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=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS 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 8BA90C282C2 for ; Wed, 13 Feb 2019 10:50:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5BDC521901 for ; Wed, 13 Feb 2019 10:50:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2391541AbfBMKu2 (ORCPT ); Wed, 13 Feb 2019 05:50:28 -0500 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:51324 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731311AbfBMKu2 (ORCPT ); Wed, 13 Feb 2019 05:50:28 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 8F295A78; Wed, 13 Feb 2019 02:50:27 -0800 (PST) Received: from [10.1.197.45] (e112298-lin.cambridge.arm.com [10.1.197.45]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 45F683F575; Wed, 13 Feb 2019 02:50:24 -0800 (PST) Subject: Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region To: Peter Zijlstra Cc: Ingo Molnar , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, mingo@redhat.com, catalin.marinas@arm.com, will.deacon@arm.com, james.morse@arm.com, hpa@zytor.com, valentin.schneider@arm.com References: <1547560709-56207-1-git-send-email-julien.thierry@arm.com> <1547560709-56207-4-git-send-email-julien.thierry@arm.com> <20190211134527.GA121589@gmail.com> <20190211135159.GC32511@hirez.programming.kicks-ass.net> <20190213103553.GO32494@hirez.programming.kicks-ass.net> From: Julien Thierry Message-ID: <1c2429a4-9df9-40a3-98e0-51577de4bd6a@arm.com> Date: Wed, 13 Feb 2019 10:50:21 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20190213103553.GO32494@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 13/02/2019 10:35, Peter Zijlstra wrote: > On Tue, Feb 12, 2019 at 09:15:13AM +0000, Julien Thierry wrote: > >>>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >>>>> index a674c7db..b1bb7e9 100644 >>>>> --- a/kernel/sched/core.c >>>>> +++ b/kernel/sched/core.c >>>>> @@ -3289,6 +3289,14 @@ static inline void schedule_debug(struct task_struct *prev) >>>>> __schedule_bug(prev); >>>>> preempt_count_set(PREEMPT_DISABLED); >>>>> } >>>>> + >>>>> + if (IS_ENABLED(CONFIG_DEBUG_UACCESS_SLEEP) && >>>>> + unlikely(unsafe_user_region_active())) { >>>>> + printk(KERN_ERR "BUG: scheduling while user_access enabled: %s/%d/0x%08x\n", >>>>> + prev->comm, prev->pid, preempt_count()); >>>>> + dump_stack(); >>>>> + } >>>>> + >>>>> rcu_sleep_check(); >>>>> >>>>> profile_hit(SCHED_PROFILING, __builtin_return_address(0)); > >> I guess I'll drop the might_resched() part of this patch if that sounds >> alright. > > I'm still confused by the schedule_debug() part. How is that not broken? Hmmm, I am not exactly sure which part you expect to be broken, I guess it's because of the nature of the uaccess unsafe accessor usage. Basically, the following is a definite no: if (user_access_begin(ptr, size)) { [...] //something that calls schedule [...] user_access_end(); } However the following is fine: - user_access_begin(ptr, size) - taking irq/exception - get preempted - get resumed at some point in time - restore state + eret - user_access_end() That's because exceptions/irq implicitly "suspend" the user access region. (That's what I'm trying to clarify with the comment) So, unsafe_user_region_active() should return false in a irq/exception context. Is this what you were concerned about? Or there still something that might be broken? Thanks, -- Julien Thierry