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 83D76C47404 for ; Mon, 7 Oct 2019 14:10:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5F4C5206C2 for ; Mon, 7 Oct 2019 14:10:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728322AbfJGOKc (ORCPT ); Mon, 7 Oct 2019 10:10:32 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:50822 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727753AbfJGOKb (ORCPT ); Mon, 7 Oct 2019 10:10:31 -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 1iHTiT-0002Ej-VB; Mon, 07 Oct 2019 14:10:26 +0000 Date: Mon, 7 Oct 2019 16:10:24 +0200 From: Christian Brauner To: Dmitry Vyukov Cc: Andrea Parri , bsingharora@gmail.com, Marco Elver , LKML , syzbot , syzkaller-bugs , stable Subject: Re: [PATCH v2] taskstats: fix data-race Message-ID: <20191007141023.tozp36ydvzqdlzd5@wittgenstein> References: <20191007104039.GA16085@andrea.guest.corp.microsoft.com> <20191007110117.1096-1-christian.brauner@ubuntu.com> <20191007131804.GA19242@andrea.guest.corp.microsoft.com> <20191007135527.qd5ibfyajnihsrsh@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: 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 04:08:41PM +0200, Dmitry Vyukov wrote: > On Mon, Oct 7, 2019 at 3:55 PM Christian Brauner > wrote: > > > > On Mon, Oct 07, 2019 at 03:50:47PM +0200, Dmitry Vyukov wrote: > > > On Mon, Oct 7, 2019 at 3:18 PM 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? > > > > > > > > > > > > > > > > > > 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? > > > > > > I would assume callers of this function access *stats. So the > > > dependency is between loading stats and accessing *stats. > > > > Right, but why READ_ONCE() and not smp_load_acquire here? > > Because if all memory accesses we need to order have data dependency > between them, READ_ONCE is enough and is cheaper on some archs (e.g. > ARM). > In our case there is a data dependency between loading of stats and > accessing *stats (only Alpha could reorder that, other arches can't > load via a pointer before loading the pointer itself (sic!)). Right, the except-Alpha-clause is well-known... Christian