Re: perf: use-after-free in perf_release

[Oleg Nesterov]

On 03/14, Peter Zijlstra wrote:
>
> On Tue, Mar 14, 2017 at 03:03:02PM +0100, Oleg Nesterov wrote:
> > On 03/14, Peter Zijlstra wrote:
> > >
> > > Yes, this looks buggy. But I cannot explain how that would result in the
> > > observed use-after-free.
> >
> > Yes...
> >
> > Suppose that copy_process() fails after perf_event_init_task(). In this
> > case perf_event_free_task() does put_ctx(), but if this ctx has another
> > reference (ctx->refcount > 1) then ctx->task will point to the already
> > freed task, copy_process() does free_task() at the end of error path.
> > And we can't replace it with put_task_struct().
> >
> > I am looking at TASK_TOMBSTONE, perhaps perf_event_free_task() should
> > use it too?
>
> The idea was that the task isn't visible when we use
> perf_event_free_task(). But I'll have a look.

I can be easily wrong, I do not understans this code.

But. perf_event_init_task() adds child_event to parent_event->child_list.

If perf_event_release_kernel(parent_event) is called before copy_process()
does perf_event_free_task() which (in particular) removes it from child_list,
perf_event_release_kernel() can find this child_event and do get_ctx(ctx)
(under the list_for_each_entry(child, &event->child_list, child_list) loop).

Then it does put_ctx(ctx), but ctx->task can be already freed by
copy_process()->free_task() in this case.

No?

Oleg.