[CVE-2026-72123][IMPORTANT] can: bcm: defer rx_op deallocation to workqueue to fix thrtimer UAF

[CVE-2026-72123][IMPORTANT] can: bcm: defer rx_op deallocation to workqueue to fix thrtimer UAF

[AL-KERNEL]

CVE: CVE-2026-72123
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: can: bcm: defer rx_op deallocation to workqueue to fix thrtimer UAF
Commit: 3cf4fd5316f449811d8baf1bc6978ef5a7b743a9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3cf4fd5316f449811d8baf1bc6978ef5a7b743a9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72123
Analysis date: Sun, 16 Aug 2026 01:58:48 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum / Strong Important candidate for manual review
Manual review required: YES

Summary:
A race in CAN_BCM RX operation teardown can leave thrtimer armed after bcm_op is freed through RCU, causing a timer callback use-after-free that is locally triggerable through SocketCAN and may lead to kernel crash or possible privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-72123 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72123

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: IMPORTANT
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-2026-72123	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in the CAN BCM RX teardown path can leave thrtimer armed after bcm_op has been scheduled for RCU freeing. When the RCU grace period completes, the object can be freed while a later hrtimer callback still dereferences the old operation and its socket pointer, resulting in a use-after-free. For the CVSS the PR:L is used for the paranoid score because a local user or local process with access to CAN_BCM sockets can create and tear down BCM operations without full administrator privileges in some deployments, although creating CAN interfaces usually requires CAP_NET_ADMIN. The issue is not directly reachable over IP networks and is primarily triggered through the local SocketCAN control path, with incoming CAN traffic only helping the race timing. Impact is at least local denial of service via kernel crash and in the worst case may allow privilege escalation because this is a real kernel use-after-free in a timer callback.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum / Strong Important candidate for manual review (with actual score 7)	YES	DANGER INIT NOMEMCHK RACE UAF IMPROVEONLY TIMER LINUS KPANIC PACKET 	YES	NO	checked

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

ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum / Strong Important candidate for manual review**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  CAN_BCM sockets can be used by a local process when SocketCAN and a CAN or vcan interface are available. Creating the interface may require CAP_NET_ADMIN, but use of an existing interface may be available to less privileged local users depending on deployment.

* Memory corruption, strong corruption primitive: +2
  The commit explicitly describes a real use-after-free where thrtimer later dereferences a freed bcm_op.

* Real lifetime corruption: +1
  This is an RCU and timer lifetime bug. The fix defers freeing to a workqueue and adds sock_hold and sock_put to protect op->sk lifetime.

* Privilege escalation plausible: +1 conservative, +2 paranoid
  Conservative scoring uses weak to moderate LPE concern because no attacker-controlled reclaim or overwrite primitive is demonstrated. Paranoid scoring uses strong LPE concern because the stale timer callback dereferences a freed kernel object and socket pointer after an attacker-influenced teardown and rearm race.

* Reliable kernel crash / strong DoS: +1
  A later hrtimer callback dereferencing freed memory is a credible kernel crash path.

Subtracted signals:

* Hard or unreliable race / special timing required: -1
  The bug requires a teardown vs packet RX fast-path race and timer rearm timing.

* Configuration dependent SocketCAN exposure: -1
  This is not ordinary IP remote exposure. It requires CAN_BCM support and a usable CAN or vcan setup.

Call-site confidence: high.
The patch and commit show the relevant lifetime path: bcm_rx_handler, bcm_rx_update_and_send, call_rcu, hrtimer callback, bcm_free_op_rcu, deferred workqueue free, and socket reference handling.

## 3. Reachability analysis

The bug is locally reachable through the SocketCAN CAN_BCM control path. A local process can create BCM RX operations, tear them down, and rely on concurrent CAN RX activity to rearm thrtimer during the RCU grace-period window. It is not directly reachable over normal IP networking.

