Re: [RFC PATCH v19 2/5] security: Add new SHOULD_EXEC_CHECK and SHOULD_EXEC_RESTRICT securebits

[Steve Dower]

On 08/07/2024 22:15, Jeff Xu wrote:
> IIUC:
> CHECK=0, RESTRICT=0: do nothing, current behavior
> CHECK=1, RESTRICT=0: permissive mode - ignore AT_CHECK results.
> CHECK=0, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, no exception.
> CHECK=1, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, except
> those in the "checked-and-allowed" list.

I had much the same question for Mickaël while working on this.

Essentially, "CHECK=0, RESTRICT=1" means to restrict without checking. 
In the context of a script or macro interpreter, this just means it will 
never interpret any scripts. Non-binary code execution is fully disabled 
in any part of the process that respects these bits.

"CHECK=1, RESTRICT=1" means to restrict unless AT_CHECK passes. This 
case is the allow list (or whatever mechanism is being used to determine 
the result of an AT_CHECK check). The actual mechanism isn't the 
business of the script interpreter at all, it just has to refuse to 
execute anything that doesn't pass the check. So a generic interpreter 
can implement a generic mechanism and leave the specifics to whoever 
configures the machine.

The other two case are more obvious. "CHECK=0, RESTRICT=0" is the 
zero-overhead case, while "CHECK=1, RESTRICT=0" might log, warn, or 
otherwise audit the result of the check, but it won't restrict execution.

Cheers,
Steve