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 6C764C43470 for ; Mon, 3 May 2021 20:25:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4DE04611CB for ; Mon, 3 May 2021 20:25:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229658AbhECU0r (ORCPT ); Mon, 3 May 2021 16:26:47 -0400 Received: from mga18.intel.com ([134.134.136.126]:1357 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229472AbhECU0n (ORCPT ); Mon, 3 May 2021 16:26:43 -0400 IronPort-SDR: zmVTVZ7wQHoUYASfMcjgug3YRPRUOQoSaIHy1AbJSi5ngJvWfhmVQAMPPnchAyaf8l2lCJPqVF edkyx36mslIA== X-IronPort-AV: E=McAfee;i="6200,9189,9973"; a="185300685" X-IronPort-AV: E=Sophos;i="5.82,270,1613462400"; d="scan'208";a="185300685" Received: from orsmga001.jf.intel.com ([10.7.209.18]) by orsmga106.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 03 May 2021 13:25:49 -0700 IronPort-SDR: j6W8Y+3CGbViGBD7u64rbmmAj2twhGDrTkWyR+4rqnaSG5yKCy1h1YedTyCLcYvHDTNJRSwYGH TRcfe9UbLFwQ== X-IronPort-AV: E=Sophos;i="5.82,270,1613462400"; d="scan'208";a="468203041" Received: from yyu32-mobl1.amr.corp.intel.com (HELO [10.251.140.183]) ([10.251.140.183]) by orsmga001-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 03 May 2021 13:25:46 -0700 Subject: Re: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack) To: Andy Lutomirski Cc: Andy Lutomirski , 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: <782ffe96-b830-d13b-db80-5b60f41ccdbf@intel.com> <2D8926E4-F1B6-433A-96EA-995A66F3F42D@amacapital.net> From: "Yu, Yu-cheng" Message-ID: Date: Mon, 3 May 2021 13:25:45 -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: <2D8926E4-F1B6-433A-96EA-995A66F3F42D@amacapital.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/3/2021 8:29 AM, Andy Lutomirski wrote: > >> On May 3, 2021, at 8:14 AM, Yu, Yu-cheng wrote: >> >> 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. > > I think we should make the ABI support an alternate shadow stack even if we don’t implement it initially. After all, some day someone might want to register a handler for shadow stack overflow. > Agree. We can probably add something in parallel of sigaltstack(), and let the user choose separately alternate normal/shadow stacks. Thanks, Yu-cheng