[U-Boot] Disable command at runtime

[Petr Kubizňák]

Dear Wolfgang,

On 08/01/2016 10:05 PM, Wolfgang Denk wrote:
> How could that ever be "safe" - in the sense of protecting against an
> attacker?  How could you perform such a "switch" between modes?  By
> setting some bit somewhere.  And it has to be in some persistent
> storage.  And the source code of your image is available to the
> public.  What should prevent an attacker from undoing your bit
> setting and switching back to "full" mode?
If it was to be an irreversible switch, a reliable way might be to 
effectively remove some parts of the program by overwriting them. Not 
that I ever have done that, perhaps it's not that easy as I imagine, but 
I believe it's possible.
> U-Boot is a boot loader, not a high security environment.  If you
> grand somebody access to the U-Boot command line interface, he owns
> the system.  If not directly, so by just pulling a few simple tricks.
You are absolutely right, whoever has access to U-Boot, can easily 
destroy the system. The main problem is perhaps in my understanding of 
the concept. I'm always tempted to keep access to U-Boot "for future 
sakes", but have not found a reliable way to deny the access to the 
others. Is there a "correct approach"?

By the way, once I read in some conversation that bad security is no 
security, so that's why U-Boot does not implement bad security. From my 
point of view, bad security (e.g. password stored in env) is strong 
enough to keep away the amateurs who just want to play with it and don't 
really know they might destroy the system. Of course it does not secure 
the system from the really evil attackers, but what does?

Best Regards,
Petr