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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 28DADC433F5 for ; Wed, 17 Nov 2021 11:52:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0F64961BD4 for ; Wed, 17 Nov 2021 11:52:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237056AbhKQLzl (ORCPT ); Wed, 17 Nov 2021 06:55:41 -0500 Received: from foss.arm.com ([217.140.110.172]:55764 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237051AbhKQLzg (ORCPT ); Wed, 17 Nov 2021 06:55:36 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 2B860ED1; Wed, 17 Nov 2021 03:52:38 -0800 (PST) Received: from C02TD0UTHF1T.local (unknown [10.57.46.245]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 88A5A3F70D; Wed, 17 Nov 2021 03:52:36 -0800 (PST) Date: Wed, 17 Nov 2021 11:52:34 +0000 From: Mark Rutland To: Qian Cai Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, catalin.marinas@arm.com, dvyukov@google.com, peterz@infradead.org, valentin.schneider@arm.com, will@kernel.org, woodylin@google.com Subject: Re: [PATCH] Reset task stack state in bringup_cpu() Message-ID: <20211117115234.GB41542@C02TD0UTHF1T.local> References: <20211115113310.35693-1-mark.rutland@arm.com> 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 Tue, Nov 16, 2021 at 11:31:40AM -0500, Qian Cai wrote: > On Mon, Nov 15, 2021 at 11:33:10AM +0000, Mark Rutland wrote: > > To hot unplug a CPU, the idle task on that CPU calls a few layers of C > > code before finally leaving the kernel. When KASAN is in use, poisoned > > shadow is left around for each of the active stack frames, and when > > shadow call stacks are in use. When shadow call stacks are in use the > > task's SCS SP is left pointing at an arbitrary point within the task's > > shadow call stack. > > > > When an offlines CPU is hotlpugged back into the kernel, this stale > > state can adversely affect the newly onlined CPU. Stale KASAN shadow can > > alias new stackframes and result in bogus KASAN warnings. A stale SCS SP > > is effectively a memory leak, and prevents a portion of the shadow call > > stack being used. Across a number of hotplug cycles the task's entire > > shadow call stack can become unusable. > > > > We previously fixed the KASAN issue in commit: > > > > e1b77c92981a5222 ("sched/kasan: remove stale KASAN poison after hotplug") > > > > In commit: > > > > f1a0a376ca0c4ef1 ("sched/core: Initialize the idle task with preemption disabled") > > > > ... we broke both KASAN and SCS, with SCS being fixed up in commit: > > > > 63acd42c0d4942f7 ("sched/scs: Reset the shadow stack when idle_task_exit") > > > > ... but as this runs in the context of the idle task being offlines it's > > potentially fragile. > > > > Fix both of these consistently and more robustly by resetting the SCS SP > > and KASAN shadow immediately before we online a CPU. This ensures the > > idle task always has a consistent state, and removes the need to do so > > when initializing an idle task or when unplugging an idle task. > > > > I've tested this with both GCC and clang, with reelvant options enabled, > > offlining and online CPUs with: > > > > | while true; do > > | for C in /sys/devices/system/cpu/cpu*/online; do > > | echo 0 > $C; > > | echo 1 > $C; > > | done > > | done > > > > Link: https://lore.kernel.org/lkml/20211012083521.973587-1-woodylin@google.com/ > > Link: https://lore.kernel.org/linux-arm-kernel/YY9ECKyPtDbD9q8q@qian-HP-Z2-SFF-G5-Workstation/ > > Fixes: 1a0a376ca0c4ef1 ("sched/core: Initialize the idle task with preemption disabled") > > Reported-by: Qian Cai > > Signed-off-by: Mark Rutland > > Thanks for fixing this quickly, Mark. Triggering an user-after-free in > user namespace but don't think it is related. I'll investigate that > first since it is blocking the rest of regression testing. Cool; are you happy to provide a Tested-by tag for this patch? :) Thanks, Mark.