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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,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 C6B8FC43462 for ; Fri, 30 Apr 2021 18:32:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 888CE613FA for ; Fri, 30 Apr 2021 18:32:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231652AbhD3Sdk (ORCPT ); Fri, 30 Apr 2021 14:33:40 -0400 Received: from mga06.intel.com ([134.134.136.31]:44389 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231204AbhD3Sdh (ORCPT ); Fri, 30 Apr 2021 14:33:37 -0400 IronPort-SDR: cDH+ydm3F3DplhIR4xQRkjN44Uz0eqO4qXpLj66XWX7W6MDjH6a9FzzKYJIfaD3ggGbwZXs7jH u0o6Q9Z9+Hxg== X-IronPort-AV: E=McAfee;i="6200,9189,9970"; a="258620576" X-IronPort-AV: E=Sophos;i="5.82,263,1613462400"; d="scan'208";a="258620576" Received: from orsmga004.jf.intel.com ([10.7.209.38]) by orsmga104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Apr 2021 11:32:48 -0700 IronPort-SDR: BuEEwoZVskVpKur8QODMKzjWucceyRG+8FXzCydVCLpFg3qZhfZ48pTkcyex1irrACKguT+/Lo QORo15MQgF7A== X-IronPort-AV: E=Sophos;i="5.82,263,1613462400"; d="scan'208";a="537852516" Received: from yyu32-mobl1.amr.corp.intel.com (HELO [10.212.119.226]) ([10.212.119.226]) by orsmga004-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Apr 2021 11:32:46 -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: Date: Fri, 30 Apr 2021 11:32:45 -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: 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 4/30/2021 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: >>> >>> The kernel has: >>> >>> struct rt_sigframe { >>> char __user *pretcode; >>> struct ucontext uc; >>> struct siginfo info; >>> /* fp state follows here */ >>> }; >>> >>> This is roughly the actual signal frame. But userspace does not have >>> this struct declared, and user code does not know the sizes of the >>> fields. So it's accessed in a nonsensical way. The signal handler >>> function is passed a pointer to the whole sigframe implicitly in RSP, >>> a pointer to &frame->info in RSI, anda pointer to &frame->uc in RDX. >>> User code can *find* the fp state by following a pointer from >>> mcontext, which is, in turn, found via uc: >>> >>> struct ucontext { >>> unsigned long uc_flags; >>> struct ucontext *uc_link; >>> stack_t uc_stack; >>> struct sigcontext uc_mcontext; <-- fp pointer is in here >>> sigset_t uc_sigmask; /* mask last for extensibility */ >>> }; >>> >>> The kernel, in sigreturn, works a bit differently. The sigreturn >>> variants know the base address of the frame but don't have the benefit >>> of receiving pointers to the fields. So instead the kernel takes >>> advantage of the fact that it knows the offset to uc and parses uc >>> accordingly. And the kernel follows the pointer in mcontext to find >>> the fp state. The latter bit is quite important later. The kernel >>> does not parse info at all. >>> >>> The fp state is its own mess. When XSAVE happened, Intel kindly (?) >>> gave us a software defined area between the "legacy" x87 region and >>> the modern supposedly extensible part. Linux sticks the following >>> structure in that hole: >>> >>> struct _fpx_sw_bytes { >>> /* >>> * If set to FP_XSTATE_MAGIC1 then this is an xstate context. >>> * 0 if a legacy frame. >>> */ >>> __u32 magic1; >>> >>> /* >>> * Total size of the fpstate area: >>> * >>> * - if magic1 == 0 then it's sizeof(struct _fpstate) >>> * - if magic1 == FP_XSTATE_MAGIC1 then it's sizeof(struct _xstate) >>> * plus extensions (if any) >>> */ >>> __u32 extended_size; >>> >>> /* >>> * Feature bit mask (including FP/SSE/extended state) that is present >>> * in the memory layout: >>> */ >>> __u64 xfeatures; >>> >>> /* >>> * Actual XSAVE state size, based on the xfeatures saved in the layout. >>> * 'extended_size' is greater than 'xstate_size': >>> */ >>> __u32 xstate_size; >>> >>> /* For future use: */ >>> __u32 padding[7]; >>> }; >>> >>> >>> 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. > >> >> However, the WAIT_ENDBR status needs to be saved/restored for signals. >> Since IBT is now dependent on shadow stack, we can use a spare bit of >> the shadow stack restore token for that. > > That seems like unnecessary ABI coupling. We have plenty of bits in > uc_flags, and we have an entire reserved word in sigcontext. How > about just sticking this bit in one of those places? Yes, I will make it UC_WAIT_ENDBR. Thanks, Yu-cheng