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

[daw@cs.berkeley.edu (David Wagner)]

Kees Cook  wrote:
>On Mon, Oct 06, 2008 at 08:00:21AM +0200, Andi Kleen wrote:
>> 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.

Using /dev/urandom does seem like exactly the right thing to do.
(Andi Kleen's criticisms would be relevant if get_random_bytes() acted
like reading from /dev/random.)

I don't think it would be wise to use less than crypto strength
pseudorandom numbers for glibc -- at least, not without very thorough
analysis.  glibc is using this for security, so it has to be right.
When people say "oh, we don't need crypto-strength randomness", in
my experience it's too common to end up with something insecure.