From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751644AbaJOToT (ORCPT ); Wed, 15 Oct 2014 15:44:19 -0400 Received: from mx1.redhat.com ([209.132.183.28]:26431 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751586AbaJOToS (ORCPT ); Wed, 15 Oct 2014 15:44:18 -0400 Date: Wed, 15 Oct 2014 21:40:44 +0200 From: Oleg Nesterov To: Kirill Tkhai Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Ingo Molnar , Vladimir Davydov , Kirill Tkhai Subject: Re: [PATCH RFC] sched: Revert delayed_put_task_struct() and fix use after free Message-ID: <20141015194044.GA4557@redhat.com> References: <1413376300.24793.55.camel@tkhai> <20141015150641.GA2755@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141015150641.GA2755@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/15, Oleg Nesterov wrote: > > On 10/15, Kirill Tkhai wrote: > > > > Regarding to scheduler this may be a reason of use-after-free. > > > > task_numa_compare() schedule() > > rcu_read_lock() ... > > cur = ACCESS_ONCE(dst_rq->curr) ... > > ... rq->curr = next; > > ... context_switch() > > ... finish_task_switch() > > ... put_task_struct() > > ... __put_task_struct() > > ... free_task_struct() > > task_numa_assign() ... > > get_task_struct() ... > > Agreed. I don't understand this code (will try to take another look later), > but at first glance this looks wrong. > > At least the code like > > rcu_read_lock(); > get_task_struct(foreign_rq->curr); > rcu_read_unlock(); > > is certainly wrong. And _probably_ the problem should be fixed here. Perhaps > we can add try_to_get_task_struct() which does atomic_inc_not_zero() ... Yes, but perhaps in this particular case another simple fix makes more sense. The patch below needs a comment to explain that we check PF_EXITING because: 1. It doesn't make sense to migrate the exiting task. Although perhaps we could check ->mm == NULL instead. But let me repeat that I do not understand this code, I am not sure we can equally treat is_idle_task() and PF_EXITING here... 2. If PF_EXITING is not set (or ->mm != NULL) then delayed_put_task_struct() won't be called until we drop rcu_read_lock(), and thus get_task_struct() is safe. And. it seems that there is another problem? Can't task_h_load(cur) race with itself if 2 CPU's call task_numa_migrate() and inspect the same rq in parallel? Again, I don't understand this code, but update_cfs_rq_h_load() doesn't look "atomic". In fact I am not even sure about task_h_load(env->p), p == current but we do not disable preemption. What do you think? Oleg. --- x/kernel/sched/fair.c +++ x/kernel/sched/fair.c @@ -1165,7 +1165,7 @@ static void task_numa_compare(struct tas rcu_read_lock(); cur = ACCESS_ONCE(dst_rq->curr); - if (cur->pid == 0) /* idle */ + if (is_idle_task(cur) || (curr->flags & PF_EXITING)) cur = NULL; /*