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=-0.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 99232C54FCB for ; Wed, 22 Apr 2020 17:51:24 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id ECB2B20882 for ; Wed, 22 Apr 2020 17:51:23 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="SbiBDMBj" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org ECB2B20882 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=chromium.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-18606-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 30632 invoked by uid 550); 22 Apr 2020 17:51:17 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 30604 invoked from network); 22 Apr 2020 17:51:16 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=ZF6h4B8lRkEW+PGdoi5ViWh9NdqvBm+plj3edg+BLxs=; b=SbiBDMBjNOJtZvz674h6CaZCEGdmafbK0qSIOSSMTCWnqd5jMXbDXqcct8K8mtTChe 4BmBYx34rurFf5imZkH6NB9jvIu0M0140rWbXixJsCfD/+8yfkJPCm/IHIsm3ZoRWfwb VIyeJQzY1bEBF+t0X5Jqa/2kR8k/V4WASdOqg= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=ZF6h4B8lRkEW+PGdoi5ViWh9NdqvBm+plj3edg+BLxs=; b=B6CZ/FxoHBBnKTORE7Pdxlx+gYZBMEMfTLbwpLGPaBcVhIqoKFhSq5Avd/zymU/xjs SWbUmDjaJ2Mgsmqh92Go/i3hgRPCwUpdfqQpb213VPyQ23J7zd+KHmLaX4nhHylYPuQP RrCuxCF54WkgOqbQgQavj58QuQxpljrS60w5nlVHLeZ1unkvzASlxt2Eg54hw2/rQir8 TpsrOi4I+WX9PPVcz3wReECorZrJcM7bH9rftTZRCDq4SryMcavIoUTB3eMk4leD/Im7 96TkSDvQ6Tv9OFCOzEDBXyLpmHlvI8zoark+6hGTdngNJgpYJAFPzDvaZfSYBSzBpl5b kTWA== X-Gm-Message-State: AGi0PubnqBBNpgR2LWRYFh2SyAVwaazfIik/O4j8KQQH2zS1H8Lmf/+S yn5r+uesNnhp2qXcWqqkCgkNIA== X-Google-Smtp-Source: APiQypKgukJRS2MpdwcmV4gR8X/qlLkQhVOoNqLyqZFCT1UTaoEDKt7OxFkygmvQ9SLxUbFYDjP/kg== X-Received: by 2002:a63:9e54:: with SMTP id r20mr141098pgo.301.1587577864965; Wed, 22 Apr 2020 10:51:04 -0700 (PDT) Date: Wed, 22 Apr 2020 10:51:02 -0700 From: Kees Cook To: Will Deacon 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: <202004221047.3AEAECC1@keescook> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200422173938.GA3069@willie-the-truck> 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(). -- Kees Cook