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

On 2018/10/26 23:39, Oleg Nesterov wrote:

On 10/26, Tetsuo Handa wrote:

quoted

Suppose p1 == p2->real_parent and p2 == p3->real_parent, and p1 exited
when someone tried to attach on p2, p2->real_parent was pointing to already
(or about to be) freed p1.

I don't see a difference.

If p1 exits it will re-parent p2, p2->real_parent will be updated.

quoted

So, the puzzle part is why p2->real_parent was still pointing p1 even after
p1 was freed...

I don't understand the question.

Once again. TASK->real_parent can point to the freed mem only if a) TASK exits,
and b) _after_ that its parent TASK->real_parent exits too.

Oh, p2 exited and then p1 also exited when someone tried to attach on p2.
Then, p2->real_parent can point to already (or about to be) freed p1.

quoted

quoted

Again, did you read my previous email?

Yes. But I still can't be convinced that pid_alive() test helps.

Well, I don't understand which part of my explanations is not clear to you.

OK. Checking pid_alive() should help.

(By the way, if p->real_parent were updated to point to init_task when p exits,
we could omit pid_alive() check?)