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virtio: rng: gracefully handle 0 byte returns

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According to the virtio v1.x "entropy device" specification, a virtio-rng
device is supposed to always return at least one byte of entropy.
However the virtio v0.9 spec does not mention such a requirement.

The Arm Fixed Virtual Platform (FVP) implementation of virtio-rng always
returns 8 bytes less of entropy than requested. If 8 bytes or less are
requested, it will return 0 bytes.
This behaviour makes U-Boot's virtio_rng_read() implementation go into an
endless loop, hanging the system.

Work around this problem by always requesting 8 bytes more than needed,
but only if a previous call to virtqueue_get_buf() returned 0 bytes.

This should never trigger on a v1.x spec compliant implementation, but
fixes the hang on the Arm FVP.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reported-by: Peter Hoyes <peter.hoyes@arm.com>
This commit is contained in:
Andre Przywara 2023-11-07 16:09:00 +00:00 committed by Tom Rini
parent 741d1e9d3f
commit 45c4b276f0
1 changed files with 7 additions and 2 deletions
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9
drivers/virtio/virtio_rng.c
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@ -20,7 +20,7 @@ struct virtio_rng_priv {
static int virtio_rng_read(struct udevice *dev, void *data, size_t len)
{
int ret;
unsigned int rsize;
unsigned int rsize = 1;
unsigned char buf[BUFFER_SIZE] __aligned(4);
unsigned char *ptr = data;
struct virtio_sg sg;
@ -29,7 +29,12 @@ static int virtio_rng_read(struct udevice *dev, void *data, size_t len)
while (len) {
sg.addr = buf;
sg.length = min(len, sizeof(buf));
/*
* Work around implementations which always return 8 bytes
* less than requested, down to 0 bytes, which would
* cause an endless loop otherwise.
*/
sg.length = min(rsize ? len : len + 8, sizeof(buf));
sgs[0] = &sg;
ret = virtqueue_add(priv->rng_vq, sgs, 0, 1);