Speed and security of /dev/urandom

[Mateusz Guzik]

On Sat, Jul 19, 2014 at 11:53:50PM +0300, Konstantin Belousov wrote:
> On Sat, Jul 19, 2014 at 09:47:12PM +0100, Steven Chamberlain wrote:
> > On 19/07/14 20:26, Konstantin Belousov wrote:
> > > I think that using sysctl for non-management functionality is wrong.
> > > If this feature is for the libraries and applications, and not for
> > > system management and introspection utilities, it should be normal
> > > syscall.
> > 
> > If this is only to seed the arc4random in userland (with ~256 bytes or
> > so), it would be just like OpenBSD getentropy(2)?
> > 
> > Just yesterday, something very similar is proposed for Linux, called
> > getrandom(2):
> > http://lists.openwall.net/linux-kernel/2014/07/18/329
> 
> We, in fact, do not use sysctl for seeding SSP canary.  Kernel puts
> random bytes on stack, and libc fetches them.  But it is 64 bytes for
> 64-bit platforms, 32 bytes for 32-bit.
> 
> Yes, the interface of the getrandom(2) from the link above looks
> reasonable.  The big question is, indeed, about its supposed use
> models.  For one-time seeding of RNG with fixed amount of bytes,
> the ELF aux vector mechanism is much less intrusive and faster.

I believe the idea here is to have reliable source for reseeding after
fork.

-- 
Mateusz Guzik <mjguzik gmail.com>