From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A1A83C3A59E for ; Mon, 2 Sep 2019 14:44:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7D7B12087E for ; Mon, 2 Sep 2019 14:44:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731682AbfIBOo2 (ORCPT ); Mon, 2 Sep 2019 10:44:28 -0400 Received: from mx1.redhat.com ([209.132.183.28]:46412 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726382AbfIBOo2 (ORCPT ); Mon, 2 Sep 2019 10:44:28 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 04A7A10C6975; Mon, 2 Sep 2019 14:44:28 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.63]) by smtp.corp.redhat.com (Postfix) with SMTP id 9F7FD5D9C3; Mon, 2 Sep 2019 14:44:25 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Mon, 2 Sep 2019 16:44:27 +0200 (CEST) Date: Mon, 2 Sep 2019 16:44:24 +0200 From: Oleg Nesterov To: Peter Zijlstra Cc: "Eric W. Biederman" , Linus Torvalds , Russell King - ARM Linux admin , Chris Metcalf , Christoph Lameter , Kirill Tkhai , Mike Galbraith , Thomas Gleixner , Ingo Molnar , Linux List Kernel Mailing Subject: Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference() without checking return value Message-ID: <20190902144424.GB14770@redhat.com> References: <20190830140805.GD13294@shell.armlinux.org.uk> <20190830160957.GC2634@redhat.com> <87o906wimo.fsf@x220.int.ebiederm.org> <20190902134003.GA14770@redhat.com> <20190902135315.GR2369@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190902135315.GR2369@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.65]); Mon, 02 Sep 2019 14:44:28 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/02, Peter Zijlstra wrote: > > On Mon, Sep 02, 2019 at 03:40:03PM +0200, Oleg Nesterov wrote: > > diff --git a/include/linux/sched.h b/include/linux/sched.h > > index 8dc1811..1f9b021 100644 > > --- a/include/linux/sched.h > > +++ b/include/linux/sched.h > > @@ -1134,7 +1134,10 @@ struct task_struct { > > > > struct tlbflush_unmap_batch tlb_ubc; > > > > - struct rcu_head rcu; > > + union { > > + bool xxx; > > + struct rcu_head rcu; > > + }; > > > > /* Cache last used pipe for splice(): */ > > struct pipe_inode_info *splice_pipe; > > diff --git a/kernel/exit.c b/kernel/exit.c > > index a75b6a7..baacfce 100644 > > --- a/kernel/exit.c > > +++ b/kernel/exit.c > > @@ -182,6 +182,11 @@ static void delayed_put_task_struct(struct rcu_head *rhp) > > put_task_struct(tsk); > > } > > > > +void call_delayed_put_task_struct(struct task_struct *p) > > +{ > > + if (xchg(&p->xxx, 1)) > > + call_rcu(&p->rcu, delayed_put_task_struct); > > +} > > I think this is the first usage of xchg() on _Bool, also not all archs > implement xchg8() I didn't even notice I used "bool" ;) speaking of the users of task_rcu_dereference(), membarrier_global_expedited() does rcu_read_lock(); p = task_rcu_dereference(&cpu_rq(cpu)->curr); if (p && p->mm && (atomic_read(&p->mm->membarrier_state) & MEMBARRIER_STATE_GLOBAL_EXPEDITED)) { This asks for READ_ONCE, but this is minor. Why can't p->mm be freed? I guess it is fine to read the garbage from &p->mm->membarrier_state if we race with the exiting task, but in theory this looks unsafe if CONFIG_DEBUG_PAGEALLOC. Another possible user of probe_slab_address() or I am totally confused? Oleg.