On Wed, Jan 15, 2020 at 09:16:16AM +0000, Will Deacon wrote:

On Wed, Jan 15, 2020 at 08:48:46AM +0100, Ard Biesheuvel wrote:

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Note that we are only adding data here, it will be mixed into the pool
but won't be credited as entropy. There are currently no suitable
interfaces for that at present - extending the random code to provide

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This is slightly unfortunate, as this way, we lose the ability to use
random.trust_cpu=1 to get the entropy credited and initialize CRNG
early.

Right.  OTOH that's a bit of a mess to do, I do have some
thoughts but it's a bit of a mess trying to do it tastefully,
especially when considering that you probably don't want an
interface that it's easy for something to misuse.  The effort
involved certainly seems large enough to handle separately.

Agreed. Do you think we should wait for that support before merging the
series? Given that I don't know of any CPUs implementing this extension,
we can probably afford not to rush this in.

It's implemented in at least the fast models already, not checked
any of the other emulators, so there's some possibility of people
using it while developing other things and hopefully at least
some of the various CI systems will be including emulated
platforms with newer extensions in their coverage so might gain
some benefit from it.  Frankly the only reason I'm looking at
this at all is that I'd written patch 3 because I was getting fed
up with KASLR initialization being easily disabled when I was
trying to test E0PD on the models (especially before I added the
status print at boot to KASLR so this happened silently), having
this in mainline would've helped considerably when working on
that.

I don't see any downside to having the code in mainline as is,
even though it's not ideal it does make things better since if
for some reason anyone does end up running this code on a system
that has the feature they'll get at least some benefit from it
even if nothing else happens.  The bulk of the code isn't going
to change when the early init stuff gets improved and includes
tables like cpufeature.h that make it annoying to hold out of
tree, the bits that are going to change can just as well be
worked on incrementally as held out of tree entirely and having
the rest in means there's less friction doing that.