[CVE-2026-89965][MODERATE 7.0] nvdimm/btt: reject an arena whose nfree is below the lane count

[CVE-2026-89965][MODERATE 7.0] nvdimm/btt: reject an arena whose nfree is below the lane count

[AL-KERNEL]

CVE: CVE-2026-89965
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nvdimm/btt: reject an arena whose nfree is below the lane count
Commit: eb491343d2d1784121ad9af91c25b338c28913c3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=eb491343d2d1784121ad9af91c25b338c28913c3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89965
Analysis date: Wed, 16 Sep 2026 12:49:21 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89965 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89965

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-2026-89965	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89965 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;*CWE-20;CWE-119;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'A kernel out-of-bounds write can occur in the nvdimm/BTT path because the BTT arena nfree value is read from persistent media without validating that it is at least the NVDIMM lane count. The freelist and rtt arrays are sized by nfree, while the I/O path indexes them by a lane value bounded by nd_region num_lanes. A crafted or foreign BTT arena with nfree smaller than the lane count can therefore make freelist(lane) or rtt(lane) access run past the allocation. For the CVSS the PR:L is used in the paranoid score because a local user or delegated storage management context may be able to make the kernel discover or use crafted BTT metadata without full host administrator privileges. The issue is not network reachable and normally requires local storage or persistent memory metadata control. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality and integrity impact or privilege escalation because the primitive is an out-of-bounds write in kernel memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 6)	YES	WRITE OOB SIMPLEFIX ERRORPATH KPANIC MEMORY  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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

ActionableScore=7
ActionableScoreLower=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5. No floor or cap applied.
* Raw paranoid score: 7
* Paranoid score: 7. No floor or cap applied.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

Conservative signals:

* +2 Strong corruption primitive. The commit explicitly establishes an out-of-bounds write because freelist[] and rtt[] are allocated according to arena->nfree but later indexed by lane values bounded by nd_region->num_lanes.
* +1 Reliable kernel crash / strong DoS. An out-of-bounds kernel write in an I/O path can realistically crash the kernel.
* +1 Integrity plausible. The primitive is a write past a kernel allocation and can corrupt kernel state, even though no arbitrary write is demonstrated.
* +1 Important storage path. The bug is in nvdimm/BTT persistent memory block translation metadata used by the I/O path.

Paranoid additional signals:

* +1 Memory layout invariant restoration. The patch enforces the required invariant that nfree must be at least the lane count before per-lane arrays are allocated and later indexed.
* +1 Weak LPE concern. The bug is more than a generic crash because the attacker-influenced on-media metadata controls an allocation/indexing mismatch that produces a kernel OOB write. Controlled reclaim, arbitrary write, and demonstrated privilege escalation are not established.

Non-awarded signals:

* Remote reachability not awarded. This is not network-triggered.
* RealUAF not awarded. The patch establishes OOB write, not use-after-free.
* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Confidentiality not awarded because no read-back or disclosure path is established.
* Strong LPE +2 not separately awarded because no controlled target, callback, credential overwrite, or demonstrated exploit chain is supplied.
* Extra availability not awarded because the same likely crash is already counted once.
* Repeated release/refcount analysis is not applicable.

3. Reachability analysis

The attacker must cause the kernel to discover or use a crafted or foreign BTT arena whose nfree value is smaller than the NVDIMM lane count. The input does not positively establish that an ordinary local user can complete this operation without host administration rights. It also does not positively establish that host-admin-equivalent privilege is always required, because crafted persistent-memory metadata, delegated storage management, virtualized storage, or foreign media scenarios may exist.

HostAdminRequired is therefore UNKNOWN. NamespaceScopedPrivilege is NO because no user namespace or namespace-scoped capability path is shown. Prior host setup may be required in common deployments because nvdimm/BTT requires persistent memory namespace configuration, but the evidence does not justify a strict host-admin penalty. No race is involved. No special timing requirement is shown. The subsystem is hardware/storage-specific, but the supplied evidence establishes a concrete memory corruption primitive, so no niche-subsystem downgrade is applied.

4. Primitive and severity interpretation

The exact primitive is a kernel out-of-bounds write. arena->nfree is read from persistent media and used to size per-lane freelist[] and rtt[] arrays. Later I/O processing indexes those arrays by a lane number bounded by nd_region->num_lanes rather than by nfree. If nfree is smaller than the lane count, freelist[lane] or rtt[lane] can write beyond the allocation.

This is active kernel memory corruption, not a validation-only issue and not a pure DoS/resource leak. It is not a UAF and there is no concrete repeated release/refcount operation. The demonstrated consequence is kernel memory corruption with likely crash. The speculative consequence is privilege escalation or broader kernel compromise if the OOB write can be steered into useful adjacent state, but controlled target selection is not demonstrated.

PrimitiveClass is CLASS_A because an attacker-influenced OOB write is directly established. ImportantCandidate is YES because the primitive is strong enough to require Important-vs-Moderate review even without a complete exploit chain. AutoDowngradeAllowed is NO because automatically closing this as ordinary Moderate would discard a concrete kernel write corruption primitive.

5. One-sentence report phrase

A crafted or foreign nvdimm/BTT arena with nfree smaller than the lane count can make the kernel write past per-lane freelist or rtt allocations during I/O, causing kernel memory corruption and likely DoS with unresolved privilege escalation risk.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a concrete kernel out-of-bounds write driven by on-media metadata and affecting an active storage I/O path, with unresolved exploitation leverage beyond crash.

7. Call-site confidence

Call-site confidence: medium

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

Patch: nvdimm/btt: reject an arena whose nfree is below the lane count
Commit: eb491343d2d1784121ad9af91c25b338c28913c3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=eb491343d2d1784121ad9af91c25b338c28913c3

Commit description:

The BTT info block's nfree field, the number of reserve free blocks, is
read from the medium without validation.  btt_freelist_init() and
btt_rtt_init() size the per-lane freelist[] and rtt[] arrays by nfree,
but the I/O path indexes them by the lane from nd_region_acquire_lane(),
which is bounded by nd_region->num_lanes (ND_MAX_LANES), not by nfree.
A crafted or foreign arena whose nfree is below the lane count makes
freelist[lane]/rtt[lane] run past the allocation: an out-of-bounds write.

btt.rst documents the nlanes = min(nfree, num_cpus) invariant, which the
code does not currently honor: num_lanes is ND_MAX_LANES regardless of
nfree.  Reject an arena whose nfree is below num_lanes at discovery,
before the per-lane arrays are allocated, enforcing that invariant.

Fixes: 5212e11 ("nd_btt: atomic sector updates")
Cc: stable@vger.kernel.org
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Tested-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/20260620-b4-disp-88b2514b-v1-1-3834e707d232@proton.me
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/nvdimm/btt.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=eb491343d2d1784121ad9af91c25b338c28913c3

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

The original Linux kernel CVE announcement for CVE-2026-89965 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89965

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 5
  Paranoid ActionableScore: 7

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).