[PATCH][RFC] arm64: kaslr: add pseudo-RNG in kernel

[mark.rutland@arm.com (Mark Rutland)]

On Fri, Feb 26, 2016 at 12:51:25PM +0100, Ard Biesheuvel wrote:
> On 26 February 2016 at 12:01, Laurentiu Tudor <laurentiu.tudor@nxp.com> wrote:
> > In case the bootloader doesn't provide randomness
> > (through device tree or by uefi protocol) generate
> > a pseudo-random seed based on the timer counter.
> > People might find this "week rng" approach convenient
> > as it gets rid of the bootloader dependency.
> >
> > The patch tries to mimic the x86's rdtsc
> > based implementation authored by Kees Cook.
> >
> > Signed-off-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
> > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> > Cc: Kees Cook <keescook@chromium.org>
> 
> Hi Laurentiu,
> 
> I appreciate the interest in this work, but to be honest, I don't like
> this at all. I went out of my way to ensure that
> a) the kernel itself does not take part in generating the random bits, and
> b) the random bits are used in such a way that there is no correlation
> between the randomization of the core kernel, the module region and
> the linear region if there is no correlation between the random bits.
> 
> The limited entropy of the cycle counter at early boot defeats that,
> and even worse, it will not encourage platform vendors to implement
> this properly in their boot code, given that it will appear to work,
> and the only thing more dangerous than no security is a false sense of
> security imo.

I agree that a false sense of security is a worry here.

Either way, I think we need numbers. It's non-obvious how much entropy
we can acquire through counters or other means at early boot time.

Has anyone done an analysis of environmental entropy available (through
any means) at early boot, VM vs native?

It's also not obvious that vendors will correctly implement the EFI RNG
protocol; depending on the above we may want to mix in additional entropy
regardless.

> What I would ack, for development purposes, is something similar to
> what Mark Rutland implemented for randomizing TEXT_OFFSET, so that
> developers get to exercise this code even if their boot environment
> does not provide any entropy. Anything beyond that is a nack as far as
> I am concerned.

FWIW, I would not like to see that approach. I can easily see a build-time
constant KASLR seed being abused to give a false sense of security. Having a
bootloader or hypervisor provide different random seeds to the same Image gives
you a much better turnaround time for testing regardless (vs rebuilding,
copying, etc).

The TEXT_OFFSET fuzzing was for fuzzing the bootloader, which is a bit
different to fuzzing the kernel itself.

Mark.