exit: reparent: fix the cross-namespace PR_SET_CHILD_SUBREAPER reparenting

find_new_reaper() assumes that "has_child_subreaper" logic is safe as
long as we are not the exiting ->child_reaper and this is doubly wrong:

1. In fact it is safe if "pid_ns->child_reaper == father"; there must
   be no children after zap_pid_ns_processes() returns, so it doesn't
   matter what we return in this case and even pid_ns->child_reaper is
   wrong otherwise: we can't reparent to ->child_reaper == current.

   This is not a bug, but this is confusing.

2. It is not safe if we are not pid_ns->child_reaper but from the same
   thread group. We drop tasklist_lock before zap_pid_ns_processes(),
   so another thread can lock it and choose the new reaper from the
   upper namespace if has_child_subreaper == T, and this is obviously
   wrong.

   This is not that bad, zap_pid_ns_processes() won't return until the
   the new reaper reaps all zombies, but this should be fixed anyway.

We could change for_each_thread() loop to use ->exit_state instead of
PF_EXITING which we had to use until 8aac6270, or we could change
copy_signal() to check CLONE_NEWPID before setting has_child_subreaper,
but lets change this code so that it is clear we can't look outside of
our namespace, otherwise same_thread_group(reaper, child_reaper) check
will look wrong and confusing anyway.

We can simply start from "father" and fix the problem. We can't wrongly
return a thread from the same thread group if ->is_child_subreaper == T,
we know that all threads have PF_EXITING set.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Sterling Alexander <stalexan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>