linux-kernel - Re: [PATCH] ELF: implement AT

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 7 Oct 2008 01:28:10 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Kees Cook <kees.cook@...onical.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, Jakub Jelinek <jakub@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>, libc-alpha@...rceware.org
Subject: Re: [PATCH] ELF: implement AT_RANDOM for future glibc use

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@...ux.intel.com
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/