From: Stephan Mueller Subject: Re: [RFC PATCH] char: random: stir the output pools differently when the random_write lenght allows splitting the seed Date: Fri, 10 Jan 2014 13:15:34 +0100 Message-ID: <5743525.Bz4yhBJ8Tl@tauon> References: <42f6dc88535d5ecdab13b1376db7ccbbbc550600.1389297772.git.aquini@redhat.com> <1510904.XsOZgVgixq@tauon> <52CFDB76.2030207@ladisch.de> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Cc: Rafael Aquini , Theodore Ts'o , Arnd Bergmann , Greg Kroah-Hartman , linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org To: Clemens Ladisch Return-path: Received: from mail.atsec.com ([195.30.99.214]:37405 "EHLO mail.atsec.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751858AbaAJMPl (ORCPT ); Fri, 10 Jan 2014 07:15:41 -0500 Received: from mail.atsec.com (localhost [127.0.0.1]) by mail.atsec.com (Postfix) with ESMTP id 585D478079 for ; Fri, 10 Jan 2014 13:15:39 +0100 (CET) In-Reply-To: <52CFDB76.2030207@ladisch.de> Sender: linux-crypto-owner@vger.kernel.org List-ID: Am Freitag, 10. Januar 2014, 12:37:26 schrieb Clemens Ladisch: Hi Clemens, >Stephan Mueller wrote: >> Am Freitag, 10. Januar 2014, 09:13:57 schrieb Clemens Ladisch: >>> Rafael Aquini wrote: >>>> This patch introduces changes to the random_write method so it can >>>> split the given seed and completely stir the output pools with >>>> different halves of it, when seed lenght allows us doing so. >>>> >>>> - ret = write_pool(&blocking_pool, buffer, count); >>>> + ret = write_pool(pool1, buffer, count1); >>>> >>>> if (ret) >>>> >>>> return ret; >>>> >>>> - ret = write_pool(&nonblocking_pool, buffer, count); >>>> + ret = write_pool(pool2, buffer + offset, count2); >>> >>> Doesn't this assume that both halves of the buffer contain some >>> (uncredited) entropy? In other words, wouldn't this result in worse >>> randomness for pool2 if the second half of the buffer contains just >>> zero padding? >> >> [...] >> Coming back to your concern: sure, the caller can pad any data >> injected into /dev/?random with zeros. > >Assume that the userspace of an embedded device wants to do the same >kind of initialization that a call to add_device_randomness() does, and >that it has some data like "char serial_number[256]". The padding >wouldn't be done intentionally, it's just a property of the data (and >it wouldn't have mattered before this patch). > >> But as writing to the character files is allowed to every user, this >> per definition must not matter (e.g. an attacker may simply write >> zeros or other known data into the character file). And the random.c >> driver handles that case appropriately by not increasing the entropy >> estimator when receiving data. > >The problem is not with the entropy estimate. > >> All the patch tries to achieve is to ensure that both pools are not >> always mixed with the same values. > >Before this patch, both pools got mixed with the same values. After >this patch, both pools indeed get mixed with different values, but now >one pool gets mixed with a known value if one half of the buffer >happens to be known. Do you imply in your example above that the serial number is unknown? Anything that unprivileged user space tries to inject into /dev/?random should be considered data with known value. Ciao Stephan