Re: [PATCH, RFC] random: introduce getrandom(2) system call

On Do, 2014-07-17 at 08:52 -0400, Theodore Ts'o wrote:

On Thu, Jul 17, 2014 at 12:57:07PM +0200, Hannes Frederic Sowa wrote:

quoted

Btw. couldn't libressl etc. fall back to binary_sysctl
kernel.random.uuid and seed with that as a last resort? We have it
available for few more years.

Yes, they could.  But trying to avoid more uses of binary_sysctl seems
to be a good thing, I think.  The other thing is that is that this
interface provides is the ability to block until the entropy pool is
initialized, which isn't a big deal for x86 systems, but might be
useful as a gentle forcing function to force ARM systems to figure out
good ways of making sure the entropy pools are initialized (i.e., by
actually providing !@#!@ cycle counter) without breaking userspace
compatibility --- since this is a new interface.

I am not questioning this new interface - I like it - just wanted to
mention there is already a safe fallback for LibreSSL in the way they
already seem to do it in OpenBSD (via sysctl).

quoted

quoted

+	if (count > 256)
+		return -EINVAL;
+

Why this "arbitrary" limitation? Couldn't we just check for > SSIZE_MAX
or to be more conservative to INT_MAX?

I'm not wedded to this limitation.  OpenBSD's getentropy(2) has an
architected arbitrary limit of 128 bytes.  I haven't made a final
decision if the right answer is to hard code some value, or make this
limit be configurable, or remote the limit entirely (which in practice
would be SSIZE_MAX or INT_MAX).

The main argument I can see for putting in a limit is to encourage the
"proper" use of the interface.  In practice, anything larger than 128
probably means the interface is getting misused, either due to a bug
or some other kind of oversight.

For example, when I started instrumenting /dev/urandom, I caught
Google Chrome pulling 4k out of /dev/urandom --- twice --- at startup
time.  It turns out it was the fault of the NSS library, which was
using fopen() to access /dev/urandom.  (Sigh.)

In the end people would just recall getentropy in a loop and fetch 256
bytes each time. I don't think the artificial limit does make any sense.
I agree that this allows a potential misuse of the interface, but
doesn't a warning in dmesg suffice?

It also makes it easier to port applications from open("/dev/*random"),
read(...) to getentropy() by reusing the same limits.

I would vote for warning (at about 256 bytes) + no limit.

Thanks,
Hannes