The 386 and up supports no-exec, but only on differing segments. Most OS
systems aren't properly implemented on the 386+ architecture. The 386+
supports read-only pages in the paging architecture, but to separate
executable code from stack and data, you have to point the segment
registers at differing memory areas. If they overlap, which simplifies
memory management, then the code, data and stack are all shared. An
entire segment, according to the Intel documentation, can set aside as
non-executable.
I think one of the major problems with the Linux implementation, and
apparently windows too, is that noone pays attention to the added security
provided by segmentation (at least to the point of putting the stack on a
different segment?) I've not delved into that portion of the Linux
source, but maybe I will someday if I have some free time. (I got my info
from a copy of Intel's pentium family processor architecture and
programming manual.)
(I'm talking about the code and data bits in the segment registers.)
Chris Rhodes
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On Fri, 26 Nov 1999 [EMAIL PROTECTED] wrote:
> I seem to recall a Linux kernel guru explaining that the x86 MMU doesn't actually
> support non-exec pages, or some such. It doesn't support it, or it just doesn't
> work right. I remember bringing up the issue of noexec and that was the answer.
>
> --Perry
>
> > Ok, here it is, on page 58, it's talking about Access Control of virtual
> > pages, and it says, literally if a page can be read, it can be executed. I
> > remember that this took my attention for some days, then I forgot about it, until
> > you mentioned it.
> >
> > richie
>
> --
> Perry Harrington Director of zelur xuniL ()
> [EMAIL PROTECTED] System Architecture Think Blue. /\
>
