Few comments after reading Wiki

[Werner Almesberger]

Marcin Wiacek wrote:
> In worst case device should start with default parameters and without
> additional apps.

Our idea is that you can at least load a new boot loader, kernel,
etc.  over DFU. That's the minimum "sane" unbricking requirement.
Anything else would require more space. (We may actually have
enough space for also putting the kernel and a bit of user space
there. But that wouldn't be a "regular" environment, but more
something like kboot or linuxbios.)

Note that there's also the option to do an "emergency boot" from
the boot menu. This is for more benign screw-ups, e.g., incorrect
touch screen calibration making the GUI unusable. For the "end
user", this level of protection, combined with a bit of DFU
hardening will be enough.

> IMHO, the best is separating memory to two phisical chips and have
> main software in first (with protection) and additional software/"HDD"
> inside second (or protection should block writing to chip below specified
> address).

Per-block protection is tricky. There are only very few companies
out there who have chips with this, and even fewer whose chips we
could actually use. I don't know of any Flash chip with useful zone
protection (you get a lot that protect about 16 kB, but that's not
enough). Just protecting the whole chip won't do, since we expect
our users (hackers) to install their own boot loaders, kernels, and
such.

With the MCU we'll use in the future, you also get (useful) zone
protection. We think it can be circumvented (by malware but also
by well-meant tools), so it's not the "100% carefree" solution
we're looking for either. However, it will allow for more flexible
use of the "recovery" Flash chip for those who choose to remove
the bulletproof protection. (E.g., if they have a debug board.)

- Werner

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 / Werner Almesberger, Buenos Aires, Argentina     werner at almesberger.net /
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