CVE-2026-99244: pid_namespace: prevent TIF_NOTIFY_SIGNAL from interrupting the reaper

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

pidnamespace: prevent TIFNOTIFYSIGNAL from interrupting the reaper

Since commit 7fea700e04bd ("zappidnsprocesses: clear TIFNOTIFYSIGNAL along with TIFSIGPENDING") zappidnsprocesses() clears TIFNOTIFYSIGNAL before every kernelwait4() call. Otherwise kernelwait4() doesn't sleep and returns -ERESTARTSYS if signalpending() is true and the reaper busy-waits.

But clearing TIFNOTIFYSIGNAL doesn't fix it. It can get raised again. For example, klpsendsignals() sets it raw against any task that hasn't transitioned yet. And it keeps sending it until the livepatch finishes.

The sleep loop waiting for pidallocated to drop has the same problem and it doesn't even clear anything. So a schedule() in TASKINTERRUPTIBLE returns immediately.

Use a nonotifysignal guard. signalpending() will ignore TIFNOTIFYSIGNAL and the pid namespace reaper sleeps until a child exits or freepid() wakes it.