Re: [f-cpu] little feed-back from the libre softawre meeting

[Cedric BAIL <cedric.bail@free.fr>]

> > 2) Supervisor/user bits :
> >     - 's' : some instructions which are considered as priviledged requires
> > this bit set.
> 
> Inside the TLB?

Question: which instruction, normally the only one is get and put the other
didn't affect the CPU ?
 
> > But I aggree with you : we should have like 'sr','sw','ur','uw' for data
> > memory and 'sx','ux' for code memory (that way we can protect some user
> > applications for accessing supervisor code pages with 'sx = 1' and 'ux' = 0
> > in fact) if we want a more fined-grain protection.

you must have rwx bits that must be differencied. I mean that on a x86, you
have difficult to sy that a page is only executable and not readable, in fact
it was the only big recommandation, we must have 3 bits to say rwx rights.

> > 5) Ring bits : well instead of Supervisor/user bit we have several bits to
> > encode a level ring. A page must have a ring of lower priority to be
> > accessed.
> 
> Big can of worms. If privileged code always has at least the same
> access rights as unprivileged code, you have a built-in security hole
> (see Intel). Contrary to popular belief, one must be able to give
> privileged code *less* access rights than unprivileged code.

In fact ring a really not useful, I think that on the majority of the
processor where it's implemented it is not used. (in hurd, and linux it's
not used, i don't know for bsd).

> > 6) Inheritance bits : mostly a software issue I think, so I wouldn't detail
> > them here.
> 
> Who inherits what?

I didn't understand the meaning of inheritance bits.

Cedric
 
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