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.
