[CVE-2026-89737][MODERATE 7.0] usb: typec: thunderbolt: Disable work before freeing tbt on remove

[CVE-2026-89737][MODERATE 7.0] usb: typec: thunderbolt: Disable work before freeing tbt on remove

[AL-KERNEL]

CVE: CVE-2026-89737
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: usb: typec: thunderbolt: Disable work before freeing tbt on remove
Commit: ebb840d982a612261cfc8a7687735a140c6c7024
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ebb840d982a612261cfc8a7687735a140c6c7024
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89737
Analysis date: Fri, 11 Sep 2026 21:17:19 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89737 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89737

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-2026-89737	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89737 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:P/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.4';DESCR 'A use-after-free candidate exists in the USB Type-C Thunderbolt alternate mode teardown path because tbt work can still run or be queued after plug and cable references are dropped. The work function dereferences those references and can also requeue itself on an error path, while VDM callbacks can queue the same work item during teardown. A malicious or controlled Thunderbolt capable Type-C device may be able to trigger the race through hotplug and VDM activity, causing a kernel crash or other memory safety impact. For the CVSS the PR:N is selected because the paranoid scenario does not require a local account on the victim when the attacker has physical access to the port or controls the attached device. The issue is not network reachable. Impact is at least denial of service via kernel crash, and in the paranoid case may include confidentiality and integrity impact because the patch context supports a UAF race rather than a pure DoS-only bug.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	USB SIMPLEFIX ERRORPATH  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 4
* Applied conservative floor/cap: floor 4 applied because a real async workqueue lifetime violation is established and host-admin-equivalent privilege is not required.
* Raw paranoid score: 3
* Paranoid score: 5
* Applied paranoid floor/cap: floor 5 applied because the stale work item is active asynchronous processing, can be queued by VDM callbacks, and can requeue itself during teardown.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2 Real UAF / active lifetime violation. `tbt_altmode_remove()` drops plug and cable references while `tbt->work` may still run and dereference those references.
* +1 Reliable kernel crash / strong DoS. A workqueue dereference after reference drop can realistically cause a kernel fault or crash.
* +1 Device-mediated external influence. A controlled Thunderbolt-capable Type-C device and VDM activity can influence scheduling of the affected work item.
* +1 paranoid only: active async work / requeue surface. The work item can requeue itself and VDM callbacks can queue it during teardown, making this more than a single passive stale metadata read.

Negative signals:

* -1 Teardown race / timing constraint. The vulnerable condition depends on ordering between remove teardown and already queued or newly queued work.
* -1 Physical/peripheral reachability constraint. The plausible attacker model requires physical or device-level access to the Type-C/Thunderbolt path.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary read/write not demonstrated.
* Callback or function-pointer control not demonstrated.
* Remote network reachability not established.
* Extra Availability not awarded because the crash/DoS is already counted once.
* Confidentiality and Integrity are not awarded as independent ActionableScore points because no concrete disclosure, write primitive, or controlled reuse is shown.
* Concrete repeated release/refcount signal not awarded. The patch shows reference drops followed by possible stale dereference, not a concrete second put/release/refcount mutation on the exact same dangling reference.
* Weak LPE and structured stale-object leverage are not separately awarded beyond the paranoid async-work surface because there is no demonstrated controlled replacement or post-free attacker-shaped object consumption.

Repeated-release/refcount checklist:

* First release can end lifetime: yes, `typec_altmode_put_plug()` may drop references to plug objects.
* Exact same reference survives: likely yes in `tbt` fields until teardown completes.
* Second operation is a real lifetime/refcount mutation: no, supplied evidence says the work dereferences the references, not that it performs a second put/release.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage awarded: no.
* FLOOR 5 applied: yes, but due active async workqueue stale-use surface, not due repeated release/refcount.

3. Reachability analysis

The most plausible attacker is a physically present attacker or a malicious/controlled Thunderbolt-capable USB-C device that can influence VDM callback activity and hotplug/remove timing. No host administrator privileges are required for the physical-device attacker model, so `HostAdminRequired=NO`.

This is not namespace-scoped. Network namespaces and delegated capabilities are not relevant to the trigger, so `NamespaceScopedPrivilege=NO`.

The issue is not directly network reachable. It requires the Type-C/Thunderbolt alternate mode path and a remove/teardown transition while work may still be queued or running. Prior host setup may be required in the practical sense that the system must have the affected Type-C/Thunderbolt support enabled and a relevant device path present, but no separate privileged configuration step is established as mandatory for the attacker once physical device interaction is possible.

Race repeatability is moderate. The bug is timing-dependent, but the work item can be queued by callbacks and can requeue itself, which improves practical reachability compared with a purely one-shot cleanup fault.

4. Primitive and severity interpretation

The exact primitive is an async workqueue lifetime violation: `tbt->work` can run after `tbt_altmode_remove()` drops plug and cable references used by that work. This is a real UAF / stale-reference candidate involving active structured objects, not merely a passive bookkeeping mismatch.

The demonstrated consequence is kernel crash or DoS. The speculative consequence is stronger memory-safety impact if stale plug or cable references are reused in a way that affects active workqueue processing, but controlled reclaim or attacker-shaped replacement is not demonstrated.

This is `CLASS_B`: a real kernel lifetime primitive requiring exploitation-leverage review. It is not `CLASS_A` because there is no demonstrated controlled reclaim, arbitrary write, callback hijack, privilege escalation, or security-boundary bypass.

`ImportantCandidate=YES` because active async work can survive teardown, can be queued by VDM callbacks, and can requeue itself, leaving unresolved exploitation leverage beyond a trivial local cleanup-only crash. `AutoDowngradeAllowed=NO` because automatically closing it as ordinary DoS would discard a concrete kernel lifetime primitive in an externally influenced device path.

Likely operational severity is Actionable Moderate, with manual review needed to decide whether the stale work dereference can be shaped into stronger impact than DoS.

5. One-sentence report phrase

A teardown race in the USB Type-C Thunderbolt alternate mode driver allows `tbt->work` to run or be queued after plug and cable references are dropped, creating a UAF candidate that can be triggered through physical device and VDM activity and can cause at least a kernel crash.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real async workqueue lifetime bug with stale dereference after reference release, externally influenced by Type-C/Thunderbolt device activity, and the work item can requeue itself.

7. Call-site confidence

Call-site confidence: medium

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

Patch: usb: typec: thunderbolt: Disable work before freeing tbt on remove
Commit: ebb840d982a612261cfc8a7687735a140c6c7024
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ebb840d982a612261cfc8a7687735a140c6c7024

Commit description:

tbt_altmode_remove() drops the plug and cable references without
draining tbt->work. The work function dereferences those references,
and can also requeue itself in its error path. The VDM callbacks can
queue the same work item.

Disable and drain tbt->work before dropping the references. This waits
for an existing invocation and prevents subsequent schedule_work()
calls from queueing it during teardown.

This issue was found by an in-house static analysis tool and confirmed
by manual code review.

Fixes: 100e257 ("usb: typec: Add driver for Thunderbolt 3 Alternate Mode")
Cc: stable@vger.kernel.org
Assisted-by: Codex:gpt-5.6
Signed-off-by: Fan Wu <fanwu01@zju.edu.cn>
Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://patch.msgid.link/20260802014959.416687-1-fanwu01@zju.edu.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/typec/altmodes/thunderbolt.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=ebb840d982a612261cfc8a7687735a140c6c7024

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

The original Linux kernel CVE announcement for CVE-2026-89737 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89737

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:P/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:P/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 6.4

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).