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Re: Buffer overflow prevention
From: pageexec () freemail hu
Date: Mon, 18 Aug 2003 11:18:45 +0200

Subject:  Re: Buffer overflow prevention
From:     Theo de Raadt <deraadt () cvs ! openbsd ! org>
Date:     2003-08-14 21:43:10

It's not difficult at all on x86, but having non-overlapping Segments
for Code and Data/Stack would limit the virtual address space.

I am not sure if you have heard of this neat technology called "shared
libraries".  Either you have never heard of them, or you are unaware
of they work on an x86.  Let me be completely blunt.  What you are
suggesting is unfeasable.  Please go do some learning before making
any more utterly ridiculous proposals.

Oh, the strong words of a strong man ;-). Seriously, you are wrong, the
segmentation based non-executable pages feature of PaX (SEGMEXEC [1]) does
exactly this, it creates separate (non-overlapping) segments for data/code
and has no problems with coping with shared libraries (the key to this is
vma mirroring [2]).

Anyways, on an i386 you can do W^X somewhat.  Not as perfectly as you
can on cpus that have a per-page X bit...

You are wrong again, PaX provides perfect per-page non-executable pages
using segmentation (SEGMEXEC), there are no restrictions on the ordering
of data/code pages like in OpenBSD.

This would give per-page execution stuff like we have on better cpus.
We've not worked on this yet; it is less valuable since I think it is
only newer Xeons and high-end AMD cpus which support this.  And we've
never found documentation for it either :)

Page 286 in [3] and section 5.6 in [4] have enough information about this
feature (and of course linux 2.4/2.6 source code).

[1] http://pageexec.virtualave.net/docs/segmexec.txt
[2] http://pageexec.virtualave.net/docs/vmmirror.txt
[3] http://www.amd.com/us-
[4] http://www.amd.com/us-

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