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_PASS autolearn=ham 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 B0D28C43219 for ; Fri, 3 May 2019 16:41:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8A4762087F for ; Fri, 3 May 2019 16:41:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728488AbfECQk7 convert rfc822-to-8bit (ORCPT ); Fri, 3 May 2019 12:40:59 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:58966 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727780AbfECQk6 (ORCPT ); Fri, 3 May 2019 12:40:58 -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-192-OaTw3byYN0Kl1wNCyn9k8g-1; Fri, 03 May 2019 17:40:55 +0100 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; Fri, 3 May 2019 17:40:53 +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; Fri, 3 May 2019 17:40:53 +0100 From: David Laight To: "'Reshetova, Elena'" , Ingo Molnar CC: Andy Lutomirski , Theodore Ts'o , "Eric Biggers" , "ebiggers@google.com" , "herbert@gondor.apana.org.au" , Peter Zijlstra , "keescook@chromium.org" , Daniel Borkmann , "linux-kernel@vger.kernel.org" , "jpoimboe@redhat.com" , "jannh@google.com" , "Perla, Enrico" , "mingo@redhat.com" , "bp@alien8.de" , "tglx@linutronix.de" , "gregkh@linuxfoundation.org" , "Edgecombe, Rick P" , Linus Torvalds , Peter Zijlstra Subject: RE: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall Thread-Topic: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall Thread-Index: AQHU9E1UquBTkhVACE2y3BuRFoekIqY8wW2AgAAdM1CAAXexAIAANZ3ggAAW1gCAAApRgIAAMeKAgAAd+PCAAQuGgIAAYQuAgAAKhwCACsPi4IADJTwAgAAcagCAAExngIAEBbGAgACIbACAAbyQ8IAA626AgAGZfXCAAARpgIAAWpuAgAAF74CAABf/AIAAAvkAgAGZnrCAAAQUUA== Date: Fri, 3 May 2019 16:40:53 +0000 Message-ID: References: <57357E35-3D9B-4CA7-BAB9-0BE89E0094D2@amacapital.net> <2236FBA76BA1254E88B949DDB74E612BA4C66A8A@IRSMSX102.ger.corp.intel.com> <6860856C-6A92-4569-9CD8-FF6C5C441F30@amacapital.net> <2236FBA76BA1254E88B949DDB74E612BA4C6A4D7@IRSMSX102.ger.corp.intel.com> <303fc4ee5ac04e4fac104df1188952e8@AcuMS.aculab.com> <2236FBA76BA1254E88B949DDB74E612BA4C6C2C3@IRSMSX102.ger.corp.intel.com> <2e55aeb3b39440c0bebf47f0f9522dd8@AcuMS.aculab.com> <20190502150853.GA16779@gmail.com> <20190502164524.GB115950@gmail.com> <2236FBA76BA1254E88B949DDB74E612BA4C6F523@IRSMSX102.ger.corp.intel.com> In-Reply-To: <2236FBA76BA1254E88B949DDB74E612BA4C6F523@IRSMSX102.ger.corp.intel.com> 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 X-MC-Unique: OaTw3byYN0Kl1wNCyn9k8g-1 X-Mimecast-Spam-Score: 0 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: Reshetova, Elena > Sent: 03 May 2019 17:17 ... > rdrand (calling every 8 syscalls): Simple syscall: 0.0795 microseconds You could try something like: u64 rand_val = cpu_var->syscall_rand while (unlikely(rand_val == 0)) rand_val = rdrand64(); stack_offset = rand_val & 0xff; rand_val >>= 6; if (likely(rand_val >= 4)) cpu_var->syscall_rand = rand_val; else cpu_var->syscall_rand = rdrand64(); return stack_offset; That gives you 10 system calls per rdrand instruction and mostly takes the latency out of line. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)