On Sep 20, 2019, at 11:15 AM, Alexander E. Patrakov [off-list ref] wrote:

20.09.2019 22:52, Andy Lutomirski пишет:

quoted

I think that, given existing software, we should make two or three
changes to fix the basic problems here:
1. Add GRND_INSECURE: at least let new applications do the right thing
going forward.
2. Fix what is arguably a straight up kernel bug, not even an ABI
issue: when a user program is blocking in getrandom(..., 0), the
kernel happily sits there doing absolutely nothing and deadlocks the
system as a result.  This IMO isn't an ABI issue -- it's an
implementation problem.  How about we make getrandom() (probably
actually wait_for_random_bytes()) do something useful to try to seed
the RNG if the system is otherwise not doing IO.
3. Optionally, entirely in user code: Get glibc to add new *library*
functions: getentropy_secure_blocking() and getentropy_insecure() or
whatever they want to call them.  Deprecate getentropy().
I think #2 is critical.  Right now, suppose someone has a system that
neets to do a secure network request (a la Red Hat's Clevis).  I have
no idea what Clevis actually does, but it wouldn't be particularly
crazy to do a DH exchange or sign with an EC key to ask some network
server to help unlock a dm-crypt volume.  If the system does this at
boot, it needs to use getrandom(..., 0), GRND_EXPLICIT, or whatever,
because it NEEDS a secure random number.  No about of ABI fiddling
will change this.  The kernel should *work* in this case rather than
deadlocking.

Let me express a little bit of disagreement with the logic here.

I do agree that #2 is critical, and the Clevis use case is a perfect example why it is important. I doubt that it is solvable without trusting jitter entropy, or without provoking a dummy read on a random block device, just for timings, or maybe some other interaction with the external world - but Willy already said "it seems fishy". However, _if_ it is solved, then we don't need GRND_INSECURE, because solving #2 is equivalent to magically making secure random numbers always available.

I beg to differ. There is a big difference between “do your best *right now*” and “give me a real secure result in a vaguely timely manner”.

For example, the former is useful for ASLR or hash table randomization. The latter is not.