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

[Ard Biesheuvel]

On 26 February 2016 at 13:18, Mark Rutland <mark.rutland at arm.com> wrote:
> On Fri, Feb 26, 2016 at 12:51:25PM +0100, Ard Biesheuvel wrote:
>> On 26 February 2016 at 12:01, Laurentiu Tudor <laurentiu.tudor at 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 at nxp.com>
>> > Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> > Cc: Kees Cook <keescook at 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?
>

Define 'environmental'. The only architected thing you can rely on is
the timer, which really does not hold a lot of entropy on modern solid
state platforms. I don't have the numbers, but I do have the
experience to back this up, unfortunately (at my former employer).

You could argue that KASLR does not require strong entropy like key
generation, but I would prefer to simply steer clear of that entire
debate. The bootloader simply needs to do the best job it can, either
based on some peripheral that the kernel has no awareness of this
early in the boot, or perhaps by other means if it doesn't (which
could include storing a random seed in the file system, or even in a
EFI variable). Punting it to the kernel is really the last resort.

> 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.
>

True, but that is not the point. The main risk I see is that vendors
will not bother at all once the digits start to look random to the
human eye.

>> 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).
>

If it would be *instead* of the ordinary handling, with a big fat
blurb saying that KASLR is disabled, I would not mind.

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