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=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, 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 B2B6EECE58D for ; Mon, 7 Oct 2019 13:18:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 88CB921721 for ; Mon, 7 Oct 2019 13:18:13 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="W5R84jvA" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728427AbfJGNSN (ORCPT ); Mon, 7 Oct 2019 09:18:13 -0400 Received: from mail-wm1-f66.google.com ([209.85.128.66]:37964 "EHLO mail-wm1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727490AbfJGNSM (ORCPT ); Mon, 7 Oct 2019 09:18:12 -0400 Received: by mail-wm1-f66.google.com with SMTP id 3so12308043wmi.3; Mon, 07 Oct 2019 06:18:11 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=Lvbiovo34sbcPGsBmiq1Qy97fup2BnoiwtVZlbVjSGQ=; b=W5R84jvAFVa4k/eE7K/BQiTfZVF3QzirWTst5VDVb0+ef7p1k2xDL6pVJUeJvuVj+F qWJK25c0rBszEiDrqHbaTfFYq7m0WXmEhKd+UrTx6DD04IbAyE9xqB5cPBKZiDhm436o qDBUoqeVPCdWdQ8IjAzYf2/hA4nLj/I25DNLh6+0FLBIkT4q/Hgb+BkeBMZQO4LM8LYG +kDNvaXUr3pDOxI4WLfTwQAjQKve+Y43J33Twi91qu8+92uxfXaRYJunQm8lq5iJjyxC JzstWwLiFZXX4XTS2gddT9qu6FW5sPEOfA9Upk6+e2mB1HUfI0nbzKD3+aHICD6l/wkC Q1oA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=Lvbiovo34sbcPGsBmiq1Qy97fup2BnoiwtVZlbVjSGQ=; b=RSD92n7Qofy9akdvzrnF1+RuSzCHQcOYycuBauoYodLU9ZYawJ8HNxhX4BhhCcWlNC EmioGoA9oprhnZ//Dmi+plfdWuop2CSA5uHAmqcYU5izW+tD+j0F5KQtI5rUtLyImk7w am40cWQ9pXcU9FjdyWldsEJ206dq7YECJgiZs1tcpHo28gdvZbbHfTuvrQC0mBP03q2y PCnl7LZxfnQ8dwp+Nfb6lJxgEPtCzlkiLciR6jyVwcPFwKvBMfzSuAqVFrc2C10DJFmc 1i9BN712fBSksHzirikVHVabvqpi+UOh0MaRbh86AHm95pqNSj90u3qe+AM3Vvk8oUfd 4pwQ== X-Gm-Message-State: APjAAAWhgIIG/d3Q6H3BfcGJGyI1sb7xmdh0z9yf7ujujIllzwSAaar3 wp5tCnflUk10PLZEEbSp+PU= X-Google-Smtp-Source: APXvYqy5FKq/arq9suAylL8FCDD31yWal4T1+5SMCI8jKldI7qcarOQyqJJrccvJnsIPc0PrlGqw6g== X-Received: by 2002:a1c:4108:: with SMTP id o8mr21745194wma.129.1570454290392; Mon, 07 Oct 2019 06:18:10 -0700 (PDT) Received: from andrea.guest.corp.microsoft.com ([2a01:110:8012:1012:69a4:816:a2c:e717]) by smtp.gmail.com with ESMTPSA id o9sm35726857wrh.46.2019.10.07.06.18.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 07 Oct 2019 06:18:09 -0700 (PDT) Date: Mon, 7 Oct 2019 15:18:04 +0200 From: Andrea Parri To: Christian Brauner 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: <20191007131804.GA19242@andrea.guest.corp.microsoft.com> References: <20191007104039.GA16085@andrea.guest.corp.microsoft.com> <20191007110117.1096-1-christian.brauner@ubuntu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191007110117.1096-1-christian.brauner@ubuntu.com> 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 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? > + 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? Thanks, Andrea > + stats_new = NULL; > } > spin_unlock_irq(&tsk->sighand->siglock); > > - if (stats) > - kmem_cache_free(taskstats_cache, stats); > -ret: > + if (stats_new) > + kmem_cache_free(taskstats_cache, stats_new); > + > return sig->stats; > } > > -- > 2.23.0 >