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=-9.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_GIT 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 E1E74CA9EB7 for ; Mon, 21 Oct 2019 11:33:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C3C172089C for ; Mon, 21 Oct 2019 11:33:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728488AbfJULdc (ORCPT ); Mon, 21 Oct 2019 07:33:32 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:57248 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727685AbfJULdc (ORCPT ); Mon, 21 Oct 2019 07:33:32 -0400 Received: from [213.220.153.21] (helo=localhost.localdomain) by youngberry.canonical.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1iMVwI-0007A3-CJ; Mon, 21 Oct 2019 11:33:30 +0000 From: Christian Brauner To: Will Deacon , linux-kernel@vger.kernel.org, christian.brauner@ubuntu.com Cc: 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: [PATCH v6] taskstats: fix data-race Date: Mon, 21 Oct 2019 13:33:27 +0200 Message-Id: <20191021113327.22365-1-christian.brauner@ubuntu.com> X-Mailer: git-send-email 2.23.0 In-Reply-To: <20191009114809.8643-1-christian.brauner@ubuntu.com> References: <20191009114809.8643-1-christian.brauner@ubuntu.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 without holding sighand lock. 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. 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. Reported-by: syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com Fixes: 34ec12349c8a ("taskstats: cleanup ->signal->stats allocation") Cc: Will Deacon Cc: Marco Elver Cc: Andrea Parri Cc: Dmitry Vyukov Cc: stable@vger.kernel.org Signed-off-by: Christian Brauner --- /* v1 */ Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@ubuntu.com /* v2 */ Link: https://lore.kernel.org/r/20191006235216.7483-1-christian.brauner@ubuntu.com - Dmitry Vyukov , Marco Elver : - fix the original double-checked locking using memory barriers /* v3 */ Link: https://lore.kernel.org/r/20191007110117.1096-1-christian.brauner@ubuntu.com - Andrea Parri : - document memory barriers to make checkpatch happy /* v4 */ Link: https://lore.kernel.org/r/20191009113134.5171-1-christian.brauner@ubuntu.com - Andrea Parri : - use smp_load_acquire(), not READ_ONCE() - update commit message /* v5 */ Link: https://lore.kernel.org/r/20191009114809.8643-1-christian.brauner@ubuntu.com - Andrea Parri : - fix typo in smp_load_acquire() /* 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; } spin_unlock_irq(&tsk->sighand->siglock); - if (stats) - kmem_cache_free(taskstats_cache, stats); -ret: - return sig->stats; + if (stats_new) + kmem_cache_free(taskstats_cache, stats_new); + + return stats; } /* Send pid data out on exit */ -- 2.23.0