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=-1.0 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 1143DC433E0 for ; Tue, 23 Jun 2020 13:42:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E86AA2070E for ; Tue, 23 Jun 2020 13:42:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732732AbgFWNmf convert rfc822-to-8bit (ORCPT ); Tue, 23 Jun 2020 09:42:35 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:43182 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732631AbgFWNme (ORCPT ); Tue, 23 Jun 2020 09:42:34 -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-42-tuWEcT22NmmyTjcRjBXV1g-1; Tue, 23 Jun 2020 14:42:31 +0100 X-MC-Unique: tuWEcT22NmmyTjcRjBXV1g-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; Tue, 23 Jun 2020 14:42:30 +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; Tue, 23 Jun 2020 14:42:30 +0100 From: David Laight To: 'Kees Cook' , Arvind Sankar CC: Thomas Gleixner , Elena Reshetova , "x86@kernel.org" , "Andy Lutomirski" , Peter Zijlstra , "Catalin Marinas" , Will Deacon , "Mark Rutland" , Alexander Potapenko , Alexander Popov , Ard Biesheuvel , Jann Horn , "kernel-hardening@lists.openwall.com" , "linux-arm-kernel@lists.infradead.org" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" Subject: RE: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset each syscall Thread-Topic: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset each syscall Thread-Index: AQHWSPkh/p0bc5D3iUelrjbiR4wWCajmNFDA Date: Tue, 23 Jun 2020 13:42:30 +0000 Message-ID: <917458f43d194385a760d75292d4eb47@AcuMS.aculab.com> References: <20200622193146.2985288-1-keescook@chromium.org> <20200622193146.2985288-4-keescook@chromium.org> <20200622225615.GA3511702@rani.riverdale.lan> <202006221604.871B13DE3@keescook> <20200623000510.GA3542245@rani.riverdale.lan> <202006221748.DA27A7FFC@keescook> In-Reply-To: <202006221748.DA27A7FFC@keescook> 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 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com 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: Kees Cook > Sent: 23 June 2020 01:56 > On Mon, Jun 22, 2020 at 08:05:10PM -0400, Arvind Sankar wrote: > > But I still don't see anything _stopping_ the compiler from optimizing > > this better in the future. The "=m" is not a barrier: it just informs > > the compiler that the asm produces an output value in *ptr (and no other > > outputs). If nothing can consume that output, it doesn't stop the > > compiler from freeing the allocation immediately after the asm instead > > of at the end of the function. > > Ah, yeah, I get what you mean. > > > I'm talking about something like > > asm volatile("" : : "r" (ptr) : "memory"); > > which tells the compiler that the asm may change memory arbitrarily. > > Yeah, I will adjust it. > > > Here, we don't use it really as a barrier, but to tell the compiler that > > the asm may have stashed the value of ptr somewhere in memory, so it's > > not free to reuse the space that it pointed to until the function > > returns (unless it can prove that nothing accesses memory, not just that > > nothing accesses ptr). Do you need another asm volatile("" : : "r" (ptr) : "memory"); (or similar) at the bottom of the function - that the compiler thinks might access the memory whose address it thought got saved earlier? I wonder if it would be easier to allocate the stack space in the asm wrapper? At least as an architecture option. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)