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

Neil Horman
2011-09-08T12:52:47Z

On Thu, Sep 08, 2011 at 08:41:57AM +0200, Tomas Mraz wrote:
> On Wed, 2011-09-07 at 19:57 -0400, Neil Horman wrote:
> > On Wed, Sep 07, 2011 at 04:56:49PM -0400, Steve Grubb wrote:
> > > On Wednesday, September 07, 2011 04:37:57 PM Sasha Levin wrote:
> > > > 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.
> > >
> > Where are you getting that number from?
> >
> > You may not need it, but there are other people using this facility as well that
> > you're not considering. If you assume that in the example Sasha has given, if
> > conservatively, you have a modern disk with 4k sectors, and you fill each 4k
> > sector with the value obtained from a 4 byte read from /dev/urandom, You will:
> >
> > 1) Generate an interrupt for every page you write, which in turn will add at
> > most 12 bits to the entropy pool
> >
> > 2) Extract 32 bits from the entropy pool
> >
> > Thats just a loosing proposition. Barring further entropy generation from
> > another source, this is bound to stall with this feature enabled.
> Why so? In the case the blocking limit is on 8MBits of data read
> from /dev/urandom per every 1 bit added to the entropy pool (this is not
> the exact way how the patch behaves but we can approximate that) I do
> not see the /dev/urandom can block if the bytes read from it are written
Easy, all you have to do is read 8MB of data out of /dev/urandom (plus whatever
other conditions are needed to first drain the entropy pool), prior to that bit
of entropy getting added.

> to disk device - of course only if the device adds entropy into the
> primary pool when there are writes on the device.
Yes, and thats a problem. We're assuming in the above case that writes to disk
generate interrupts which in turn generate entropy in the pool. If that
happens, then yes, it can be difficult (though far from impossible) to block on
urandom with this patch and a sufficiently high blocking threshold. But
interrupt randomness is only added for interrupts flagged with
IRQF_SAMPLE_RANDOM, and if you look, almost no hard irqs add that flag. So its
possible (and even likely) that writing to disk will not generate additional
entropy.

>
> Of course you can still easily make the /dev/urandom to occasionally
> block with this patch, just read the data and drop it.
>
> But you have to understand that the value that will be set with the
> sysctl added by this patch will be large in the order of millions of
> bits.
>
You can guarantee that? This sysctl allows for a setting of 2 just as easily as
it allows for a setting of 8,000,000. And the former is sure to break or
otherwise adversely affect applications that expect urandom to never block.
Thats what Sasha was referring to, saying that patch makes it easy for admins to
make serious mistakes.

Neil

> --
> Tomas Mraz
> No matter how far down the wrong road you've gone, turn back.
> Turkish proverb
>