[CVE-2026-90215][LOW] mtd: ubi: Release device reference on busy detach [ Upstream

[CVE-2026-90215][LOW] mtd: ubi: Release device reference on busy detach [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90215
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: mtd: ubi: Release device reference on busy detach [ Upstream
Commit: 87812858c4c8df979cb70bcd0040b925a543df6f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=87812858c4c8df979cb70bcd0040b925a543df6f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90215
Analysis date: Fri, 18 Sep 2026 07:25:46 -0400
ActionableScore: 0
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-2026-90215 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90215

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-2026-90215	LOW	https://www.kernelcve.org/list/?q=CVE-2026-90215 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';*CWE-772;CWE-404;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '3.3';DESCR 'A reference management error in ubi_detach_mtd_dev leaves the device reference obtained by ubi_get_device held when a busy UBI device causes an EBUSY return. The leaked reference can prevent the UBI device object from being released after a later detach and can interfere with removal or reuse of the affected device. For the CVSS the PR:L value is used for the paranoid score because access may be delegated to a restricted service or container with control over the UBI device. Normal deployments usually require administrative privileges and are represented by PR:H in the base score. The issue is local and is not network reachable. Impact is limited availability loss for the affected UBI device. There is no demonstrated memory corruption, information disclosure, kernel crash, or privilege escalation primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)  SKIP CVE-2026-90215 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	SIMPLEFIX HARDWARE LINUS 	-	-	checked

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

ActionableScore=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: -1.
* Conservative score: 0.
* Raw paranoid score: -1.
* Paranoid score: 0.
* Applied lower bound for both scores: -1 -> 0.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive signals:

* +1 Concrete persistent kernel resource/reference leak. `ubi_get_device()` acquires a device reference, but the `-EBUSY` path previously returned without the matching `put_device()`.
* No separate availability point. Preventing release of the affected UBI device is the direct consequence of the same leaked reference and is not an independent host-wide DoS mechanism.

Negative signals:

* -2 Full host-admin-equivalent privilege. The normal UBI detach control path is guarded by `CAP_SYS_RESOURCE` through the UBI control ioctl, and this capability check is not an attacker-obtainable namespace-scoped detach path.

Important non-awarded signals:

* No UAF, double release, dangling dereference, or post-lifetime access is established.
* No memory corruption, controlled reclaim, arbitrary write, information disclosure, or privilege escalation is demonstrated.
* No reliable kernel crash or host-wide resource exhaustion is established.
* Repeated busy detach calls leak additional references, but they do not allocate a new device object or establish realistic host-wide memory exhaustion.
* Remote reachability is not established.
* No race or timing dependency exists.
* The concrete repeated release/refcount signal does not apply. This is a missing `put_device()`, not a second mutation through a potentially dangling reference.

3. Reachability analysis

The vulnerable path is reached while detaching a busy UBI device. Normal userspace entry is the privileged UBI control ioctl, whose detach operation requires host-level `CAP_SYS_RESOURCE`. An ordinary user cannot instantiate and detach the underlying MTD-backed UBI device through an attacker-created user namespace.

The complete operation is therefore not namespace-scoped. It requires an existing UBI attachment backed by MTD storage and administrative control of the UBI subsystem. No network path, special race, or remote input is involved. UBI requires relevant storage configuration or hardware, but no additional rarity penalty is applied because specialization alone is insufficient.

4. Primitive and severity interpretation

The exact defect is an unmatched acquired device reference on the `-EBUSY` return path. It pins the device object after a later detach and can interfere with device removal or reuse.

This is a resource-lifetime leak, not a stale post-lifetime use. The device remains alive too long rather than being freed while still referenced. Consequently:

* Primitive class is CLASS_C.
* RealUAF is NO.
* The demonstrated consequence is device-local lifecycle and availability degradation.
* Stronger consequences such as kernel corruption, host-wide exhaustion, or privilege escalation are unsupported.
* ImportantCandidate is NO because no Important-grade corruption or security-boundary primitive exists.
* AutoDowngradeAllowed is YES because the issue is privilege-gated, device-local, and contains no unresolved corruption primitive.

Likely operational severity is Low-like or ordinary Moderate, depending on how important reliable UBI detach and reattachment are to the affected product.

5. One-sentence report phrase

A missing put_device call on the busy UBI detach path leaks a device reference and can prevent later release or reuse of the affected UBI device, but requires privileged local device management and provides no demonstrated memory corruption or privilege escalation primitive.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a straightforward privileged reference leak with limited device-local impact and no unresolved memory-corruption or security-boundary mechanism.

7. Call-site confidence

Call-site confidence: medium

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

Patch: mtd: ubi: Release device reference on busy detach [ Upstream
Commit: 87812858c4c8df979cb70bcd0040b925a543df6f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=87812858c4c8df979cb70bcd0040b925a543df6f

Changed files:
  drivers/mtd/ubi/build.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=87812858c4c8df979cb70bcd0040b925a543df6f

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

The original Linux kernel CVE announcement for CVE-2026-90215 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90215

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:L/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS score: 3.3

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: 0

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).