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amelius parent
So the problem is very practical, not theoretical. Someone just needs to do the plumbing, and get that entropy from host to guest OS.
orra
> get that entropy from host to guest OS.

There are some ways, although that doesn't mean they're always used.

I read this week QEMU has provides a virtio RNG device to the guest, that reads from the host. That's good. What I'm less clear about is other hypervisors, or whether x86 hypervisors tend to provide RDRAND support.

als0
> or whether x86 hypervisors tend to provide RDRAND support.

One issue that I've been wondering about is how safe this is. Presumably you can't let a VM use the RDRAND directly because it could starve the entropy of other VMs. Assuming that presumption is true, a hypervisor would need to manage that shared resource fairly amongst the VMs.

asveikau
I don't know this for sure, but I'm not under the impression that RDRAND has the same "entropy as a scarce resource" or "block waiting on entropy" issue that the Linux apis have.
dragontamer
RDSEED is actually what people want most of the time. RDRAND may return before sufficient entropy has been gathered.

Basically, RDRAND was made first to be something like a hardware /dev/urandom. Of course, people started using RDRAND to generate RSA-keys or whatever, but the entropy may be depleted between uses.

So you're supposed to use RDSEED whenever you need something that'd actually be truly random with a full set of entropy.

als0
Ah I realise I meant RDSEED
eternityforest
AFAIK part of the problem is Linux doesn't fully trust RDRAND. If it did, people would riot because they don't trust that Intel hasn't backdoored it.
PeterStuer
If you don't trust the CPU wouldn't you have bigger and more fish to fry than just RDRAND?
eternityforest
Yes but RDRAND is easy to hide. No test can tell the difference between a good and a backdoored RNG, you don't know what to look for, the whole point of randomness is that you don't know what it will do, so you can only trust reasoning based on understanding where the numbers come from.

I'm pretty sure rdrand is fine, and would not bat an eye if Linux trusted it, but I'm not sure I would bet some nuclear launch codes on it. Linux is trying to be as safe as practically possible, not just "Probably safe".

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