On Fri, Dec 27, 2019 at 06:06:56PM -0800, Andy Lutomirski wrote:

I'm thinking of having a real class device and chardev for each hwrng
device.  Authentication is entirely in userspace: whatever user code
is involved can look at the sysfs hierarchy and decide to what extent
it trusts a given source.  This could be done based on bus topology or
based on anything else.

Yes, that's what I was thinking.  Another project on my "when I can
get a round tuit" list is to change how drivers/char/random.c taps
into the hwrng devices, mixing in a bit from each of these devies in a
round-robin fashion, instead of just feeding from a single hwrng.

The kernel could also separately expose various noise sources, and the
user code can do whatever it wants with them.  But these should be
explicitly unconditioned, un-entropy-extracted sources -- user code
can run its favorite algorithm to extract something it believes to be
useful.  The only conceptually tricky bit is keeping user code like
this from interfering with the in-kernel RNG.

The other problem is the unconditioned values of the noise sources may
leak unacceptable amounts of information about system operation.  The
most obvious example of this would be keyboard and mouse sources,
where today we mix in not only the timing information, but the actual
input values (e.g., the keyboard scancodes) into the entropy pool.
Exposing this to userspace, even if it is via a privileged system
call, would be... unwise.

						- Ted