From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751160AbdHOIBm (ORCPT ); Tue, 15 Aug 2017 04:01:42 -0400 Received: from wtarreau.pck.nerim.net ([62.212.114.60]:35479 "EHLO 1wt.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750924AbdHOIBl (ORCPT ); Tue, 15 Aug 2017 04:01:41 -0400 Date: Tue, 15 Aug 2017 10:01:28 +0200 From: Willy Tarreau To: Ingo Molnar Cc: "Theodore Ts'o" , Borislav Petkov , Linus Torvalds , x86-ml , "Jason A. Donenfeld" , lkml Subject: Re: early x86 unseeded randomness Message-ID: <20170815080128.GA2288@1wt.eu> References: <20170814173515.n3vcpz37xh33ayuq@pd.tnic> <20170814180048.m3igiaiunlyb5wur@pd.tnic> <20170814190013.zixopgjyq26ukxcj@pd.tnic> <20170815013124.2afytkibspxrikdn@thunk.org> <20170815064437.GA1986@1wt.eu> <20170815074254.6byayhspc5tdtjb5@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170815074254.6byayhspc5tdtjb5@gmail.com> User-Agent: Mutt/1.6.1 (2016-04-27) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 15, 2017 at 09:42:54AM +0200, Ingo Molnar wrote: > > * Willy Tarreau wrote: > > > Nowadays we could use similar methods using RDTSC providing more accurate > > counting. This doesn't provide a lot of entropy of course, given that a > > 2 GHz machine will at most count 31 bits there. But I tend to think that > > what matters during early boot is to transform something highly predictable > > into something unlikely to be predicted (ie: an exploit having to scan 2^31 > > possible addresses will not be really usable). It's also possible to do the > > same with the PIT0 counter ticking at 18.2 Hz without any correlation with > > the RTC by the way, and roughly provide 25 more bits. And if you expect > > that the BIOS has emitted a 800 Hz beep at boot, you could still have a > > divider of 1491 in PIT2 providing 10 more bits, though with a bit of > > correlation with PIT0 since they use the same 1.19 MHz source. These > > methods increase the boot time by up to one second though, but my point > > here is that when you have nothing it's always a bit better. > > One other thing besides trying to extract entropy via timing would be to utilize > more of the machine's environment in seeding the random number generator. > > For example on x86 the E820 table is available very early on and its addresses > could be mixed into the random pool. An external attacker often would not know the > precise hardware configuration. > > Likewise the boot parameters string could be mixed into the initial random pool as > well - and this way distributions could create per installation seed simply by > appending a random number to the boot string. > > Both methods should be very fast and robust. Definitely, just like a simple MD5SUM on the first MB of RAM including the BIOS, and on the CMOS RAM contents, which also differ quite a bit between systems. Willy