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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable 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 9094AC47404 for ; Mon, 7 Oct 2019 13:28:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 70D07206C2 for ; Mon, 7 Oct 2019 13:28:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728009AbfJGN24 (ORCPT ); Mon, 7 Oct 2019 09:28:56 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:49348 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727324AbfJGN24 (ORCPT ); Mon, 7 Oct 2019 09:28:56 -0400 Received: from [185.66.195.251] (helo=wittgenstein) by youngberry.canonical.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1iHT4G-0007M9-S3; Mon, 07 Oct 2019 13:28:53 +0000 Date: Mon, 7 Oct 2019 15:28:51 +0200 From: Christian Brauner To: Andrea Parri Cc: bsingharora@gmail.com, dvyukov@google.com, elver@google.com, linux-kernel@vger.kernel.org, syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com, stable@vger.kernel.org Subject: Re: [PATCH v2] taskstats: fix data-race Message-ID: <20191007132850.u4iwjh5c2or4p2dz@wittgenstein> References: <20191007104039.GA16085@andrea.guest.corp.microsoft.com> <20191007110117.1096-1-christian.brauner@ubuntu.com> <20191007131804.GA19242@andrea.guest.corp.microsoft.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20191007131804.GA19242@andrea.guest.corp.microsoft.com> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 07, 2019 at 03:18:04PM +0200, Andrea Parri wrote: > On Mon, Oct 07, 2019 at 01:01:17PM +0200, Christian Brauner wrote: > > When assiging and testing taskstats in taskstats_exit() there's a race > > when writing and reading sig->stats when a thread-group with more than > > one thread exits: > > > > cpu0: > > thread catches fatal signal and whole thread-group gets taken down > > do_exit() > > do_group_exit() > > taskstats_exit() > > taskstats_tgid_alloc() > > The tasks reads sig->stats holding sighand lock seeing garbage. > > You meant "without holding sighand lock" here, right? Correct, thanks for noticing! > > > > > > cpu1: > > task calls exit_group() > > do_exit() > > do_group_exit() > > taskstats_exit() > > taskstats_tgid_alloc() > > The task takes sighand lock and assigns new stats to sig->stats. > > > > Fix this by using READ_ONCE() and smp_store_release(). > > > > Reported-by: syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com > > Fixes: 34ec12349c8a ("taskstats: cleanup ->signal->stats allocation") > > Cc: stable@vger.kernel.org > > Signed-off-by: Christian Brauner > > Reviewed-by: Dmitry Vyukov > > Link: https://lore.kernel.org/r/20191006235216.7483-1-christian.brauner@ubuntu.com > > --- > > /* v1 */ > > Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@ubuntu.com > > > > /* v2 */ > > - Dmitry Vyukov , Marco Elver : > > - fix the original double-checked locking using memory barriers > > > > /* v3 */ > > - Andrea Parri : > > - document memory barriers to make checkpatch happy > > --- > > kernel/taskstats.c | 21 ++++++++++++--------- > > 1 file changed, 12 insertions(+), 9 deletions(-) > > > > diff --git a/kernel/taskstats.c b/kernel/taskstats.c > > index 13a0f2e6ebc2..978d7931fb65 100644 > > --- a/kernel/taskstats.c > > +++ b/kernel/taskstats.c > > @@ -554,24 +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; > > + /* Pairs with smp_store_release() below. */ > > + stats = READ_ONCE(sig->stats); > > This pairing suggests that the READ_ONCE() is heading an address > dependency, but I fail to identify it: what is the target memory > access of such a (putative) dependency? > > > > + 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; > > + /* Pairs with READ_ONCE() above. */ > > + smp_store_release(&sig->stats, stats_new); > > This is intended to 'order' the _zalloc() (zero initializazion) > before the update of sig->stats, right? what else am I missing? Right, I should've mentioned that. I'll change the comment. But I thought this also paired with smp_read_barrier_depends() that's placed alongside READ_ONCE()? Christian