On Mon, Oct 06, 2008 at 03:07:59PM -0700, Kees Cook wrote:
quoted text
> On Mon, Oct 06, 2008 at 09:26:41PM +0200, Andi Kleen wrote:
> > > 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)
> > 
> > Yes exactly that's the problem. Think about it: do you really 
> > need the same cryptographic strength for your mmap placement
> > as you need for your SSL session keys?
> > 
> > And if you need true entropy for your session keys do you
> > still get it when it was all used for low security 
> > purposes first?
> 
> Off-list I was just shown random32().  If AT_RANDOM used that instead,
> would that be acceptable?

random32() is not a cryptographically strong RNG. I suspect it would
be pretty easy to reverse engineer its seed given some state. It hasn't
been designed to be protected against that.

While I suspect this wouldn't be a serious threat to the security
model for mmap (to break the mmap placement you would still need quite a lot of
addresses before you can predict some and I presume most apps do not leak
addresses) it would seem unnecessarily 
weak to me because using a better algorithm is not very costly.
Also it might be a problem for some of the other potential users.

cryptographically strong RNGs are especially designed to make this
reverse engineering of the state hard. 

Simple ones can be just a cryptographic hash + counter + secret or
the same with a encryption algorithm like AES, but there are 
also algorithms who are especially designed for this like yarrow/fortuna

See
http://en.wikipedia.org/wiki/Cryptographically_secure_pseudo-random_number_generator

-Andi

-- 
ak@linux.intel.com
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