CVE-2026-99218: coredump: hold RCU while releasing parked threads

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

coredump: hold RCU while releasing parked threads

coredumpfinish() releases the parked threads by clearing ->task in their corethread entry and calling wakeupprocess() on each of them. It does that without holding a reference on the task and without being inside rcu.

But calling wakeupprocess(task) without rcu here isn't safe. A parked thread doesn't need that wakeup to leave. A spurious wakeup or a preemption after the store is enough for the task to go away.

So if the coredump client is preempted between the store and wakeupprocess() the thread can exit and be freed in the meantime and trytowakeup() takes pilock in freed memory.

Hold rcu across the loop.