From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8D222C433EF for ; Sat, 12 Mar 2022 20:17:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231974AbiCLUST (ORCPT ); Sat, 12 Mar 2022 15:18:19 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59376 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230017AbiCLUSS (ORCPT ); Sat, 12 Mar 2022 15:18:18 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C8C25205972; Sat, 12 Mar 2022 12:17:11 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 66C8EB80AFD; Sat, 12 Mar 2022 20:17:10 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5579EC340EB; Sat, 12 Mar 2022 20:17:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1647116229; bh=GadeI9krdmz72UAKX4LbaW8MXzY58uGXhigxFEem6Ec=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=jE3/CwikWAHLy/vqmyy6WAdYAkZIYo18cP1UTBbNI+1qsTH8/ey4mJDVOwp5KnOJD CctZ0fpjWAK9ar+FEBg8naCczl9EI0rh/I2HBSa8IuhdJaDaoputRSDhpjkQEwRUYz f1PLc/A01nyLo7xirNHN8cObJVcjFjcrePXcinOLYdLyW7dLu/B9wBfijr3ZvMl9qj hASD7WSQe5Li02dTiC7rM+tULT+ShfOi7u+4AWvO5SNpLn2pMlnEMLi5Az+1vzVFiH Hx0dvNjx7Qsw9YhSkQcMg1UxBt9O1Qi83GgaTfR2LdHMZAwVCUwSiOcBmmA5/GURvu h02OwujwM4Rww== Date: Sat, 12 Mar 2022 12:17:06 -0800 From: Eric Biggers To: "Jason A. Donenfeld" Cc: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, linux-arch@vger.kernel.org, Dinh Nguyen , Nick Hu , Max Filippov , Palmer Dabbelt , "David S . Miller" , Yoshinori Sato , Michal Simek , Borislav Petkov , Guo Ren , Geert Uytterhoeven , Joshua Kinard , David Laight , Dominik Brodowski , Ard Biesheuvel , Arnd Bergmann , Thomas Gleixner , Andy Lutomirski , Kees Cook , Lennart Poettering , Konstantin Ryabitsev , Linus Torvalds , Greg Kroah-Hartman , Theodore Ts'o Subject: Re: [PATCH v1] random: block in /dev/urandom Message-ID: References: <20220217162848.303601-1-Jason@zx2c4.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220217162848.303601-1-Jason@zx2c4.com> Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Thu, Feb 17, 2022 at 05:28:48PM +0100, Jason A. Donenfeld wrote: > This topic has come up countless times, and usually doesn't go anywhere. > This time I thought I'd bring it up with a slightly narrower focus, > updated for some developments over the last three years: we finally can > make /dev/urandom always secure, in light of the fact that our RNG is > now always seeded. > > Ever since Linus' 50ee7529ec45 ("random: try to actively add entropy > rather than passively wait for it"), the RNG does a haveged-style jitter > dance around the scheduler, in order to produce entropy (and credit it) > for the case when we're stuck in wait_for_random_bytes(). How ever you > feel about the Linus Jitter Dance is beside the point: it's been there > for three years and usually gets the RNG initialized in a second or so. > > As a matter of fact, this is what happens currently when people use > getrandom(). It's already there and working, and most people have been > using it for years without realizing. > > So, given that the kernel has grown this mechanism for seeding itself > from nothing, and that this procedure happens pretty fast, maybe there's > no point any longer in having /dev/urandom give insecure bytes. In the > past we didn't want the boot process to deadlock, which was > understandable. But now, in the worst case, a second goes by, and the > problem is resolved. It seems like maybe we're finally at a point when > we can get rid of the infamous "urandom read hole". > > The one slight drawback is that the Linus Jitter Dance relies on random_ > get_entropy() being implemented. The first lines of try_to_generate_ > entropy() are: > > stack.now = random_get_entropy(); > if (stack.now == random_get_entropy()) > return; > > On most platforms, random_get_entropy() is simply aliased to get_cycles(). > The number of machines without a cycle counter or some other > implementation of random_get_entropy() in 2022, which can also run a > mainline kernel, and at the same time have a both broken and out of date > userspace that relies on /dev/urandom never blocking at boot is thought > to be exceedingly low. And to be clear: those museum pieces without > cycle counters will continue to run Linux just fine, and even > /dev/urandom will be operable just like before; the RNG just needs to be > seeded first through the usual means, which should already be the case > now. > > On systems that really do want unseeded randomness, we already offer > getrandom(GRND_INSECURE), which is in use by, e.g., systemd for seeding > their hash tables at boot. Nothing in this commit would affect > GRND_INSECURE, and it remains the means of getting those types of random > numbers. > > This patch goes a long way toward eliminating a long overdue userspace > crypto footgun. After several decades of endless user confusion, we will > finally be able to say, "use any single one of our random interfaces and > you'll be fine. They're all the same. It doesn't matter." And that, I > think, is really something. Finally all of those blog posts and > disagreeing forums and contradictory articles will all become correct > about whatever they happened to recommend, and along with it, a whole > class of vulnerabilities eliminated. > > With very minimal downside, we're finally in a position where we can > make this change. > > Cc: Dinh Nguyen > Cc: Nick Hu > Cc: Max Filippov > Cc: Palmer Dabbelt > Cc: David S. Miller > Cc: Yoshinori Sato > Cc: Michal Simek > Cc: Borislav Petkov > Cc: Guo Ren > Cc: Geert Uytterhoeven > Cc: Joshua Kinard > Cc: David Laight > Cc: Dominik Brodowski > Cc: Eric Biggers > Cc: Ard Biesheuvel > Cc: Arnd Bergmann > Cc: Thomas Gleixner > Cc: Andy Lutomirski > Cc: Kees Cook > Cc: Lennart Poettering > Cc: Konstantin Ryabitsev > Cc: Linus Torvalds > Cc: Greg Kroah-Hartman > Cc: Theodore Ts'o > Signed-off-by: Jason A. Donenfeld > --- > Having learned that MIPS32 isn't affected by this (initially my largest > worry), and then heartened today upon reading LWN's summary of our > previous discussion ("it would seem there are no huge barriers to > removing the final distinction between /dev/random and /dev/urandom"), I > figured I'd go ahead and submit a v1 of this. It seems at least worth > trying and seeing if somebody arrives with legitimate complaints. To > that end I've also widened the CC list quite a bit. > > Changes v0->v1: > - We no longer touch GRND_INSECURE at all, in anyway. Lennart (and to an > extent, Andy) pointed out that getting insecure numbers immediately at > boot is still something that has legitimate use cases, so this patch > no longer touches that code. > > drivers/char/mem.c | 2 +- > drivers/char/random.c | 51 ++++++------------------------------------ > include/linux/random.h | 2 +- > 3 files changed, 9 insertions(+), 46 deletions(-) > Just a small nit: the comments above rng_is_initialized() and wait_for_random_bytes() still imply that /dev/urandom is nonblocking. - Eric