From patchwork Tue May 21 14:11:30 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Laurent Vivier X-Patchwork-Id: 1102815 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Authentication-Results: ozlabs.org; spf=pass (mailfrom) smtp.mailfrom=nongnu.org (client-ip=209.51.188.17; helo=lists.gnu.org; envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org; receiver=) Authentication-Results: ozlabs.org; dmarc=fail (p=none dis=none) header.from=redhat.com Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 457d8433TDz9s6w for ; Wed, 22 May 2019 00:15:16 +1000 (AEST) Received: from localhost ([127.0.0.1]:54548 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hT5Xu-0006yo-F2 for incoming@patchwork.ozlabs.org; Tue, 21 May 2019 10:15:14 -0400 Received: from eggs.gnu.org ([209.51.188.92]:39596) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hT5Uh-0005Ak-DO for qemu-devel@nongnu.org; Tue, 21 May 2019 10:11:56 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hT5Uf-0000kf-D3 for qemu-devel@nongnu.org; Tue, 21 May 2019 10:11:55 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48664) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hT5Ud-0000j5-FP for qemu-devel@nongnu.org; Tue, 21 May 2019 10:11:51 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id BA2B4308402A; Tue, 21 May 2019 14:11:44 +0000 (UTC) Received: from thinkpad.redhat.com (unknown [10.40.205.57]) by smtp.corp.redhat.com (Postfix) with ESMTP id 0746E5E7AF; Tue, 21 May 2019 14:11:39 +0000 (UTC) From: Laurent Vivier To: qemu-devel@nongnu.org Date: Tue, 21 May 2019 16:11:30 +0200 Message-Id: <20190521141133.27380-2-lvivier@redhat.com> In-Reply-To: <20190521141133.27380-1-lvivier@redhat.com> References: <20190521141133.27380-1-lvivier@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.40]); Tue, 21 May 2019 14:11:44 +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: [Qemu-devel] [PATCH v6 1/4] 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: , Cc: Laurent Vivier , =?utf-8?q?Daniel_P_=2E_Berrang?= =?utf-8?b?w6k=?= , Amit Shah , "Michael S. Tsirkin" , Richard Henderson , Markus Armbruster , "Richard W . M . Jones" , Stefan Hajnoczi , Kashyap Chamarthy Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" From: Kashyap Chamarthy 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 on Linux 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`. What about other OSes? ---------------------- `/dev/urandom` exists and works on OS-X, FreeBSD, DragonFlyBSD, NetBSD and OpenBSD, which cover 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' backend by default, as that will work on Windows. - - - Given the above, change the entropy source for VirtIO-RNG device to `/dev/urandom`. Related discussion in these[1][2] past threads. [1] https://lists.nongnu.org/archive/html/qemu-devel/2018-06/msg08335.html -- "RNG: Any reason QEMU doesn't default to `/dev/urandom`?" [2] https://lists.nongnu.org/archive/html/qemu-devel/2018-09/msg02724.html -- "[RFC] Virtio RNG: Consider changing the default entropy source to /dev/urandom" Signed-off-by: Kashyap Chamarthy Reviewed-by: Daniel P. Berrangé Reviewed-by: Stefan Hajnoczi Reviewed-by: Markus Armbruster Signed-off-by: Laurent Vivier Acked-by: Ed Maste --- backends/rng-random.c | 2 +- qemu-options.hx | 2 +- 2 files changed, 2 insertions(+), 2 deletions(-) diff --git a/backends/rng-random.c b/backends/rng-random.c index e2a49b0571d7..eff36ef14084 100644 --- a/backends/rng-random.c +++ b/backends/rng-random.c @@ -112,7 +112,7 @@ static void rng_random_init(Object *obj) rng_random_set_filename, NULL); - s->filename = g_strdup("/dev/random"); + s->filename = g_strdup("/dev/urandom"); s->fd = -1; } diff --git a/qemu-options.hx b/qemu-options.hx index 7ae3373a0094..73046f8274d1 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -4326,7 +4326,7 @@ Creates a random number generator backend which obtains entropy from a device on the host. The @option{id} parameter is a unique ID that will be used to reference this entropy backend from the @option{virtio-rng} device. The @option{filename} parameter specifies which file to obtain -entropy from and if omitted defaults to @option{/dev/random}. +entropy from and if omitted defaults to @option{/dev/urandom}. @item -object rng-egd,id=@var{id},chardev=@var{chardevid}