Received: by 2002:a05:6358:16cc:b0:ea:6187:17c9 with SMTP id r12csp6423113rwl; Mon, 9 Jan 2023 08:09:48 -0800 (PST) X-Google-Smtp-Source: AMrXdXtBZxc7SKc9jEsr/GQIzm6YzP0p3W/yPEsYoUjwC6x3QCVavbmHPAMPxYADV8DHokcaQ+xB X-Received: by 2002:a17:90a:4f4d:b0:225:c711:c40f with SMTP id w13-20020a17090a4f4d00b00225c711c40fmr59467587pjl.30.1673280588566; Mon, 09 Jan 2023 08:09:48 -0800 (PST) ARC-Seal: i=1; a=rsa-sha256; t=1673280588; cv=none; d=google.com; s=arc-20160816; b=QdnAz8EaM9Q2hgCVHjiFHEh3pjVsHSxPaSDlKEFNUt/+Dprh71SguGzVn2tp0j03+V f6XtBKDtc5vsumovKw3GJUbz6ggsD3SkYNEkOx2hdoQlXsBYF+ER7WTixMtvYe2QTx5o RRvulepxn520eG4RxBTsVifLnZ/iUYNyrAdceDnbgdveESovNzwuESPQ3uQKBb/QGPZS 6vsM+GOOdcQ/VcYPSFrBheUm4DbZ5wT7i2UHUMOJvuvE7B8MKiA/emQ2NXvb0DtOsRkZ Cgm3ecMLTwJSu7Ra/pkwNF/pcNx6pnkc/DIu4o/OeeENxPzFCcQsZCCAXbBiAGSSKJfS pQfQ== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:in-reply-to:content-disposition:mime-version :references:reply-to:message-id:subject:cc:to:from:date :dkim-signature; bh=3k88UcLPT1xKDuJEN/2t06llf7EdMUClfoLIbpSzFXg=; b=h2L38XG4K/CMdKF8R6VOlNEagc0h1jwg/HNOZVwFIFYdZOE+NmYTIInqy8pmZgvWla tcDV1ukdnvijCRDmEadW5uuf4+Wrs0Hj9f+Zs6AqdZ1nz0btAYuAQd8hjCen4mNJ5vVV Fvylv75f8PFaN+QYOCev8oaQ5kPjDrq/t37LcTsDdCZrtqXC4arWw44F9TEjS4N8GRiZ oQrzoWyS6INjvEbbs9CMTT762uyJBTYF74/m623D4eZOFowUoS2RcufDPX4HzNOQzPbK PxU/nr9hp4xZeXV8YyBzqYAW82DtIxEeuBzgNkBPErKRnpanzjtpMGmQqmKNhV5oyj30 8glg== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@kernel.org header.s=k20201202 header.b=I7yoMCVm; 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 Return-Path: Received: from out1.vger.email (out1.vger.email. [2620:137:e000::1:20]) by mx.google.com with ESMTP id mw14-20020a17090b4d0e00b00218de7c19efsi14495404pjb.108.2023.01.09.08.09.42; Mon, 09 Jan 2023 08:09:48 -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=I7yoMCVm; 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 S234756AbjAIPW5 (ORCPT + 53 others); Mon, 9 Jan 2023 10:22:57 -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: 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 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) {