Re: [PATCH] ELF: implement AT_RANDOM for future glibc use

[Kees Cook <kees.cook@canonical.com>]

On Mon, Oct 06, 2008 at 08:00:21AM +0200, Andi Kleen wrote:
> Kees Cook <kees.cook@canonical.com> writes:
> 
> > While discussing[1] the need for glibc to have access to random bytes
> > during program load, it seems that an earlier attempt to implement
> > AT_RANDOM got stalled.  This implements a configurable number of random
> > bytes available to every ELF program via a new auxv AT_RANDOM vector.
> 
> While the basic idea is good using get_random_bytes() is not.
> 
> That eats precious cryptography strength entropy from the entropy
> pool, which on many systems is not adequately fed. In those cases you
> really only want to use it for real keys, not for lower grade
> applications. The applications glibc wants to use this for do not
> really require crypto strength entropy, just relatively unpredictable
> randomness.

We're already using get_random* for stack, heap, and brk.  Also,
get_random* uses the nonblocking pool, so this is the same as if userspace
had tried to pull bytes out of /dev/urandom, which (as I understand it)
is the very thing we're trying to duplicate without the VFS overhead.

> What you should instead do is to initialize some other cryptographic RNG
> regularly and use the output of that.

Can you give me some examples of this?  I thought the nonblocking
entropy pool was specifically for this purpose?

-Kees

-- 
Kees Cook
Ubuntu Security Team