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

From: Kees Cook <kees.cook@canonical.com>
To: Andi Kleen <andi@firstfloor.org>
Cc: Roland McGrath <roland@redhat.com>,
	linux-kernel@vger.kernel.org, Jakub Jelinek <jakub@redhat.com>,
	Ulrich Drepper <drepper@redhat.com>,
	libc-alpha@sourceware.org
Subject: Re: [PATCH] ELF: implement AT_RANDOM for future glibc use
Date: Mon, 6 Oct 2008 16:29:36 -0700	[thread overview]
Message-ID: <20081006232936.GR10357@outflux.net> (raw)
In-Reply-To: <20081006231942.GO3180@one.firstfloor.org>

On Tue, Oct 07, 2008 at 01:19:42AM +0200, Andi Kleen wrote:
> And you won't deny that session keys are more important than mmap
> placement, will you?

Right, I would tend to agree that session key strength is more important
than ASLR strength.

> > I'd really love to see this solved.  My goal is to get a mainline glibc
> > patch for a low-cost randomized stack guard value.
> 
> Your current implementation is high cost.
>...
> 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.

It's being used for randomness in the networking code, so it's at least
mildly random "enough".

> IMHO it needs a new class of random numbers in the kernel that use
> some cryptographically strong RNG (there are a couple of candidates
> like yarrow) which is very rarely seeded
> from the entropy pool[1] and use that for these applications.
> A couple of other users in the kernel would benefit that too,
> most users of get_random_bytes() probably should be reviewed
> for their true requirements.

Sure, but this is a larger (and pre-existing) problem.

> Ideally expose it to userland too so that dumb users like
> tmpfile can use it too. 

Would you propose that it get hooked to /dev/urandom?

-Kees

-- 
Kees Cook
Ubuntu Security Team