Re: [PATCH kvmtool 1/2] virtio/rng: switch to using /dev/urandom

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Thu, Apr 13, 2023 at 05:57:56PM +0100, Andre Przywara wrote:
> At the moment we use /dev/random as the backing device to provide random
> numbers to our virtio-rng implementation. The downside of doing so is
> that it may block indefinitely - or return EAGAIN repeatedly in our case.
> On one headless systbem without ample noise sources (no keyboard, mouse,

system

> or network traffic) I measured 30 seconds to gain one byte of randomness.
> At the moment EDK II insists in waiting for all of the requsted random
> bytes (for its EFI_RNG_PROTOCOL runtime service) to arrive, that held up
> a Linux kernel boot for more than 10 minutes(!).
> 
> According to the Internet(TM), on Linux /dev/urandom provides the same
> quality random numbers as /dev/random, it just does not block when the
> entropy estimation algorithm suggests so. For all practical purposes the
> recommendation is to just use /dev/urandom, QEMU did the switch as well
> in 2019 [1].
> 
> Use /dev/urandom instead of /dev/random when opening the file descriptor
> providing the randomness source for the virtio/rng implementation.
> 
> [1] https://gitlab.com/qemu-project/qemu/-/commit/a2230bd778d8
> 
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>

Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>

> ---
>  virtio/rng.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/virtio/rng.c b/virtio/rng.c
> index 8f85d5ec1..eab8f3ac0 100644
> --- a/virtio/rng.c
> +++ b/virtio/rng.c
> @@ -166,7 +166,7 @@ int virtio_rng__init(struct kvm *kvm)
>  	if (rdev == NULL)
>  		return -ENOMEM;
>  
> -	rdev->fd = open("/dev/random", O_RDONLY | O_NONBLOCK);
> +	rdev->fd = open("/dev/urandom", O_RDONLY | O_NONBLOCK);
>  	if (rdev->fd < 0) {
>  		r = rdev->fd;
>  		goto cleanup;
> -- 
> 2.25.1
>