remains reachable. Later userspace access to that item can trigger a use after free in kernel memory. For the CVSS the PR:L reflects a conservative deployment where a less trusted local service, reduced capability root, or delegated configfs control can create and power on null_blk devices while module initialization is in progress. The issue is not network reachable. Impact is at least local denial of service via warning or crash and in worst case may allow confidentiality or integrity impact due to the UAF primitive. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	INIT UAF HARDWARE IPV6 KPANIC 	NO	NO	checked

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

ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Applied conservative floor: 4 -> 5 due to real UAF with externally reachable configfs item and meaningful post-lifetime access surface.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2 Real UAF. The commit explicitly states that null_free_dev() kfrees a user device nullb_device while `/sys/kernel/config/nullb/<name>` remains reachable, and later userspace access triggers a UAF.
* +1 Reliable kernel crash / strong DoS. The patch documents WARN in `__add_disk()` and an access-after-free path that can realistically fault or crash the kernel.
* +1 Attacker-influenced active-object lifetime. A racing userspace `mkdir()` plus `poweron` can put a user-created null_blk device on `nullb_list` during module initialization, affecting the object that is later freed.
* +1 Paranoid structured stale-object leverage. The stale object is not only touched inside a closed cleanup path. It remains externally reachable through configfs and later userspace operations can consume freed structured kernel state.

Non-awarded signals:

* Controlled reclaim is not demonstrated.
* Arbitrary read/write is not demonstrated.
* Privilege escalation is not demonstrated.
* Remote reachability is not established.
* Concrete repeated release/refcount mutation is not established. This is a kfree followed by later stale access, not a visible second put/release on the same reference.
* Extra Availability was not awarded because the WARN/UAF crash is the same availability consequence.
* C/I impact was not awarded directly because the patch does not establish disclosure, directed write, controlled reclaim, or callback control.

Repeated-release/refcount check:

* First release can end lifetime: yes, `null_free_dev()` kfrees `nullb_device`.
* Exact same reference survives into a second lifetime mutation: not established.
* Second operation is a real lifetime/refcount mutation: no.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: yes, but due to externally reachable active structured UAF, not due to repeated release.

3. Reachability analysis

The trigger is local through configfs operations during `null_blk` module initialization. The supplied trace uses root, and null_blk configfs management is normally an administrative control-plane interface, but the complete privilege boundary is not fully proven by the supplied patch alone. Therefore HostAdminRequired is UNKNOWN rather than YES or NO.

NamespaceScopedPrivilege is NO because the relevant object is under global configfs, not a demonstrated attacker-created namespace-scoped interface. No network trigger is present. Prior module loading is a practical prerequisite when null_blk is built as a module, but the input does not fully establish whether the attacker must be the same host administrator who loads the module or whether a delegated/local service could race the exposed configfs interface.

The race is during module initialization and is timing-dependent, but the patch describes a concrete race and concrete UAF outcome. I did not subtract a hard-race penalty because the evidence does not prove a low-probability or externally uncontrollable race beyond the module-init timing window.

4. Primitive and severity interpretation

The exact primitive is a local configfs lifetime bug: a `nullb_device` can be freed while its configfs item remains reachable. This is an active structured object UAF rather than fixed stale storage. It is not merely an ordinary cleanup stale dereference because userspace can later access the still-visible configfs item after the backing object has been freed.

PrimitiveClass is CLASS_B. The patch establishes a real kernel lifetime violation and post-lifetime access, but does not establish CLASS_A evidence such as controlled reclaim, arbitrary kernel R/W, callback hijack, credential manipulation, or demonstrated LPE.

ImportantCandidate is YES because the UAF remains externally reachable through configfs and exploitation leverage beyond crash is unresolved. AutoDowngradeAllowed is NO because automatically downgrading would discard a concrete reachable kernel lifetime primitive that is stronger than a WARN-only or pure DoS bug.

Likely operational severity is Actionable Moderate, with Strong Important review warranted if further analysis shows controlled reclaim, sensitive field reuse, or a less-privileged complete trigger path.

5. One-sentence report phrase

A local configfs race in null_blk module initialization can leave a freed user-created nullb_device reachable through `/sys/kernel/config/nullb/<name>`, causing a real kernel UAF on later userspace access and requiring manual review for possible impact beyond DoS.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly documents a reachable UAF where a freed kernel object remains accessible through configfs, and the post-lifetime behavior is not fully bounded to a harmless crash-only path.

7. Call-site confidence

Call-site confidence: high

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

Patch: null_blk: register configfs subsystem after creating default devices [ Upstream
Commit: 2679b11e9256bd70a5ef41783bab6ad6c34a3db1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2679b11e9256bd70a5ef41783bab6ad6c34a3db1

Changed files:
  block/genhd.c
  drivers/block/null_blk/main.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=2679b11e9256bd70a5ef41783bab6ad6c34a3db1

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

The original Linux kernel CVE announcement for CVE-2026-90189 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90189

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: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: unknown
  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).