[CVE-2026-46140][LOW] Bluetooth: btmtk: validate WMT event SKB length before struct access

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

CVE: CVE-2026-46140
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: Bluetooth: btmtk: validate WMT event SKB length before struct access
Commit: c411cf1bfde951cfa821809cf4020ba177f76e0c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c411cf1bfde951cfa821809cf4020ba177f76e0c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46140
Analysis date: Thu, 28 May 2026 12:54:54 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A short firmware-controlled WMT event in the MediaTek Bluetooth driver can cause an out-of-bounds read from SKB tailroom during internal response parsing, with realistic impact limited to driver failure or DoS rather than privilege escalation or data disclosure.

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

The original Linux kernel CVE announcement for CVE-2026-46140 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46140

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-46140	LOW	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-125;CWE-126;*CWE-20;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L';BEST CVSS score: '4.6';DESCR 'A short firmware controlled WMT event response in the btmtk Bluetooth driver can make btmtk_usb_hci_wmt_sync read past the valid SKB data when parsing btmtk_hci_wmt_evt or btmtk_hci_wmt_evt_funcc. This is an out-of-bounds read from SKB tailroom rather than an out-of-bounds write, so the realistic impact is driver failure, KASAN detection, or denial of service for the Bluetooth device path. For the CVSS the PR:N is used because a malicious or faulty controller response does not require privileges on the victim once the affected device is present. The issue is not directly network reachable as normal Bluetooth packet parsing and is better treated as firmware or device mediated input. Confidentiality and integrity impact are not assigned because the patch shows no user visible disclosure path and no write primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2) 	MAYBE	READ OOB INIT SIMPLEFIX ERRORPATH SKB 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: **1**
* Paranoid score: **2**
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

* Firmware-mediated external influence: **+1**. The malformed input is a short WMT event response from the MediaTek Bluetooth controller or firmware, not ordinary userspace input.
* Availability impact realistic: **+1** in the paranoid score. A short response can cause an OOB read from SKB tailroom and may lead to driver failure, KASAN report, or device-path DoS.
* Weak corruption / read-only OOB: **+0**. This is an out-of-bounds read, not an OOB write, UAF, double-free, or controlled overwrite.
* No confidentiality signal: **+0**. The patch does not show a path where tailroom bytes are copied back to userspace or exposed externally.
* No integrity signal: **+0**. There is no write primitive or state corruption shown.
* Device-specific / difficult-to-reach condition: **-1**. Practical triggering requires a faulty, malicious, or compromised MediaTek Bluetooth controller or firmware response.

## 3. Reachability analysis

The bug is triggered through the `btmtk` USB Bluetooth driver while parsing firmware/controller WMT event responses. It is not a normal Bluetooth over-the-air packet parsing path and should not be treated as directly network reachable. A local user normally cannot trigger it unless they can influence firmware behavior, attach a malicious USB Bluetooth device, or otherwise control the controller response. Namespaces and containers do not materially lower the requirement because the vulnerable input comes from device firmware. The path is relevant only on systems with affected MediaTek Bluetooth hardware and the `btmtk` driver active.

## 4. Severity interpretation

This behaves like a **Low-like or ordinary Moderate** issue. The theoretical bug class is an OOB read, but the realistic primitive is limited to reading beyond valid SKB data during internal driver parsing. There is no demonstrated information leak, no write primitive, no lifetime corruption, and no plausible LPE path from the patch context. The main realistic impact is Bluetooth device failure or kernel diagnostic crash under hardened/debug configurations.

## 5. One-sentence report phrase

A short firmware-controlled WMT event in the MediaTek Bluetooth driver can cause an out-of-bounds read from SKB tailroom during internal response parsing, with realistic impact limited to driver failure or DoS rather than privilege escalation or data disclosure.

## 6. Manual review recommendation

**NO MANUAL CHECK NEEDED**

This is firmware-mediated read-only OOB access with no demonstrated disclosure path, no memory overwrite, and no realistic privilege-escalation primitive.

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

Patch: Bluetooth: btmtk: validate WMT event SKB length before struct access
Commit: c411cf1bfde951cfa821809cf4020ba177f76e0c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c411cf1bfde951cfa821809cf4020ba177f76e0c

Commit description:

btmtk_usb_hci_wmt_sync() casts the WMT event response SKB data to
struct btmtk_hci_wmt_evt (7 bytes) and struct btmtk_hci_wmt_evt_funcc
(9 bytes) without first checking that the SKB contains enough data.
A short firmware response causes out-of-bounds reads from SKB tailroom.

Use skb_pull_data() to validate and advance past the base WMT event
header. For the FUNC_CTRL case, pull the additional status field bytes
before accessing them.

Fixes: d019930 ("Bluetooth: btmtk: move btusb_mtk_hci_wmt_sync to btmtk.c")
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/bluetooth/btmtk.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=c411cf1bfde951cfa821809cf4020ba177f76e0c

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

The original Linux kernel CVE announcement for CVE-2026-46140 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46140

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:P/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.6

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: 1
  Paranoid ActionableScore: 2

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).