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().
