A css_set represents the relationship between a set of tasks and
css's.  css_set never pinned the associated css's.  This was okay
because tasks used to always disassociate immediately (in RCU sense) -
either a task is moved to a different css_set or exits and never
accesses css_set again.

Unfortunately, afcf6c8b

 ("cgroup: add cgroup_subsys->free() method
and use it to fix pids controller") and patches leading up to it made
a zombie hold onto its css_set and deref the associated css's on its
release.  Nothing pins the css's after exit and it might have already
been freed leading to use-after-free.

 general protection fault: 0000 [#1] PREEMPT SMP
 task: ffffffff81bf2500 ti: ffffffff81be4000 task.ti: ffffffff81be4000
 RIP: 0010:[<ffffffff810fa205>]  [<ffffffff810fa205>] pids_cancel.constprop.4+0x5/0x40
 ...
 Call Trace:
  <IRQ>
  [<ffffffff810fb02d>] ? pids_free+0x3d/0xa0
  [<ffffffff810f8893>] cgroup_free+0x53/0xe0
  [<ffffffff8104ed62>] __put_task_struct+0x42/0x130
  [<ffffffff81053557>] delayed_put_task_struct+0x77/0x130
  [<ffffffff810c6b34>] rcu_process_callbacks+0x2f4/0x820
  [<ffffffff810c6af3>] ? rcu_process_callbacks+0x2b3/0x820
  [<ffffffff81056e54>] __do_softirq+0xd4/0x460
  [<ffffffff81057369>] irq_exit+0x89/0xa0
  [<ffffffff81876212>] smp_apic_timer_interrupt+0x42/0x50
  [<ffffffff818747f4>] apic_timer_interrupt+0x84/0x90
  <EOI>
 ...
 Code: 5b 5d c3 48 89 df 48 c7 c2 c9 f9 ae 81 48 c7 c6 91 2c ae 81 e8 1d 94 0e 00 31 c0 5b 5d c3 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 <f0> 48 83 87 e0 00 00 00 ff 78 01 c3 80 3d 08 7a c1 00 00 74 02
 RIP  [<ffffffff810fa205>] pids_cancel.constprop.4+0x5/0x40
  RSP <ffff88001fc03e20>
 ---[ end trace 89a4a4b916b90c49 ]---
 Kernel panic - not syncing: Fatal exception in interrupt
 Kernel Offset: disabled
 ---[ end Kernel panic - not syncing: Fatal exception in interrupt

Fix it by making css_set pin the associate css's until its release.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Dave Jones <davej@codemonkey.org.uk>
Reported-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Link: http://lkml.kernel.org/g/20151120041836.GA18390@codemonkey.org.uk
Link: http://lkml.kernel.org/g/5652D448.3080002@bmw-carit.de
Fixes: afcf6c8b ("cgroup: add cgroup_subsys->free() method and use it to fix pids controller")