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.
