From mboxrd@z Thu Jan 1 00:00:00 1970 From: James Yonan Subject: Re: [PATCH] crypto_mem_not_equal: add constant-time equality testing of memory regions Date: Mon, 16 Sep 2013 11:10:59 -0600 Message-ID: <52373BA3.9090709@openvpn.net> References: <5232CDCF.50208@redhat.com> <1379259179-2677-1-git-send-email-james@openvpn.net> <878uyyks0e.fsf@mid.deneb.enyo.de> <5235E77F.1050807@openvpn.net> <5236B9A7.3090001@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Florian Weimer , Marcelo Cerri , linux-crypto@vger.kernel.org, herbert@gondor.hengli.com.au To: Daniel Borkmann Return-path: Received: from magnetar.openvpn.net ([74.52.27.18]:47223 "EHLO magnetar.openvpn.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751181Ab3IPRLM (ORCPT ); Mon, 16 Sep 2013 13:11:12 -0400 In-Reply-To: <5236B9A7.3090001@redhat.com> Sender: linux-crypto-owner@vger.kernel.org List-ID: On 16/09/2013 01:56, Daniel Borkmann wrote: > On 09/15/2013 06:59 PM, James Yonan wrote: >> On 15/09/2013 09:45, Florian Weimer wrote: >>> * James Yonan: >>> >>>> + * Constant-time equality testing of memory regions. >>>> + * Returns 0 when data is equal, non-zero otherwise. >>>> + * Fast path if size == 16. >>>> + */ >>>> +noinline unsigned long crypto_mem_not_equal(const void *a, const >>>> void *b, size_t size) >>> >>> I think this should really return unsigned or int, to reduce the risk >>> that the upper bytes are truncated because the caller uses an >>> inappropriate type, resulting in a bogus zero result. Reducing the >>> value to 0/1 probably doesn't hurt performance too much. It also >>> doesn't encode any information about the location of the difference in >>> the result value, which helps if that ever leaks. >> >> The problem with returning 0/1 within the function body of >> crypto_mem_not_equal is that it makes it easier for the compiler to >> introduce a short-circuit optimization. >> >> It might be better to move the test where the result is compared >> against 0 into an inline function: >> >> noinline unsigned long __crypto_mem_not_equal(const void *a, const >> void *b, size_t size); >> >> static inline int crypto_mem_not_equal(const void *a, const void *b, >> size_t size) { >> return __crypto_mem_not_equal(a, b, size) != 0UL ? 1 : 0; >> } >> >> This hides the fact that we are only interested in a boolean result >> from the compiler when it's compiling crypto_mem_not_equal.c, but also >> ensures type safety when users test the return value. It's also >> likely to have little or no performance impact. > > Well, the code snippet I've provided from NaCl [1] is not really > "fast-path" > as you say, but rather to prevent the compiler from doing such > optimizations > by having a transformation of the "accumulated" bits into 0 and 1 as an end > result (likely to prevent a short circuit), plus it has static size, so no > loops applied here that could screw up. > > Variable size could be done under arch/ in asm, and if not available, that > just falls back to normal memcmp that is being transformed into a same > return > value. By that, all other archs could easily migrate afterwards. What do > you > think? > > [1] http://www.spinics.net/lists/linux-crypto/msg09558.html I'm not sure that the differentbits -> 0/-1 transform in NaCl really buys us anything because we don't care very much about making the final test of differentbits != 0 constant-time. An attacker already knows whether the test succeeded or failed -- we care more about making the failure cases constant-time. To do this, we need to make sure that the compiler doesn't insert one or more early instructions to compare differentbits with 0xFF and then bypass the rest of the F(n) lines because it knows then that the value of differentbits cannot be changed by subsequent F(n) lines. It seems that all of the approaches that use |= to build up the differentbits value suffer from this problem. My proposed solution is rather than trying to outsmart the compiler with code that resists optimization, why not just turn optimization off directly with #pragma GCC optimize. Or better yet, use an optimization setting that provides reasonable speed without introducing potential short-circuit optimizations. By optimizing for size ("Os"), the compiler will need to turn off optimizations that add code for a possible speed benefit, and the kind of short-circuit optimizations that we are trying to avoid fall precisely into this class -- they add an instruction to check if the OR accumulator has all of its bits set, then if so, do an early exit. So by using Os, we still benefit from optimizations that increase speed but don't increase code size. Once we eliminate the possibility of short-circuit optimizations, then it's much more straightforward to make the function fast (e.g. by comparing 64-bits at a time) and flexible (by handling arbitrary size). My current implementation of crypto_mem_not_equal does both. James