How can I force /dev/random to block?

Question

For a class on cryptography, I am trying to drain the entropy pool in Linux (e.g. make /proc/sys/kernel/random/entropy_avail go to 0 and block a command reading from /dev/random) but I can't make it happen. I'm supposed to get reads from /dev/random to block. If I execute these two commands:

watch -n 0.5 cat /proc/sys/kernel/random/entropy_avail

to watch entropy and then:

od -d /dev/random

to dump the random pool, the value from the watch command hovers between 3700 and 3900, and gains and loses only a little while I run this command. I let both commands run for about three minutes with no discernible substantial change in the size of entropy_avail. I didn't do much on the computer during that time. From googling around I find that perhaps a hardware random number generator could be so good that the entropy won't drop but if I do:

cat /sys/devices/virtual/misc/hw_random/rng_available

I see nothing, I just get a blank line. So I have a few questions:

1. What's replenishing my entropy so well, and how can I find the specific source of randomness?
2. Is there any way to temporarily disable sources of randomness so I can force this blocking to happen?

Answer 1

There is a surprising amount of development going on around the Linux random device. The slow, blocking /dev/random is gone and replaced by a fast /dev/random that never runs out of data.

You'll have to travel back in time, like prior to linux 4.8 (which introduced a much faster crng algorithm) or possibly linux 5.6 (which introduced jitter entropy generation).

There is no way to get the original behavior back in current kernels.

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If you are seeing this issue in older versions of Linux, hwrng aside, you might be using haveged or rng-tools rngd, or similar userspace entropy providers.

Some distros install these by default to avoid hangs while waiting for a few random bits, in that case you can uninstall or disable them or try it from within an initrd / busybox shell where no other processes are running.

If the issue still persists, you might just have a very noisy piece of hardware from which kernel keeps collecting entropy naturally.

Answer 2

What's replenishing my entropy so well, and how can I find the specific source of randomness?

As of kernel 5.6, /dev/random obtains randomness from the kernel's CRNG, which is initialized during boot and does not block.

See "Removing the Linux /dev/random blocking pool".

Is there any way to temporarily disable sources of randomness so I can force this blocking to happen?

I think you would have to use a pre-5.6 kernel.

Answer 3

This can depend on your hardware if it's using the CPU features then it might be endless. Other hardware is not.

However, you could try the kernel option random.trust_cpu=off when booting.