[CVE-2026-64453][MODERATE 7.0] usb: misc: usbio: fix disconnect UAF in client teardown

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

CVE: CVE-2026-64453
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: usb: misc: usbio: fix disconnect UAF in client teardown
Commit: c40090f8d19b415e2925b22be964b5d1f695666f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c40090f8d19b415e2925b22be964b5d1f695666f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64453
Analysis date: Sat, 25 Jul 2026 13:39:03 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A confirmed use-after-free in the USBIO disconnect path allows a physical attacker or privileged local device-management path to trigger a kernel crash, with limited but nonzero concern for stronger impact due to stale list traversal after freeing `usbio_client`.

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

The original Linux kernel CVE announcement for CVE-2026-64453 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64453

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
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CVE-2026-64453	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-672;*CWE-664;Other CVSS 'AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.8';DESCR 'A use-after-free can occur in the USBIO disconnect path because usbio_disconnect iterates the client list with a non safe reverse iterator while auxiliary_device_uninit can drop the final reference and free the current usbio_client. After the loop body returns, the iterator may read the previous list pointer from freed memory and trigger a KASAN slab-use-after-free or kernel crash. For the CVSS the PR:N value is used for the paranoid score because a physical attacker with access to the USB port may trigger the vulnerable disconnect path without a local account. The issue is not network reachable and requires the affected USBIO driver and a matching device or teardown path. Impact is at least denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel UAF memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)  SKIP CVE-2026-64453 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 121a0f839dbb397af5fabb701cea3e9983223e50 YES  NO NO unknown 	MAYBE	DANGER USB UAF HARDWARE  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* +1 Firmware-mediated or external-device influence. Triggering depends on USB device presence and disconnect or teardown behavior, not network reachability.
* +2 Generic memory corruption. The report shows a real slab-use-after-free in `usbio_disconnect`.
* +1 Real lifetime corruption. `auxiliary_device_uninit()` can drop the final reference and free `struct usbio_client` while the iterator still depends on it.
* +1 Reliable kernel crash or strong DoS. KASAN reproduced a slab-use-after-free in the disconnect path.
* +1 Privileged kernel/device-management lifetime path. The bug is in USB auxiliary-device teardown and object lifetime management.
* -2 Physical-only or rare hardware-only condition. Realistic attacker influence usually requires physical USB access or control of the affected device path.
* -1 Rare and difficult-to-reach subsystem/configuration. This is specific to the Intel USBIO bridge driver, not a broad USB core path.

Paranoid additional signals:

* +1 Weak LPE concern. The stale freed list node controls the next iterator cursor in a kernel list teardown path, which is a possible corruption sink, although no attacker-controlled reclaim or overwrite primitive is demonstrated.
* +1 Availability impact realistic. A physical or local teardown trigger can produce a host kernel crash, not merely a warning.

Not counted:

* No remote/network trigger.
* No strong LPE plausibility because the patch does not show attacker-controlled reclaim, callback control, type confusion, arbitrary write, or refcount takeover.
* No Dirty-Pipe-like or page-cache primitive.

## 3. Reachability analysis

A physical attacker with access to the USB port and a matching affected USBIO device may be able to trigger the disconnect teardown path without a local account. A local privileged user can likely trigger similar paths through driver unbind or device management, but that is less security-interesting than the physical device case. Containers and namespaces do not materially lower the privilege requirement because the vulnerable path is USB device teardown in the host kernel. The path is not network reachable. Exposure is limited by the need for the affected `usbio` driver and relevant Intel USBIO bridge hardware or emulation.

Call-site confidence: high. The patch and commit message include the relevant caller behavior inside `usbio_disconnect`, the freeing operation through `auxiliary_device_uninit`, and the exact unsafe iterator use.

## 4. Severity interpretation

This is not an ordinary low-risk cleanup because it is a confirmed kernel UAF with a reproduced KASAN report. Realistically, the demonstrated impact is a physical or local DoS through kernel crash during USB disconnect teardown. Theoretical memory-corruption impact is higher than the demonstrated crash because freed list traversal can sometimes become exploitable if object reuse is controllable, but the patch does not prove such control. Therefore the conservative score is borderline manual-review level, while the paranoid score reaches Actionable Moderate.

