Re: Rep:Rep:Re: Rep:Re: [f-cpu] Stack handling

[nico <nicolas.boulay@ifrance.com>]

Yann Guidon a écrit :
> 
> hi,
> 
> Nicolas Boulay wrote:
> >
> > Another idea for security :
> >
> > One of my idea is to use a separate stack for return adress to avoid
> > buffer overflow. But this new stack are in memory with read/write right
> > by this task. If an other way is find to modify the memory place, it's
> > always possible.
> >
> >  (i should refind an article where they explain how to bypass none
> > execute right on stack by writing inside librairies address space (that
> > could be protected by a ring ?) or by executing exec() with the good
> > parameter (/sbin/sh ! ;p) ).
> >
> > So what about creating 2 stores instructions ? One manipulates data
> > visible for the user and the other one for "internal" management as for
> > return address. Then we add a new bit on the MMU to allow a page to be
> > accessed (or not) by "user" store.
> >
> > This userStore could be used when manipulating array and pointers.
> > Stacks will be manipulated with the sysStore instruction inside a
> > protected page.
> >
> > Comments ?
> 
> 1) security in FC0 is enforced through the SRs and the TLB.

It's nothing new compare to others machines.

> 
> 2) there can be only one kind of load and store instructions
>    because there are already a lot of variations around it.
>

I don't see any problem. It's just a flag that follow the store.
 
> 3) maybe a compromise would be to use the "stream" flags :
>    computers that recognize it can setup a specific right or
>    protection mechanism. Others (like embedded stuffs with no
>    security problems) could simply not care, and the SW portability
>    would work at no cost.
>

You should set a right on page to have an explicit protection !
 
> > nicO
> WHYGEE
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