Re: [RFC PATCH 04/11] x86/boot/KASLR: Introduce PRNG for faster shuffling

Kristen Carlson Accardi
2020-02-07T16:53:18Z

On Fri, 2020-02-07 at 01:05 -0800, Kees Cook wrote:
> On Fri, Feb 07, 2020 at 08:23:53AM +0100, Jean-Philippe Aumasson
> wrote:
> > On Thu, Feb 6, 2020 at 4:10 PM Jason A. Donenfeld <Jason@zx2c4.com>
> > wrote:
> >
> > > Hey Kees,
> > >
> > > On Wed, Feb 05, 2020 at 02:39:43PM -0800, Kristen Carlson Accardi
> > > wrote:
> > > > +#define rot(x, k) (((x)<<(k))|((x)>>(64-(k))))
> > > > +static u64 prng_u64(struct prng_state *x)
> > > > +{
> > > > + u64 e;
> > > > +
> > > > + e = x->a - rot(x->b, 7);
> > > > + x->a = x->b ^ rot(x->c, 13);
> > > > + x->b = x->c + rot(x->d, 37);
> > > > + x->c = x->d + e;
> > > > + x->d = e + x->a;
> > > > +
> > > > + return x->d;
> > > > +}
> > >
> > > I haven't looked closely at where the original entropy sources
> > > are
> > > coming from and how all this works, but on first glance, this
> > > prng
> > > doesn't look like an especially cryptographically secure one. I
> > > realize
> > > that isn't necessarily your intention (you're focused on speed),
> > > but
> > > actually might this be sort of important? If I understand
> > > correctly, the
> > > objective of this patch set is so that leaking the address of one
> > > function doesn't leak the address of all other functions, as is
> > > the case
> > > with fixed-offset kaslr. But if you leak the addresses of _some_
> > > set of
> > > functions, and your prng is bogus, might it be possible to figure
> > > out
> > > the rest? For some prngs, if you give me the output stream of a
> > > few
> > > numbers, I can predict the rest. For others, it's not this
> > > straight
> > > forward, but there are some varieties of similar attacks. If any
> > > of that
> > > set of concerns turns out to apply to your prng_u64 here, would
> > > that
> > > undermine kaslr in similar ways as the current fixed-offset
> > > variety? Or
> > > does it not matter because it's some kind of blinded fixed-size
> > > shuffle
> > > with complex reasoning that makes this not a problem?
> >
> > Let me share my 2 cents:
> >
> > That permutation might be safe but afaict it hasn't been analyzed
> > wrt
> > modern cryptographic techniques and there might well be
> > differential
> > characteristics, statistical biases, etc.
> >
> > What about just using SipHash's permutation, already in the kernel?
> > It
> > works on 4*u64 words too, and 6 rounds would be enough.
> >
> > Doing a basic ops count, we currently have 5 group operations and 3
> > rotations per round or 150 and 90 for the 30 init rounds. With
> > SipHash it'd
> > be 48 and 36 with the proposed 6 rounds. Probably insignificant
> > speed wise
> > as init is only done once but just to show that we'd get both
> > better
> > security assurance and better performance.
>
> Yeah, this was never meant to be anything but a POC and after timing
> tests, it seemed like an unneeded abstraction but was kept for this
> RFC so it was possible to specify a stable seed at boot for
> debugging,
> etc. I think this patch will not survive to v1. :)

That's right, I'm going to drop it and go with the ChaCha20
implementation as was suggested.