[CVE-2026-72121][MODERATE REGULAR] can: bcm: add locking when updating filter and timer values commit 749179c2e25b95d22499ed29096b3e02d6dfd2b4 upstream.

[CVE-2026-72121][MODERATE REGULAR] can: bcm: add locking when updating filter and timer values commit 749179c2e25b95d22499ed29096b3e02d6dfd2b4 upstream.

[AL-KERNEL]

CVE: CVE-2026-72121
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: can: bcm: add locking when updating filter and timer values commit 749179c2e25b95d22499ed29096b3e02d6dfd2b4 upstream.
Commit: a7eb6db1cd3f7b556a301dc1265945ad112089f7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a7eb6db1cd3f7b556a301dc1265945ad112089f7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72121
Analysis date: Sat, 15 Aug 2026 22:09:50 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended / Actionable Moderate candidate
Manual review required: YES

Summary:
A local race in CAN BCM runtime updates can expose partially updated RX filter timer and frame state to RX handlers or hrtimer callbacks, causing stale CAN replies or timer driven DoS, but the patch does not demonstrate a strong memory corruption or privilege escalation primitive.

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

The original Linux kernel CVE announcement for CVE-2026-72121 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72121

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 REGULAR
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-2026-72121	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;CWE-667;CWE-662;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H';BEST CVSS score: '5.8';DESCR 'A race condition in CAN BCM can occur when bcm_rx_setup updates RX filter timer and frame state while RX handlers or hrtimer callbacks process incoming CAN traffic at the same time. This can expose partially updated operation state and may result in stale RTR replies inconsistent timer handling or a zero interval hrtimer forward path. For the CVSS the PR:L is used because a local user or process must be able to create and update CAN BCM socket operations, while incoming CAN traffic or timer callbacks provide the concurrent execution context. The issue is not Internet reachable and is not a generic remote network attack. Impact is mainly local denial of service and limited CAN message integrity impact. A limited confidentiality impact is kept only in the paranoid score because concurrent frame snapshots can observe inconsistent data, but the patch does not show a strong UAF or OOB write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Actionable Moderate candidate (with actual score 4)  SKIP CVE-2026-72121 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: c2aba69d0c36a496ab4f2e81e9c2b271f2693fd7 YES  NO NO unknown 	MAYBE	KASAN LOCK DEADLOCK INIT NOMEMCHK RACE TIMER HARDWARE LINUS SYZBOT 	-	-	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended / Actionable Moderate candidate**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local process can interact with CAN BCM socket operations and update BCM RX state while CAN RX handlers or timers run concurrently.
* Local high-control kernel data-plane API refinement: +1
  CAN BCM allows user-controlled frame arrays, timers, flags, and runtime operation updates, which are precisely the objects involved in the race.
* Reliable kernel crash / strong DoS concern: +1
  The patch fixes races involving hrtimer state, including avoiding hrtimer_forward() with a zero interval. This supports a realistic local DoS concern, although no strong memory corruption primitive is shown.
* Integrity impact plausible: +1
  A concurrent bcm_rx_handler() could transmit a stale RTR reply with CAN_RTR_FLAG still set, causing limited CAN message integrity impact.
* Hard or unreliable race / special timing required: -1
  The issue depends on concurrent bcm_rx_setup() updates racing with RX handlers or hrtimer callbacks.

Paranoid additional signal:

* Confidentiality impact plausible: +1
  Limited only for paranoid scoring, because concurrent snapshots of partially updated frame state may expose inconsistent kernel-managed CAN frame contents. This is not treated as a strong info leak.

Not awarded:

* Generic memory corruption: +0
  The patch shows data races and inconsistent state, but no UAF, double free, OOB write, type confusion, refcount takeover, or arbitrary write.
* Privilege escalation plausible: +0
  No reclaim, callback control, object replacement, or write-after-free primitive is demonstrated.
* Remote reachable / network-triggerable: +0
  This is not Internet reachable and is not a generic remote network parsing path.

Call-site confidence: high. The relevant update path, RX path, timer callbacks, and bcm_can_tx() call sites are present in the supplied patch.

## 3. Reachability analysis

The realistic trigger is local. An attacker needs a process able to create or update CAN BCM operations and arrange concurrent incoming CAN traffic or timer activity. In many systems SocketCAN is not exposed to ordinary users by default unless CAN interfaces such as vcan or physical CAN devices are configured, but the interface is still reachable through ordinary local socket operations once available.

