Linux has an opportunity to learn from OpenBSD.
https://www.openbsd.org/papers/hackfest2014-arc4random/index...
Seems like the man page sort of notes the problem in the article:
> Writing to /dev/random or /dev/urandom will update the entropy pool with the data written, but this will not result in a higher entropy count.
But, why not, man page, why not? I feel like if I, as root, write to /dev/random, I am saying that this is acceptable input for seeding the generator, and that it is on me to not, say, seed it with the exact same stuff every boot, or seed it with straight nuls, no?
How does OpenBSD guarantee that this file exists, though? Perhaps making it a system default is wise, but what about brand new VMs/installs? Does the install just take the time to do the first generation of that file?
The OpenBSD folks also appear to trust RDRANG, which would help not blocking. But I disagree with that trust decision.
[1]: start reading here: https://www.openbsd.org/papers/hackfest2014-arc4random/mgp00...
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.
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...
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?
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
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?)
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.
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...
But, at some point nobody can save you from yourself entirely. Get a broken enough system, and every security feature will fail.
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.
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
Mixing more data into a random source cannot make it worse. OpenBSD does not use rdrand alone.
OpenBSD also controls its own boot loader, which also feeds in entropy that the kernel then leverages. Perhaps GRUB(2) should do something similar?
"Basically as of now the entropy file saved as /var/lib/systemd/random-seed will not - drumroll - add entropy to the random pool when played back during boot. Actually it will. It will just not be accounted for. So Linux doesn't know. And continues blocking getrandom(). This is obviously different from SysVinit times2 when /var/lib/urandom/random-seed (that you still have lying around on updated systems) made sure the system carried enough entropy over reboot to continue working right after enough of the system was booted."
Thing was you need the Haveged daemon to generate entropy, something that should come within the kernel itself.