[RFC PATCH 2/7] x86/sci: add core implementation for system call isolation

[Peter Zijlstra]

On Fri, Apr 26, 2019 at 12:45:49AM +0300, Mike Rapoport wrote:
> The initial SCI implementation allows access to any kernel data, but it
> limits access to the code in the following way:
> * calls and jumps to known code symbols without offset are allowed
> * calls and jumps into a known symbol with offset are allowed only if that
> symbol was already accessed and the offset is in the next page
> * all other code access are blocked

So if you have a large function and an in-function jump skips a page
you're toast.

Why not employ the instruction decoder we have and unconditionally allow
all direct JMP/CALL but verify indirect JMP/CALL and RET ?

Anyway, I'm fearing the overhead of this one, this cannot be fast.