[CVE-2026-46186][MODERATE 7.0] Bluetooth: virtio_bt: validate rx pkt_type header length

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

CVE: CVE-2026-46186
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: Bluetooth: virtio_bt: validate rx pkt_type header length
Commit: 1e1e509b6fd2a42421745bbcd98bd16daad20904
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1e1e509b6fd2a42421745bbcd98bd16daad20904
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46186
Analysis date: Thu, 28 May 2026 10:23:09 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
`virtbt_rx_handle()` failed to validate per-type HCI header lengths after stripping `pkt_type`, allowing an untrusted virtio Bluetooth backend to send short packets that trigger invalid or uninitialized reads in the guest HCI stack.

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

The original Linux kernel CVE announcement for CVE-2026-46186 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46186

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: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-46186	MODERATE	CHECK	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-125;*CWE-457;*CWE-20;CWE-754;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '6.4';DESCR 'virtio_bt accepted backend supplied RX packets after only reading the leading pkt_type byte and did not verify that the remaining payload contained the fixed HCI header required for the selected packet type. A malicious or compromised virtio Bluetooth backend can return a one byte completion so the skb length becomes zero after skb_pull, while hci_recv_frame still receives the frame and later HCI code may read uninitialized RX buffer bytes. For the CVSS the PR:N is used because the attacker is the backend or host side of the virtio device and does not need privileges inside the victim guest. The issue is not Internet reachable but can cross a host guest virtualization boundary when virtio Bluetooth is exposed. Impact is primarily guest availability through crash or malformed HCI processing, with limited confidentiality concern from uninitialized data reads and no supported arbitrary write or direct privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	VIRT ERRORPATH SKB HARDWARE PACKET  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Firmware/backend-mediated external influence: +1. The input is supplied by an untrusted virtio Bluetooth backend rather than by local guest userspace.
* Weak/indirect corruption candidate: +1. The bug causes invalid or uninitialized reads after short HCI packets are passed into the core.
* Reliable kernel crash / strong DoS: +1. Short packets can reach HCI parsing paths and may cause guest kernel faults or malformed HCI processing.
* Availability impact realistic: +1. Guest Bluetooth stack or guest kernel availability can be affected.

Paranoid additional signal:

* Cross-boundary isolation impact: +1. The bug crosses a host/backend to guest boundary when virtio Bluetooth is exposed.

Not counted:

* No generic memory corruption. The primitive is read-only invalid/uninitialized access.
* No OOB write, UAF, double free, arbitrary write, or demonstrated LPE.
* No Internet/network reachability.
* Confidentiality impact is limited and not scored strongly because the described read is internal classification of uninitialized RX-buffer bytes, not a clear leak to the attacker.

## 3. Reachability analysis

A malicious or compromised virtio Bluetooth backend can supply a too-short RX completion to the guest. The guest does not need to run attacker-controlled local code, and the attacker does not need privileges inside the guest if they control the backend side.

This is not reachable from ordinary Bluetooth radio traffic or the Internet by itself. It is relevant to virtualized environments where virtio Bluetooth is exposed and the backend is less trusted than the guest.

The trigger may require specific HCI state for the described ACL fast path, such as active CIS, BIS, or PA links, but the missing per-type header validation affects all accepted packet types.

## 4. Severity interpretation

This behaves like an actionable Moderate virtualization-boundary input validation issue. The realistic impact is guest DoS or malformed Bluetooth/HCI processing.

It does not behave like an Important memory-corruption issue because the patch supports invalid or uninitialized reads, not controlled writes, UAF, or a demonstrated privilege escalation chain.

## 5. One-sentence report phrase

`virtbt_rx_handle()` failed to validate per-type HCI header lengths after stripping `pkt_type`, allowing an untrusted virtio Bluetooth backend to send short packets that trigger invalid or uninitialized reads in the guest HCI stack.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended for deployments exposing virtio Bluetooth across host/guest trust boundaries, but current evidence supports guest availability and limited read concerns only, not LPE or arbitrary memory corruption.

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

Patch: Bluetooth: virtio_bt: validate rx pkt_type header length
Commit: 1e1e509b6fd2a42421745bbcd98bd16daad20904
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1e1e509b6fd2a42421745bbcd98bd16daad20904

Commit description:

virtbt_rx_handle() reads the leading pkt_type byte from the RX skb
and forwards the remainder to hci_recv_frame() for every
event/ACL/SCO/ISO type, without checking that the remaining payload
is at least the fixed HCI header for that type.

After the preceding patch bounds the backend-supplied used.len to
[1, VIRTBT_RX_BUF_SIZE], a one-byte completion still reaches
hci_recv_frame() with skb->len already pulled to 0. If the byte
happened to be HCI_ACLDATA_PKT, the ACL-vs-ISO classification
fast-path in hci_dev_classify_pkt_type() dereferences
hci_acl_hdr(skb)->handle whenever the HCI device has an active
CIS_LINK, BIS_LINK, or PA_LINK connection, reading two bytes of
uninitialized RX-buffer data. The same hazard exists for every
packet type the driver accepts because none of the switch cases in
virtbt_rx_handle() check skb->len against the per-type minimum HCI
header size before handing the frame to the core.

After stripping pkt_type, require skb->len to cover the fixed
header size for the selected type (event 2, ACL 4, SCO 3, ISO 4)
before calling hci_recv_frame(); drop ratelimited otherwise.
Unknown pkt_type values still take the original kfree_skb() default
path.

Use bt_dev_err_ratelimited() because both the length and pkt_type
values come from an untrusted backend that can otherwise flood the
kernel log.

Fixes: 160fbcf ("Bluetooth: virtio_bt: Use skb_put to set length")
Cc: stable@vger.kernel.org
Cc: Soenke Huster <soenke.huster@eknoes.de>
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Assisted-by: Claude:claude-opus-4-7
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/virtio_bt.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=1e1e509b6fd2a42421745bbcd98bd16daad20904

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

The original Linux kernel CVE announcement for CVE-2026-46186 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46186

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:L/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).