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=-14.4 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL 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 608EFECE58C for ; Mon, 7 Oct 2019 13:51:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2705A21721 for ; Mon, 7 Oct 2019 13:51:01 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="DWoXSOXJ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728164AbfJGNvA (ORCPT ); Mon, 7 Oct 2019 09:51:00 -0400 Received: from mail-qk1-f194.google.com ([209.85.222.194]:43931 "EHLO mail-qk1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727715AbfJGNvA (ORCPT ); Mon, 7 Oct 2019 09:51:00 -0400 Received: by mail-qk1-f194.google.com with SMTP id h126so12571882qke.10 for ; Mon, 07 Oct 2019 06:50:59 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=IYTQc9pkklqWHMouyJseXYf3fIK7lEf7q4Z1ZbUboWU=; b=DWoXSOXJ2+WloLrHOXlt4e17wDt7AR/niWGjAZjfVMddOsQ1wyWBYBhaosgPDbnc6v UyMg0Ny06xFOyH1RZj47SIcL51nYf7PbPsRVTrwd6YKcgUeXlCy6yVn615HYcKIz3Kci 1u8tIvjVXsXQLoObbMxWI8p0fSXcI473Z2JoCieUc4rg54wNoltxZjQNr+Mj6pk0o8RZ f/W3q6CNJJRuLphdRpMzNw86lXvJ77IqyX6pEWJMAWO7WpRL7mGAGcp+ZkCrrdKFH3i2 lYzr5NLCTSspZ3S4Amb+3i8JgIq0YAeRqlPbRnwbOy2lxJFqVzn4NmnHDUzdKM6h3+dG lo2g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=IYTQc9pkklqWHMouyJseXYf3fIK7lEf7q4Z1ZbUboWU=; b=tfAD/nCKY8yecgkrZwP3WNy8vRWxa0rN7WtzEYNvmcBygN8YxHGjTWHdnKIUTc61yy /zFJaOA0qHuGelP1D8+UCWm+cFGgef4CX/na1IOBeSblhwVK4hmI/RlNBZxyjfzb5Feu R3eT+2G7FpZY+Wjw3GRJz2ofWIOurz3Yal4VkpHTbZHS7Dplq5s/RWUTNyZgzEtsXMhL 3axOVEFbNA7IE/uwnGI5cctKE2DU1JjJRSkt/HLz7o5BZdFE80CbBHcEFG9ekGZN0yD/ 2GDwvJa5Aay9ukSZImxSkhVukON8Xdhm/bhwb4u6byKSW7Pfx9PRwNH7XT0x4EPMMpxy aJWw== X-Gm-Message-State: APjAAAX1SsIbF6l05QMj+6LGvuuiVct94oc9yCs+N9bTak1M7YCHjwLc xYfI8WV+OPMqq9vcoyg8fDpPn8cIybnoY0psHus7fw== X-Google-Smtp-Source: APXvYqzh/OeSTjLyIs3Dc0YlIhCuQFyOlIjf37jONr3UqmFwJP6OBS+YmngvTj21e28J/XdDCSiEYCj4ZuEOOd361n0= X-Received: by 2002:a37:985:: with SMTP id 127mr23035292qkj.43.1570456258406; Mon, 07 Oct 2019 06:50:58 -0700 (PDT) MIME-Version: 1.0 References: <20191007104039.GA16085@andrea.guest.corp.microsoft.com> <20191007110117.1096-1-christian.brauner@ubuntu.com> <20191007131804.GA19242@andrea.guest.corp.microsoft.com> In-Reply-To: <20191007131804.GA19242@andrea.guest.corp.microsoft.com> From: Dmitry Vyukov Date: Mon, 7 Oct 2019 15:50:47 +0200 Message-ID: Subject: Re: [PATCH v2] taskstats: fix data-race To: Andrea Parri Cc: Christian Brauner , bsingharora@gmail.com, Marco Elver , LKML , syzbot , syzkaller-bugs , stable Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > + 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 > >