[CVE-2026-90400][MODERATE REGULAR] md: recheck spare changes before starting sync [ Upstream

[CVE-2026-90400][MODERATE REGULAR] md: recheck spare changes before starting sync [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90400
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: md: recheck spare changes before starting sync [ Upstream
Commit: c3777d16bc3335c0ac4bdad0551c80d38c5d94cc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c3777d16bc3335c0ac4bdad0551c80d38c5d94cc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90400
Analysis date: Fri, 18 Sep 2026 09:11:21 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90400 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90400

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: YES

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

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

CVE-2026-90400	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90400 CHECK WITH IMPACT FROM ORIG NN NONE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;**CWE-476;CWE-667;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A race in md_start_sync can use a stale result from md_spares_need_change before reconfig_mutex is acquired. If rdev state changes in that window, spare removal or replacement can run while normal IO still references the array device state, leading to rdev_dec_pending(NULL) and a kernel crash. For the CVSS the PR:L is used for the paranoid score because a reduced capability admin, container root, or service account with md management access may be able to change spare state while local users generate normal IO. The issue is local and not network reachable. Impact is at least denial of service through a kernel crash. In the worst still defensible case, limited confidentiality or integrity impact is kept for manual review sensitivity because the race modifies kernel storage device references during active IO, but the patch evidence does not prove an arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	NULLPTR DISK RACE IMPROVEONLY LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	YES	NO	checked

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

ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap was applied.

2. Signal breakdown

Positive signals:

* +1 Weak / indirect corruption candidate. The patch establishes an invalid active md rdev configuration state where spare removal or replacement can race with normal I/O.
* +1 Reliable kernel crash / strong DoS. The supplied trace scenario reaches rdev_dec_pending(NULL), causing a NULL pointer dereference while decrementing nr_pending.
* +1 Important storage path. The bug is in md RAID synchronization and spare reconfiguration while normal I/O is active.

Negative signals:

* No host-admin penalty was applied. md reconfiguration is normally administrative, but the supplied evidence does not fully resolve the complete attacker-controlled trigger path.
* No race penalty was applied. The issue is a race, but the supplied text does not prove a one-shot or externally uncontrollable timing window beyond normal concurrent md operation.

Important non-awarded signals:

* RealUAF not awarded. The demonstrated terminal primitive is rdev_dec_pending(NULL), not dereference of a freed rdev.
* Concrete repeated release/refcount not awarded. The second operation is a refcount-like decrement attempt on NULL, not a concrete second lifetime mutation through the same dangling freed reference.
* Strong exploitation leverage not awarded. No controlled reclaim, object replacement under attacker control, arbitrary write, callback control, or refcount takeover is demonstrated.
* Confidentiality and integrity not awarded. The patch supports crash/DoS, not disclosure or unauthorized data modification.
* Extra availability not awarded. The same kernel crash is already counted once.
* Remote reachability not awarded. The issue is local/kernel control-plane and storage-I/O related, not network-triggered.

Repeated-release/refcount checklist:

* First release can end lifetime: not established.
* Exact same reference survives: not established as a dangling freed object.
* Second operation mutates lifetime/refcount: not in the required sense, because the demonstrated call is rdev_dec_pending(NULL).
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.

3. Reachability analysis

The vulnerable condition involves md spare configuration changes racing with normal I/O. A practical trigger likely requires an md array, spare or replacement device state changes, and concurrent normal writes. Direct md management is usually performed by root or a host-level storage management service, but the supplied patch does not prove that host-admin-equivalent privilege is the only possible way to reach the racing state.

Ordinary local users may be able to generate normal I/O against an already configured array, but the evidence does not establish that they can also cause the required spare state transition. No namespace-scoped trigger is indicated. No network trigger is indicated. Prior host setup of an md RAID array is required as an environmental prerequisite, but the scoring does not subtract a rare-configuration penalty because md RAID is a normal kernel storage subsystem.

Race repeatability is unresolved. The interleaving is concrete, but the patch does not prove that an attacker can reliably drive both sides of the race.

4. Primitive and severity interpretation

The exact demonstrated primitive is a NULL pointer dereference caused by stale decision-making in md_start_sync(). The first md_spares_need_change() result can become stale before reconfig_mutex is acquired, allowing remove_spares() or remove_and_add_spares() to run without the array being suspended while normal I/O still observes the changing rdev state.

This is an active storage-state consistency race, but the supplied evidence demonstrates crash behavior rather than a true UAF or reusable dangling object. The affected state is active structured md storage state, not fixed stale storage, but the stale operation shown is not a concrete repeated lifetime/refcount mutation on a freed object.

PrimitiveClass is CLASS_C because the established behavior is invalid state leading to NULL dereference and DoS. ImportantCandidate is NO because meaningful exploitation leverage beyond crash is not established. AutoDowngradeAllowed is YES because there is no demonstrated UAF, double release, arbitrary write, ownership violation, authorization bypass, or unresolved Class B lifetime primitive. Operational severity is borderline Moderate: worth manual review due to md storage concurrency, but not an Important-class candidate from the supplied evidence.

5. One-sentence report phrase

A race in md_start_sync can let spare removal or replacement proceed from a stale md_spares_need_change result while normal I/O is still active, leading to rdev_dec_pending(NULL) and a local kernel crash.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is an md storage concurrency bug involving active rdev replacement during normal I/O, but the supplied evidence supports NULL dereference DoS rather than UAF, repeated lifetime mutation, or privilege escalation.

7. Call-site confidence

Call-site confidence: medium

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

Patch: md: recheck spare changes before starting sync [ Upstream
Commit: c3777d16bc3335c0ac4bdad0551c80d38c5d94cc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c3777d16bc3335c0ac4bdad0551c80d38c5d94cc

Changed files:
  drivers/md/md.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=c3777d16bc3335c0ac4bdad0551c80d38c5d94cc

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

The original Linux kernel CVE announcement for CVE-2026-90400 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90400

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:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 3

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).