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

Kees Cook
2020-02-07T09:05:08Z

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. :)

--
Kees Cook