KASAN: use-after-free Read in task_is_descendant

[Tetsuo Handa]

On 2018/10/21 16:10, syzbot wrote:
> BUG: KASAN: use-after-free in __read_once_size include/linux/compiler.h:188 [inline]
> BUG: KASAN: use-after-free in task_is_descendant.part.2+0x610/0x670 security/yama/yama_lsm.c:295
> Read of size 8 at addr ffff8801c4666b20 by task syz-executor3/12722
> 
> CPU: 1 PID: 12722 Comm: syz-executor3 Not tainted 4.19.0-rc8+ #70
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x1c4/0x2b4 lib/dump_stack.c:113
>  print_address_description.cold.8+0x9/0x1ff mm/kasan/report.c:256
>  kasan_report_error mm/kasan/report.c:354 [inline]
>  kasan_report.cold.9+0x242/0x309 mm/kasan/report.c:412
>  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
>  __read_once_size include/linux/compiler.h:188 [inline]
>  task_is_descendant.part.2+0x610/0x670 security/yama/yama_lsm.c:295

Do we need to hold

  write_lock_irq(&tasklist_lock);

rather than

  rcu_read_lock();

when accessing

  "struct task_struct"->real_parent

?