[CVE-2026-98773][MODERATE 7.0] USB: cdc-acm: skip URB restart in port_shutdown if disconnected

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

CVE: CVE-2026-98773
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: USB: cdc-acm: skip URB restart in port_shutdown if disconnected
Commit: 9742905d9ed5bf23ebe4c100dec80b6bc50fb522
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9742905d9ed5bf23ebe4c100dec80b6bc50fb522
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98773
Analysis date: Sat, 24 Oct 2026 10:20:40 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A physical disconnect race can resubmit CDC-ACM URBs during teardown, allowing callbacks to access freed URB/DMA storage and potentially crash the kernel.

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

The original Linux kernel CVE announcement for CVE-2026-98773 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98773

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: NO

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-2026-98773	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98773 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=3 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	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-672;Other CVSS 'AV:P/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '6.4';DESCR 'A race between USB disconnect and final tty port shutdown can cause acm_port_shutdown to unpoison and resubmit control and read URBs after acm_disconnect has started tearing the device down. The concurrent disconnect can then free the resubmitted URBs and DMA buffers while callbacks may still access them, creating a kernel use after free with a clear crash path and a possible but unproven memory corruption consequence. For the CVSS the PR:N value reflects that a physical attacker controlling or disconnecting a matching USB device does not need an authenticated kernel user context. Exploitation requires the ALWAYS_POLL_CTRL quirk and race timing between disconnect and shutdown, so physical attack vector and high attack complexity are appropriate. Impact is at least denial of service and the UAF involving DMA backed memory warrants manual review for possible confidentiality, integrity, or privilege escalation consequences. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 6.4 and the production ActionableScore returned 0 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.0 and ActionableScore 3 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)	MAYBE	USB INIT UAF DMAorINTERRUPT HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=0
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 0.
* Conservative score: 0.
* Raw paranoid score: 0.
* Paranoid score: 0; no additional proof-supported primitive.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +2 Real UAF: shutdown can resubmit poisoned control/read URBs; disconnect then frees those URBs and DMA buffers while callbacks may still access them.
* +1 Reliable kernel crash/DoS: callback access to freed DMA-backed storage plausibly causes an Oops or host crash.
* -2 Physical/specific-hardware requirement: triggering requires physical control or disconnection of a matching USB CDC-ACM device with `ALWAYS_POLL_CTRL`.
* -1 Hard race: free must overlap the resubmitted in-flight URBs; supplied evidence specifically identifies high-complexity disconnect/shutdown timing.
* No extra availability point: it would duplicate the same crash.
* No controlled reclaim, attacker-selected target/payload, callback-pointer consumption, arbitrary write, disclosure, authorization bypass, or LPE is shown.
* The legacy C:H/I:H possibilities are not scored; access to freed DMA memory does not establish either impact.

3. Reachability analysis

A physical attacker controlling or disconnecting the affected USB device can induce disconnect and the resulting tty hangup/shutdown sequence without host UID 0 or host-administrator capability. No namespace capability is involved. The path requires affected hardware/quirk state and a race between port shutdown, URB resubmission, and disconnect teardown; repeatable timing control is not demonstrated.

4. Primitive and severity reading

This is a real asynchronous UAF involving fixed URBs and DMA buffers, not an active structured object with attacker-shaped replacement. The stale callbacks access storage freed by concurrent teardown. The patch supports crash-capable post-lifetime access but does not show useful post-reclaim field consumption or a directed post-free write.

Thus it is constrained CLASS_B rather than CLASS_A. ImportantCandidate is NO because the demonstrated path is physical, hardware-specific, timing-dependent, and presently crash-oriented. Auto-downgrade remains allowed, although review is recommended because DMA-backed post-free activity deserves driver-lifetime verification.

5. One-sentence report phrase

A physical disconnect race can resubmit CDC-ACM URBs during teardown, allowing callbacks to access freed URB/DMA storage and potentially crash the kernel.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm whether the controller can write into reallocated DMA backing before cancellation; that stronger primitive is not established by the supplied patch.

7. Call-site confidence

Call-site confidence: high

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

Patch: USB: cdc-acm: skip URB restart in port_shutdown if disconnected
Commit: 9742905d9ed5bf23ebe4c100dec80b6bc50fb522
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9742905d9ed5bf23ebe4c100dec80b6bc50fb522

Commit description:

acm_port_shutdown() is called from the tty-port machinery whenever the
last user closes the port.  acm_disconnect() sets acm->disconnected and
calls tty_port_tty_vhangup(), which eventually triggers port_shutdown().
At that point acm_disconnect() has already called acm_poison_urbs() and
is about to free the control and read URBs.

However, acm_port_shutdown() unconditionally calls acm_unpoison_urbs()
and, when the ALWAYS_POLL_CTRL quirk is set, immediately re-submits the
ctrlurb and all read URBs.  If the concurrent acm_disconnect() then
frees those URBs and their DMA buffers while they are in-flight, the
result is a use-after-free in acm_ctrl_irq() or acm_read_bulk_callback()
accessing the freed DMA memory.

Fix by checking acm->disconnected after draining the delayed anchor and
returning early: if we are in a disconnect path the URBs must remain
poisoned and must not be resubmitted.

The early return is placed after the delayed-anchor drain loop because
that loop only cancels write-side accounting (wb->use = false and
autopm counter) and does not touch the hardware path.

Fixes: f58752e ("USB: cdc-acm: Add quirks for Yoga Book 9 14IAH10 INGENIC touchscreen")
Cc: stable <stable@kernel.org>
Signed-off-by: Yuchao Zhang <ndaugoing@gmail.com>
Link: https://patch.msgid.link/20260918114309.3659635-2-ndaugoing@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/class/cdc-acm.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=9742905d9ed5bf23ebe4c100dec80b6bc50fb522

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

The original Linux kernel CVE announcement for CVE-2026-98773 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98773

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:L/I:L/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: unknown
  Paranoid ActionableScore: 0

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).