Namespaces and containers matter. If a container or reduced-privilege environment is given access to a CAN or vcan interface, the practical privilege requirement may be closer to PR:L rather than PR:H. If no CAN interface exists and the attacker must create vcan or configure CAN devices, CAP_NET_ADMIN is typically required.

The affected path is not a broad default remote attack surface, but it is a real kernel lifetime bug in a networking protocol family. Realistic exploitation is timing dependent, but the UAF is explicit and the timer callback makes the issue more actionable than a simple NULL dereference or resource leak.

## 4. Severity interpretation

This should not be auto-closed as an ordinary Moderate. The realistic impact is at least local denial of service through kernel crash. The theoretical impact includes local privilege escalation because the stale hrtimer callback can access a freed bcm_op and op->sk after RCU teardown, although the patch does not demonstrate controlled reclaim, arbitrary write, or a reliable exploitation primitive.

Overall this is best treated as Actionable Moderate at minimum, with Strong Important candidate handling for manual review.

## 5. One-sentence report phrase

A race in CAN_BCM RX operation teardown can leave thrtimer armed after bcm_op is freed through RCU, causing a timer callback use-after-free that is locally triggerable through SocketCAN and may lead to kernel crash or possible privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is an explicit kernel UAF involving RCU lifetime, hrtimer callbacks, and socket lifetime. Even though exploitation is race dependent and not remotely reachable over IP, the stale callback into freed kernel state makes it important enough for manual review.

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

Patch: can: bcm: defer rx_op deallocation to workqueue to fix thrtimer UAF
Commit: 3cf4fd5316f449811d8baf1bc6978ef5a7b743a9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3cf4fd5316f449811d8baf1bc6978ef5a7b743a9

Commit description:

Commit f1b4e32 ("can: bcm: use call_rcu() instead of costly
synchronize_rcu()") replaced synchronize_rcu() in bcm_delete_rx_op()
with call_rcu() and introduced the RX_NO_AUTOTIMER flag.

However, this flag check was omitted for thrtimer in the packet rx
fast-path. During BCM RX operation teardown, a concurrent RCU reader
(bcm_rx_handler) can race and re-arm thrtimer via
bcm_rx_update_and_send() after call_rcu() has been scheduled.  Once
the RCU grace period elapses, bcm_op is freed.  The subsequently
firing thrtimer then dereferences the deallocated op, causing a UAF.

Adding flag checks to the rx fast-path (bcm_rx_update_and_send) does not
fully close the TOCTOU race and introduces latency for every CAN frame.
Conversely, calling hrtimer_cancel() directly inside the RCU callback
(softirq context) is fatal as hrtimer_cancel() can sleep, triggering
a "scheduling while atomic" panic.

Resolve this by deferring the timer cancellation and memory free to a
dedicated unbound workqueue (bcm_wq).  The RCU callback now queues a
work item to bcm_wq, which safely cancels both timers and deallocates
memory in sleepable process context.  A dedicated workqueue is used to
prevent system-wide WQ saturation and is cleanly flushed/destroyed
on module unload to avoid rmmod page faults.

Since the deferred work can now outlive the calling context by an
unbounded amount, also take a reference on op->sk when it is assigned
and drop it only once the deferred work has cancelled both timers, so a
socket can no longer be freed out from under a still-armed timer whose
callback (bcm_send_to_user()) dereferences op->sk.

Fixes: f1b4e32 ("can: bcm: use call_rcu() instead of costly synchronize_rcu()")
Tested-by: Feng Xue <feng.xue@outlook.com>
Tested-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20260714-bcm_fixes-v15-1-562f7e3e42da@hartkopp.net
Cc: stable@kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/can/bcm.c
  net/sock.h
  net/net_namespace.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=3cf4fd5316f449811d8baf1bc6978ef5a7b743a9

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

The original Linux kernel CVE announcement for CVE-2026-72123 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72123

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:L/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: 5
  Paranoid ActionableScore: 6

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).