[CVE-2026-64340][MODERATE 7.0] USB: legousbtower: fix use-after-free on disconnect race

[CVE-2026-64340][MODERATE 7.0] USB: legousbtower: fix use-after-free on disconnect race

[AL-KERNEL]

CVE: CVE-2026-64340
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: USB: legousbtower: fix use-after-free on disconnect race
Commit: 11d069f85851997b4ea0adf242ed9672dc749b8f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=11d069f85851997b4ea0adf242ed9672dc749b8f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64340
Analysis date: Sat, 25 Jul 2026 15:41:13 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local use-after-free race in the USB legousbtower driver can occur when `release()` races with device disconnect, allowing freed driver data containing an embedded mutex to be accessed during `mutex_unlock()`, leading at least to a kernel crash and requiring manual review for possible memory-corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64340 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64340

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-2026-64340	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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 'A use-after-free race exists in the USB legousbtower driver when release() races with device disconnect. The driver data contains the mutex itself, and mutex_unlock() may still touch the mutex object after the lock is released, so freeing the driver data directly from the racing path can leave mutex_unlock() operating on freed memory. For the CVSS the PR:L is used because a local user or local process with access to the legousbtower device node can participate in the open and release path, while the disconnect side can be triggered by device removal or equivalent local device control. The issue is not network reachable and requires a local USB device context plus a timing race. Impact is at least local denial of service via kernel crash. In the paranoid interpretation, the kernel use-after-free primitive may allow confidentiality or integrity impact and possible privilege escalation, so manual review is required.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	LOCK USB INIT RACE UAF HARDWARE LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process with access to the legousbtower character device can open and release the file path involved in the race.
* Memory corruption, weak race-UAF primitive: +1. The patch explicitly fixes a use-after-free caused by freeing driver data while `mutex_unlock()` may still access the embedded mutex.
* Real lifetime corruption: +1. The fix replaces direct free with `kref` lifetime management across `open()`, `release()`, and `disconnect()`.
* Reliable kernel crash / strong DoS: +1. A stale mutex access after object free can plausibly crash the kernel.
* Hard or unreliable race / special timing required: -1. Triggering requires a race between file release and USB disconnect.
* Availability impact realistic: +1. The likely practical impact is host kernel crash.

Paranoid additional signal:

* Weak LPE concern: +1. This is not a demonstrated exploit primitive, but a kernel UAF involving freed driver data and an embedded synchronization object is enough to keep it out of auto-close territory.

Not counted:

* No remote/network reachability.
* No strong LPE bonus. The patch does not show attacker-controlled reclaim, callback control, arbitrary write, or type confusion.
* No broad/default subsystem bonus. This is a device-specific USB misc driver.

Call-site confidence: high. The relevant `open()`, `release()`, `disconnect()`, and `tower_delete()` paths are visible in the patch.

## 3. Reachability analysis

The bug is locally reachable when the affected USB device and driver are present and a user or process can access the device node. The release side is reachable through normal file descriptor close behavior, while the disconnect side requires physical removal, USB reset, driver unbind, or equivalent local device-management action. Containers and namespaces usually do not expose this USB device by default, but delegated device access can make the file operation side reachable to a less privileged process. The path is not network reachable and is not a default broad kernel attack surface.

## 4. Severity interpretation

This behaves above an ordinary Moderate because the commit explicitly fixes a real kernel use-after-free lifetime bug. Realistic exploitation evidence supports local DoS more strongly than privilege escalation. Theoretical memory corruption and LPE risk exist because freed driver data containing a mutex may be accessed after free, but the patch does not demonstrate controlled reclaim or a write primitive. Therefore the conservative result is borderline/manual-review, while the paranoid result is Actionable Moderate.

## 5. One-sentence report phrase

A local use-after-free race in the USB legousbtower driver can occur when `release()` races with device disconnect, allowing freed driver data containing an embedded mutex to be accessed during `mutex_unlock()`, leading at least to a kernel crash and requiring manual review for possible memory-corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is an explicit kernel UAF race in object lifetime management. Even though the practical demonstrated impact is likely DoS and the affected hardware path is narrow, kernel UAF bugs should not be auto-closed without review.

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

Patch: USB: legousbtower: fix use-after-free on disconnect race
Commit: 11d069f85851997b4ea0adf242ed9672dc749b8f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=11d069f85851997b4ea0adf242ed9672dc749b8f

Commit description:

mutex_unlock() may access the mutex structure after releasing the lock
and therefore cannot be used to manage lifetime of objects directly
(unlike spinlocks and refcounts). [1][2]

Use a kref to release the driver data to avoid use-after-free in
mutex_unlock() when release() races with disconnect().

[1] a51749a ("locking/mutex: Document that mutex_unlock() is
                   non-atomic")
[2] 2b9d9e0 ("locking/mutex: Clarify that mutex_unlock(), and most
                   other sleeping locks, can still use the lock object
                   after it's unlocked")

Fixes: 18bcbcf ("USB: misc: legousbtower: semaphore to mutex")
Cc: stable <stable@kernel.org>
Cc: Daniel Walker <dwalker@mvista.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://patch.msgid.link/20260622152612.116422-5-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/misc/legousbtower.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=11d069f85851997b4ea0adf242ed9672dc749b8f

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

The original Linux kernel CVE announcement for CVE-2026-64340 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64340

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).