## 5. One-sentence report phrase

A confirmed use-after-free in the USBIO disconnect path allows a physical attacker or privileged local device-management path to trigger a kernel crash, with limited but nonzero concern for stronger impact due to stale list traversal after freeing `usbio_client`.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a confirmed kernel UAF in object lifetime teardown. Even though realistic reachability is limited to physical USB or privileged device-management scenarios, stale list traversal after free is a memory-corruption pattern that should not be auto-closed without human review.

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

Patch: usb: misc: usbio: fix disconnect UAF in client teardown
Commit: c40090f8d19b415e2925b22be964b5d1f695666f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c40090f8d19b415e2925b22be964b5d1f695666f

Commit description:

usbio_disconnect() walks usbio->cli_list in reverse and uninitializes each
auxiliary device. auxiliary_device_uninit() drops the device reference, and
for an unbound child that can run usbio_auxdev_release() and free the
containing struct usbio_client.

list_for_each_entry_reverse() advances after the loop body by reading
client->link.prev. If the current client is freed by
auxiliary_device_uninit(), the iterator dereferences freed memory.

Use list_for_each_entry_safe_reverse() so the previous client is
cached before the body can drop the final reference. This preserves
reverse teardown order while keeping the next iterator cursor independent
of the current client's lifetime.

Validation reproduced this kernel report:
BUG: KASAN: slab-use-after-free in usbio_disconnect+0x12e/0x150

Call Trace:
 <TASK>
 dump_stack_lvl+0x66/0xa0
 print_report+0xce/0x630
 ? usbio_disconnect+0x12e/0x150
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? __virt_addr_valid+0x188/0x320
 ? usbio_disconnect+0x12e/0x150
 kasan_report+0xe0/0x110
 ? usbio_disconnect+0x12e/0x150
 usbio_disconnect+0x12e/0x150
 usb_unbind_interface+0xf3/0x400
 really_probe+0x316/0x660
 __driver_probe_device+0x106/0x240
 driver_probe_device+0x4a/0x110
 __device_attach_driver+0xf1/0x1a0
 ? __pfx___device_attach_driver+0x10/0x10
 bus_for_each_drv+0xf9/0x160
 ? __pfx_bus_for_each_drv+0x10/0x10
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? trace_hardirqs_on+0x18/0x130
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? _raw_spin_unlock_irqrestore+0x44/0x60
 __device_attach+0x133/0x2a0
 ? __pfx___device_attach+0x10/0x10
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? do_raw_spin_unlock+0x9a/0x100
 ? srso_alias_return_thunk+0x5/0xfbef5
 device_initial_probe+0x55/0x70
 bus_probe_device+0x4a/0xd0
 device_add+0x9b9/0xc10
 ? __pfx_device_add+0x10/0x10
 ? _raw_spin_unlock_irqrestore+0x44/0x60
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? lockdep_hardirqs_on_prepare+0xea/0x1a0
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? usb_enable_lpm+0x3c/0x260
 usb_set_configuration+0xb64/0xf20
 usb_generic_driver_probe+0x5f/0x90
 usb_probe_device+0x71/0x1b0
 really_probe+0x46b/0x660
 __driver_probe_device+0x106/0x240
 driver_probe_device+0x4a/0x110
 __device_attach_driver+0xf1/0x1a0
 ? __pfx___device_attach_driver+0x10/0x10
 bus_for_each_drv+0xf9/0x160
 ? __pfx_bus_for_each_drv+0x10/0x10
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? trace_hardirqs_on+0x18/0x130
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? _raw_spin_unlock_irqrestore+0x44/0x60
 __device_attach+0x133/0x2a0
 ? __pfx___device_attach+0x10/0x10
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? do_raw_spin_unlock+0x9a/0x100
 ? srso_alias_return_thunk+0x5/0xfbef5
 device_initial_probe+0x55/0x70
 bus_probe_device+0x4a/0xd0
 device_add+0x9b9/0xc10
 ? __pfx_device_add+0x10/0x10
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? add_device_randomness+0xb7/0xf0
 usb_new_device+0x492/0x870

[Commit description truncated; see the upstream URL below]

Changed files:
  drivers/usb/misc/usbio.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=c40090f8d19b415e2925b22be964b5d1f695666f

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

The original Linux kernel CVE announcement for CVE-2026-64453 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64453

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:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 6.8

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: 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: NO
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).