[CVE-2026-53194][IMPORTANT] USB: serial: kl5kusb105: fix bulk-out buffer overflow

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

CVE: CVE-2026-53194
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: USB: serial: kl5kusb105: fix bulk-out buffer overflow
Commit: 60af1fd82983c26604102e63a3fcc822c186cceb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=60af1fd82983c26604102e63a3fcc822c186cceb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53194
Analysis date: Thu, 25 Jun 2026 07:41:46 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local user with write access to a kl5kusb105 USB serial tty can trigger a 2 byte attacker influenced slab out of bounds write in the bulk out transmit buffer, causing at least kernel crash risk and requiring manual review for possible privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-53194 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53194

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: IMPORTANT
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-2026-53194	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-122;CWE-119;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'A slab out of bounds write in the USB serial kl5kusb105 transmit path occurs because klsi_105_prepare_write_buffer copies up to the full bulk out buffer size into dest plus KLSI_HDR_LEN, leaving no room for the two byte header. A local user or process that can write bulk_out_size or more bytes to the matching tty device can overflow the 64 byte bulk out buffer by KLSI_HDR_LEN bytes and corrupt adjacent slab memory. For the CVSS the PR:L is used because reliable triggering requires local access to the tty or an equivalent process context with permission to write to the USB serial port, not full administrator privileges in common dialout style deployments. The issue is not network reachable. Impact is at least kernel crash and denial of service, and because the primitive is a controlled kernel slab out of bounds write, a privilege escalation risk is plausible and should be manually reviewed.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	WRITE KASAN OOB USB PRINTF-TTY HARDWARE LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local user or process that can write to the matching tty device can trigger the overflow.
* Constrained kernel buffer overwrite: +1
  The overflow is a 2 byte slab out of bounds write past a 64 byte USB serial bulk out buffer.
* Weak LPE concern: +1
  The write data is attacker influenced, but the overwrite size and target selection are constrained.
* Reliable kernel crash / strong DoS: +1
  KASAN confirms slab out of bounds write in the transmit path, and non KASAN systems may crash or corrupt adjacent slab state.
* Confidentiality impact plausible: +1
  Limited confidentiality impact is plausible only through possible kernel heap corruption, not directly demonstrated.
* Integrity impact plausible: +1
  Limited integrity impact is plausible because this is a write beyond a kernel heap buffer.
* Rare/device specific exposure: -1
  The vulnerable path requires the kl5kusb105 USB serial driver and a matching or emulated device.

Paranoid adjustment:

* Memory corruption, strong corruption primitive: +2 instead of constrained +1
  The bug is a real slab OOB write with attacker influenced payload bytes.
* Privilege escalation plausible: +2 instead of weak +1
  In the paranoid interpretation, controlled kernel slab corruption can potentially be shaped into LPE and should not be auto closed without manual analysis.

## 3. Reachability analysis

The bug is not network reachable. It is triggered through the local tty write path for a kl5kusb105 compatible USB serial device. Full root is not necessarily required because ttyUSB devices are commonly writable by users in groups such as dialout, or by service accounts that legitimately access serial ports. Namespaces do not normally make this broadly reachable unless the USB serial device node is passed into a container. Practical exploitation requires the vulnerable driver to be loaded and the attacker to have write access to the device node.

## 4. Severity interpretation

This is not an ordinary Moderate resource leak or warning. It is a concrete kernel slab out of bounds write, but the overwrite is small and occurs in a device specific transfer buffer, so the conservative interpretation is Actionable Moderate. The paranoid interpretation is Strong Important candidate because local attacker influenced heap corruption in kernel space can sometimes be converted into privilege escalation even when the initial report only demonstrates KASAN corruption or crash.

## 5. One-sentence report phrase

A local user with write access to a kl5kusb105 USB serial tty can trigger a 2 byte attacker influenced slab out of bounds write in the bulk out transmit buffer, causing at least kernel crash risk and requiring manual review for possible privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should be manually reviewed because the patch fixes a real kernel slab OOB write with attacker influenced data, not merely a validation error or DoS-only condition.

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

Patch: USB: serial: kl5kusb105: fix bulk-out buffer overflow
Commit: 60af1fd82983c26604102e63a3fcc822c186cceb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=60af1fd82983c26604102e63a3fcc822c186cceb

Commit description:

klsi_105_prepare_write_buffer() is called by the generic write path
with the bulk-out buffer and its size (bulk_out_size, 64 bytes). It
stores a two-byte length header at the start of the buffer and copies
the payload from the write fifo starting at buf + KLSI_HDR_LEN, but
passes the full buffer size as the number of bytes to copy:

  count = kfifo_out_locked(&port->write_fifo, buf + KLSI_HDR_LEN,
                           size, &port->lock);

When the fifo holds at least size bytes, size bytes are copied starting
two bytes into the size-byte buffer, writing KLSI_HDR_LEN bytes past its
end. Copy at most size - KLSI_HDR_LEN bytes instead, leaving room for
the header as safe_serial already does.

Writing bulk_out_size or more bytes to the tty triggers a slab
out-of-bounds write, observed with KASAN by emulating the device with
dummy_hcd and raw-gadget:

  BUG: KASAN: slab-out-of-bounds in kfifo_copy_out+0x83/0xc0
  Write of size 64 at addr ffff888112c62202 by task python3
   kfifo_copy_out
   klsi_105_prepare_write_buffer [kl5kusb105]
   usb_serial_generic_write_start [usbserial]
  Allocated by task 139:
   usb_serial_probe [usbserial]
  The buggy address is located 2 bytes inside of allocated 64-byte region

The out-of-bounds write no longer occurs with this change applied.

Fixes: 60b3013 ("USB: kl5usb105: reimplement using generic framework")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: HyeongJun An <sammiee5311@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/serial/kl5kusb105.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=60af1fd82983c26604102e63a3fcc822c186cceb

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

The original Linux kernel CVE announcement for CVE-2026-53194 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53194

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:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).