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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 15FCDC43334 for ; Thu, 21 Jul 2022 08:22:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231730AbiGUIWC convert rfc822-to-8bit (ORCPT ); Thu, 21 Jul 2022 04:22:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53362 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231185AbiGUIWA (ORCPT ); Thu, 21 Jul 2022 04:22:00 -0400 Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.85.151]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 5DEDC7E012 for ; Thu, 21 Jul 2022 01:21:56 -0700 (PDT) Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-244-mS6qzph4P96-D1ySmr3PJw-1; Thu, 21 Jul 2022 09:21:53 +0100 X-MC-Unique: mS6qzph4P96-D1ySmr3PJw-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 15.0.1497.36; Thu, 21 Jul 2022 09:21:52 +0100 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.036; Thu, 21 Jul 2022 09:21:52 +0100 From: David Laight To: 'Peter Zijlstra' , Sami Tolvanen CC: Linus Torvalds , Thomas Gleixner , Joao Moreira , LKML , the arch/x86 maintainers , Tim Chen , Josh Poimboeuf , "Cooper, Andrew" , Pawan Gupta , Johannes Wikner , Alyssa Milburn , Jann Horn , "H.J. Lu" , "Moreira, Joao" , "Nuzman, Joseph" , "Steven Rostedt" , "Gross, Jurgen" , "Masami Hiramatsu" , Alexei Starovoitov , "Daniel Borkmann" , Peter Collingbourne , "Kees Cook" Subject: RE: [patch 00/38] x86/retbleed: Call depth tracking mitigation Thread-Topic: [patch 00/38] x86/retbleed: Call depth tracking mitigation Thread-Index: AQHYnH2Hi03gNC/QIkePUsvbFv4MOq2Ie8qw Date: Thu, 21 Jul 2022 08:21:52 +0000 Message-ID: <8177b804cfcb461d9f1f40d242cd84db@AcuMS.aculab.com> References: <87lesqukm5.ffs@tglx> <2f7f899cb75b79b08b0662ff4d2cb877@overdrivepizza.com> <87fsiyuhyz.ffs@tglx> In-Reply-To: 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-kernel@vger.kernel.org From: Peter Zijlstra > Sent: 20 July 2022 22:13 ... > The prettiest option to obscure the immediate at the callsite I could > conjure up is something like: > > kcfi_caller_linus: > movl $0x12345600, %r10d > movb $0x78, %r10b > cmpl %r10d, -OFFSET(%r11) > je 1f > ud2 > 1: call __x86_thunk_indirect_r11 > > Which comes to around 22 bytes (+5 over the original). You'd be better doing: movl $0x12345678-0xaa, %r10d addl $0xaa, %r10d so that the immediate is obscured even if the low bits are zero. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)