Re: repeatable boot randomness inside KVM guest

[Matthew Wilcox <willy@infradead.org>]

On Tue, Apr 17, 2018 at 12:57:12PM +0100, James Bottomley wrote:
> On Tue, 2018-04-17 at 04:47 -0700, Matthew Wilcox wrote:
> > On Tue, Apr 17, 2018 at 10:13:34AM +0100, James Bottomley wrote:
> > > On Sat, 2018-04-14 at 17:41 -0700, Matthew Wilcox wrote:
> > > > On Sat, Apr 14, 2018 at 06:44:19PM -0400, Theodore Y. Ts'o wrote:
> > > > > What needs to happen is freelist should get randomized much
> > > > > later in the boot sequence.  Doing it later will require
> > > > > locking; I don't know enough about the slab/slub code to know
> > > > > whether the slab_mutex would be sufficient, or some other lock
> > > > > might need to be added.
> > > > 
> > > > Could we have the bootloader pass in some initial randomness?
> > > 
> > > Where would the bootloader get it from (securely) that the kernel
> > > can't?
> > 
> > In this particular case, qemu is booting the kernel, so it can apply
> > to /dev/random for some entropy.
> 
> Well, yes, but wouldn't qemu virtualize /dev/random anyway so the guest
>  kernel can get it from the HWRNG provided by qemu?

The part of Ted's mail that I snipped explained that virtio-rng relies on
being able to kmalloc memory, so by definition it can't provide entropy
before kmalloc is initialised.

> > I thought our model was that if somebody had compromised the
> > bootloader, all bets were off.
> 
> You don't have to compromise the bootloader to influence this, you
> merely have to trick it into providing the random number you wanted. 
> The bigger you make the attack surface (the more inputs) the more
> likelihood of finding a trick that works.
> 
> >   And also that we were free to mix in as many untrustworthy bytes of
> > alleged entropy into the random pool as we liked.
> 
> No, entropy mixing ensures that all you do with bad entropy is degrade
> the quality, but if the quality degrades to zero (as it might at boot
> when you've no other entropy sources so you feed in 100% bad entropy),
> then the random sequences become predictable.

I don't understand that.  If I estimate that I have 'k' bytes of entropy
in my pool, and then I mix in 'n' entirely predictable bytes, I should
still have k bytes of entropy in the pool.  If I withdraw k bytes from
the pool, then yes the future output from the pool may be entirely
predictable, but I have to know what those k bytes were.