Re: [PATCH RFC] sched: Revert delayed_put_task_struct() and fix use after free

[Kirill Tkhai]

Yeah, you're sure about initial patch. Thanks for signal explanation.

On 15.10.2014 23:40, Oleg Nesterov wrote:
> On 10/15, Oleg Nesterov wrote:
>>
>> On 10/15, Kirill Tkhai wrote:
>>>
>>> Regarding to scheduler this may be a reason of use-after-free.
>>>
>>>     task_numa_compare()                    schedule()
>>>         rcu_read_lock()                        ...
>>>         cur = ACCESS_ONCE(dst_rq->curr)        ...
>>>             ...                                rq->curr = next;
>>>             ...                                    context_switch()
>>>             ...                                        finish_task_switch()
>>>             ...                                            put_task_struct()
>>>             ...                                                __put_task_struct()
>>>             ...                                                    free_task_struct()
>>>             task_numa_assign()                                     ...
>>>                 get_task_struct()                                  ...
>>
>> Agreed. I don't understand this code (will try to take another look later),
>> but at first glance this looks wrong.
>>
>> At least the code like
>>
>> 	rcu_read_lock();
>> 	get_task_struct(foreign_rq->curr);
>> 	rcu_read_unlock();
>>
>> is certainly wrong. And _probably_ the problem should be fixed here. Perhaps
>> we can add try_to_get_task_struct() which does atomic_inc_not_zero() ...
> 
> Yes, but perhaps in this particular case another simple fix makes more
> sense. The patch below needs a comment to explain that we check PF_EXITING
> because:
> 
> 	1. It doesn't make sense to migrate the exiting task. Although perhaps
> 	   we could check ->mm == NULL instead.
> 
> 	   But let me repeat that I do not understand this code, I am not sure
> 	   we can equally treat is_idle_task() and PF_EXITING here...
> 
> 	2. If PF_EXITING is not set (or ->mm != NULL) then delayed_put_task_struct()
> 	   won't be called until we drop rcu_read_lock(), and thus get_task_struct()
> 	   is safe.
> 

Cool! Elegant fix. We set PF_EXITING in exit_signals(), which is earlier
than release_task() is called.

Shouldn't we use smp_rmb/smp_wmb here?

> And. it seems that there is another problem? Can't task_h_load(cur) race
> with itself if 2 CPU's call task_numa_migrate() and inspect the same rq
> in parallel? Again, I don't understand this code, but update_cfs_rq_h_load()
> doesn't look "atomic". In fact I am not even sure about task_h_load(env->p),
> p == current but we do not disable preemption.
> 
> What do you think?

We use it completely unlocked, so nothing good is here. Also we work
with pointers.

As I understand in update_cfs_rq_h_load() we go from bottom to top,
and then from top to bottom. We set cfs_rq::h_load_next to be able
to do top-bottom passage (top is a root of "tree").

Yeah, this "way" may be overwritten by competitor. Also, task may change
its cfs_rq.

> --- x/kernel/sched/fair.c
> +++ x/kernel/sched/fair.c
> @@ -1165,7 +1165,7 @@ static void task_numa_compare(struct tas
>  
>  	rcu_read_lock();
>  	cur = ACCESS_ONCE(dst_rq->curr);
> -	if (cur->pid == 0) /* idle */
> +	if (is_idle_task(cur) || (curr->flags & PF_EXITING))
>  		cur = NULL;
>  
>  	/*
> 

Looks like, we have to use the same fix for task_numa_group().

grp = rcu_dereference(tsk->numa_group);

Below we dereference grp->nr_tasks.

Also, the same in rt.c and deadline.c, but we do no take second
reference there. Wrong pointer dereference is not possible there,
not so bad.

Kirill
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