[PATCH] Use arc4random() to permutate talloc_magic number

[Andreas Schneider]

On Friday, 16 February 2018 03:32:55 CET Timur I. Bakeyev via samba-technical 
wrote:
> Hi, Andrew!
> 
> On 14 February 2018 at 04:51, Andrew Bartlett <abartlet at samba.org> wrote:
> > On Wed, 2018-02-14 at 03:59 +0100, Timur I. Bakeyev via samba-technical
> > 
> > wrote:
> > > Hi!
> > > 
> > > I've noticed that talloc testsuite fails on FreeBSD cause talloc_magic
> > > is
> > > always a predictable number, derived from the function address of the
> > > loaded library, which is unfortunately always the same(contrary to
> > 
> > OpenBSD,
> > 
> > > for example). The existing talloc code uses Linux/glibc specific calls
> > > to
> > > obtaining random bytes to permutate talloc_magic. *BSD has their own set
> > 
> > of
> > 
> > > arc4*  functions that can provide cryptographically applicable PRNG.
> > > 
> > > So, this small patch adds support for the usage of arc4random() to
> > > obtain
> > > random talloc_magic signature per invocation.
> > 
> > The problem I have with this being the default is that unlike on *BSD
> > arc4random on in libbsd relies on the recent getentropy() stuff or goes
> > back to /dev/urandom.
> > 
> > Now, you don't link against libbsd intentionally, but I can see that
> > happening.
> > 
> > I chose the getauxval() because we don't want the library constructor
> > for talloc looking to open files.
> > 
> > Sorry,
> 
> I understand your concerns about possibility of some misuse of arc4random()
> on a
> Linux platform, although not very likely to happen. But what shall FreeBSD
> and other
> *BSD users do in that case? Just stay without any protection from pool
> hijacking?
> 
> I think it's not very fair in regards to *BSD community. Can we find some
> solution that
> would make everyone happy?
> 
> Is your main concern regarding this patch that it makes arc4random()
> preferable choice
> over getauxval()? That can be fixed with some code re-arrangement and macro
> magic.

Doesn't FreeBSD offer getentropy()?

-- 
Andreas Schneider                   GPG-ID: CC014E3D
Samba Team                             asn at samba.org
www.samba.org