Re: KASAN: use-after-free Read in task_is_descendant

[Tetsuo Handa]

On 2018/10/22 18:54, Oleg Nesterov wrote:
> 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:
>>> Â__dump_stack lib/dump_stack.c:77 [inline]
>>> Âdump_stack+0x1c4/0x2b4 lib/dump_stack.c:113
>>> Âprint_address_description.cold.8+0x9/0x1ff mm/kasan/report.c:256
>>> Âkasan_report_error mm/kasan/report.c:354 [inline]
>>> Âkasan_report.cold.9+0x242/0x309 mm/kasan/report.c:412
>>> Â__asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
>>> Â__read_once_size include/linux/compiler.h:188 [inline]
>>> Âtask_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.

OK.

> 
> 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() ?

Since the caller (ptrace() path) called get_task_struct(child), child itself can't be
released. Do we still need pid_alive(child) ?