CVE-2026-63942

In the Linux kernel, the following vulnerability has been resolved:

parport: Fix race between port and client registration

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.

Weakness

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/15b1723c1472e802f9f7e69ae4e64f7dbf588848
• https://git.kernel.org/stable/c/290f515c5e3b3900bc2fe24f179999fd08d23bfa
• https://git.kernel.org/stable/c/51026cff1f4f3b762a0b5a07c727bd59cef45320
• https://git.kernel.org/stable/c/74d6aae1df45d3414178986be743f946988fddf6
• https://git.kernel.org/stable/c/a1e81b58da0179531bedf0b9f2811f5f992d5c4b
• https://git.kernel.org/stable/c/d16548be2ea5058227d79799e81dab61c9bca8ec
• https://git.kernel.org/stable/c/ef15ccbb3e8640a723c42ad90eaf81d66ae02017
• https://git.kernel.org/stable/c/f3378b0d7bd4605de89b083b2900788157a181cc