Re: [Qemu-devel] [PATCH] VirtIO-RNG: Update default entropy source to `/dev/urandom`

[Richard W.M. Jones]

On Fri, May 03, 2019 at 05:46:13PM +0200, Kashyap Chamarthy wrote:
> 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).
> 
> So change the entropy source to the recommended `/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 <address@hidden>
> ---
>  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 e2a49b0571..eff36ef140 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 51802cbb26..a525609149 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4276,7 +4276,7 @@ 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,address@hidden,address@hidden

I think this is a very sensible change, removing a bit of superstition
about randomness.

Reviewed-by: Richard W.M. Jones <address@hidden>

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
http://fedoraproject.org/wiki/MinGW