exit: hold a reference to thread_pid across proc_flush_pid

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

exit: hold a reference to thread_pid across proc_flush_pid

Commit 0a36bad01731 ("release_task: kill the no longer needed
get/put_pid(thread_pid)") removed the reference around proc_flush_pid().
It assumed that free_pids(post.pids) at the end of release_task() would
keep thread_pid alive until then.

That assumption is wrong. __change_pid() only records a detached PID in
post.pids when pid_has_task() is false for every PIDTYPE. If another task
still uses the exiting task's PID as its process group or session ID,
__unhash_process() removes the exiting task's PIDTYPE_PID link but leaves
the PID out of post.pids. release_task() therefore holds no reference to
it after dropping tasklist_lock.

The other task can then remove the remaining PIDTYPE links. Its
free_pids() call schedules delayed_put_pid(), and the RCU callback can free
the PID before the first release_task() reaches proc_flush_pid().

An unprivileged reproducer races wait4(-1) against setsid() to trigger this
ordering. Three of three fresh v7.2 KASAN boots reported:

BUG: KASAN: slab-use-after-free in
proc_invalidate_siblings_dcache+0x3e2/0x3f0
Read of size 8 by task h7_pid_reaper/1921

Call Trace:
proc_invalidate_siblings_dcache
release_task
wait_consider_task
__do_wait
do_wait
kernel_wait4

Freed by task 0:
kmem_cache_free
put_pid
delayed_put_pid
rcu_core

Last potentially related work creation:
__call_rcu_common
free_pids
ksys_setsid

KASAN identified a 144-byte object from the pid cache and located the bad
read 80 bytes into the freed object, matching pid->inodes. With an
explicit reference, three of three fresh boots completed without a KASAN
report. The concurrent RCU callback dropped its reference while
proc_flush_pid() was protected, and the balancing put_pid() performed the
final free afterward.

Take a reference before __unhash_process() clears p->thread_pid and release
it after proc_flush_pid() completes.

A tested source reproducer is available privately on request. No
controlled read or write, information leak, or privilege escalation is
claimed. The mainline patch applies directly to v6.19.y and newer;
v6.16.y through v6.18.y need a context-adjusted backport.