diff mbox series

[kvmtool,v2,1/2] virtio/rng: switch to using /dev/urandom

Message ID 20230419170136.1883584-2-andre.przywara@arm.com (mailing list archive)
State New, archived
Headers show
Series Fix virtio/rng handling in low entropy situations | expand

Commit Message

Andre Przywara April 19, 2023, 5:01 p.m. UTC 
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 system without ample noise sources (no keyboard, mouse,
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.
Due to a special behaviour documented on the urandom(4) manpage, a read
from /dev/urandom will never block, so we can drop the O_NONBLOCK flag.

[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 mbox series

Patch

diff --git a/virtio/rng.c b/virtio/rng.c
index 8f85d5ec1..e6e70ced3 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);
 	if (rdev->fd < 0) {
 		r = rdev->fd;
 		goto cleanup;