rewriter: Omit register scrubbing when caller/target pkey are zero

[ayrtonm]

If a call gate's caller has pkey zero then its memory is accessible to all compartments and we shouldn't care about scrubbing registers before calling the target since its memory is not protected anyway. If the call gate's target has pkey zero then the same argument holds for the register scrubbing after calling the target function.

[ayrtonm]

To summarize our conversation on security issues yesterday, by omitting dead register (i.e. those not used for args/return val) scrubbing compartment A could write to B's memory. This could either happen if a function in B uses inline assembly to write a nominally dead register (dead according to function B's ABI) to memory or if a signal handler is invoked while in compartment B and all registers must be written to the stack in B.

However if compartment A wanted to write to B's memory it could always just use a live register which B spills to the stack. To me the two cases above seem much more difficult to achieve than using live registers (which must always be an option since that's the bare minimum we need for compartments to be able to interact) so I don't think we should have register scrubbing just for these scenarios. That's why I see register scrubbing as more important for preventing unintended data from exiting a compartment (e.g. leaking sensitive information) rather than preventing unintended data from entering a compartment (e.g. injecting malicious values into a compartment). @fw-immunant feel free to add anything I may have misinterpreted. @rinon thoughts?

[rinon]

This LGTM, I took a look through the resulting assembly and things look right to me. We are effectively assuming that any necessary scrubs happen before transitioning to compartment zero. I agree on scrubs being necessary for confidentiality, not integrity, i.e. we don't care if values end up spilled to the stack as long as they aren't secret values.