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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 51E62C3A5A1 for ; Wed, 28 Aug 2019 15:53:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 307672064A for ; Wed, 28 Aug 2019 15:53:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726917AbfH1Pxa (ORCPT ); Wed, 28 Aug 2019 11:53:30 -0400 Received: from mx2.suse.de ([195.135.220.15]:48680 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726763AbfH1Pxa (ORCPT ); Wed, 28 Aug 2019 11:53:30 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 1F846AD85; Wed, 28 Aug 2019 15:53:28 +0000 (UTC) Received: by ds.suse.cz (Postfix, from userid 10065) id 16F82DA809; Wed, 28 Aug 2019 17:53:49 +0200 (CEST) Date: Wed, 28 Aug 2019 17:53:49 +0200 From: David Sterba To: dsterba@suse.cz, Borislav Petkov , Linus Torvalds , Pu Wen , Thomas Gleixner , Tom Lendacky , Linux List Kernel Mailing , the arch/x86 maintainers Subject: Re: [GIT pull] x86/urgent for 5.3-rc5 Message-ID: <20190828155348.GD2752@twin.jikos.cz> Reply-To: dsterba@suse.cz Mail-Followup-To: dsterba@suse.cz, Borislav Petkov , Linus Torvalds , Pu Wen , Thomas Gleixner , Tom Lendacky , Linux List Kernel Mailing , the arch/x86 maintainers References: <20190825193218.GD20639@zn.tnic> <20190825194912.GF20639@zn.tnic> <20190825201723.GG20639@zn.tnic> <20190826125342.GC28610@zn.tnic> <20190827173955.GI29752@zn.tnic> <20190828152040.GC2752@twin.jikos.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190828152040.GC2752@twin.jikos.cz> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 28, 2019 at 05:20:41PM +0200, David Sterba wrote: > On Tue, Aug 27, 2019 at 07:39:55PM +0200, Borislav Petkov wrote: > > @@ -42,5 +43,24 @@ void x86_init_rdrand(struct cpuinfo_x86 *c) > > return; > > } > > } > > + > > + /* > > + * Stupid sanity-check whether RDRAND does *actually* generate > > + * some at least random-looking data. > > + */ > > + prev = tmp; > > + for (i = 0; i < SANITY_CHECK_LOOPS; i++) { > > + if (rdrand_long(&tmp)) { > > + if (prev != tmp) > > + changed++; > > You could do some sort of weak statistical test like > > if (popcnt(prev ^ tmp) < BITS_PER_LONG / 3) > bad++; > > if (bad > TOO_BAD) > WARN(...); > > this should catch same value, increments you mentioned and possibly > other trivial classes of not-so-random values. So the average popcount seems to be a more reliable check as I don't have a good estimate for TOO_BAD. Ie. calculate average popcount and if it's less than third of bit width, consider it broken. The script below can be used to demonstrate the behaviour on a good random generator. --- #!/usr/bin/python3 import random popsum = 0 popcount = 0 prev = random.randrange(0, 2**64) for i in range(16): x = return random.randrange(0, 2**64) pop = bin(x ^ prev).count("1") popsum += pop popcount += 1 print("PREV=%d X=%d popcnt=%d" % (prev, x, pop)) prev = x print("Average popcnt: %d" % (1.0 * popsum / popcount)) --- PREV=8900625479737950182 X=12846979731852325434 popcnt=32 PREV=12846979731852325434 X=8887925161619955047 popcnt=35 PREV=8887925161619955047 X=988349339658261072 popcnt=36 PREV=988349339658261072 X=431141664953398919 popcnt=32 PREV=431141664953398919 X=13830962168734488538 popcnt=33 PREV=13830962168734488538 X=16591763919535693884 popcnt=33 PREV=16591763919535693884 X=7388685098481568010 popcnt=22 PREV=7388685098481568010 X=3526579832640281911 popcnt=33 PREV=3526579832640281911 X=2069567414175453497 popcnt=27 PREV=2069567414175453497 X=5562304115464083982 popcnt=28 PREV=5562304115464083982 X=14604545499285704604 popcnt=31 PREV=14604545499285704604 X=10602551277613833090 popcnt=31 PREV=10602551277613833090 X=6431137842853826307 popcnt=32 PREV=6431137842853826307 X=16231642336281616741 popcnt=41 PREV=16231642336281616741 X=520921733225029500 popcnt=38 PREV=520921733225029500 X=12014110422974389822 popcnt=21 Average popcnt: 31