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Subject: Re: Linux 5.3-rc8
To:     Linus Torvalds <torvalds@linux-foundation.org>,
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From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
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Date:   Wed, 18 Sep 2019 11:33:39 +0200
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On 17/09/2019 22.58, Linus Torvalds wrote:
> Side note, and entirely unrelated to this particular problem, but
> _because_ I was looking at the entropy init and sources of randomness
> we have, I notice that we still don't use the ToD clock as a source.

And unrelated to the non-use of the RTC (which I agree seems weird), but
because there's no better place in this thread: How "random" is the
contents of RAM after boot? Sure, for virtualized environments one
probably always gets zeroed pages from the host (otherwise the host has
a problem...), and on PCs maybe the BIOS interferes.

But for cheap embedded devices with non-ECC RAM and not a lot of
value-add firmware between power-on and start_kernel(), would it make
sense to read a few MB of memory outside of where the kernel was loaded
and feed those to add_device_randomness() (of course, doing it as early
as possible, maybe first thing in start_kernel())? Or do the reading in
the bootloader and pass on the sha256() in the DT/rng-seed property?

A quick "kitchen-table" experiment with the board I have on my desk
shows that there are at least some randomness to be had after a cold boot.

Maybe this has already been suggested and rejected?

Rasmus