Re: [Qemu-devel] [PATCH v2] rng-builtin: add an RNG backend that uses qemu_guest_getrandom()

[Markus Armbruster]

Laurent Vivier <address@hidden> writes:

> Add a new RNG backend using QEMU builtin getrandom function.
>
> It can be created and used with something like:
>
>     ... -object rng-builtin,id=rng0 -device virtio-rng,rng=rng0 ...
>
> Reviewed-by: Richard Henderson <address@hidden>
> Signed-off-by: Laurent Vivier <address@hidden>
> ---
>
> Notes:
>     This patch applies on top of
>     "[PATCH v5 00/24] Add qemu_getrandom and ARMv8.5-RNG etc"
>     Based-on: address@hidden
>     
>     v2: Update qemu-options.hx
>         describe the new backend and specify virtio-rng uses the
>         rng-random by default (do we want to change this?)
>
>  backends/Makefile.objs |  2 +-
>  backends/rng-builtin.c | 56 ++++++++++++++++++++++++++++++++++++++++++
>  qemu-options.hx        | 10 +++++++-
>  3 files changed, 66 insertions(+), 2 deletions(-)
>  create mode 100644 backends/rng-builtin.c
>
> diff --git a/backends/Makefile.objs b/backends/Makefile.objs
> index ff619d31b461..8da4a508d97b 100644
> --- a/backends/Makefile.objs
> +++ b/backends/Makefile.objs
> @@ -1,4 +1,4 @@
> -common-obj-y += rng.o rng-egd.o
> +common-obj-y += rng.o rng-egd.o rng-builtin.o
>  common-obj-$(CONFIG_POSIX) += rng-random.o
>  
>  common-obj-$(CONFIG_TPM) += tpm.o
> diff --git a/backends/rng-builtin.c b/backends/rng-builtin.c
> new file mode 100644
> index 000000000000..b1264b745407
> --- /dev/null
> +++ b/backends/rng-builtin.c
> @@ -0,0 +1,56 @@
> +/*
> + * QEMU Builtin Random Number Generator Backend
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +#include "qemu/osdep.h"
> +#include "sysemu/rng.h"
> +#include "qapi/error.h"
> +#include "qapi/qmp/qerror.h"
> +#include "qemu/main-loop.h"
> +#include "qemu/guest-random.h"
> +
> +#define TYPE_RNG_BUILTIN "rng-builtin"
> +#define RNG_BUILTIN(obj) OBJECT_CHECK(RngBuiltin, (obj), TYPE_RNG_BUILTIN)
> +
> +typedef struct RngBuiltin {
> +    RngBackend parent;
> +} RngBuiltin;
> +
> +static void rng_builtin_request_entropy(RngBackend *b, RngRequest *req)
> +{
> +    RngBuiltin *s = RNG_BUILTIN(b);
> +
> +    while (!QSIMPLEQ_EMPTY(&s->parent.requests)) {
> +        RngRequest *req = QSIMPLEQ_FIRST(&s->parent.requests);
> +
> +        qemu_guest_getrandom_nofail(req->data, req->size);
> +
> +        req->receive_entropy(req->opaque, req->data, req->size);
> +
> +        rng_backend_finalize_request(&s->parent, req);
> +    }
> +}
> +
> +static void rng_builtin_class_init(ObjectClass *klass, void *data)
> +{
> +    RngBackendClass *rbc = RNG_BACKEND_CLASS(klass);
> +
> +    rbc->request_entropy = rng_builtin_request_entropy;
> +}
> +
> +static const TypeInfo rng_builtin_info = {
> +    .name = TYPE_RNG_BUILTIN,
> +    .parent = TYPE_RNG_BACKEND,
> +    .instance_size = sizeof(RngBuiltin),
> +    .class_init = rng_builtin_class_init,
> +};
> +
> +static void register_types(void)
> +{
> +    type_register_static(&rng_builtin_info);
> +}
> +
> +type_init(register_types);
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 0191ef8b1eb7..3e2a51c691b0 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4280,13 +4280,21 @@ other options.
>  
>  The @option{share} boolean option is @var{on} by default with memfd.
>  
> address@hidden -object rng-builtin,address@hidden
> +
> +Creates a random number generator backend which obtains entropy from
> +QEMU builtin functions. The @option{id} parameter is a unique ID that
> +will be used to reference this entropy backend from the @option{virtio-rng}
> +device.
> +
>  @item -object rng-random,address@hidden,address@hidden/dev/random}
>  
>  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.

There's also the "spapr-rng" device, I think.

>          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/random}. By default,
> +the @option{virtio-rng} device uses this RNG backend.
>  
>  @item -object rng-egd,address@hidden,address@hidden

Trivial conflict with Kashyap's "[PATCH v2] VirtIO-RNG: Update default
entropy source to `/dev/urandom`".

virtio-rng indeed creates an rng-random backend when the user doesn't
specify one.  I consider having device model frontends create backends a
bad idea.  Not this patch's fault, of course.

That said, would rng-builtin be a better default?  For starters, it's
available when !CONFIG_POSIX.  I suspect virtio-rng crashes when it
tries to create an rng-random that isn't available.

The new rng-builtin is considerably simpler than both rng-random and
rng-egd.  Moreover, it just works, whereas rng-random is limited to
CONFIG_POSIX, and rng-egd needs egd running (which I suspect basically
nobody does).  Have we considered deprecating these two backends in
favor of rng-builtin?