From: James Yonan Subject: Re: [PATCH] crypto: more robust crypto_memneq Date: Mon, 25 Nov 2013 08:59:03 -0700 Message-ID: <529373C7.10201@openvpn.net> References: <1385327535-27991-1-git-send-email-cesarb@cesarb.eti.br> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Herbert Xu , "David S. Miller" , Daniel Borkmann , Florian Weimer , linux-kernel@vger.kernel.org To: Cesar Eduardo Barros , linux-crypto@vger.kernel.org Return-path: Received: from mail.yonan.net ([54.244.116.145]:49823 "EHLO mail.yonan.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751809Ab3KYQIH (ORCPT ); Mon, 25 Nov 2013 11:08:07 -0500 In-Reply-To: <1385327535-27991-1-git-send-email-cesarb@cesarb.eti.br> Sender: linux-crypto-owner@vger.kernel.org List-ID: On 24/11/2013 14:12, Cesar Eduardo Barros wrote: > Disabling compiler optimizations can be fragile, since a new > optimization could be added to -O0 or -Os that breaks the assumptions > the code is making. > > Instead of disabling compiler optimizations, use a dummy inline assembly > (based on RELOC_HIDE) to block the problematic kinds of optimization, > while still allowing other optimizations to be applied to the code. > > The dummy inline assembly is added after every OR, and has the > accumulator variable as its input and output. The compiler is forced to > assume that the dummy inline assembly could both depend on the > accumulator variable and change the accumulator variable, so it is > forced to compute the value correctly before the inline assembly, and > cannot assume anything about its value after the inline assembly. > > This change should be enough to make crypto_memneq work correctly (with > data-independent timing) even if it is inlined at its call sites. That > can be done later in a followup patch. > > Compile-tested on x86_64. > > Signed-off-by: Cesar Eduardo Barros This approach using __asm__ ("" : "=r" (var) : "0" (var)) to try to prevent compiler optimizations of var is interesting. I like the fact that it's finer-grained than -Os and doesn't preclude inlining. One concern would be that __asm__ could be optimized out unless __volatile__ is present. James