Re: uprobes/perf: KASAN: use-after-free in uprobe_perf_close

[Oleg Nesterov <oleg@redhat.com>]

On 04/09, Peter Zijlstra wrote:
>
> On Tue, Mar 06, 2018 at 06:49:10PM +0900, Prashant Bhole wrote:
> > Sorry for late reply. I tried these changes. It didn't fix the problem. With
>
> He, sorry for completely forgetting about this one :/

me too, sorry Prashant,

> > these changes, the use-after-free access of task_struct occurs at
> > _free_event() for the last remaining event.

Heh, I didn't even try to compile the "patch" I sent, I am not surprised it is
not correct. But unless I forget again, I'll try to make the working version.

> > In your changes, I tried keeping get/put_task_struct() in
> > perf_alloc_context()/put_ctx() intact and The problem did not occur. Change
> > are mentioned below.
>
> Yes, I think you're right in that this is the cleanest solution; it add
> reference counting to the exact pointer we're using.

OK, agreed, lets make the minimal fix for now.

But I still think that we should (try to) remove put_task_struct() from put_ctx().

Quite possibly I missed something, but I think it only adds some confusion. Once
again, even if ctx can't go away you can't use ctx->task without TASK_TOMBSTONE
check, exactly because this task can exit. So why perf_event_context should add
another reference?

Oleg.