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 CB231C433ED for ; Tue, 4 May 2021 20:49:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9AD54613C7 for ; Tue, 4 May 2021 20:49:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232443AbhEDUuA (ORCPT ); Tue, 4 May 2021 16:50:00 -0400 Received: from mga12.intel.com ([192.55.52.136]:25517 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230150AbhEDUuA (ORCPT ); Tue, 4 May 2021 16:50:00 -0400 IronPort-SDR: BiTzOCpr/sRVblL917FJK67gdIB0IIsdm8d5yabb1QarmW8cV+3HtMntt3EPHU48yrwk2kNSV9 TNvhmUbOZfVw== X-IronPort-AV: E=McAfee;i="6200,9189,9974"; a="177614963" X-IronPort-AV: E=Sophos;i="5.82,272,1613462400"; d="scan'208";a="177614963" Received: from orsmga003.jf.intel.com ([10.7.209.27]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 May 2021 13:49:05 -0700 IronPort-SDR: 6KDt1n8pfcb9gJH0NzrYGgA5EZXyDWXzCYb41j9NgDFcQB0vnbTXvsm/VpOSZC2ClLdLm+iNKj OCE0Yn8pJhDw== X-IronPort-AV: E=Sophos;i="5.82,272,1613462400"; d="scan'208";a="388940055" Received: from yyu32-mobl1.amr.corp.intel.com (HELO [10.251.140.235]) ([10.251.140.235]) by orsmga003-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 May 2021 13:49:03 -0700 Subject: Re: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack) From: "Yu, Yu-cheng" 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> Message-ID: <5fc5dea4-0705-2aad-cf8f-7ff78a5e518a@intel.com> Date: Tue, 4 May 2021 13:49:02 -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: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On 4/30/2021 11:32 AM, Yu, Yu-cheng wrote: > 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. Personally, I think an explicit flag is cleaner than using a reserved word somewhere. However, there is a small issue: ia32 has no uc_flags. This series can support legacy apps up to now. But, instead of creating too many special cases, perhaps we should drop CET support of ia32? Thoughts? Thanks, Yu-cheng