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 X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=FROM_EXCESS_BASE64, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED, USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2DDCCC04A6B for ; Fri, 10 May 2019 12:12:11 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id EDD3120850 for ; Fri, 10 May 2019 12:12:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EDD3120850 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([127.0.0.1]:42438 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hP4Nm-0005yR-5y for qemu-devel@archiver.kernel.org; Fri, 10 May 2019 08:12:10 -0400 Received: from eggs.gnu.org ([209.51.188.92]:45140) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hP4My-00058I-KE for qemu-devel@nongnu.org; Fri, 10 May 2019 08:11:21 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hP4Mx-0003sj-Dx for qemu-devel@nongnu.org; Fri, 10 May 2019 08:11:20 -0400 Received: from mx1.redhat.com ([209.132.183.28]:18495) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hP4Mx-0003s5-3I for qemu-devel@nongnu.org; Fri, 10 May 2019 08:11:19 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 7C921307D852; Fri, 10 May 2019 12:11:17 +0000 (UTC) Received: from redhat.com (ovpn-112-68.ams2.redhat.com [10.36.112.68]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 54DF65D6A9; Fri, 10 May 2019 12:11:13 +0000 (UTC) Date: Fri, 10 May 2019 13:11:10 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Markus Armbruster Message-ID: <20190510121110.GJ7671@redhat.com> References: <20190510081526.15507-1-kchamart@redhat.com> <874l62sei2.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <874l62sei2.fsf@dusky.pond.sub.org> User-Agent: Mutt/1.11.4 (2019-03-13) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.48]); Fri, 10 May 2019 12:11:17 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [PATCH v2] VirtIO-RNG: Update default entropy source to `/dev/urandom` X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: qemu-devel@nongnu.org, amit@kernel.org, rjones@redhat.com, stefanha@redhat.com, Kashyap Chamarthy Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, May 10, 2019 at 02:03:33PM +0200, Markus Armbruster wrote: > Kashyap Chamarthy writes: > > > When QEMU exposes a VirtIO-RNG device to the guest, that device needs a > > source of entropy, and that source needs to be "non-blocking", like > > `/dev/urandom`. However, currently QEMU defaults to the problematic > > `/dev/random`, which is "blocking" (as in, it waits until sufficient > > entropy is available). > > > > Why prefer `/dev/urandom` over `/dev/random`? > > --------------------------------------------- > > > > The man pages of urandom(4) and random(4) state: > > "The /dev/random device is a legacy interface which dates back to a > > time where the cryptographic primitives used in the implementation > > of /dev/urandom were not widely trusted. It will return random > > bytes only within the estimated number of bits of fresh noise in the > > entropy pool, blocking if necessary. /dev/random is suitable for > > applications that need high quality randomness, and can afford > > indeterminate delays." > > > > Further, the "Usage" section of the said man pages state: > > > > "The /dev/random interface is considered a legacy interface, and > > /dev/urandom is preferred and sufficient in all use cases, with the > > exception of applications which require randomness during early boot > > time; for these applications, getrandom(2) must be used instead, > > because it will block until the entropy pool is initialized. > > > > "If a seed file is saved across reboots as recommended below (all > > major Linux distributions have done this since 2000 at least), the > > output is cryptographically secure against attackers without local > > root access as soon as it is reloaded in the boot sequence, and > > perfectly adequate for network encryption session keys. Since reads > > from /dev/random may block, users will usually want to open it in > > nonblocking mode (or perform a read with timeout), and provide some > > sort of user notification if the desired entropy is not immediately > > available." > > > > And refer to random(7) for a comparison of `/dev/random` and > > `/dev/urandom`. > > This is Linux. What about other supported POSIX[*] hosts? If any such > host has /dev/random that works here, but not /dev/urandom, we regress. It exists on OS-X, FreeBSD, DragonFlyBSD, NetBSD and OpenBSD, which covers all the non-Linux platforms we explicitly support, aside from Windows. On Windows /dev/random doesn't work either so we don't regress. This is actually another argument in favour of using the newly proposed rng-builtin by default, as that will work on Windows. > *If* there's an actual regression risk: a simple & stupid way to reduce > it risk could be falling back to /dev/random when opening /dev/urandom > fails. Perhaps only when it fails with ENOENT. Unless I missed something, I think we'll be ok without the fallback though I wouldn't object to having a fallback as you describe. > Possible implementation: instead of setting a default filename in > rng_random_init(), change rng_random_opened() to try /dev/urandom, then > /dev/random when filename is still null. > > Aside: "opened" sounds like a predicate. Goes back to commit > a9b7b2ad7b0. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|