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 400B8C43334 for ; Tue, 26 Jul 2022 12:08:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238070AbiGZMIb (ORCPT ); Tue, 26 Jul 2022 08:08:31 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49584 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232810AbiGZMI0 (ORCPT ); Tue, 26 Jul 2022 08:08:26 -0400 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B50592B252; Tue, 26 Jul 2022 05:08:23 -0700 (PDT) Received: from dggemv704-chm.china.huawei.com (unknown [172.30.72.57]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4LsbDl1C8HzWfWq; Tue, 26 Jul 2022 20:04:23 +0800 (CST) Received: from kwepemm600010.china.huawei.com (7.193.23.86) by dggemv704-chm.china.huawei.com (10.3.19.47) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Tue, 26 Jul 2022 20:08:15 +0800 Received: from [10.67.110.237] (10.67.110.237) by kwepemm600010.china.huawei.com (7.193.23.86) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Tue, 26 Jul 2022 20:08:14 +0800 Subject: Re: [PATCH 3/5] ARM: stacktrace: Allow stack trace saving for non-current tasks To: "Russell King (Oracle)" CC: Linus Walleij , , , , , , , , , , , , , , , , , , , , , References: <20220712021527.109921-1-lihuafei1@huawei.com> <20220712021527.109921-4-lihuafei1@huawei.com> <1288c73b-cf29-707d-47cb-4e2737300a29@huawei.com> From: Li Huafei Message-ID: Date: Tue, 26 Jul 2022 20:08:14 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.67.110.237] X-ClientProxiedBy: dggems702-chm.china.huawei.com (10.3.19.179) To kwepemm600010.china.huawei.com (7.193.23.86) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/7/26 17:49, Russell King (Oracle) wrote: > On Tue, Jul 26, 2022 at 05:12:39PM +0800, Li Huafei wrote: >> >> >> On 2022/7/18 17:07, Linus Walleij wrote: >>> On Tue, Jul 12, 2022 at 4:18 AM Li Huafei wrote: >>> >>>> The current ARM implementation of save_stack_trace_tsk() does not allow >>>> saving stack trace for non-current tasks, which may limit the scenarios >>>> in which stack_trace_save_tsk() can be used. Like other architectures, >>>> or like ARM's unwind_backtrace(), we can leave it up to the caller to >>>> ensure that the task that needs to be unwound is not running. >>>> >>>> Signed-off-by: Li Huafei >>> >>> That sounds good, but: >>> >>>> if (tsk != current) { >>>> -#ifdef CONFIG_SMP >>>> - /* >>>> - * What guarantees do we have here that 'tsk' is not >>>> - * running on another CPU? For now, ignore it as we >>>> - * can't guarantee we won't explode. >>>> - */ >>>> - return; >>>> -#else >>>> + /* task blocked in __switch_to */ >>> >>> The commit text is not consistent with the comment you are removing. >>> >>> The commit is talking about "non-current" tasks which is one thing, >>> but the code is avoiding any tasks under SMP because they may be >>> running on another CPU. So you need to update the commit >>> message to say something like "non-current or running on another CPU". >>> >>> If this condition will be checked at call sites in following patches, >>> then mention >>> that in the commit as well, so we know the end result is that we do >>> not break it, >> >> The generic code stack_trace_save_tsk() does not have this check, and by >> 'caller' I mean the caller of stack_trace_save_tsk(), expecting the >> 'caller' to ensure that the task is not running. So in effect this check >> has been dropped and there is no more guarantee. Sorry for not >> clarifying the change here. > > Can you prove in every case that the thread we're being asked to unwind > is not running? I don't think you can. > > There are things like proc_pid_stack() in procfs and the stack traces > in sysrq-t which have attempted to unwind everything whether it's > running or not. > > So no, there is no guarantee that the thread is blocked in > __switch_to(). > Yes, I agree. >> But can we assume that the user should know that the stacktrace is >> unreliable for a task that is running on another CPU? If not, I should >> remove this patch and keep the check. > > It's not about "unreliable" stack traces, it's about the unwinder > killing the kernel. > > The hint is this: > > frame.fp = thread_saved_fp(tsk); > frame.sp = thread_saved_sp(tsk); > frame.lr = 0; /* recovered from the stack */ > frame.pc = thread_saved_pc(tsk); > > These access the context saved by the scheduler when the task is > sleeping. When the thread is running, these saved values will be > the state when the thread last slept. However, with the thread > running, the stack could now contain any data what so ever, and > could change at any moment. > I get it. For example, since the data on the stack is changing, '*(unsigned long *)fp' could access any illegal address and crash the kernel. > Whether the unwind-table unwinder is truely safe in such a > situation is unknown - we try to ensure that it won't do anything > stupid, but proving that is a hard task, and we've recently had > issues with the unwinder even without that. > > So, allowing this feels like we're opening the door to DoS attacks > from userspace, where userspace sits there reading /proc/*/stack of > some thread running on a different CPU waiting for the kernel to > oops itself, possibly holding a lock, resulting in the system > dying. > > These decisions need to be made by architecture code not generic > code, particularly where the method of unwinding is architecture > specific and thus may have criteria defining when its safe to do so. > Thank you for your comments, I'll remove the patch and keep the check. Thanks, Huafei