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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 3EB53C54FCB for ; Wed, 22 Apr 2020 18:02:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 205EE2082E for ; Wed, 22 Apr 2020 18:02:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1587578523; bh=FzQTGQ2Ix7C7yR7IfBH/DXbB5Ch65ICfo4tHoOW7nnI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=OxV7Xk8y+G6zLJRiYGCozCGY6DPRBGgw8aIlloulHf2Sq35NmnvSh0H4SkT2J84oF k8b03vtnDQ9ZyneXQzi7/JwLtafFX+tMWd9LCLC3w5637S1bRS2p8cqRNQ0aFlvjuF hM79iOlloaRGIh45WghJ2OKHMt211w0b0aFd5Mgk= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726851AbgDVSCC (ORCPT ); Wed, 22 Apr 2020 14:02:02 -0400 Received: from mail.kernel.org ([198.145.29.99]:55320 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726698AbgDVSCA (ORCPT ); Wed, 22 Apr 2020 14:02:00 -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 824A1214AF; Wed, 22 Apr 2020 18:01:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1587578520; bh=FzQTGQ2Ix7C7yR7IfBH/DXbB5Ch65ICfo4tHoOW7nnI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=0A/YQX2JzsYbuIiD4/c6g+qjcG+aB9pdJitG9h83SJHB7+jTxNwY5IDmnL47zNJ6V qkPfXvBBgDtNOmFzFEVREiwbBFjVhVldeCzCnsxfJ4wJIjmCpYs/DAm781XyRGitZZ 7lfgUik1elBgd5oRg2rtr0BDR5SgI2rZBf6TZ7fc= Date: Wed, 22 Apr 2020 19:01:53 +0100 From: Will Deacon To: Kees Cook Cc: Sami Tolvanen , Catalin Marinas , James Morse , Steven Rostedt , Ard Biesheuvel , Mark Rutland , Masahiro Yamada , Michal Marek , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dave Martin , Laura Abbott , Marc Zyngier , Masami Hiramatsu , Nick Desaulniers , Jann Horn , Miguel Ojeda , clang-built-linux@googlegroups.com, kernel-hardening@lists.openwall.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v11 01/12] add support for Clang's Shadow Call Stack (SCS) Message-ID: <20200422180153.GD3121@willie-the-truck> References: <20191018161033.261971-1-samitolvanen@google.com> <20200416161245.148813-1-samitolvanen@google.com> <20200416161245.148813-2-samitolvanen@google.com> <20200420171727.GB24386@willie-the-truck> <20200420211830.GA5081@google.com> <20200422173938.GA3069@willie-the-truck> <202004221047.3AEAECC1@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <202004221047.3AEAECC1@keescook> 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 Wed, Apr 22, 2020 at 10:51:02AM -0700, Kees Cook wrote: > On Wed, Apr 22, 2020 at 06:39:47PM +0100, Will Deacon wrote: > > On Mon, Apr 20, 2020 at 02:18:30PM -0700, Sami Tolvanen wrote: > > > On Mon, Apr 20, 2020 at 06:17:28PM +0100, Will Deacon wrote: > > > > > + * The shadow call stack is aligned to SCS_SIZE, and grows > > > > > + * upwards, so we can mask out the low bits to extract the base > > > > > + * when the task is not running. > > > > > + */ > > > > > + return (void *)((unsigned long)task_scs(tsk) & ~(SCS_SIZE - 1)); > > > > > > > > Could we avoid forcing this alignment it we stored the SCS pointer as a > > > > (base,offset) pair instead? That might be friendlier on the allocations > > > > later on. > > > > > > The idea is to avoid storing the current task's shadow stack address in > > > memory, which is why I would rather not store the base address either. > > > > What I mean is that, instead of storing the current shadow stack pointer, > > we instead store a base and an offset. We can still clear the base, as you > > do with the pointer today, and I don't see that the offset is useful to > > an attacker on its own. > > > > But more generally, is it really worthwhile to do this clearing at all? Can > > you (or Kees?) provide some justification for it, please? We don't do it > > for anything else, e.g. the pointer authentication keys, so something > > feels amiss here. > > It's a hardening step to just reduce the lifetime of a valid address > exposed in memory. In fact, since there is a cache, I think it should be > wiped even in scs_release(). But we don't do this for /anything/ else and it forces alignment restrictions on the SCS allocation. Please either do it consistently, or not at all. Will