[kvmtool,v2,0/2] Fix virtio/rng handling in low entropy situations

Message

Andre Przywara April 19, 2023, 5:01 p.m. UTC

At the moment kvmtool uses the /dev/random device to back the randomness
provided by our virtio/rng implementation. We run it in non-blocking
mode, so are not affected by the nasty "can block indefinitely"
behaviour of that file. However:
- If /dev/random WOULD block, it returns EAGAIN, and we reflect that by
  adding 0 bytes of entropy to the virtio queue. However the virtio 1.x
  spec clearly says this is not allowed, and that we should always provide
  at least one random byte.
- If the guest is waiting for the random numbers, we still run into an
  effective blocking situation, because the buffer will only be filled
  very slowly, effectively stalling or blocking the guest. EDK II shows
  that behaviour, when servicing the EFI_RNG_PROTOCOL runtime service
  call, called by the kernel very early on boot.

Those two patches fix those problems, and allow to boot a Linux kernel
MUCH quicker when the host lacks good entropy sources. On a particular
system the kernel took 10 minutes to boot because of /dev/random
effectively blocking, this runs now at full speed.

The block is avoided by using /dev/urandom, there is a proper rabbit
hole in the internet out there why this is safe, even for cryptographic
applications.

Patch 2 aims to fix the corner case when the /dev/urandom read fails for
whatever reason: we just try once more in this case, since it should
only happen when the call is interrupted by a signal. This is not 100%
bullet proof, I am happy to hear any suggestions or whether we just
don't care about that very rare case.

Please have a look!

Cheers,
Andre

Changelog v1 ... v2:
- Drop O_NONBLOCK from the /dev/urandom open() call
- Drop block/unblock sequence after failed read, just retry once

Andre Przywara (2):
  virtio/rng: switch to using /dev/urandom
  virtio/rng: return at least one byte of entropy

 virtio/rng.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)