CVE-2026-63942: parport: Fix race between port and client registration

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 parportpc to fail probing:

--- a/drivers/parport/parportpc.c +++ b/drivers/parport/parportpc.c @@ -2069,7 +2069,7 @@ static struct parport parportpcprobeport(unsigned long int base, if (!p) goto out3; - baseres = requestregion(base, 3, p->name); + baseres = NULL; if (!baseres) goto out4;

and then running:

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

for a few seconds.

In the long term I think port registration should be changed to put the call to deviceadd() inside parportannounceport(), 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.