Namespaces can matter. If an untrusted container or service is given access to a CAN network device or a prepared vcan setup, the practical privilege level may be closer to PR:L than PR:H. If only host administrators can configure and access CAN devices, the operational risk is lower.

The path is not broadly default Internet exposed. It is specific to SocketCAN BCM, but it is not an obscure debug-only path and syzbot/KCSAN coverage indicates the race is reachable in practice under test conditions.

## 4. Severity interpretation

This behaves like an actionable Moderate or borderline manual-review issue, not an Important-class vulnerability on the supplied evidence.

Theoretical risk comes from concurrent modification of kernel operation state used by RX and timer callbacks. However, the patch does not show a concrete memory lifetime violation, UAF reclaim, OOB write, attacker-controlled overwrite, or privilege escalation primitive. The strongest realistic impacts are local DoS and limited CAN message integrity problems due to stale or partially updated BCM state.

The paranoid score keeps the issue from being auto-closed because it is a kernel race in a user-controlled socket subsystem with timer interaction, but it should not be escalated to Important without additional evidence of memory corruption or LPE.

## 5. One-sentence report phrase

A local race in CAN BCM runtime updates can expose partially updated RX filter timer and frame state to RX handlers or hrtimer callbacks, causing stale CAN replies or timer driven DoS, but the patch does not demonstrate a strong memory corruption or privilege escalation primitive.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a syzbot/KCSAN-reported kernel race in a socket data path with timer interaction and user-controlled operation state. It is not a clear auto-close case, but current evidence supports Actionable Moderate rather than Important.

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

Patch: can: bcm: add locking when updating filter and timer values commit 749179c2e25b95d22499ed29096b3e02d6dfd2b4 upstream.
Commit: a7eb6db1cd3f7b556a301dc1265945ad112089f7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a7eb6db1cd3f7b556a301dc1265945ad112089f7

Commit description:

KCSAN detected a simultaneous access to timer values that can be
overwritten in bcm_rx_setup() when updating timer and filter content
while bcm_rx_handler(), bcm_rx_timeout_handler() or bcm_rx_thr_handler()
run concurrently on incoming CAN traffic.

Protect the timer (ival1/ival2/kt_ival1/kt_ival2/kt_lastmsg) and filter
(nframes/flags/frames/last_frames) updates in bcm_rx_setup() with a new
per-op bcm_rx_update_lock, taken with the matching scope in the RX
handlers. memcpy_from_msg() is staged into a temporary buffer before the
lock is taken, since it can sleep and must not run under a spinlock.

hrtimer_cancel() is always called without bcm_rx_update_lock held, since
bcm_rx_timeout_handler()/bcm_rx_thr_handler() take the same lock and a
running callback would otherwise deadlock against the canceller.

Also close a related race: bcm_rx_setup() cleared the RTR flag in the
stored reply frame's can_id as a separate, unprotected step after the
frame content was already installed, so a concurrent bcm_rx_handler()
could transmit a stale reply with CAN_RTR_FLAG still set. Fold that
normalization into the initial frame preparation instead (on the staged
buffer for updates, directly on op->frames pre-registration for new
ops), so the installed frame is always atomically self-consistent.

bcm_rx_handler()'s RX_RTR_FRAME check now takes a lock-protected
snapshot of op->flags before deciding whether to call bcm_can_tx(),
but does not hold the lock across that call.

Also take a lock-protected snapshot of the currframe in bcm_can_tx()
to avoid partly overwrites by content updates in bcm_tx_setup().
Finally check if a TX_RESET_MULTI_IDX/SETTIMER might have reset
op->currframe between the two locked sections in bcm_can_tx().

Omit calling hrtimer_forward() with zero interval in bcm_rx_thr_handler().
kt_ival2 may have been concurrently cleared by bcm_rx_setup() before it
cancels this timer, so check kt_ival2 inside the bcm_rx_update_lock.

Fixes: c2aba69d0c36 ("can: bcm: add locking for bcm_op runtime updates")
Reported-by: syzbot+75e5e4ae00c3b4bb544e@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-can/6975d5cf.a00a0220.33ccc7.0022.GAE@google.com/
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20260714-bcm_fixes-v15-3-562f7e3e42da@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>
Diffstat -rw-r--r-- net/can/bcm.c 176

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=a7eb6db1cd3f7b556a301dc1265945ad112089f7

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

The original Linux kernel CVE announcement for CVE-2026-72121 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72121

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:N/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 3
  Paranoid ActionableScore: 4

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 REGULAR

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).