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 C9ABCC433E0 for ; Tue, 23 Jun 2020 13:42:52 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 3325A2070E for ; Tue, 23 Jun 2020 13:42:52 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3325A2070E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=ACULAB.COM Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19066-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 12025 invoked by uid 550); 23 Jun 2020 13:42:45 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 11988 invoked from network); 23 Jun 2020 13:42:44 -0000 X-MC-Unique: tuWEcT22NmmyTjcRjBXV1g-1 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: quoted-printable 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 "=3Dm" is not a barrier: it just informs > > the compiler that the asm produces an output value in *ptr (and no othe= r > > 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. >=20 > Ah, yeah, I get what you mean. >=20 > > I'm talking about something like > > =09asm volatile("" : : "r" (ptr) : "memory"); > > which tells the compiler that the asm may change memory arbitrarily. >=20 > Yeah, I will adjust it. >=20 > > Here, we don't use it really as a barrier, but to tell the compiler tha= t > > 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 tha= t > > 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. =09David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)