From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932782Ab3BLKXi (ORCPT ); Tue, 12 Feb 2013 05:23:38 -0500 Received: from ka.mail.enyo.de ([87.106.162.201]:33984 "EHLO ka.mail.enyo.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755677Ab3BLKXg (ORCPT ); Tue, 12 Feb 2013 05:23:36 -0500 From: Florian Weimer To: Daniel Borkmann Cc: gregkh@linuxfoundation.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] lib: memcmp_nta: add timing-attack secure memcmp References: <87zjzallp5.fsf@mid.deneb.enyo.de> <5119777E.6030005@iogearbox.net> Date: Tue, 12 Feb 2013 11:23:29 +0100 In-Reply-To: <5119777E.6030005@iogearbox.net> (Daniel Borkmann's message of "Mon, 11 Feb 2013 23:58:06 +0100") Message-ID: <87y5et7rvi.fsf@mid.deneb.enyo.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Daniel Borkmann: > On 02/11/2013 08:00 PM, Florian Weimer wrote: >> * Daniel Borkmann: > > Thanks for your feedback, Florian! > >>> + * memcmp_nta - memcmp that is secure against timing attacks >> >> It's not providing an ordering, so it should not have "cmp" in the >> name. > > I agree. What would you suggest? Probably, it would make sense to > integrate this into the Linux crypto API and name it sth like ... > > crypto_mem_verify(const void *,const void *,__kernel_size_t) > > ... which returns: > > == 0 - mem regions equal each other > != 0 - mem regions do not equal each other crypto_mem_equal or crypto_mem_equals should be fine. Or anything else which matches an existing function name with similar function. >>> + for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--) >>> + res |= (*su1 ^ *su2); >> >> The compiler could still short-circuit this loop. Unlikely at >> present, but this looks like a maintenance hazard. > > So then better we leave out '|' as a possible candidate and rewrite it as: > > + for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--) > + res += (*su1 ^ *su2); That will cause false matches for long inputs. If we had only four platforms to support, I would write this function in assembler because it will be considerably easier to read.