Re: [PATCH cgroup/for-4.4-fixes] cgroup: make css_set pin its css's to avoid use-afer-free

[Tejun Heo <tj@kernel.org>]

On Mon, Nov 23, 2015 at 02:55:41PM -0500, Tejun Heo wrote:
> 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, afcf6c8b7544 ("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: afcf6c8b7544 ("cgroup: add cgroup_subsys->free() method and use it to fix pids controller")

Applied to cgroup/for-4.4-fixes.

-- 
tejun