[CVE-2026-46123][MODERATE 7.0] Bluetooth: virtio_bt: clamp rx length before skb_put

[CVE-2026-46123][MODERATE 7.0] Bluetooth: virtio_bt: clamp rx length before skb_put

[AL-KERNEL]

CVE: CVE-2026-46123
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: Bluetooth: virtio_bt: clamp rx length before skb_put
Commit: ed41c81d30b211a671667259c3b5feeba0e062d5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ed41c81d30b211a671667259c3b5feeba0e062d5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46123
Analysis date: Thu, 28 May 2026 14:05:00 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious virtio Bluetooth backend can report an invalid RX length and cause the guest kernel to include uninitialized heap bytes in an skb or process an empty packet, leading to possible information exposure and DoS.

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

The original Linux kernel CVE announcement for CVE-2026-46123 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46123

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-2026-46123	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H';*CWE-20;CWE-908;*CWE-457;*CWE-200;Other CVSS 'AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '7.7';DESCR 'virtio_bt RX handling trusted the used length reported by the virtio backend and passed it to skb_put() even though only 1000 bytes were exposed to the device. A malicious or buggy backend can report a length greater than 1000 but still within skb tailroom, causing the skb to include uninitialized guest kernel heap bytes that were never written by the device. A zero length completion can also reach packet handling and make the driver read an uninitialized packet type byte. For the CVSS the PR:N is used because the attacker does not need privileges inside the guest when controlling the virtio backend, but this is not network reachable and requires a malicious or compromised host or device model. Impact is mainly confidentiality exposure and possible denial of service, with no shown write primitive or direct privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	OOB VIRT INIT ERRORPATH SKB HARDWARE KPANIC  KPANIC INCREASED_TO_MODERATE7_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

* Firmware/device-mediated external influence: **+1**. The length comes from an untrusted virtio backend or device model.
* Confidentiality impact plausible: **+1**. A backend-reported length larger than the actually exposed 1000-byte buffer can cause uninitialized guest kernel heap bytes to be included in the skb.
* Availability impact realistic: **+1**. Invalid lengths or zero-length completions can lead to malformed packet handling and possible driver or Bluetooth stack disruption.
* Privileged device/backend path: **+1**. The issue is in a trusted virtio device receive path where backend-provided metadata controls guest kernel skb state.
* Broad virtualization relevance, paranoid only: **+1**. Virtio is common in virtualized environments, though `virtio_bt` itself is more specialized.
* No strong memory corruption: **+0**. This is not an OOB write, UAF, double-free, or controlled overwrite.
* No direct LPE primitive: **+0**. The patch does not show privilege escalation or arbitrary kernel write.
* Niche device exposure considered: **-1 conservative**. Practical exposure requires a virtio Bluetooth device and a malicious or buggy backend.

## 3. Reachability analysis

The direct attacker is a malicious or compromised virtio backend, host device model, or equivalent device-side component. The issue is not reachable over normal Bluetooth radio traffic and is not a network packet parsing bug. It does not require privileges inside the guest if the attacker controls the backend, but it does require the guest to use `virtio_bt`. Containers inside the guest are not the main boundary unless they can observe resulting Bluetooth/HCI traffic. In confidential or device-isolation threat models, this is more relevant because the host/backend should not be able to make the guest expose unrelated kernel heap contents.

## 4. Severity interpretation

This is best treated as **Actionable Moderate**. The realistic primitive is uninitialized guest kernel heap exposure and possible DoS caused by trusting a backend-reported receive length. It is not a proven memory-corruption-to-LPE issue, because there is no write primitive or lifetime corruption. However, backend-controlled length influencing skb contents is security-relevant and should not be auto-closed as a harmless functional bug.

## 5. One-sentence report phrase

A malicious virtio Bluetooth backend can report an invalid RX length and cause the guest kernel to include uninitialized heap bytes in an skb or process an empty packet, leading to possible information exposure and DoS.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Manual review is required because this is an untrusted virtio backend controlled length bug with plausible guest kernel heap information exposure, even though no direct LPE or write primitive is shown.

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

Patch: Bluetooth: virtio_bt: clamp rx length before skb_put
Commit: ed41c81d30b211a671667259c3b5feeba0e062d5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ed41c81d30b211a671667259c3b5feeba0e062d5

Commit description:

virtbt_rx_work() calls skb_put(skb, len) where len comes directly
from virtqueue_get_buf() with no validation against the buffer we
posted to the device. The RX skb is allocated in virtbt_add_inbuf()
and exposed to virtio as exactly 1000 bytes via sg_init_one().

Checking len against skb_tailroom(skb) is not sufficient because
alloc_skb() can leave more tailroom than the 1000 bytes actually
handed to the device. A malicious or buggy backend can therefore
report used.len between 1001 and skb_tailroom(skb), causing skb_put()
to include uninitialized kernel heap bytes that were never written by
the device.

The same path also accepts len == 0, in which case skb_put(skb, 0)
leaves the skb empty but virtbt_rx_handle() still reads the pkt_type
byte from skb->data, consuming uninitialized memory.

Define VIRTBT_RX_BUF_SIZE once and reuse it in alloc_skb() and
sg_init_one(), and gate virtbt_rx_work() on that same constant so
the bound checked matches the buffer actually exposed to the device.
Reject used.len == 0 in the same gate so an empty completion can
no longer reach virtbt_rx_handle().

Use bt_dev_err_ratelimited() because the length value comes from an
untrusted backend that can otherwise flood the kernel log.

Same class of bug as commit c04db81 ("net/9p: Fix buffer
overflow in USB transport layer"), which hardened the USB 9p
transport against unchecked device-reported length.

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
  net/bluetooth/hci_core.h

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=ed41c81d30b211a671667259c3b5feeba0e062d5

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

The original Linux kernel CVE announcement for CVE-2026-46123 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46123

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:N/UI:N/S:U/C:L/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
  The Best / paranoid CVSS score: 7.7

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).