[CVE-2026-64343][MODERATE REGULAR] USB: ldusb: fix use-after-free on disconnect race

[CVE-2026-64343][MODERATE REGULAR] USB: ldusb: fix use-after-free on disconnect race

[AL-KERNEL]

CVE: CVE-2026-64343
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: USB: ldusb: fix use-after-free on disconnect race
Commit: fc55923a972e715f9a27187b47d4920709e23d85
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fc55923a972e715f9a27187b47d4920709e23d85
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64343
Analysis date: Sat, 25 Jul 2026 10:18:03 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review / Actionable Moderate candidate
Manual review required: YES

Summary:
A race between ldusb `release()` and USB `disconnect()` can free driver data while `mutex_unlock()` may still access the embedded mutex, leading to a local kernel use-after-free and likely DoS.

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

The original Linux kernel CVE announcement for CVE-2026-64343 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64343

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 REGULAR
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-2026-64343	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-672;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 can occur in the USB ldusb driver when release() races with disconnect() and the driver data is freed while mutex_unlock() may still access the mutex object. A local user who can open the ldusb device and trigger or race a USB disconnect can potentially crash the kernel. For the CVSS the PR:L is used because reliable triggering requires a local user context with access to the device node or an equivalent local path, not full administrator privileges in the paranoid interpretation. The issue is not network reachable. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality and integrity impact due to kernel use after free, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review / Actionable Moderate candidate (with actual score 4)	YES	LOCK USB INIT RACE UAF HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review / Actionable Moderate candidate**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process that can open the ldusb character device can participate in the release path.
* Memory corruption, weak primitive: +1
  The patch explicitly fixes a kernel use-after-free, but it is a race-only lifetime bug without shown attacker-controlled reclaim, overwrite, callback control, or type confusion.
* Real lifetime corruption: +1
  The lifetime of `struct ld_usb` was managed incorrectly across `release()` and `disconnect()`.
* Reliable kernel crash / strong DoS: +1
  A UAF in a mutex-containing object can plausibly crash the kernel.

Negative signals:

* Hard or unreliable race / special timing required: -1
  The bug requires a race between file release and USB disconnect.
* Rare or hardware-dependent reachability: -1 in the conservative score
  Triggering requires an affected ldusb device and access to its device node or equivalent USB disconnect path.

Paranoid adjustment:

* The paranoid score does not upgrade this to strong LPE because the patch does not show controlled reclaim, stale callback use, write-after-free, type confusion, or refcount takeover.
* The score is kept below Important-class range under the race-UAF downgrade rule.

## 3. Reachability analysis

The bug is local and not network reachable. A realistic trigger requires access to an ldusb device file and a concurrent disconnect event, either physical unplug, device reset, USB unbind, or another local device-management path. In typical deployments this is not broadly exposed unless the specific USB device is present and accessible to the user. Containers do not automatically improve reachability unless the USB device node is passed through and device cgroup policy allows access.

## 4. Severity interpretation

This behaves more like a manual-review Moderate than a clear Important issue. Theoretical memory corruption exists because this is a kernel UAF, but the exploitability evidence is weak: the patch shows a lifetime race around `mutex_unlock()` and object freeing, not a controlled reuse or write primitive. Realistic impact is primarily local kernel crash / DoS, with privilege escalation only a paranoid concern.

## 5. One-sentence report phrase

A race between ldusb `release()` and USB `disconnect()` can free driver data while `mutex_unlock()` may still access the embedded mutex, leading to a local kernel use-after-free and likely DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is an explicit kernel UAF lifetime race. Even though the demonstrated primitive appears weak and hardware-dependent, UAF bugs should not be auto-closed without manual review.

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

Patch: USB: ldusb: fix use-after-free on disconnect race
Commit: fc55923a972e715f9a27187b47d4920709e23d85
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fc55923a972e715f9a27187b47d4920709e23d85

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: ce0d7d3 ("usb: ldusb: ld_usb 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/20260622152612.116422-4-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/misc/ldusb.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=fc55923a972e715f9a27187b47d4920709e23d85

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

The original Linux kernel CVE announcement for CVE-2026-64343 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64343

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: 3
  Paranoid ActionableScore: 4

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 REGULAR

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).