[CVE-2026-72113][MODERATE 7.0] can: bcm: add missing device refcount for CAN filter removal

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

CVE: CVE-2026-72113
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: can: bcm: add missing device refcount for CAN filter removal
Commit: 84aa4807816e405c1bf87114fc63e06d244281ef
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=84aa4807816e405c1bf87114fc63e06d244281ef
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72113
Analysis date: Sun, 16 Aug 2026 00:01:36 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in CAN BCM filter removal can leave a stale receive filter pointing to a freed `bcm_op` or socket after concurrent netdevice unregister, creating a local UAF candidate with likely DoS impact and possible privilege escalation concerns in namespace or delegated CAP_NET_ADMIN environments.

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

The original Linux kernel CVE announcement for CVE-2026-72113 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72113

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
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CVE-2026-72113	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'CAN BCM filter removal can leave a stale receive filter when bcm_release or bcm_delete_rx_op tries to re-find the net device by ifindex after a concurrent NETDEV_UNREGISTER already unlisted it. The stale filter can keep a callback with a pointer to a soon-to-be-freed bcm_op or socket, making this a race based use-after-free candidate rather than a simple leak. For the CVSS the PR:L is used for the paranoid score because a local user or reduced capability network admin context may be able to create CAN_BCM state and race device removal in a namespace or managed environment. The issue is not directly reachable over an IP network. External CAN bus traffic alone is not sufficient without the local kernel state and unregister race. Impact is at least local denial of service via crash and in the worst case may allow confidentiality or integrity impact due to use-after-free memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	INIT LEAK UAF HARDWARE LINUS  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local user can create CAN BCM socket state and RX subscriptions if PF_CAN and CAN_BCM are available, although reliable racing with device unregister may need additional control.

* Memory corruption, strong corruption primitive: +2
  The stale CAN receive filter can retain a callback/private pointer to a soon-to-be-freed `bcm_op` or socket. This is a UAF candidate with callback-based reachability, not just a leak.

* Real lifetime corruption: +1
  The bug is specifically a missing netdevice lifetime reference around filter registration and removal.

* Reliable kernel crash / strong DoS: +1
  A stale receive filter pointing at freed BCM/socket state can plausibly produce kernel crash behavior when later hit by CAN receive processing.

* Weak LPE concern: +1 in paranoid interpretation
  Privilege escalation is not demonstrated, but stale callback/private-data UAF in a socket receive path is enough to require manual review.

Subtracted signals:

* Hard or unreliable race / special timing required: -1
  The issue depends on concurrent `NETDEV_UNREGISTER` racing with BCM filter cleanup.

* Requires CAP_NET_ADMIN or equivalent for reliable device unregister in typical deployments: -1 to -2
  Conservative handling applies stronger privilege friction. Paranoid handling uses a reduced penalty because CAP_NET_ADMIN may be delegated in namespaces or containerized environments.

## 3. Reachability analysis

The bug is local, not directly IP-network reachable. An attacker needs local ability to create CAN BCM state and a realistic way to race CAN device unregistration or namespace/device teardown. In normal host deployments, unregistering CAN or vcan devices usually requires CAP_NET_ADMIN or administrative control. In container or user namespace setups, a reduced-capability network admin context may make the trigger more reachable than full host root. The affected path is not a broad default Internet-facing path, but PF_CAN and CAN BCM are ordinary kernel interfaces on systems using CAN.

Call-site confidence: high. The patch shows registration, cleanup, and release paths directly.

## 4. Severity interpretation

This is more than an ordinary Moderate cleanup bug because the skipped `can_rx_unregister()` leaves a stale filter referencing objects that may be freed. Realistic exploitation is constrained by the race and by device-management privileges, so the conservative score remains borderline. The paranoid score reaches Actionable Moderate because the primitive is a lifetime/UAF candidate with callback reachability in a kernel socket receive path. Practical privilege escalation is not proven, but the bug should not be auto-closed as DoS-only.

## 5. One-sentence report phrase

A race in CAN BCM filter removal can leave a stale receive filter pointing to a freed `bcm_op` or socket after concurrent netdevice unregister, creating a local UAF candidate with likely DoS impact and possible privilege escalation concerns in namespace or delegated CAP_NET_ADMIN environments.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: stale callback/private-data UAF candidate in a networking socket path, with namespace-dependent privilege assumptions and possible LPE relevance despite no demonstrated exploit primitive.

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

Patch: can: bcm: add missing device refcount for CAN filter removal
Commit: 84aa4807816e405c1bf87114fc63e06d244281ef
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=84aa4807816e405c1bf87114fc63e06d244281ef

Commit description:

sashiko-bot remarked a problem with a concurrent device unregistration
in isotp.c which also is present in the bcm.c code. A former fix for raw.c
commit c275a17 ("can: raw: add missing refcount for memory leak fix")
introduced a netdevice_tracker which solves the issue for bcm.c too.

bcm_release(), bcm_delete_rx_op() and bcm_notifier() relied on
dev_get_by_index(ifindex) to re-find the device for an rx_op before
unregistering its filter. If a concurrent NETDEV_UNREGISTER has already
unlisted the device from the ifindex table, that lookup fails and
can_rx_unregister() is silently skipped, leaving a stale CAN filter
pointing at the soon-to-be-freed bcm_op/socket.

Hold a netdev_hold()/netdev_put() tracked reference on op->rx_reg_dev
from the moment the rx filter is registered in bcm_rx_setup() until it
is unregistered in bcm_rx_unreg(), and use that reference directly in
bcm_release() and bcm_delete_rx_op() instead of re-looking the device
up by ifindex.

Reported-by: sashiko-bot@kernel.org
Closes: https://sashiko.dev/#/patchset/20260707094716.63578-1-socketcan@hartkopp.net
Fixes: ffd980f ("[CAN]: Add broadcast manager (bcm) protocol")
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20260714-bcm_fixes-v15-8-562f7e3e42da@hartkopp.net
Cc: stable@kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  net/can/bcm.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=84aa4807816e405c1bf87114fc63e06d244281ef

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

The original Linux kernel CVE announcement for CVE-2026-72113 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72113

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:H/PR:H/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: NO
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).