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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,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 737BBC43462 for ; Mon, 3 May 2021 15:14:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4213F6121D for ; Mon, 3 May 2021 15:14:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230330AbhECPOy (ORCPT ); Mon, 3 May 2021 11:14:54 -0400 Received: from mga12.intel.com ([192.55.52.136]:2810 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229717AbhECPOw (ORCPT ); Mon, 3 May 2021 11:14:52 -0400 IronPort-SDR: mQQqslQ6I2Eg5q0jPFgJ5fgJmbuvyCnqbEQS+G44c1t+zN1RrSZUI4q/bayx4ytp6gXCq1NuAr XhtXIfVrigpA== X-IronPort-AV: E=McAfee;i="6200,9189,9973"; a="177294938" X-IronPort-AV: E=Sophos;i="5.82,270,1613462400"; d="scan'208";a="177294938" Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 03 May 2021 08:13:59 -0700 IronPort-SDR: pWcsJXyFsd7b5TPpL4Vw5oqr/3WXl+APLR9msm7QUhLFaWu0dgX6ieePzgaMhnw05H/y9AQ1zU zwVegWqY9wEQ== X-IronPort-AV: E=Sophos;i="5.82,270,1613462400"; d="scan'208";a="462597827" Received: from ijholwag-mobl1.amr.corp.intel.com (HELO [10.251.140.183]) ([10.251.140.183]) by fmsmga002-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 03 May 2021 08:13:58 -0700 Subject: Re: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack) To: Andy Lutomirski Cc: linux-arch , X86 ML , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , LKML , "open list:DOCUMENTATION" , Linux-MM , Linux API , Arnd Bergmann , Balbir Singh , Borislav Petkov , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H.J. Lu" , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra , Randy Dunlap , "Ravi V. Shankar" , Vedvyas Shanbhogue , Dave Martin , Weijiang Yang , Pengfei Xu , Haitao Huang References: <20210427204315.24153-1-yu-cheng.yu@intel.com> <20210427204315.24153-26-yu-cheng.yu@intel.com> <8fd86049-930d-c9b7-379c-56c02a12cd77@intel.com> From: "Yu, Yu-cheng" Message-ID: <782ffe96-b830-d13b-db80-5b60f41ccdbf@intel.com> Date: Mon, 3 May 2021 08:13:57 -0700 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/2/2021 4:23 PM, Andy Lutomirski wrote: > On Fri, Apr 30, 2021 at 10:47 AM Andy Lutomirski wrote: >> >> On Fri, Apr 30, 2021 at 10:00 AM Yu, Yu-cheng wrote: >>> >>> On 4/28/2021 4:03 PM, Andy Lutomirski wrote: >>>> On Tue, Apr 27, 2021 at 1:44 PM Yu-cheng Yu wrote: >>>>> >>>>> When shadow stack is enabled, a task's shadow stack states must be saved >>>>> along with the signal context and later restored in sigreturn. However, >>>>> currently there is no systematic facility for extending a signal context. >>>>> There is some space left in the ucontext, but changing ucontext is likely >>>>> to create compatibility issues and there is not enough space for further >>>>> extensions. >>>>> >>>>> Introduce a signal context extension struct 'sc_ext', which is used to save >>>>> shadow stack restore token address. The extension is located above the fpu >>>>> states, plus alignment. The struct can be extended (such as the ibt's >>>>> wait_endbr status to be introduced later), and sc_ext.total_size field >>>>> keeps track of total size. >>>> >>>> I still don't like this. >>>> >>>> Here's how the signal layout works, for better or for worse: >>>> [...] >>>> >>>> That's where we are right now upstream. The kernel has a parser for >>>> the FPU state that is bugs piled upon bugs and is going to have to be >>>> rewritten sometime soon. On top of all this, we have two upcoming >>>> features, both of which require different kinds of extensions: >>>> >>>> 1. AVX-512. (Yeah, you thought this story was over a few years ago, >>>> but no. And AMX makes it worse.) To make a long story short, we >>>> promised user code many years ago that a signal frame fit in 2048 >>>> bytes with some room to spare. With AVX-512 this is false. With AMX >>>> it's so wrong it's not even funny. The only way out of the mess >>>> anyone has come up with involves making the length of the FPU state >>>> vary depending on which features are INIT, i.e. making it more compact >>>> than "compact" mode is. This has a side effect: it's no longer >>>> possible to modify the state in place, because enabling a feature with >>>> no space allocated will make the structure bigger, and the stack won't >>>> have room. Fortunately, one can relocate the entire FPU state, update >>>> the pointer in mcontext, and the kernel will happily follow the >>>> pointer. So new code on a new kernel using a super-compact state >>>> could expand the state by allocating new memory (on the heap? very >>>> awkwardly on the stack?) and changing the pointer. For all we know, >>>> some code already fiddles with the pointer. This is great, except >>>> that your patch sticks more data at the end of the FPU block that no >>>> one is expecting, and your sigreturn code follows that pointer, and >>>> will read off into lala land. >>>> >>> >>> Then, what about we don't do that at all. Is it possible from now on we >>> don't stick more data at the end, and take the relocating-fpu approach? >>> >>>> 2. CET. CET wants us to find a few more bytes somewhere, and those >>>> bytes logically belong in ucontext, and here we are. >>>> >>> >>> Fortunately, we can spare CET the need of ucontext extension. When the >>> kernel handles sigreturn, the user-mode shadow stack pointer is right at >>> the restore token. There is no need to put that in ucontext. >> >> That seems entirely reasonable. This might also avoid needing to >> teach CRIU about CET at all. > > Wait, what's the actual shadow stack token format? And is the token > on the new stack or the old stack when sigaltstack is in use? For > that matter, is there any support for an alternate shadow stack for > signals? > The restore token is a pointer pointing directly above itself and bit[0] indicates 64-bit mode. Because the shadow stack stores only return addresses, there is no alternate shadow stack. However, the application can allocate and switch to a new shadow stack. Yu-cheng