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> The OpenBSD folks also appear to trust RDRANG, which would help not blocking. But I disagree with that trust decision.

It feeds the entropy pool, mixed in with many sources. OpenBSD also feeds in data from the AMD CCP as well. The point is it doesn't matter.

https://man.openbsd.org/ccp

And anyone running "echo badapples > /dev/random" in a loop, it still doesn't matter.

https://man.openbsd.org/random.4

https://man.openbsd.org/arc4random.9

> How does OpenBSD guarantee that this file exists, though?

If you keep reading, OpenBSD's random subsystem always mixes old with new data from many sources. If the random.seed file is not available (for example, boot media), it is not fatal. There is a warning displayed. But the system rc(8) scripts handle creating the seed file at both boot and shutdown.

https://www.openbsd.org/papers/hackfest2014-arc4random/mgp00...

> If you keep reading, OpenBSD's random subsystem always mixes old with new data from many sources.

Sure. But say we don't trust RDRAND. It's the first boot, so you haven't got a seed file yet. Assuming you haven't got a HW generator at hand, what other source are there that don't take significant time to collect from? (I.e., I know you can get it from network timings, keyboard/mouse if they exist, etc., but that takes time, and would cause a call not wanting to return prior to there being sufficient data to block. If you're only mixing in unready or untrusted sources, you might get lucky and make it hard for an attacker, or worse, if your other sources are blocking b/c they're collecting data… you might not, but regardless it hardly seems principled?)

It'd be willing to write off the first boot, except that I feel like a lot of things do potentially get initialized then; e.g., in a VM in the cloud, I think that's when host keys are initialized, for example, and certainly whatever application that VM might be hosting could have further requirements. Not blocking and issuing a warning would be to supply data prior to having sufficiently initialized a generator, no?

> Sure. But say we don't trust RDRAND. It's the first boot, so you haven't got a seed file yet.

Why not? The install media should be collecting entropy as it does the install, xoring it with a pool of random data built into it, and writing that to disk as part of the install.

Take a look at https://github.com/openbsd/src/blob/master/distrib/miniroot/..., specifically, feed_random and store_random

>Why not? The install media should be collecting entropy as it does the install, xoring it with a pool of random data built into it, and writing that to disk as part of the install.

and during first reboot you image the disk and flash it into millions of routers, resulting in something like https://devcraft.io/posts/2017/07/21/tp-link-archer-c9-admin...

Thank god OpenBSD mixes in data from hardware RNGs, and the bootloader collects entropy as part of the boot process, effectively turning that exploit vector into a state level attack.

But if you're imaging a drive and also RDRAND fails, OpenBSD is just as vulnerable to attack. See also AMD for RDRAND failing entirely.

Yeah, that's why the bootloader also gathers entropy as it's coming up.

But, at some point nobody can save you from yourself entirely. Get a broken enough system, and every security feature will fail.

The bootloader collects some, but I doubt it reliably collects enough all by itself.

How much do you think is enough, and why do you think the amount collected during boot is insufficient?

Let's say 300 bits.

I'm assuming a bootloader can't collect more in three seconds than an OS can collect in multiple minutes. Is that assumption wrong?

If it's the design of the code that lets it collect entropy faster, then it's that code that needs to be ported, and it doesn't matter if it's in a bootloader or not except to save a second or two.

300 true random bits is a lot. Even something like 128 bits is unbreakable with available hardware.

Okay, that's fair for install media installing to a physical machine such as a desktop.

What about virtual machines / machines booting off an image? We can't put a seed in the image, or it'll get distributed to all downstream consumers.

Also, there's the case posed in response to my first comment, about IOT devices which would ship with some factory-installed image.

(I suppose there are some novel ways one could work around this such as somehow keeping a pool of one-shot images ready w/ just the seed added to them, but I don't feel like this is how real-world systems work. E.g., an AMI in AWS?)

> What about virtual machines / machines booting off an image? We can't put a seed in the image, or it'll get distributed to all downstream consumers.

Mitigations within mitigations.

Having one random seed per VM image that gets installed is better than having nothing at all -- now, an attacker needs to have your install image. Then the hardware RNG and the virtio random drivers help mix into that seed.

Given that presumably you've already booted the image for testing, you've probably already generated the SSH keys that you need to be concerned about -- so you'd probably need to take care to regenerate them securely in any case, from a running system that has started to gather entropy from all the sources it can get its hands on, including the boot loader.

And, ideally part of the imaging process would write some random data to the image. But I agree, most people wouldn't even think of doing that.

http://man.openbsd.org/man4/viornd.4

HW rng is essential then

/dev/random and /dev/urandom are world-writeable by design. This allows anyone to contribute to randomness via "cat ~/my_secrets > /dev/random". Of course, my_secrets might not be very secret, or controlled by the attacker. So there is no credit given to that.

There is a way for a root process, like systemd, to contribute trusted entropy to the entropy pool, using an ioctl. Of course, then it is systemd which will be taking on the responsibility if it turns out that /var/lib/random_seed turns out to be replicated onto a zillion static images on some IOT device, so it turns out /var/lib/random_seed wasn't actually all that random to begin with

> The OpenBSD folks also appear to trust RDRANG, which would help not blocking. But I disagree with that trust decision.

Mixing more data into a random source cannot make it worse. OpenBSD does not use rdrand alone.

This is unlikely, but wouldn't it be conceivable that a malicious hardware "RNG" that lives in the CPU and has access to the internal state of the OS RNG (and knows common OS algorithms) could always supply the worst possible value to counteract whatever is in the pool?

Yeah, but you don't need to use rdrand in that scenario -- the cpu could also just tamper with the pool at will, regardless of the RDRAND instruction.

djb has noted that RDRAND is a particularly attractable means to exfiltration back in 2014 [1]. So it is not simply irrelevant.

[1] https://blog.cr.yp.to/20140205-entropy.html

I believe they xor the output of rdrand with that of other randomness sources; meaning, they don't have to trust that its output is actually random.

There is an ioctl to write with an ascribed entropy from Linux.