On 10/21, Tetsuo Handa wrote:
>
> 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
Well, if "task" is stable (can't exit), then I think
rcu_dereference(task->real_parent)
is fine, we know that ->real_parent did not pass exit_notif() yet.
However, task_is_descendant() looks unnecessarily complicated, it could be
static int task_is_descendant(struct task_struct *parent,
struct task_struct *child)
{
int rc = 0;
struct task_struct *walker;
if (!parent || !child)
return 0;
rcu_read_lock();
for (walker = child; walker->pid; walker =
rcu_dereference(walker->real_parent))
if (same_thread_group(parent, walker)) {
rc = 1;
break;
}
rcu_read_unlock();
return rc;
}
And again, I do not know how/if yama ensures that child is rcu-protected,
perhaps
task_is_descendant() needs to check pid_alive(child) right after
rcu_read_lock() ?
Oleg.
