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- en/assets/content_type/white_papers_and_tech_docs/26094.PDF [4] http://www.amd.com/us- en/assets/content_type/white_papers_and_tech_docs/24593.pdf
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