On Wed, 2020-07-15 at 00:00 +0200, mwi...@suse.com wrote:
> From: Martin Wilck <mwi...@suse.com>
>
> If a program opens /dev/hwrng with O_NONBLOCK and uses poll() and
> non-blocking read() to retrieve random data, it ends up in a tight
> loop with poll() always returning POLLIN and read() returning EAGAIN.
> This repeats forever until some process makes a blocking read() call.
> The reason is that virtio_read() always returns 0 in non-blocking
> mode,
> even if data is available.
>
> The following test program illustrates the behavior.
...
> This can be observed in the real word e.g. with nested qemu/KVM
> virtual
> machines, if both the "outer" and "inner" VMs have a virtio-rng
> device.
> If the "inner" VM requests random data, qemu running in the "outer"
> VM
> uses this device in a non-blocking manner like the test program
> above.
>
> Fix it by returning available data if it exists.
>
> Signed-off-by: Martin Wilck <mwi...@suse.com>
> ---
> drivers/char/hw_random/virtio-rng.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/drivers/char/hw_random/virtio-rng.c
> b/drivers/char/hw_random/virtio-rng.c
> index 79a6e47b5fbc..94806308d814 100644
> --- a/drivers/char/hw_random/virtio-rng.c
> +++ b/drivers/char/hw_random/virtio-rng.c
> @@ -59,6 +59,9 @@ static int virtio_read(struct hwrng *rng, void
> *buf, size_t size, bool wait)
> if (vi->hwrng_removed)
> return -ENODEV;
>
> + if (vi->data_avail >= size || (vi->data_avail && !wait))
> + return vi->data_avail;
> +
> if (!vi->busy) {
> vi->busy = true;
> reinit_completion(&vi->have_data);
This patch was nonsense. I'm sorry. Looking into it again.
Martin
