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 4C4B8C433EF for ; Thu, 21 Jul 2022 10:56:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232326AbiGUK4w convert rfc822-to-8bit (ORCPT ); Thu, 21 Jul 2022 06:56:52 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52392 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229661AbiGUK4u (ORCPT ); Thu, 21 Jul 2022 06:56:50 -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 0501E7E803 for ; Thu, 21 Jul 2022 03:56:49 -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-320-fQeKd-7MMEaPmiYsKAFFNQ-1; Thu, 21 Jul 2022 11:56:46 +0100 X-MC-Unique: fQeKd-7MMEaPmiYsKAFFNQ-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 11:56:45 +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 11:56:45 +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/QIkePUsvbFv4MOq2Ie8qwgAAqTiA= Date: Thu, 21 Jul 2022 10:56:45 +0000 Message-ID: References: <87lesqukm5.ffs@tglx> <2f7f899cb75b79b08b0662ff4d2cb877@overdrivepizza.com> <87fsiyuhyz.ffs@tglx> <8177b804cfcb461d9f1f40d242cd84db@AcuMS.aculab.com> In-Reply-To: <8177b804cfcb461d9f1f40d242cd84db@AcuMS.aculab.com> 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: David Laight > Sent: 21 July 2022 09:22 > > 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. Actually, can't you use %eax instead of %r10d? IIRC it is only used for the number of FP registers in a varargs call - and that isn't used in the kernel. That removes the 3 'REG' prefixes and lets you use the 2-byte 04-xx instruction to add to %al. Although I'm sure I remember something about a penalty for accessing %al just after the full register. So the 3-byte sign extending add may be better. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)