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=-5.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 753D7C432C3 for ; Fri, 29 Nov 2019 17:56:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3C980216F4 for ; Fri, 29 Nov 2019 17:56:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1575050173; bh=NB5Gf1Q3vSZTblBj+r+anX1Gd6+985MhbvLhEw+wH68=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=es7wbeiaQ3Nwn0aEVcJ3wYRCkjZxJMY3xCZ5I8h38duKRXxZtG3sKfBCUBfABcaov 2MJniDug2Bb70Q2tdyPVjOX854F5nJo1A3gQr1epRYp98wenUhT+MujjD6s/KKfOm1 IpEyvnsG3au2ZFyJzyPRDrq62CGsvT9v1tIQjrLk= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727043AbfK2R4M (ORCPT ); Fri, 29 Nov 2019 12:56:12 -0500 Received: from mail.kernel.org ([198.145.29.99]:48032 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726970AbfK2R4M (ORCPT ); Fri, 29 Nov 2019 12:56:12 -0500 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id B69242158A; Fri, 29 Nov 2019 17:56:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1575050170; bh=NB5Gf1Q3vSZTblBj+r+anX1Gd6+985MhbvLhEw+wH68=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=SRXGlV5RVcc+XmFeQm3pDrl7JXNeolayQezj9ZbtKE0XVQBPKE7YLSYtvNXvLEWqI zJuURxnHYvQT7mewsRGOYFJu2UHO2Lrzg4E9KZmEeEuYRNwfRgJLyGITiHQCLyt3h0 jF4+weFrW/rmQdpVPFVBBqUMajzeyfaKARGv6Pio= Date: Fri, 29 Nov 2019 17:56:05 +0000 From: Will Deacon To: Christian Brauner Cc: Rasmus Villemoes , linux-kernel@vger.kernel.org, bsingharora@gmail.com, dvyukov@google.com, elver@google.com, parri.andrea@gmail.com, stable@vger.kernel.org, syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com Subject: Re: [PATCH v6] taskstats: fix data-race Message-ID: <20191129175604.GA29789@willie-the-truck> References: <20191009114809.8643-1-christian.brauner@ubuntu.com> <20191021113327.22365-1-christian.brauner@ubuntu.com> <20191021130417.5yi7pxpigsydz5po@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191021130417.5yi7pxpigsydz5po@wittgenstein> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 21, 2019 at 03:04:18PM +0200, Christian Brauner wrote: > On Mon, Oct 21, 2019 at 02:19:01PM +0200, Rasmus Villemoes wrote: > > On 21/10/2019 13.33, Christian Brauner wrote: > > > The first approach used smp_load_acquire() and smp_store_release(). > > > However, after having discussed this it seems that the data dependency > > > for kmem_cache_alloc() would be fixed by WRITE_ONCE(). > > > Furthermore, the smp_load_acquire() would only manage to order the stats > > > check before the thread_group_empty() check. So it seems just using > > > READ_ONCE() and WRITE_ONCE() will do the job and I wanted to bring this > > > up for discussion at least. > > > > > > /* v6 */ > > > - Christian Brauner : > > > - bring up READ_ONCE()/WRITE_ONCE() approach for discussion > > > --- > > > kernel/taskstats.c | 26 +++++++++++++++----------- > > > 1 file changed, 15 insertions(+), 11 deletions(-) > > > > > > diff --git a/kernel/taskstats.c b/kernel/taskstats.c > > > index 13a0f2e6ebc2..111bb4139aa2 100644 > > > --- a/kernel/taskstats.c > > > +++ b/kernel/taskstats.c > > > @@ -554,25 +554,29 @@ static int taskstats_user_cmd(struct sk_buff *skb, struct genl_info *info) > > > static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk) > > > { > > > struct signal_struct *sig = tsk->signal; > > > - struct taskstats *stats; > > > + struct taskstats *stats_new, *stats; > > > > > > - if (sig->stats || thread_group_empty(tsk)) > > > - goto ret; > > > + /* Pairs with WRITE_ONCE() below. */ > > > + stats = READ_ONCE(sig->stats); > > > + if (stats || thread_group_empty(tsk)) > > > + return stats; > > > > > > /* No problem if kmem_cache_zalloc() fails */ > > > - stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL); > > > + stats_new = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL); > > > > > > spin_lock_irq(&tsk->sighand->siglock); > > > - if (!sig->stats) { > > > - sig->stats = stats; > > > - stats = NULL; > > > + if (!stats) { > > > + stats = stats_new; > > > + /* Pairs with READ_ONCE() above. */ > > > + WRITE_ONCE(sig->stats, stats_new); > > > + stats_new = NULL; > > > > No idea about the memory ordering issues, but don't you need to > > load/check sig->stats again? Otherwise it seems that two threads might > > both see !sig->stats, both allocate a stats_new, and both > > unconditionally in turn assign their stats_new to sig->stats. Then the > > first assignment ends up becoming a memory leak (and any writes through > > that pointer done by the caller end up in /dev/null...) > > Trigger hand too fast. I guess you're thinking sm like: > > diff --git a/kernel/taskstats.c b/kernel/taskstats.c > index 13a0f2e6ebc2..c4e1ed11e785 100644 > --- a/kernel/taskstats.c > +++ b/kernel/taskstats.c > @@ -554,25 +554,27 @@ static int taskstats_user_cmd(struct sk_buff *skb, struct genl_info *info) > static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk) > { > struct signal_struct *sig = tsk->signal; > - struct taskstats *stats; > + struct taskstats *stats_new, *stats; > > - if (sig->stats || thread_group_empty(tsk)) > - goto ret; > + stats = READ_ONCE(sig->stats); This probably wants to be an acquire, since both the memcpy() later on in taskstats_exit() and the accesses in {b,x}acct_add_tsk() appear to read from the taskstats structure without the sighand->siglock held and therefore may miss zeroed allocation from the zalloc() below, I think. > + if (stats || thread_group_empty(tsk)) > + return stats; > > - /* No problem if kmem_cache_zalloc() fails */ > - stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL); > + stats_new = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL); > > spin_lock_irq(&tsk->sighand->siglock); > - if (!sig->stats) { > - sig->stats = stats; > - stats = NULL; > + stats = READ_ONCE(sig->stats); You hold the spinlock here, so I don't think you need the READ_ONCE(). > + if (!stats) { > + stats = stats_new; > + WRITE_ONCE(sig->stats, stats_new); You probably want a release here to publish the zeroes from the zalloc() (back to my first comment). With those changes: Reviewed-by: Will Deacon However, this caused me to look at do_group_exit() and we appear to have racy accesses on sig->flags there thanks to signal_group_exit(). I worry that might run quite deep, and can probably be looked at separately. Will