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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9A997C61DB3 for ; Mon, 9 Jan 2023 15:23:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237110AbjAIPW6 (ORCPT ); Mon, 9 Jan 2023 10:22:58 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35396 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237060AbjAIPW1 (ORCPT ); Mon, 9 Jan 2023 10:22:27 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5E75937527; Mon, 9 Jan 2023 07:22:08 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id D4F1061196; Mon, 9 Jan 2023 15:22:07 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 40427C433EF; Mon, 9 Jan 2023 15:22:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1673277727; bh=HH3u76bj1eMiMDOuqUoDInmJot4wY57kASPDBAy7ekQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=I7yoMCVmpEUf5HcEGNkT9P+aQ1vr0b6SYK5CpGbniaqfkEFXiaRzNdmsfkHWPbgsG dUw7AlUvklvRyN7TYHClnTrRX36Cg69COPnZl6JNLZvUFmAHfgVHAux080xdaX4FcJ X3pUEkY7dycmzZuepi73EmDFw+w3kBbxATO6kKEtYTRPsx3VyGLDN0Tgw6zlVYLbKv GFqpbpvivASMn8AgZvB/Rl8jTsANiw6XYbd+woZMJwKkiDE/sg40QkL1lS+OU+opTb Tf4IbVcpCIJDxPBMRDbLDKJOSW9iTSOdazHmkBrFWE9kDS/EfwWAOfJyz3CCArrlyX SuGB+WPGORYaA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id CB5435C090A; Mon, 9 Jan 2023 07:22:06 -0800 (PST) Date: Mon, 9 Jan 2023 07:22:06 -0800 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: Zqiang , quic_neeraju@quicinc.com, joel@joelfernandes.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] rcu: Safe access to rcu_node structure's->exp_tasks Message-ID: <20230109152206.GP4028633@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20221224052553.263199-1-qiang1.zhang@intel.com> <20230106034146.GM4028633@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 09, 2023 at 02:21:01PM +0100, Frederic Weisbecker wrote: > On Thu, Jan 05, 2023 at 07:41:46PM -0800, Paul E. McKenney wrote: > > On Sat, Dec 24, 2022 at 01:25:53PM +0800, Zqiang wrote: > > > For kernels built with CONFIG_PREEMPT_RCU=y, the following scenario > > > can result system oops. > > > > > > CPU1 CPU2 > > > rcu_preempt_deferred_qs_irqrestore rcu_print_task_exp_stall > > > if (special.b.blocked) READ_ONCE(rnp->exp_tasks) != NULL > > > raw_spin_lock_rcu_node > > > np = rcu_next_node_entry(t, rnp) > > > if (&t->rcu_node_entry == rnp->exp_tasks) > > > WRITE_ONCE(rnp->exp_tasks, np) > > > .... > > > raw_spin_unlock_irqrestore_rcu_node > > > raw_spin_lock_irqsave_rcu_node > > > t = list_entry(rnp->exp_tasks->prev, > > > struct task_struct, rcu_node_entry) > > > (if rnp->exp_tasks is NULL > > > will trigger oops) > > > > > > This problem is that CPU2 accesses rcu_node structure's->exp_tasks > > > without holding the rcu_node structure's ->lock and CPU2 did not > > > observe CPU1's change to rcu_node structure's->exp_tasks in time, > > > if rcu_node structure's->exp_tasks is set null pointer by CPU1, after > > > that CPU2 accesses members of rcu_node structure's->exp_tasks will > > > trigger oops. > > > > > > This commit therefore allows rcu_node structure's->exp_tasks to be > > > accessed while holding rcu_node structure's ->lock. > > > > > > Signed-off-by: Zqiang > > > > Apologies for the delay and thank you for the reminder! > > > > Please check the wordsmithed version below, which I have queued. > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > commit 389b0eafd72829fd63548f7ff4e8d6ac90fa1f98 > > Author: Zqiang > > Date: Sat Dec 24 13:25:53 2022 +0800 > > > > rcu: Protect rcu_print_task_exp_stall() ->exp_tasks access > > > > For kernels built with CONFIG_PREEMPT_RCU=y, the following scenario can > > result in a NULL-pointer dereference: > > > > CPU1 CPU2 > > rcu_preempt_deferred_qs_irqrestore rcu_print_task_exp_stall > > if (special.b.blocked) READ_ONCE(rnp->exp_tasks) != NULL > > raw_spin_lock_rcu_node > > np = rcu_next_node_entry(t, rnp) > > if (&t->rcu_node_entry == rnp->exp_tasks) > > WRITE_ONCE(rnp->exp_tasks, np) > > .... > > raw_spin_unlock_irqrestore_rcu_node > > raw_spin_lock_irqsave_rcu_node > > t = list_entry(rnp->exp_tasks->prev, > > struct task_struct, rcu_node_entry) > > (if rnp->exp_tasks is NULL, this > > will dereference a NULL pointer) > > > > The problem is that CPU2 accesses the rcu_node structure's->exp_tasks > > field without holding the rcu_node structure's ->lock and CPU2 did > > not observe CPU1's change to rcu_node structure's ->exp_tasks in time. > > Therefore, if CPU1 sets rcu_node structure's->exp_tasks pointer to NULL, > > then CPU2 might dereference that NULL pointer. > > > > This commit therefore holds the rcu_node structure's ->lock while > > accessing that structure's->exp_tasks field. > > > > Signed-off-by: Zqiang > > Signed-off-by: Paul E. McKenney > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > index 7cc4856da0817..902e7c8709c7e 100644 > > --- a/kernel/rcu/tree_exp.h > > +++ b/kernel/rcu/tree_exp.h > > @@ -803,9 +803,11 @@ static int rcu_print_task_exp_stall(struct rcu_node *rnp) > > int ndetected = 0; > > struct task_struct *t; > > > > - if (!READ_ONCE(rnp->exp_tasks)) > > - return 0; > > raw_spin_lock_irqsave_rcu_node(rnp, flags); > > + if (!READ_ONCE(rnp->exp_tasks)) { > > Does it have to be READ_ONCE then? Good point, that should not be necessary. I will drop the READ_ONCE on my next rebase. (Unless someone tells me there is something subtle that I am missing.) Thanx, Paul > Thanks. > > > + raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > > + return 0; > > + } > > t = list_entry(rnp->exp_tasks->prev, > > struct task_struct, rcu_node_entry); > > list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {