From a2230bd778d8445231d2120b87be3ed262bb4f8a Mon Sep 17 00:00:00 2001 From: Kashyap Chamarthy Date: Wed, 29 May 2019 16:31:03 +0200 Subject: VirtIO-RNG: Update default entropy source to `/dev/urandom` MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Message-Id: <20190529143106.11789-2-lvivier@redhat.com> Signed-off-by: Laurent Vivier --- 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 aa5b9285f0..a810581393 100644 --- a/backends/rng-random.c +++ b/backends/rng-random.c @@ -113,7 +113,7 @@ 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 0d8beb4afd..2aae19b0f9 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -4328,7 +4328,7 @@ 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} -- cgit v1.2.3 From 2cd925dac80d708d01f124b26de52fb447a333aa Mon Sep 17 00:00:00 2001 From: Liam Merwick Date: Fri, 28 Jun 2019 14:08:37 +0100 Subject: docs/devel/testing: Fix typo in dockerfile path MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Signed-off-by: Liam Merwick Reviewed-by: Philippe Mathieu-Daudé Reviewed-by: Alex Bennée Reviewed-by: Li Qiang Message-Id: <1561727317-30655-1-git-send-email-liam.merwick@oracle.com> Signed-off-by: Laurent Vivier --- docs/devel/testing.rst | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/docs/devel/testing.rst b/docs/devel/testing.rst index da2d0fc964..3ef50a61db 100644 --- a/docs/devel/testing.rst +++ b/docs/devel/testing.rst @@ -327,7 +327,7 @@ Images ------ Along with many other images, the ``min-glib`` image is defined in a Dockerfile -in ``tests/docker/dockefiles/``, called ``min-glib.docker``. ``make docker`` +in ``tests/docker/dockerfiles/``, called ``min-glib.docker``. ``make docker`` command will list all the available images. To add a new image, simply create a new ``.docker`` file under the -- cgit v1.2.3