Re: [PATCH] random: add blocking facility to urandom

Steve Grubb
2011-09-07T20:57:07Z

On Wednesday, September 07, 2011 04:37:57 PM Sasha Levin wrote:
> On Wed, 2011-09-07 at 16:30 -0400, Steve Grubb wrote:
> > On Wednesday, September 07, 2011 04:23:13 PM Sasha Levin wrote:
> > > On Wed, 2011-09-07 at 16:02 -0400, Steve Grubb wrote:
> > > > On Wednesday, September 07, 2011 03:27:37 PM Ted Ts'o wrote:
> > > > > On Wed, Sep 07, 2011 at 02:26:35PM -0400, Jarod Wilson wrote:
> > > > > > We're looking for a generic solution here that doesn't require
> > > > > > re-educating every single piece of userspace. And anything done
> > > > > > in userspace is going to be full of possible holes -- there
> > > > > > needs to be something in place that actually *enforces* the
> > > > > > policy, and centralized accounting/tracking, lest you wind up
> > > > > > with multiple processes racing to grab the entropy.
> > > > >
> > > > > Yeah, but there are userspace programs that depend on urandom not
> > > > > blocking... so your proposed change would break them.
> > > >
> > > > The only time this kicks in is when a system is under attack. If you
> > > > have set this and the system is running as normal, you will never
> > > > notice it even there. Almost all uses of urandom grab 4 bytes and
> > > > seed openssl or libgcrypt or nss. It then uses those libraries.
> > > > There are the odd cases where something uses urandom to generate a
> > > > key or otherwise grab a chunk of bytes, but these are still small
> > > > reads in the scheme of things. Can you think of any legitimate use
> > > > of urandom that grabs 100K or 1M from urandom? Even those numbers
> > > > still won't hit the sysctl on a normally function system.
> > >
> > > As far as I remember, several wipe utilities are using /dev/urandom to
> > > overwrite disks (possibly several times).
> >
> > Which should generate disk activity and feed entropy to urandom.
>
> I thought you need to feed random, not urandom.

I think they draw from the same pool.

> Anyway, it won't happen fast enough to actually not block.
>
> Writing 1TB of urandom into a disk won't generate 1TB (or anything close
> to that) of randomness to cover for itself.

We don't need a 1:1 mapping of RNG used to entropy acquired. Its more on the scale of
8,000,000:1 or higher.

> > > Something similar probably happens for getting junk on disks before
> > > creating an encrypted filesystem on top of them.
> >
> > During system install, this sysctl is not likely to be applied.
>
> It may happen at any time you need to create a new filesystem, which
> won't necessarily happen during system install.
>
> See for example the instructions on how to set up a LUKS filesystem:
> https://wiki.archlinux.org/index.php/System_Encryption_with_LUKS#Preparatio
> n_and_mapping

Those instructions might need to be changed. That is one way of many to get random
numbers on the disk. Anyone really needing the security to have the sysctl on will
also probably accept that its doing its job and keeping the numbers random. Again, no
effect unless you turn it on.

-Steve