[CVE-2026-64455][MODERATE REGULAR] USB: chaoskey: Fix slab-use-after-free in chaoskey_release()

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

CVE: CVE-2026-64455
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: USB: chaoskey: Fix slab-use-after-free in chaoskey_release()
Commit: fe7a0f4be283b40dd592540027279735120d0d6f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fe7a0f4be283b40dd592540027279735120d0d6f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64455
Analysis date: Sat, 25 Jul 2026 07:19:36 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline manual review recommended. Not an Actionable Moderate by score
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-64455 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64455

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-2026-64455	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';*CWE-416;CWE-825;*CWE-672;Other CVSS 'AV:P/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.8';DESCR 'A use-after-free can occur in the USB chaoskey release path when a user closes the device file after the USB device has already been unplugged. The release routine may pass a freed usb_interface pointer to a debug logging path, and KASAN reports a slab use after free in dev_driver_string. For the CVSS the PR:L is used because a local user or process must be able to open the chaoskey device node and trigger the close path, while the realistic trigger also depends on USB hot unplug or an equivalent local device removal event. The issue is not network reachable. Impact is at least a local denial of service or diagnostic crash scenario, and in the paranoid view may justify limited confidentiality or integrity impact because the bug class is UAF, but the observed access is a debug read rather than a demonstrated write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended. Not an Actionable Moderate by score (with actual score 3)	YES	READ DANGER USB UAF HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2

1. ActionableScore
* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline manual review recommended. Not an Actionable Moderate by score**

2. Signal breakdown
* Local unprivileged trigger: +1. A local process that can open the chaoskey device node can later close the file descriptor after device removal.
* Memory corruption, weak or indirect primitive: +1. The bug is a stale pointer read from freed usb_interface memory, but only through a debug logging path and without demonstrated write, reclaim, callback, or type confusion.
* Real lifetime corruption: +1. The interface lifetime is invalid after chaoskey_free drops the last reference, and chaoskey_release still passes the stale pointer to usb_dbg.
* Reliable kernel crash / strong DoS: +1 only for the paranoid score. KASAN reports slab-use-after-free, but production impact depends on debug logging and kernel debug or panic behavior, so this is not counted conservatively.
* Special local device condition: -1. Triggering requires a chaoskey device or equivalent USB emulation and a close-after-unplug sequence.
* No confidentiality or integrity bonus. The observed primitive is a read inside dev_driver_string for logging, with no user-visible disclosure, write primitive, attacker-controlled reclaim, or object replacement shown.

3. Reachability analysis
A local user or process must be able to open the chaoskey character device and keep the file descriptor open while the USB device is unplugged or otherwise removed. This is not network reachable. Containers and namespaces usually do not expose arbitrary USB device nodes unless explicitly passed through, so container reachability depends on device delegation. The affected driver is hardware-specific and not a broad default kernel data path. Call-site confidence: high, because the stack trace and changed release path are present in the input.

4. Severity interpretation
This behaves more like a low-end or borderline Moderate UAF than an Important-class vulnerability. The theoretical concern is that it is a real use-after-free in kernel code, which justifies manual review. The realistic evidence points to a narrow stale read in an unimportant debug statement after USB unplug, with no demonstrated privilege escalation, information leak, controlled corruption, or reliable production crash path.

5. One-sentence report phrase
A local use-after-free in the USB chaoskey release path can occur when a device file is closed after unplug, causing a stale usb_interface pointer to be used by debug logging and potentially leading to a diagnostic crash or limited DoS under debug conditions.

6. Manual review recommendation
MANUAL CHECK REQUIRED. The score is low, but the bug class is UAF and should be reviewed once to confirm that the stale pointer is not reachable outside the debug logging path and does not create a reclaim or disclosure primitive.

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

Patch: USB: chaoskey: Fix slab-use-after-free in chaoskey_release()
Commit: fe7a0f4be283b40dd592540027279735120d0d6f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fe7a0f4be283b40dd592540027279735120d0d6f

Commit description:

The chaoskey driver has a use-after-free bug in its release routine.
If the user closes the device file after the USB device has been
unplugged, a debugging log statement will try to access the
usb_interface structure after it has been deallocated:

	BUG: KASAN: slab-use-after-free in dev_driver_string (drivers/base/core.c:2406)
	Read of size 8 at addr ffff888168e8a0b8 by task chaoskey_raw_re/10106

	Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
	Call Trace:
	 <TASK>
	 dump_stack_lvl (lib/dump_stack.c:94 lib/dump_stack.c:120)
	 print_report (mm/kasan/report.c:378 mm/kasan/report.c:482)
	 kasan_report (mm/kasan/report.c:595)
	 dev_driver_string (drivers/base/core.c:2406)
	 __dynamic_dev_dbg (lib/dynamic_debug.c:906)
	 chaoskey_release (drivers/usb/misc/chaoskey.c:323)
	 __fput (fs/file_table.c:510)
	 fput_close_sync (fs/file_table.c:615)
	 __x64_sys_close (fs/open.c:1507 fs/open.c:1492 fs/open.c:1492)
	 do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
	 entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)

The driver's last reference to the interface structure is dropped in
the chaoskey_free() routine, so the code must not use the interface --
even in a debugging statement -- after that routine returns.
(Exception: If we know that another reference is held by someone else,
such as the device core while the disconnect routine runs, there's no
problem.  Thanks to Johan Hovold for pointing this out.)

Since the bad access is part of an unimportant debugging statement,
we can fix the problem simply by removing the whole statement.

Reported-by: Shuangpeng Bai <shuangpeng.kernel@gmail.com>
Closes: https://lore.kernel.org/linux-usb/20EC9664-054E-438B-B411-2145D347F97B@gmail.com/
Tested-by: Shuangpeng Bai <shuangpeng.kernel@gmail.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Fixes: 66e3e59 ("usb: Add driver for Altus Metrum ChaosKey device (v2)")
Cc: stable <stable@kernel.org>
Reviewed-by: Johan Hovold <johan@kernel.org>
Link: https://patch.msgid.link/bb5b1dc6-eb59-43e1-8d26-51e658e88bbe@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/base/core.c
  lib/dump_stack.c
  mm/kasan/report.c
  lib/dynamic_debug.c
  drivers/usb/misc/chaoskey.c
  fs/file_table.c
  fs/open.c
  arch/x86/entry/syscall_64.c
  arch/x86/entry/entry_64.S

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=fe7a0f4be283b40dd592540027279735120d0d6f

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

The original Linux kernel CVE announcement for CVE-2026-64455 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64455

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:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 2
  Paranoid ActionableScore: 3

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).