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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 61579C433DF for ; Mon, 18 May 2020 13:23:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3760F20709 for ; Mon, 18 May 2020 13:23:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589808234; bh=VRHqp4EH6ZCXx3q/nKzQbhc50sXdlhPesaPcLuWElWU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=mhyQT6nm18g6SFNFzmLPAIOJ0tbYTI08FTt7g66qTd6A5XdP0RaCFUB45Mi5bdhvn GphuFtxS3MYbb4xbqjw+rvWsScxKplUDv0WYLsVS65VVRp/gUuwF16nq5uvX3108fU 0vkjxzyIb0b+qAPJkd7ILa+DhqeI+Y1a5Nioq2iw= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727989AbgERNXx (ORCPT ); Mon, 18 May 2020 09:23:53 -0400 Received: from mail.kernel.org ([198.145.29.99]:56096 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726800AbgERNXw (ORCPT ); Mon, 18 May 2020 09:23:52 -0400 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 73656207F9; Mon, 18 May 2020 13:23:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589808232; bh=VRHqp4EH6ZCXx3q/nKzQbhc50sXdlhPesaPcLuWElWU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=JK8h+I3656sP2fg7WK8m+s26PlqWyYK5YmqtE6IW3Op9VK8D8B/UqDER4PdbsMSQ5 p5sP8nzfJLTy2vv+9qvzJqkifgwB0IuSdn+9i8AFFs0qbrVy6rF0Gc0WQSn7rvJYtN sDiipVGvZG87aUDUNlQjHd0PvJY3D6ZvzjmJje/g= Date: Mon, 18 May 2020 14:23:47 +0100 From: Will Deacon To: Mark Rutland Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Sami Tolvanen , Kees Cook , Catalin Marinas , Mark Rutland , Jann Horn , Ard Biesheuvel , Peter Zijlstra , kernel-team@android.com Subject: Re: [PATCH 4/6] scs: Move scs_overflow_check() out of architecture code Message-ID: <20200518132346.GD32394@willie-the-truck> References: <20200515172756.27185-1-will@kernel.org> <20200515172756.27185-5-will@kernel.org> <20200518121210.GD1957@C02TD0UTHF1T.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200518121210.GD1957@C02TD0UTHF1T.local> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 18, 2020 at 01:12:10PM +0100, Mark Rutland wrote: > On Fri, May 15, 2020 at 06:27:54PM +0100, Will Deacon wrote: > > There is nothing architecture-specific about scs_overflow_check() as > > it's just a trivial wrapper around scs_corrupted(). > > > > For parity with task_stack_end_corrupted(), rename scs_corrupted() to > > task_scs_end_corrupted() and call it from schedule_debug() when > > CONFIG_SCHED_STACK_END_CHECK_is enabled. Finally, remove the unused > > scs_overflow_check() function entirely. > > > > This has absolutely no impact on architectures that do not support SCS > > (currently arm64 only). > > > > Signed-off-by: Will Deacon > > Pulling this out of arch code seems sane to me, and the arch-specific > chanes look sound. However, I have a concern with the changes within the > scheduler context-switch. > > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > > index a35d3318492c..56be4cbf771f 100644 > > --- a/arch/arm64/kernel/process.c > > +++ b/arch/arm64/kernel/process.c > > @@ -52,7 +52,6 @@ > > #include > > #include > > #include > > -#include > > #include > > > > #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK) > > @@ -516,7 +515,6 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev, > > entry_task_switch(next); > > uao_thread_switch(next); > > ssbs_thread_switch(next); > > - scs_overflow_check(next); > > Prior to this patch, we'd never switch to a task whose SCS had already > been corrupted. > > With this patch, we only check that when switching away from a task, and > only when CONFIG_SCHED_STACK_END_CHECK is selected, which at first > glance seems to weaken that. Yes, ignoring vmap'd stacks, this patch brings the SCS checking in-line with the main stack checking when CONFIG_SCHED_STACK_END_CHECK=y. > Arguably: > > * If the next task's SCS was corrupted by that task while it was > running, we had already lost at that point. With this change, we'll at least catch this one sooner, and that might be useful if a bug has caused us to overflow the SCS but not the main stack. > * If the next task's SCS was corrupted by another task, then that could > also happen immediately after the check (though timing to avoid the > check but affect the process could be harder). We're only checking the magic end value, so the cross-task case is basically if you overrun your own SCS as above, but then continue to overrun entire SCSs for other tasks as well. It's probably not very useful in that case. > ... and a VMAP'd SCS would be much nicer in this regard. > > Do we think this is weakening the check, or do we think it wasn't all > that helpful to begin with? I see it as a debug check to catch SCS overflow, rather than a hardening feature, and I agree that using something like vmap stack for the SCS would be better because we could have a guard page instead. This is something I would like to revisit, but we need more information from Sami about why Android rejected the larger allocation size, since I don't think there's an awful lot of point merging this series if Android doesn't pick it up. Will