On Tue, Mar 14, 2017 at 04:02:41PM +0100, Peter Zijlstra wrote:
> On Tue, Mar 14, 2017 at 03:30:11PM +0100, Oleg Nesterov wrote:
>
> > 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).
>
> Right; the child_list is the only thing that is exposed. And yes, it
> looks like that can interleave just right.
>
> > Then it does put_ctx(ctx), but ctx->task can be already freed by
> > copy_process()->free_task() in this case.
>
>
> Task1 Task2
>
> fork()
> perf_event_init_task()
> /* ... */
> goto bad_fork_$foo;
> /* ... */
> perf_event_free_task()
> mutex_lock(ctx->lock)
> perf_free_event(B)
>
> perf_event_release_kernel(A)
> mutex_lock(A->child_mutex)
> list_for_each_entry(child, ...) {
> /* child == B */
> ctx = B->ctx;
> get_ctx(ctx);
> mutex_unlock(A->child_mutex);
>
> mutex_lock(A->child_mutex)
> list_del_init(B->child_list)
> mutex_unlock(A->child_mutex)
>
> /* ... */
>
> mutex_unlock(ctx->lock);
> put_ctx() /* >0 */
> free_task();
> mutex_lock(ctx->lock);
> mutex_lock(A->child_mutex);
> /* ... */
> mutex_unlock(A->child_mutex);
> mutex_unlock(ctx->lock)
> put_ctx() /* 0 */
> ctx->task && !TOMBSTONE
> put_task_struct() /* UAF */
>
>
> Something like that, right?
>
>
> Let me see if it makes sense to retain perf_event_free_task() at all;
> maybe we should always do perf_event_exit_task().
Do we want a WARN_ON_ONCE(atomic_read(&tsk->usage)); in free_task()?
Because in the above scenario we're freeing it with references on.
