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=-6.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 E2EC8C433ED for ; Thu, 6 May 2021 23:31:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C089B613B5 for ; Thu, 6 May 2021 23:31:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232391AbhEFXcW (ORCPT ); Thu, 6 May 2021 19:32:22 -0400 Received: from mail.kernel.org ([198.145.29.99]:52910 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232209AbhEFXcS (ORCPT ); Thu, 6 May 2021 19:32:18 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 04051613E9 for ; Thu, 6 May 2021 23:31:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1620343880; bh=IITCyoi4hOnQGgAjRVqRr7RtIsMxc4kk9HMuRUc4kAA=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=dzOaN+y3YvGy/qiIgwPQ336zp8h4Qgn5S+x18V/7U5Z832vjobSnQaneW+3LoIEKJ 2a2jNix2ICu4wrie+QaMY1ZYi5JeLqwvsNAzAkbGZZ/nUllk3sHS9tkSZvZjgqpkC9 kCK+x05evvBumC0UY5a9mo0bnq7OW0c0GyYLbShG175267sg3d+ec8jXISGY66r0AR AbpkldqZtEenweVoVFX9nG8O6guWKtwnusnBxzl23d3PVIca/bebwBdDsB6yXoXDDJ ExL5i7BsuB+eGTaLABjvVKpR9zfL2ZkRE2jCVJbxEAjQIK6W00wxgMqVOOgqE5CPd4 7TyKigSjS9thg== Received: by mail-ej1-f46.google.com with SMTP id l4so10759129ejc.10 for ; Thu, 06 May 2021 16:31:19 -0700 (PDT) X-Gm-Message-State: AOAM532s4j3TIzD/9MztPQoNOHXGq+o6x71O788KJBIvBYJ3IrGpAkHB bibssITBLIpTITQQCfFKYkan3EIzVecGbMcJs1/u9Q== X-Google-Smtp-Source: ABdhPJy/HzUeyVBBOPMQzBeIl7qYyB1Cn0Bw3qIUSRljI0+cxPtL7Jufl+V2qYgyD4Kr53bGa8ynkwEWn9q1gDRzC2M= X-Received: by 2002:a17:906:c010:: with SMTP id e16mr7123175ejz.214.1620343878143; Thu, 06 May 2021 16:31:18 -0700 (PDT) MIME-Version: 1.0 References: <20210427204315.24153-1-yu-cheng.yu@intel.com> <20210427204315.24153-26-yu-cheng.yu@intel.com> <8fd86049-930d-c9b7-379c-56c02a12cd77@intel.com> <5fc5dea4-0705-2aad-cf8f-7ff78a5e518a@intel.com> In-Reply-To: From: Andy Lutomirski Date: Thu, 6 May 2021 16:31:06 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack) To: "Yu, Yu-cheng" 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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 6, 2021 at 3:05 PM Yu, Yu-cheng wrote: > > On 5/4/2021 1:49 PM, Yu, Yu-cheng wrote: > > 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. > >>>>> > > [...] > > >>>>> > >>>>> 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? I'm really not thrilled about coupling IBT and SHSTK like this. Here are a couple of possible solutions: - Don't support IBT in 32-bit mode, or maybe just don't support IBT with legacy 32-bit signals. The actual mechanics of this could be awkward. Maybe we would reject the sigaction() call or the IBT-enabling request if they conflict? - Find some space in the signal frame for these flags. Looking around a bit, sigframe_ia32 has fpstate_unused, but I can imagine things like CRIU getting very confused if it stops being unused. sigframe_ia32 uses sigcontext_32, which has a bunch of reserved space in __gsh, __fsh, etc. rt_sigframe_ia32 has uc_flags, so this isn't a real problem. I don't have a brilliant solution here.