Re: comments on PMP enhancements

John Hauser

Nick Kossifidis wrote:
You don't need to do a ROP attack on BootROM to remove the rules, any
code that runs on M-mode can remove them in a few instructions. I don't
see how we can have a security control on M mode without also locking
the rules for preventing further modifications from the next boot
stages. [...]
I'd like to point out that my own proposal does always allow for the
locking of regions for the next boot stages, when that's appropriate.

When defining security controls there are guarantees we put in place.
Let's say that we did everything correctly, both in hw and sw, up to the
point where we place the PMP rule there to prevent "other software"
running on M-mode from accessing some memory region. Can we guarantee
that this restriction will be enforced and that "other software" can't
simply bypass it ? I don't believe we can provide such a guarantee,
hence we can't claim this approach to be a security control. If the code
we are trying to restrict has direct control over the security control,
then it's not a security control, it's an ilusion of a security control.
To the contrary it may pose a security threat.
If unlocked M-mode PMP entries can't be called security controls, then
fine, let's call them something else. As I see it, the point is that
there is a demand/need for such an ability, it obviously involves PMP,
and furthermore it's been shown that the physical mechanisms are very
close to one another. If you ignore this other need and adopt the
current proposal, there's a very good chance we'll end up having to
retrofit the PMP table to support this other need later, creating a
bigger mess than if we learn to accomodate it now.

I understand you're afraid that offering any compromises in the PMP
mechanism will allow developers to choose the not-totally-secure
options and delude themselves that it gives them true security.
But that's not a hardware-mechanisms problem; that's a problem for
software policies and their enforcement. That concern needs to be
dealt with in a different way than by hobbling the hardware.

For my part, I will try to be much more careful about my use of the
words "secure" and "security" going forward.


- John Hauser

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