[CVE-2026-63942][MODERATE REGULAR] parport: Fix race between port and client registration

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

CVE: CVE-2026-63942
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: parport: Fix race between port and client registration
Commit: f3378b0d7bd4605de89b083b2900788157a181cc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f3378b0d7bd4605de89b083b2900788157a181cc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63942
Analysis date: Sun, 19 Jul 2026 22:12:31 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline manual review recommended
Manual review required: YES

Summary:
A parport registration race can allow client drivers such as lp to attach to a port before it is fully announced or while it is being removed, causing a local kernel crash under module load timing conditions.

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

The original Linux kernel CVE announcement for CVE-2026-63942 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63942

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-2026-63942	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-362;CWE-665;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'parport has a registration race where a client driver such as lp can attach to a port after the device object is visible but before the port is fully announced, or while the port is being removed. This can let client callbacks operate on incomplete or unstable port state and has been observed as a boot time crash when parport and client modules are loaded together. For the CVSS the PR:L is used in the paranoid score because a lower privileged local actor may be able to influence module autoload or a delegated service path in some deployments, although reliable reproduction normally requires module load and unload control. The issue is not network reachable. Impact is at least local denial of service via kernel crash and in worst case may allow limited confidentiality or integrity impact if the lifecycle race exposes stale port state, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended (with actual score 2)	YES	DANGER RACE HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The commit reports an occasional crash when parport and client modules are loaded around the same time.
* Real lifetime / initialization-state race concern: +1. Client drivers can attach before the port is fully initialized or while it is being torn down.
* Hard or unreliable race / special timing required: -1. The trigger depends on a registration race and module load timing.
* Requires admin/root/CAP_SYS_MODULE in typical deployments: -2. The demonstrated reproducer uses modprobe and rmmod.
* Legacy / non-default execution mode: -1. parport is a legacy hardware-oriented subsystem and the issue depends on relevant modules and parallel-port style setup.

Paranoid additions / differences:

* Local unprivileged trigger: +1. In some deployments, module autoloading or delegated service paths might allow a lower-privileged local actor to influence driver loading indirectly.
* Weak/indirect corruption candidate: +1. The race can expose client callbacks to incomplete or teardown port state, but no attacker-controlled reclaim, overwrite, callback hijack, or concrete UAF primitive is shown.

No strong memory corruption or strong LPE signal is awarded.

## 3. Reachability analysis

The realistic trigger is local and usually privileged, requiring control over parport and client module loading or unloading. Namespace/container impact is limited unless a container or service is delegated enough device/module management to influence parport autoloading, which is not typical. The path is not network reachable. The affected subsystem is not a broad default attack surface on modern systems, but it can be present during boot or on systems with parport/lp modules and PC-style parallel port support.

Call-site confidence: high. The patch shows the direct attach path through port_check(), parport_announce_port(), and parport_remove_port().

## 4. Severity interpretation

This behaves more like an ordinary Moderate / borderline manual-review issue than an Important-class vulnerability. Theoretical impact beyond DoS exists only as a weak lifecycle-state concern because clients may see incomplete or tearing-down objects. Realistic evidence points to kernel crash under a timing race, not privilege escalation or attacker-controlled memory corruption.

## 5. One-sentence report phrase

A parport registration race can allow client drivers such as lp to attach to a port before it is fully announced or while it is being removed, causing a local kernel crash under module load timing conditions.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a lifecycle race involving partially initialized or teardown state, but the demonstrated trigger is local, timing-dependent, usually privileged, non-network-reachable, and does not show a concrete UAF reclaim or privilege-escalation primitive.

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

Patch: parport: Fix race between port and client registration
Commit: f3378b0d7bd4605de89b083b2900788157a181cc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f3378b0d7bd4605de89b083b2900788157a181cc

Commit description:

The parport subsystem registers port devices before they are fully
initialised, resulting in a race condition where client drivers such
as lp can attach to ports that are not completely initialised or even
being torn down.

When the port and client drivers are built as modules and loaded
around the same time during boot, this occasionally results in a
crash.  I was able to make this happen reliably in a VM with a
PC-style parallel port by patching parport_pc to fail probing:

> --- a/drivers/parport/parport_pc.c
> +++ b/drivers/parport/parport_pc.c
> @@ -2069,7 +2069,7 @@ static struct parport *__parport_pc_probe_port(unsigned long int base,
>  	if (!p)
>  		goto out3;
>
> -	base_res = request_region(base, 3, p->name);
> +	base_res = NULL;
>  	if (!base_res)
>  		goto out4;
>

and then running:

    while true; do
        modprobe lp & modprobe parport_pc
	wait
	rmmod lp parport_pc
    done

for a few seconds.

In the long term I think port registration should be changed to put
the call to device_add() inside parport_announce_port(), but since the
latter currently cannot fail this will require changing all port
drivers.

For now, add a flag to indicate whether a port has been "announced"
and only try to attach client drivers to ports when the flag is set.

Fixes: 6fa45a2 ("parport: add device-model to parport subsystem")
Closes: https://bugs.debian.org/1130365
Closes: https://lore.kernel.org/all/6ba903ad-9897-42bb-8c2d-337385cc3746@molgen.mpg.de/
Cc: stable <stable@kernel.org>
Signed-off-by: Ben Hutchings <benh@debian.org>
Acked-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Link: https://patch.msgid.link/afo6uBv68GDevbMD@decadent.org.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/parport/parport_pc.c
  drivers/parport/share.c
  include/linux/parport.h

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=f3378b0d7bd4605de89b083b2900788157a181cc

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

The original Linux kernel CVE announcement for CVE-2026-63942 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63942

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:H/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: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: 1
  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).