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 1CFCDC55191 for ; Thu, 23 Apr 2020 18:29:04 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 746D92071C for ; Thu, 23 Apr 2020 18:29:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="b6zdPxuh" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 746D92071C 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-18618-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 28317 invoked by uid 550); 23 Apr 2020 18:28:55 -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 28291 invoked from network); 23 Apr 2020 18:28:54 -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=6Y/9ITAGgGN1ObZLXZNElUJooxFBQUxzx7tXw0tLGrw=; b=b6zdPxuhwvf6mbGggB4PksAI1ex0t7K0elOYcg58ZwrTjn8AjcWwZGTTkOt8xtEop1 QgJUSGvj+wJ1HD5HRDh4RvxxaMJq6yj8Ef8yIBSVdRMjVYw9bGTWpKc07ah08JcYCbF3 RdvWKOb7jr8FUoUPlrx507urw3KMBQUv7F7bI= 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=6Y/9ITAGgGN1ObZLXZNElUJooxFBQUxzx7tXw0tLGrw=; b=NzsefbLPs3s5EmqSd/5w28PWd9Q5Mq2bUZhaZTA6DJ8RKFIqKe7ueaIZJNtChMDfTD 9k+s8C8q4pkd0mpFPFD99hGjDRebj30YI4a1HYZyWREpdM8n64aBEZNlmqXmglvdTPdK X/YmTiBSvKFkJO19eawpxwsi0E4Yl5bPpT4sRIpT9YRczawGLRR0wZFsutO8kZ6Ch67C SZ31Tt4HuZH6ZyoN2oqMMIfkRMw7wI9uUziuTfUxaGj+d3jz7jOgm4k+ehDIRpu8jZKV A+kPj//8BG0JWscLLeJWqzmWNwGUnhCMpSt/biEN8XwBoTFndPjtv0DOMbfm3GDIcs61 ngeg== X-Gm-Message-State: AGi0PuYGWMHVMXgnaXb3ovLCswD67avVhNqtOPiKMLHU6Zd4KEv4A4cM CgTFLuZKOybBI6WhC35+I2vLhA== X-Google-Smtp-Source: APiQypL3X1xlYzw2iOcrrAvDuDiryUSnxNiAeJYrDux+iWD+qj2k8HUN3EnuJVtWslaYhtMemGHUDw== X-Received: by 2002:a62:1c97:: with SMTP id c145mr5322854pfc.68.1587666522523; Thu, 23 Apr 2020 11:28:42 -0700 (PDT) Date: Thu, 23 Apr 2020 11:28:40 -0700 From: Kees Cook To: Sami Tolvanen Cc: Will Deacon , 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: <202004231121.A13FDA100@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> <20200422235134.GA211149@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200422235134.GA211149@google.com> On Wed, Apr 22, 2020 at 04:51:34PM -0700, Sami Tolvanen 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. > > I see what you mean. However, even if we store the base address + > the offset, we still need aligned allocation if we want to clear > the address. This would basically just move __scs_base() logic to > cpu_switch_to() / scs_save(). Okay, so, I feel like this has gotten off into the weeds, or I'm really dense (or both). :) Going back to the original comment: > > > > 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. I think there was some confusion about mixing the "we want to be able to wipe the value" combined with the masking in __scs_base(). These are unrelated, as was correctly observed with "We can still clear the base". What I don't understand here is the suggestion to store two values: Why is two better than storing one? With one, we only need a single access. Why would storing the base be "friendlier on the allocations later on"? This is coming out of a single kmem cache, in 1K chunks. They will be naturally aligned to 1K (unless redzoing has been turned on for some slab debugging reason). The base masking is a way to avoid needing to store two values, and only happens at task death. Storing two values eats memory for all tasks for seemingly no meaningful common benefit. What am I missing here? -- Kees Cook