virtio/rng: return at least one byte of entropy

In contrast to the original v0.9 virtio spec (which was rather vague),
the virtio 1.0+ spec demands that a RNG request returns at least one
byte:
"The device MUST place one or more random bytes into the buffer, but it
MAY use less than the entire buffer length."

Our current implementation does not prevent returning zero bytes, which
upsets an assert in EDK II. /dev/urandom should always return at least
256 bytes of entropy, unless interrupted by a signal.

Repeat the read if that happens, and give up if that fails as well.
This makes sure we return some entropy and become spec compliant.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reported-by: Sami Mujawar <sami.mujawar@arm.com>
Link: https://lore.kernel.org/r/20230524112207.586101-3-andre.przywara@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 15 insertions, 3 deletions

diff --git a/virtio/rng.c b/virtio/rng.c
index e6e70ced..77a3a113 100644
--- a/virtio/rng.c
+++ b/virtio/rng.c
@@ -61,13 +61,25 @@ static u64 get_host_features(struct kvm *kvm, void *dev)
 static bool virtio_rng_do_io_request(struct kvm *kvm, struct rng_dev *rdev, struct virt_queue *queue)
 {
 	struct iovec iov[VIRTIO_RNG_QUEUE_SIZE];
-	ssize_t len = 0;
+	ssize_t len;
 	u16 out, in, head;
 
 	head	= virt_queue__get_iov(queue, iov, &out, &in, kvm);
 	len	= readv(rdev->fd, iov, in);
-	if (len < 0 && errno == EAGAIN)
-		len = 0;
+	if (len < 0 && (errno == EAGAIN || errno == EINTR)) {
+		/*
+		 * The virtio 1.0 spec demands at least one byte of entropy,
+		 * so we cannot just return with 0 if something goes wrong.
+		 * The urandom(4) manpage mentions that a read from /dev/urandom
+		 * should always return at least 256 bytes of randomness, so
+		 * just retry here, with the requested size clamped to that
+		 * maximum, in case we were interrupted by a signal.
+		 */
+		iov[0].iov_len = min(iov[0].iov_len, 256UL);
+		len = readv(rdev->fd, iov, 1);
+		if (len < 1)
+			return false;
+	}
 
 	virt_queue__set_used_elem(queue, head, len);