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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F2CE9C433EF for ; Thu, 18 Nov 2021 10:29:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CF36661B3E for ; Thu, 18 Nov 2021 10:29:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S245220AbhKRKcx (ORCPT ); Thu, 18 Nov 2021 05:32:53 -0500 Received: from foss.arm.com ([217.140.110.172]:39198 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243723AbhKRKcf (ORCPT ); Thu, 18 Nov 2021 05:32:35 -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 B84241FB; Thu, 18 Nov 2021 02:29:33 -0800 (PST) Received: from lakrids.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 7E6CE3F5A1; Thu, 18 Nov 2021 02:29:32 -0800 (PST) From: Mark Rutland To: linux-kernel@vger.kernel.org Cc: catalin.marinas@arm.com, dvyukov@google.com, mark.rutland@arm.com, peterz@infradead.org, quic_qiancai@quicinc.com, valentin.schneider@arm.com, will@kernel.org, woodylin@google.com Subject: [PATCH v2] Reset task stack state in bringup_cpu() Date: Thu, 18 Nov 2021 10:29:27 +0000 Message-Id: <20211118102927.4854-1-mark.rutland@arm.com> X-Mailer: git-send-email 2.11.0 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 saved SCS SP is left pointing at an arbitrary point within the task's shadow call stack. When a CPU is offlined than onlined back into the kernel, this stale state can adversely affect execution. 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 idle 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") ... by removing any stale KASAN stack poison immediately prior to onlining a CPU. Subsequently in commit: f1a0a376ca0c4ef1 ("sched/core: Initialize the idle task with preemption disabled") ... the refactoring left the KASAN and SCS cleanup in one-time idle thread initialization code rather than something invoked prior to each CPU being onlined, breaking both as above. We fixed SCS (but not KASAN) 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 offlined it's potentially fragile. To fix these consistently and more robustly, reset the SCS SP and KASAN shadow of a CPU's idle task immediately before we online that CPU in bringup_cpu(). This ensures the idle task always has a consistent state when it is running, and removes the need to so so when exiting an idle task. Whenever any thread is created, dup_task_struct() will give the task a stack which is free of KASAN shadow, and initialize the task's SCS SP, so there's no need to specially initialize either for idle thread within init_idle(), as this was only necessary to handle hotplug cycles. I've tested this with both GCC and clang, with relevant options enabled, offlining and onlining 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/ Link: https://lore.kernel.org/lkml/20211115113310.35693-1-mark.rutland@arm.com/ Fixes: 1a0a376ca0c4ef1 ("sched/core: Initialize the idle task with preemption disabled") Reported-by: Qian Cai Signed-off-by: Mark Rutland Reviewed-by: Valentin Schneider Tested-by: Qian Cai Cc: Catalin Marinas Cc: Dmitry Vyukov Cc: Peter Zijlstra Cc: Will Deacon Cc: Woody Lin --- kernel/cpu.c | 7 +++++++ kernel/sched/core.c | 4 ---- 2 files changed, 7 insertions(+), 4 deletions(-) Since v1 [1]: * Clarify commit message * Fix typos * Accumulate tags [1] https://lore.kernel.org/lkml/20211115113310.35693-1-mark.rutland@arm.com/ diff --git a/kernel/cpu.c b/kernel/cpu.c index 192e43a87407..407a2568f35e 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -31,6 +31,7 @@ #include #include #include +#include #include #include @@ -588,6 +589,12 @@ static int bringup_cpu(unsigned int cpu) int ret; /* + * Reset stale stack state from the last time this CPU was online. + */ + scs_task_reset(idle); + kasan_unpoison_task_stack(idle); + + /* * Some architectures have to walk the irq descriptors to * setup the vector space for the cpu which comes online. * Prevent irq alloc/free across the bringup. diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 3c9b0fda64ac..76f9deeaa942 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -8619,9 +8619,6 @@ void __init init_idle(struct task_struct *idle, int cpu) idle->flags |= PF_IDLE | PF_KTHREAD | PF_NO_SETAFFINITY; kthread_set_per_cpu(idle, cpu); - scs_task_reset(idle); - kasan_unpoison_task_stack(idle); - #ifdef CONFIG_SMP /* * It's possible that init_idle() gets called multiple times on a task, @@ -8777,7 +8774,6 @@ void idle_task_exit(void) finish_arch_post_lock_switch(); } - scs_task_reset(current); /* finish_cpu(), as ran on the BP, will clean up the active_mm state */ } -- 2.11.0