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[2620:137:e000::1:20]) by mx.google.com with ESMTP id qf38-20020a1709077f2600b007c365fad0basi18682ejc.152.2023.01.05.19.56.32; Thu, 05 Jan 2023 19:56:45 -0800 (PST) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20; Authentication-Results: mx.google.com; dkim=pass header.i=@kernel.org header.s=k20201202 header.b=PCi6o7ET; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230267AbjAFDlu (ORCPT + 55 others); Thu, 5 Jan 2023 22:41:50 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43888 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230194AbjAFDlt (ORCPT ); Thu, 5 Jan 2023 22:41:49 -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 5CE4B3E868; Thu, 5 Jan 2023 19:41:48 -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 01DB161CD7; Fri, 6 Jan 2023 03:41:48 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 58993C433EF; Fri, 6 Jan 2023 03:41:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1672976507; bh=/NJ6JHgebWXiCCli7ngAfOmm/46OZs0BToYL8An7kCA=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=PCi6o7ET/ARK+6SYQaihKx1uTFLPqqHhoet5YsBmvmK70R913R2ik23KPjsVPBOHC kMaiGd8gtgQE7vfJj70y1mFAiKalqqDdMynA0yovvKJUE/gfcKPeDQMv0CLAB4HYAQ BgXGyuYEp+4vJJ8tLHtE3Kk7OdhsnO0M7KA6LIP0qONTxsm5oxcLzqi1fm1TjW4Y86 1JoAKSw8X++GijX2MKEPNkPSXMrrG7KsCuBGRCfJMeh8i2+7pmipQCpqujp7IwkD27 A1FZLOO+j2zFqfp+35RfgFcIXOrkjZgxicnz+JSTDKOF3qL8mUHbnhSN+s5bfYQuYQ jVucrMw0dA/Pg== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 0171E5C0544; Thu, 5 Jan 2023 19:41:46 -0800 (PST) Date: Thu, 5 Jan 2023 19:41:46 -0800 From: "Paul E. McKenney" To: Zqiang Cc: frederic@kernel.org, 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: <20230106034146.GM4028633@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20221224052553.263199-1-qiang1.zhang@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221224052553.263199-1-qiang1.zhang@intel.com> X-Spam-Status: No, score=-7.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_HI, SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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)) { + 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) {