wireguard: queueing: always return valid online CPU in wg_cpumask_choose_online()

The function gets number of online CPUS, and uses it to search for
Nth cpu in cpu_online_mask.

If id == num_online_cpus() - 1, and one CPU gets offlined between
calling num_online_cpus() -> cpumask_nth(), there's a chance for
cpumask_nth() to find nothing and return >= nr_cpu_ids.

The caller code in __queue_work() tries to avoid that by checking the
returned CPU against WORK_CPU_UNBOUND, which is NR_CPUS. It's not the
same as '>= nr_cpu_ids'. On a typical Ubuntu desktop, NR_CPUS is 8192,
while nr_cpu_ids is the actual number of possible CPUs, say 8.

The non-existing cpu may later be passed to rcu_dereference() and
corrupt the logic. Fix it by switching from 'if' to 'while'.

Suggested-by: Jason A. Donenfeld <[e-mail]>
Signed-off-by: Yury Norov (NVIDIA) <[e-mail]>
Signed-off-by: Jason A. Donenfeld <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/wireguard/queueing.h

···
 {
 	unsigned int cpu = *stored_cpu;
 
-	if (unlikely(cpu >= nr_cpu_ids || !cpu_online(cpu)))
+	while (unlikely(cpu >= nr_cpu_ids || !cpu_online(cpu)))
 		cpu = *stored_cpu = cpumask_nth(id % num_online_cpus(), cpu_online_mask);
 
 	return cpu;