From mboxrd@z Thu Jan 1 00:00:00 1970 From: mancha Subject: [BUG/PATCH] kernel RNG and its secrets Date: Wed, 18 Mar 2015 09:53:45 +0000 Message-ID: <20150318095345.GA12923@zoho.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="lrZ03NoBR/3+SXJZ" Cc: linux-crypto@vger.kernel.org, herbert@gondor.apana.org.au, dborkman@redhat.com To: tytso@mit.edu, linux-kernel@vger.kernel.org Return-path: Received: from sender1.zohomail.com ([74.201.84.157]:38693 "EHLO sender1.zohomail.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754850AbbCRKMl (ORCPT ); Wed, 18 Mar 2015 06:12:41 -0400 Content-Disposition: inline Sender: linux-crypto-owner@vger.kernel.org List-ID: --lrZ03NoBR/3+SXJZ Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Hi. The kernel RNG introduced memzero_explicit in d4c5efdb9777 to protect memory cleansing against things like dead store optimization: void memzero_explicit(void *s, size_t count) { memset(s, 0, count); OPTIMIZER_HIDE_VAR(s); } OPTIMIZER_HIDE_VAR, introduced in fe8c8a126806 to protect crypto_memneq against timing analysis, is defined when using gcc as: #define OPTIMIZER_HIDE_VAR(var) __asm__ ("" : "=3Dr" (var) : "0" (var)) My tests with gcc 4.8.2 on x86 find it insufficient to prevent gcc from optimizing out memset (i.e. secrets remain in memory). Two things that do work: __asm__ __volatile__ ("" : "=3Dr" (var) : "0" (var)) and __asm__ __volatile__("": : :"memory") The first is OPTIMIZER_HIDE_VAR plus a volatile qualifier and the second is barrier() [as defined when using gcc]. I propose memzero_explicit use barrier(). --- a/lib/string.c +++ b/lib/string.c @@ -616,7 +616,7 @@ EXPORT_SYMBOL(memset); void memzero_explicit(void *s, size_t count) { memset(s, 0, count); - OPTIMIZER_HIDE_VAR(s); + barrier(); } EXPORT_SYMBOL(memzero_explicit); =20 For any attribution deemed necessary, please use "mancha security". Please CC me on replies. --mancha PS CC'ing Herbert Xu in case this impacts crypto_memneq. --lrZ03NoBR/3+SXJZ Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQIcBAEBCgAGBQJVCUsoAAoJEB4VYy8JqhaDAsQP/0jhWuUk4FpYLjukvqOYKZip Wa7rjVCVgR1/E017MqAo/fILSnw4VJTN+nYXYuuEiWAf9u46DEul66ZRU3unORjd RZRaF73aV9y1cbLGuZAEmHXS/TGrVRiOSV5z4wxYywkSO2zjMGhB+fO+Nog7GeX3 AB5Tw4akFmuf4JCPsW7FbAweDGC9OGXscGvF2iSu8YjBZZTBP0PvWTHcx7Hb1jT5 tY3kCG7wLOkmtQRuWxcxcDa2aQ+Br05i1FF/SEis4xUoOsmu7XJnz6vUZTOhiObf npZ0MBUOChovhJg9zsI3zqYMnM7pQZTQ0sXrV+OsLzaCPm0swrZyK6U+JzbEjLAh n99WnEhdiBv9uEmAKRlw75MldI9maEevwu2ucWfBQdl8BHtMqHKxfZjVYRJvhzIK 5Q956OqxrG6qt6U21gSwiHkrkQr9H90z2RNJkS7w0F/o/KtCwbMPqbXTncCm14Rh 4sV4elcCU77Azc9/qwMzhNAWqMVXjUGIzRmAi+uK8dYiV3+DNAfCE73ojc2oyOEx b2j5FN3OpMftCGhhEC8lBwmMqS0olYDdoUm/meTMU2Jr5q05T9KFMIXk2NHJWuaY i9btBLwt1+eHAVwpo56rHqYG4Y6p586SHvNlaqS1lYO4YEBMqIw3eXFf0AC30JXP 2rCkgLkQITl2GJ80nXBw =LU8A -----END PGP SIGNATURE----- --lrZ03NoBR/3+SXJZ--