[CVE-2026-89953][LOW] mtd: mtdoops: free page bitmap when the backing MTD is removed

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

CVE: CVE-2026-89953
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: mtd: mtdoops: free page bitmap when the backing MTD is removed
Commit: 477d61d54e097e90f748aa18d013805abd56bcbd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=477d61d54e097e90f748aa18d013805abd56bcbd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89953
Analysis date: Wed, 16 Sep 2026 11:28:10 -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-2026-89953 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89953

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-2026-89953	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89953 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.1';DESCR 'mtdoops can leak one vmalloc bitmap allocation per backing MTD remove/add cycle because oops_page_used remains allocated after the device is detached. The issue is only exposed when the backing MTD device can disappear and later be registered again while mtdoops remains loaded, so common static MTD deployments are usually not affected in practice. For the CVSS the PR:H is selected because reliable triggering normally requires administrative control over MTD device removal, module state, or equivalent device management privileges. The issue is not network reachable and there is no evidence of memory corruption, information disclosure, or privilege escalation. Impact is denial of service only through gradual memory exhaustion under repeated remove/add cycles.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)  SKIP CVE-2026-89953 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	SIMPLEFIX LEAK HARDWARE LINUS 	-	-	checked

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

ActionableScore=0
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: -1
* Conservative score: 0
* Raw paranoid score: 0
* Paranoid score: 0
* Applied conservative floor/cap: raw score clamped from -1 to 0
* Applied paranoid floor/cap: none
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Conservative signals:

* +1 Concrete persistent kernel resource/reference leak. `oops_page_used` is allocated on MTD registration and was not freed on MTD removal, so the old vmalloc allocation is lost if the device is registered again.
* -2 Host-admin-equivalent privilege required. Reliable triggering requires control over backing MTD device removal/re-registration, module/device lifecycle, or equivalent host device-management authority.

Paranoid signals:

* +1 Concrete persistent kernel resource/reference leak. Same as conservative.
* +1 Sustained resource exhaustion concern. Repeated remove/add cycles can accumulate vmalloc leaks and may eventually cause memory pressure or DoS.
* -2 Host-admin-equivalent privilege required. The complete trigger still depends on privileged MTD/device lifecycle control.

Important non-awarded signals:

* No RealUAF. The patch frees a leaked allocation and clears the pointer; it does not establish post-free use.
* No concrete repeated release/refcount mutation. There is no first release followed by a second lifetime mutation on the same dangling reference.
* No controlled reclaim, arbitrary write, OOB access, type confusion, callback control, or privilege-escalation primitive is established.
* No confidentiality or integrity signal is awarded because this is a resource leak only.
* No reliable kernel crash point is awarded. The demonstrated issue is gradual leakage, not an immediate crash or panic.
* No extra availability point is awarded beyond the paranoid sustained-exhaustion concern.
* No remote reachability is established.
* No namespace-scoped privilege is established.

3. Reachability analysis

The vulnerable path is reached when `mtdoops` remains loaded while its configured backing MTD device disappears and is later registered again. The usual static MTD deployment does not expose the issue because the remove/add cycle does not occur.

Reliable triggering appears to require host-level control over MTD device lifecycle, module state, hotplug-like behavior, or equivalent privileged hardware/device management. This supports `HostAdminRequired=YES`.

No evidence shows that an ordinary local user can perform the complete remove/add cycle. No evidence shows that the complete operation is available through attacker-created namespaces. `NamespaceScopedPrivilege=NO` is appropriate for the supplied MTD device-management context.

There is no network trigger. There is no race requirement. The required condition is a specific device lifecycle sequence rather than a timing window.

4. Primitive and severity interpretation

The exact primitive is a persistent vmalloc memory leak: `mtdoops_notify_remove()` detached from the backing MTD device but did not free `cxt->oops_page_used`. A later add path could allocate a new bitmap and overwrite the old pointer, losing the old allocation.

This is not active structured UAF and not fixed stale storage. It is a resource-management bug with possible gradual memory exhaustion under repeated privileged remove/add cycles.

PrimitiveClass is `NONE` because no meaningful corruption, stale post-lifetime access, ownership violation, authorization bypass, or security-boundary primitive is established. The demonstrated consequence is resource leakage. The speculative consequence is host memory exhaustion if a privileged actor repeatedly cycles the backing MTD device.

ImportantCandidate is `NO` because there is no unresolved memory-corruption or boundary-bypass primitive. AutoDowngradeAllowed is `YES` because this is a host-admin-gated resource leak with no UAF, no C/I impact, and no demonstrated non-DoS exploitation leverage.

Likely operational severity is Low-like or ordinary Moderate at most.

5. One-sentence report phrase

mtdoops leaks one vmalloc bitmap allocation per backing MTD remove/add cycle when mtdoops remains loaded, but reliable triggering requires host-level MTD device management and the impact is limited to gradual resource exhaustion.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a concrete but host-admin-gated resource leak with no memory corruption, no authorization bypass, no network exposure, and no plausible Important-class primitive in the supplied evidence.

7. Call-site confidence

Call-site confidence: high

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

Patch: mtd: mtdoops: free page bitmap when the backing MTD is removed
Commit: 477d61d54e097e90f748aa18d013805abd56bcbd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=477d61d54e097e90f748aa18d013805abd56bcbd

Commit description:

mtdoops_notify_add() allocates oops_page_used when the configured MTD
device is registered.  mtdoops_notify_remove() detaches from that device
but leaves the bitmap allocated.  If the same MTD device is later
registered again, the add path allocates a new bitmap and overwrites the
old pointer, leaking one vmalloc allocation per remove/add cycle.

This is only visible when the backing MTD device can disappear and be
registered again while mtdoops remains loaded, so the usual static MTD
case does not expose it.

Free the bitmap after unregistering the dumper and flushing the pending
workers, then clear the pointer and page count before a later attach can
allocate fresh state.  Clearing the pointer also keeps the module exit
path from freeing the same bitmap a second time after a remove event.

Fixes: be95745 ("mtd: mtdoops: keep track of used/unused pages in an array")
Cc: stable@vger.kernel.org
Signed-off-by: Xu Rao <raoxu@uniontech.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/mtd/mtdoops.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=477d61d54e097e90f748aa18d013805abd56bcbd

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

The original Linux kernel CVE announcement for CVE-2026-89953 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89953

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.1

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).