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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 EF0D4C83003 for ; Wed, 29 Apr 2020 08:39:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D09DD20731 for ; Wed, 29 Apr 2020 08:39:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726477AbgD2Ijk convert rfc822-to-8bit (ORCPT ); Wed, 29 Apr 2020 04:39:40 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([146.101.78.151]:57053 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726345AbgD2Ijj (ORCPT ); Wed, 29 Apr 2020 04:39:39 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-153-r75nPERROFeEoEJIwkJHxw-1; Wed, 29 Apr 2020 09:39:34 +0100 X-MC-Unique: r75nPERROFeEoEJIwkJHxw-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Wed, 29 Apr 2020 09:39:33 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Wed, 29 Apr 2020 09:39:33 +0100 From: David Laight To: 'Sami Tolvanen' , Ard Biesheuvel CC: Will Deacon , Catalin Marinas , James Morse , Steven Rostedt , "Ard Biesheuvel" , Mark Rutland , Masahiro Yamada , Michal Marek , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Kees Cook , Jann Horn , Marc Zyngier , "kernel-hardening@lists.openwall.com" , Nick Desaulniers , Linux Kernel Mailing List , Miguel Ojeda , Masami Hiramatsu , clang-built-linux , Laura Abbott , Dave Martin , Linux ARM Subject: RE: [PATCH v13 00/12] add support for Clang's Shadow Call Stack Thread-Topic: [PATCH v13 00/12] add support for Clang's Shadow Call Stack Thread-Index: AQHWHOCVWHSK1xvpOUef91FgkS/f7KiPxyTg Date: Wed, 29 Apr 2020 08:39:33 +0000 Message-ID: <6762b8d0974d49de80c3b398d714b3fb@AcuMS.aculab.com> References: <20191018161033.261971-1-samitolvanen@google.com> <20200427160018.243569-1-samitolvanen@google.com> <20200427220942.GB80713@google.com> In-Reply-To: <20200427220942.GB80713@google.com> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Sami Tolvanen > Sent: 27 April 2020 23:10 ... > > > Alternatively, I wonder if there is a way we could let the SCS and > > > ordinary stack share the [bottom of] the vmap'ed region. That would > > > give rather nasty results if the ordinary stack overflows into the > > > SCS, but for cases where we really recurse out of control, we could > > > catch this occurrence on either stack, whichever one occurs first. And > > > the nastiness -when it does occur- will not corrupt any state beyond > > > the stack of the current task. > > > > Hmm, I guess that would make it quite hard to keep the SCS address > > secret though :-( > > Yes, and the stack potentially overflowing into the SCS sort of defeats > the purpose. I'm fine with increasing the SCS size to something safer, > but using a vmapped shadow stack seems like the correct solution to this > problem, at least on devices where allocating a full page isn't an issue. Wouldn't you do it the other way around - so shadow stack overflow corrupts the bottom of the normal stack? That can be detected 'after the fact' in a few places (eg process switch and return to user) Actually you might want to do syscall entry at the base of stack area, then (effectively) allocate an on-stack buffer for the shadow stack. I'd have though that kernel code could be the shadow stack address by just reading r18? Userspace isn't supposed to be able to get the main kernel stack address either. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)