[CVE-2026-72126][MODERATE 7.0] can: isotp: use unconditional synchronize_rcu() in isotp_release()

[CVE-2026-72126][MODERATE 7.0] can: isotp: use unconditional synchronize_rcu() in isotp_release()

[AL-KERNEL]

CVE: CVE-2026-72126
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: can: isotp: use unconditional synchronize_rcu() in isotp_release()
Commit: 945d9894502cd9124f5d676181c542ed2000f7c0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=945d9894502cd9124f5d676181c542ed2000f7c0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72126
Analysis date: Sat, 15 Aug 2026 03:23:24 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in CAN ISO-TP socket teardown can let an in-flight RCU receive path re-arm a timer after the socket is freed, causing a use-after-free timer callback and potential local kernel crash, with privilege-escalation risk requiring manual review.

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

The original Linux kernel CVE announcement for CVE-2026-72126 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72126

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
======================================================================

CVE-2026-72126	MODERATE	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:N/I:N/A:H';BEST CVSS score: '7';DESCR 'CAN ISOTP has an RCU race where isotp_notify() can unregister CAN filters and clear so bound while an in flight isotp_rcv() is still running. isotp_release() used so bound to decide whether synchronize_rcu() was needed, so it could skip the grace period and cancel rxtimer while isotp_rcv() was about to re arm it through isotp_send_fc(). This creates a use after free timer callback on a freed socket and can crash the kernel. For the CVSS the PR:L is used for the paranoid score because a local user that can create CAN ISO-TP sockets and operate inside a network namespace with delegated NET_ADMIN may be able to coordinate socket release with netdevice unregister. The issue is not directly Internet reachable and requires local CAN or virtual CAN control paths. Impact is at least local denial of service and in worst case may allow confidentiality or integrity impact due to a UAF on a kernel socket object. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	INIT RACE UAF TIMER LINUS KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6	YES	NO	checked

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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:

* Generic memory corruption / UAF: +2
  The commit explicitly describes a use-after-free timer callback on a freed ISO-TP socket.

* Real lifetime corruption: +1
  This is an RCU lifetime bug involving in-flight `isotp_rcv()`, socket release, and timer cancellation.

* Reliable kernel crash / strong DoS: +1
  A stale timer callback on a freed socket is a credible kernel crash path.

* Weak LPE concern: +1
  The UAF involves an asynchronous timer callback on a freed socket object, which is more concerning than a plain stale read, but no controlled reclaim or object replacement is shown.

* Hard or unreliable race: -1
  Triggering requires ordering between `NETDEV_UNREGISTER`, in-flight receive processing, socket release, and timer re-arming.

* Privilege / control-plane dependency: -1
  Reliable triggering likely needs control over CAN or virtual CAN device unregister paths, often requiring `CAP_NET_ADMIN`, but namespaces or delegated network administration can reduce this in practice.

Paranoid adjustment:

* Local unprivileged or reduced-privilege trigger in delegated namespace: +1
  If untrusted users can create/use CAN ISO-TP sockets and operate in a network namespace with delegated `CAP_NET_ADMIN`, the practical trigger becomes closer to PR:L than full host-root.

Not awarded:

* Remote network-triggerable: +0
  This is not Internet-remote. It requires local CAN or virtual CAN control paths.

* Strong LPE plausibility: +0
  No attacker-controlled reclaim, controlled overwrite, type confusion, or arbitrary write primitive is demonstrated.

* Confidentiality / integrity impact as separate signals: +0
  Possible in a paranoid review due to UAF, but not sufficiently demonstrated for additional ActionableScore points.

## 3. Reachability analysis

A local actor needs access to CAN ISO-TP socket activity and a way to race socket release with CAN netdevice unregister. In typical host deployments, unregistering a CAN or vcan device requires `CAP_NET_ADMIN`, so this is not a simple fully unprivileged trigger. In container or namespace setups where network administration is delegated, the effective privilege requirement may be lower. The path is not remotely reachable over ordinary IP networking. Realistic exploitation requires precise timing, but the affected lifetime pattern is security-relevant because it can leave an RCU receive path able to re-arm a timer after socket teardown.

Call-site confidence: high, because the patch and commit directly describe the relevant call sequence: `isotp_notify()` clears `so->bound`, `isotp_release()` skips `synchronize_rcu()`, `isotp_rcv()` can call `isotp_send_fc()`, and `rxtimer` can be re-armed after teardown.

## 4. Severity interpretation

This is stronger than an ordinary Moderate DoS because it is an explicit kernel UAF involving RCU, socket lifetime, and an async timer callback. Realistically, the demonstrated impact is local kernel crash / DoS with hard race conditions and likely network-admin involvement. Theoretical privilege escalation is plausible enough to require manual review, but current evidence does not support treating it as a strong Important candidate without additional reclaim or callback-control details.

## 5. One-sentence report phrase

A race in CAN ISO-TP socket teardown can let an in-flight RCU receive path re-arm a timer after the socket is freed, causing a use-after-free timer callback and potential local kernel crash, with privilege-escalation risk requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly fixes a UAF on a freed kernel socket object with asynchronous timer callback behavior. Even though exploitation appears local, race-dependent, and likely tied to CAN/vcan control privileges, this class should not be auto-closed without manual analysis.

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

Patch: can: isotp: use unconditional synchronize_rcu() in isotp_release()
Commit: 945d9894502cd9124f5d676181c542ed2000f7c0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=945d9894502cd9124f5d676181c542ed2000f7c0

Commit description:

isotp_notify() unregisters the (RCU) CAN filters via can_rx_unregister()
and clears so->bound without waiting for a grace period. isotp_release()
uses so->bound to decide whether it needs to call synchronize_rcu()
before cancelling so->rxtimer, so when NETDEV_UNREGISTER runs first it
skips that synchronize_rcu() and can cancel the timer while an
in-flight isotp_rcv() is still executing and about to re-arm it via
isotp_send_fc(), leading to a use-after-free timer callback on the
freed socket.

sakisho-bot remarked a problem with rtnl_lock held in isotp_notify(),
therefore make isotp_release() always call synchronize_rcu() before
cancelling the timers, regardless of so->bound. This still closes the
original race (isotp_notify() clearing so->bound without waiting for
in-flight isotp_rcv() callers before isotp_release() cancels the RX
timer) without adding any RCU wait to the netdevice notifier path.

Fixes: 14a4696 ("can: isotp: isotp_release(): omit unintended hrtimer restart on socket release")
Closes: https://lore.kernel.org/linux-can/20260707085210.6B6C01F000E9@smtp.kernel.org/
Reported-by: Nico Yip <zdi-disclosures@trendmicro.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20260712-isotp-fixes-v10-1-793a1b1ce17f@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/isotp.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=945d9894502cd9124f5d676181c542ed2000f7c0

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

The original Linux kernel CVE announcement for CVE-2026-72126 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72126

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:N/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: 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).