[BUG 6.4] sched/core: Possible buffer overflow in do_set_cpu_allowed

[Joe Korty]

In commit 9a5418bc48bab ("sched/core: Use kfree_rcu() in
do_set_cpus_allowed()"), a kfree_rcu() is used to free a cpu mask.
However, cpu masks can be as short as 8 bytes and this is a problem,
as kfree_rcu requires the to-be freed buffer to be at least 16 bytes.
Thus there is a chance of buffer overflow corruption when the number of
possible cpus in the system is 64 or less.

I have not seen this corruption in the wild.  I only noticed this possibility
when reviewing the scheduler differences between 6.1 and 6.4.

Regards,
Joe Korty