Re: perf: use-after-free in perf_release

[Oleg Nesterov]

On 03/14, Peter Zijlstra wrote:
>
> 	    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?

Yes, exactly.

> Let me see if it makes sense to retain perf_event_free_task() at all;
> maybe we should always do perf_event_exit_task().

Yes, perhaps... but this needs changes too. Say, WARN_ON_ONCE(child != current)
in perf_event_exit_task_context(). And even perf_event_task(new => F) does not
look right in this case. In fact it would be simply buggy to do this, this task
was not fully constructed yet, so even perf_event_pid(task) is not safe.

Oleg.