Messages in this thread | | | Date | Tue, 7 Oct 2008 01:28:10 +0200 | From | Andi Kleen <> | Subject | Re: [PATCH] ELF: implement AT_RANDOM for future glibc use |
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On Mon, Oct 06, 2008 at 03:07:59PM -0700, Kees Cook wrote: > 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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