[CVE-2026-90256][MODERATE 7.0] Bluetooth: L2CAP: use proto_lock for l2cap_data to fix l2cap_disconn_ind [ Upstream

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

CVE: CVE-2026-90256
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: Bluetooth: L2CAP: use proto_lock for l2cap_data to fix l2cap_disconn_ind [ Upstream
Commit: b495a3a9b33bc4e4613e685bf5c96c136caa22d8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b495a3a9b33bc4e4613e685bf5c96c136caa22d8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90256
Analysis date: Fri, 18 Sep 2026 07:33:53 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90256 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90256

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.

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

CVE-2026-90256	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90256 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.4';DESCR 'Bluetooth L2CAP can hit a use after free because hci_conn::l2cap_data is read in l2cap_disconn_ind without the same locking used when l2cap_conn is deleted. A nearby Bluetooth peer or a local actor able to drive Bluetooth connection and disconnection timing may trigger the race while disc_work is processing a disconnect indication, causing l2cap_disconn_ind to read conn disc_reason from a freed l2cap_conn. For the CVSS the PR:N is used for the paranoid score because an adjacent Bluetooth attacker does not need an account or local privileges on the victim if the affected Bluetooth path is reachable. The issue is not Internet routable and should be treated as adjacent network reachable rather than AV:N. Impact is at least denial of service via kernel warning or crash. In worst case it may allow limited confidentiality or integrity impact due to a kernel UAF read pattern and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	REMOTE LOCK DANGER UAF NETWORK SYZBOT KPANIC  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS 	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 4
* Applied conservative floor/cap: Real UAF floor 4 applied, 3 -> 4
* Raw paranoid score: 5
* Paranoid score: 5
* Applied paranoid floor/cap: none
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2 Real UAF / active lifetime violation: `hci_conn::l2cap_data` can be read in `l2cap_disconn_ind()` while the referenced `l2cap_conn` is concurrently deleted.
* +1 Reliable kernel crash / strong DoS: the stale pointer dereference can plausibly lead to KASAN splat, Oops, or kernel crash.
* +1 Adjacent network reachable trigger: a nearby Bluetooth peer can plausibly influence L2CAP connection and disconnection timing without a local account on the victim. This is adjacent network exposure, not Internet routable AV:N exposure.
* +1 Paranoid attacker-influenced repeatable lifetime manipulation: Bluetooth connection and disconnection activity can be repeated by an adjacent peer or local Bluetooth actor, and that repeated lifetime churn is relevant to the stale `l2cap_data` race.

Negative signals:

* -1 Conservative hard race / timing: the bug requires concurrent `l2cap_conn` deletion and `disc_work` disconnect indication processing. The conservative score treats this as a narrow timing race.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity impact are not awarded in the conservative score because the visible stale use reads `conn->disc_reason`.
* Structured stale-object exploitation leverage is not separately awarded because the visible stale consumption is shallow and no attacker-shaped replacement object is shown.
* Weak LPE is not awarded solely because this is a UAF.
* Extra Availability is not awarded because the same crash is already counted.
* Concrete repeated release/refcount signal is not awarded. The patch shows a stale read of `l2cap_data`, not a second `put`, `release`, `free`, or refcount mutation through the same dangling reference.
* Paranoid repeated-refcount leverage is withheld because no second lifetime/refcount mutation is present.
* FLOOR 5 is not applied as a rule. The paranoid score reaches 5 from adjacent reachability plus repeatable lifetime manipulation, not from a repeated-release floor.

3. Reachability analysis

The evaluated attacker is a nearby Bluetooth peer or a local actor able to drive Bluetooth connection and disconnection behavior. No host administrator privilege is established as required for the Bluetooth peer model, so `HostAdminRequired=NO`.

This is not namespace-scoped. Network namespaces are not relevant to the Bluetooth radio/L2CAP path shown here, so `NamespaceScopedPrivilege=NO`.

The issue is not Internet routable. It should be treated as adjacent network reachable because the attacker needs Bluetooth proximity and a reachable affected Bluetooth connection path. Pairing, profile exposure, controller state, and local Bluetooth policy may affect practical reachability, but the supplied evidence supports below-host-admin adjacent reachability.

Prior host setup may exist in the sense that Bluetooth must be enabled and reachable, but the patch does not establish a host-admin-only setup requirement for the vulnerable operation.

Race repeatability is plausible because connection/disconnection attempts can be repeated, but precise timing between `disc_work` and `l2cap_conn_del()` is still the main uncertainty.

4. Primitive and severity interpretation

The exact primitive is a real kernel use-after-free: `l2cap_disconn_ind()` reads `hcon->l2cap_data` without the lock that protects deletion, and can then read `conn->disc_reason` after `l2cap_conn` has been deleted.

This is an active structured object rather than fixed stale storage, but the visible stale operation is shallow: it reads a disconnect reason field. The patch does not show callback use, attacker-controlled replacement contents, arbitrary read/write, or a second lifetime/refcount mutation.

This is therefore `CLASS_B`: a real kernel lifetime primitive requiring exploitation-leverage review, but not a demonstrated `CLASS_A` exploitation primitive.

Demonstrated consequence is denial of service via kernel warning, Oops, or crash. Speculative consequence is limited C/I impact if reuse of the freed `l2cap_conn` storage could be made meaningful, but that is not established by the patch.

`ImportantCandidate=YES` because this is a real adjacent-reachable kernel UAF race in a protocol path, and automatic downgrade could miss unresolved exploitation leverage. It is not a strong Important finding by itself because controlled reclaim, arbitrary write, callback control, or demonstrated LPE is absent.

`AutoDowngradeAllowed=NO` because the combination of real UAF, adjacent reachability, and repeatable lifetime manipulation deserves manual triage rather than automatic closure as ordinary DoS.

Likely operational severity: Actionable Moderate, with Important review warranted.

5. One-sentence report phrase

Bluetooth L2CAP can hit a use-after-free because `hci_conn::l2cap_data` is read without `proto_lock` in `l2cap_disconn_ind()` while `l2cap_conn` may be deleted concurrently, allowing an adjacent Bluetooth attacker to trigger a kernel crash and requiring manual review for unresolved UAF impact.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: real kernel UAF in an adjacent network protocol path, with below-host-admin reachability and unresolved exploitation leverage beyond the demonstrated crash.

7. Call-site confidence

Call-site confidence: high

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

Patch: Bluetooth: L2CAP: use proto_lock for l2cap_data to fix l2cap_disconn_ind [ Upstream
Commit: b495a3a9b33bc4e4613e685bf5c96c136caa22d8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b495a3a9b33bc4e4613e685bf5c96c136caa22d8

Changed files:
  net/bluetooth/l2cap_core.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=b495a3a9b33bc4e4613e685bf5c96c136caa22d8

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

The original Linux kernel CVE announcement for CVE-2026-90256 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90256

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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.4

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: 4
  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).