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From: Lawrence D'Oliveiro <ldo@nz.invalid>
Newsgroups: comp.arch
Subject: Re: Privilege Levels Below User
Date: Sat, 8 Jun 2024 00:06:41 -0000 (UTC)
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On Fri, 07 Jun 2024 12:03:03 -0600, John Savard wrote:

> So I am thinking it might be useful to have, for example, two states
> less privileged than the user state, and some mechanism for user
> programs to call subroutines which are in that state until they return -
> the return instruction being limited, sort of like a supervisor call, so
> it can only return in a proper manner.

MULTICS lives!

That was the next-generation kitchen-sink OS from the latter 1960s that 
was taking so long to develop, Bell Labs pulled out of the project and set 
about creating their own, much less ambitious OS instead, which they 
initially called “UNICS” (to indicate it was the opposite of “MULTICS”).

MULTICS required hardware with 8 different privilege levels (rings), from 
0 (most privileged) to 7 (least privileged).

User code normally ran at ring 4. This left 5, 6 and 7 available for 
ordinary users to impose their own additional isolation on code they 
didn’t quite trust.

Another option, less of a hierarchy and more of a privilege matrix, would 
be to use capabilities. I think I mentioned CHERI in this newsgroup 
previously.