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=-3.8 required=3.0 tests=BAYES_00, 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 ADD6AC433B4 for ; Thu, 1 Apr 2021 17:46:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8CF7F61284 for ; Thu, 1 Apr 2021 17:46:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235492AbhDARpx convert rfc822-to-8bit (ORCPT ); Thu, 1 Apr 2021 13:45:53 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:34696 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234796AbhDARkQ (ORCPT ); Thu, 1 Apr 2021 13:40:16 -0400 Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-236-55FOPC1yNrCSB69XR94FFw-1; Thu, 01 Apr 2021 12:15:44 +0100 X-MC-Unique: 55FOPC1yNrCSB69XR94FFw-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) by AcuMS.aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Thu, 1 Apr 2021 12:15:43 +0100 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.012; Thu, 1 Apr 2021 12:15:43 +0100 From: David Laight To: 'Will Deacon' , Kees Cook CC: Thomas Gleixner , Elena Reshetova , "x86@kernel.org" , "Andy Lutomirski" , Peter Zijlstra , "Catalin Marinas" , Mark Rutland , Alexander Potapenko , Alexander Popov , Ard Biesheuvel , Jann Horn , Vlastimil Babka , David Hildenbrand , Mike Rapoport , Andrew Morton , Jonathan Corbet , Randy Dunlap , "kernel-hardening@lists.openwall.com" , "linux-hardening@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" Subject: RE: [PATCH v8 3/6] stack: Optionally randomize kernel stack offset each syscall Thread-Topic: [PATCH v8 3/6] stack: Optionally randomize kernel stack offset each syscall Thread-Index: AQHXJtFkunedSLxfaUq504kizLaNqaqfgZOw Date: Thu, 1 Apr 2021 11:15:43 +0000 Message-ID: <61ae9398a03d4fe7868b68c9026d5998@AcuMS.aculab.com> References: <20210330205750.428816-1-keescook@chromium.org> <20210330205750.428816-4-keescook@chromium.org> <20210401083034.GA8554@willie-the-truck> In-Reply-To: <20210401083034.GA8554@willie-the-truck> Accept-Language: en-GB, 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-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-hardening@vger.kernel.org From: Will Deacon > Sent: 01 April 2021 09:31 ... > > +/* > > + * These macros must be used during syscall entry when interrupts and > > + * preempt are disabled, and after user registers have been stored to > > + * the stack. > > + */ > > +#define add_random_kstack_offset() do { \ > > + if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ > > + &randomize_kstack_offset)) { \ > > + u32 offset = __this_cpu_read(kstack_offset); \ > > + u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset)); \ > > + asm volatile("" : "=m"(*ptr) :: "memory"); \ > > Using the "m" constraint here is dangerous if you don't actually evaluate it > inside the asm. For example, if the compiler decides to generate an > addressing mode relative to the stack but with writeback (autodecrement), then > the stack pointer will be off by 8 bytes. Can you use "o" instead? Is it allowed to use such a mode? It would have to know that the "m" was substituted exactly once. I think there are quite a few examples with 'strange' uses of memory asm arguments. However, in this case, isn't it enough to ensure the address is 'saved'? So: asm volatile("" : "=r"(ptr) ); should be enough. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)