[CVE-2026-64336][MODERATE REGULAR] USB: serial: keyspan_pda: fix information leak

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

CVE: CVE-2026-64336
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: USB: serial: keyspan_pda: fix information leak
Commit: b069b7029862fafaff331d4c664d97d4ae828d6d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b069b7029862fafaff331d4c664d97d4ae828d6d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64336
Analysis date: Sat, 25 Jul 2026 13:48:57 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline manual review recommended / Ordinary Moderate with actionable info leak concern
Manual review required: YES

Summary:
A local information leak in the USB serial keyspan_pda write path can cause bytes beyond the tty write buffer to be sent to the attached serial device when the driver reports more accepted bytes than were actually provided.

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

The original Linux kernel CVE announcement for CVE-2026-64336 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64336

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-2026-64336	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:N/A:N';*CWE-200;CWE-125;CWE-201;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.5';DESCR 'An information leak in the USB serial keyspan_pda write path can occur because keyspan_pda_write_start() may report more bytes accepted than were provided by the current tty write buffer. The tty layer may then continue writing data from beyond that buffer, causing adjacent memory contents to be sent to the USB serial device. For the CVSS the PR:L is used because triggering requires a local user or process with permission to write to the affected ttyUSB device, not remote network access. The issue is not network reachable and normally requires access to the local serial device or to the receiving side of the USB serial link. Impact is confidentiality loss only in the base assessment, but the paranoid score uses C:H because repeated kernel memory disclosure can expose sensitive data or kernel addresses and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Ordinary Moderate with actionable info leak concern (with actual score 3)	YES	WRITE OOB USB PRINTF-TTY SIMPLEFIX LEAK HARDWARE LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  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 / Ordinary Moderate with actionable info leak concern**::

## 2. Signal breakdown

* Local unprivileged trigger: +1
  A local user or process with permission to write to the affected ttyUSB device can trigger the path.

* Firmware-mediated or device-mediated external sink: +1 in paranoid score
  The leaked bytes are sent to the attached USB serial device or serial peer. This is not network remote, but the device side can observe the leaked data.

* Confidentiality impact plausible: +1
  The tty layer may read beyond the provided write buffer and send adjacent memory contents to the device.

* Rare and device-specific exposure: -1 in conservative score
  The issue is limited to the keyspan_pda USB serial driver and requires the affected device or driver path to be present.

* No generic memory corruption: +0
  The patch supports an out-of-bounds read / information disclosure primitive, not UAF, OOB write, double free, type confusion, or arbitrary write.

* No privilege escalation signal: +0
  There is no supported path to LPE from the patch. The realistic impact is confidentiality disclosure.

## 3. Reachability analysis

The bug is reachable through the local tty write path for the affected USB serial device. It is not network reachable. Typical triggering requires local access to the device node, often controlled by permissions such as root, dialout group, or an application that writes to the serial device. Containers or namespaces do not automatically make this reachable unless the USB serial device node is passed through. The practical exposure is narrower than common kernel subsystems because it depends on a specific USB serial driver and device, but the leak is real when the path is present.

Call-site confidence: high. The commit message directly describes the tty line discipline continuing to write from beyond the tty write buffer due to the wrong callback return value.

## 4. Severity interpretation

This behaves like an ordinary Moderate issue with a real information leak, not an Important-class memory corruption issue. The theoretical risk is repeated kernel memory disclosure to the serial device, which may expose sensitive bytes or kernel addresses. Realistic exploitation is constrained by local device access and specific hardware or driver usage. It should not be treated as LPE or generic memory corruption based on the provided patch.

## 5. One-sentence report phrase

A local information leak in the USB serial keyspan_pda write path can cause bytes beyond the tty write buffer to be sent to the attached serial device when the driver reports more accepted bytes than were actually provided.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a concrete kernel memory disclosure to an external device path, even though it is device-specific, local, and does not show memory corruption or privilege escalation.

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

Patch: USB: serial: keyspan_pda: fix information leak
Commit: b069b7029862fafaff331d4c664d97d4ae828d6d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b069b7029862fafaff331d4c664d97d4ae828d6d

Commit description:

The write() callback is supposed to return the number of characters
accepted or a negative errno. Since the addition of write fifo support
the keyspan_pda implementation will however return the number characters
submitted to the device if the write urb is not already in use. If this
number is larger than the number of characters passed to write(), the
line discipline continues writing data from beyond the tty write buffer.

Fix the information leak by making sure that keyspan_pda_write_start()
returns zero on success as intended.

Fixes: 034e38e ("USB: serial: keyspan_pda: add write-fifo support")
Cc: stable@vger.kernel.org	# 5.11
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/serial/keyspan_pda.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=b069b7029862fafaff331d4c664d97d4ae828d6d

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

The original Linux kernel CVE announcement for CVE-2026-64336 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64336

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:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS score: 5.5

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).