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Re: Buffer overflow prevention
From: Patrick Dolan <dolan () cc admin unt edu>
Date: Wed, 13 Aug 2003 13:20:03 -0500

There is a flag for the Gnu C/C++ compilers, -fstack-protector, that will 
implement ProPolice stack protection.  It should prevent stack smashing 

On Wednesday 13 August 2003 05:28 am, Eygene A. Ryabinkin wrote:
 I have an idea on buffer overflow prevention. I doubt that it's new, but I
haven't seen an implementation of it in any freely distributable Un*x
system. So, I hardly need your comments on it.

 Preliminary: I'm talking about Intel x86 architecture, but maybe it will
be applicable to others as well.

 The idea itself: all (correct me if I'm wrong) buffer overflows are based
on the fact that we're using the stack, referenced by SS:ESP pair, both for
procedure return address and for local variables. It seems to me, that
would we have two stacks -- one for real stack and one for variables -- it
will solve a bunch of problems. So, my suggestion: let us organise two
segments: one for normal stack, growing downwards, referenced by SS:ESP
pair and the second one, for local variables, referenced by GS:EBP pair,
with either upwards or downwards growing. Now, if we use first segment for
passing variables and procedure return addresses (normal stack usage), and
second segment only for local procedure variables, we will have the
following advantages:
 1) Local variables and return address will be physically (by means of CPU)
    divided and it will not be possible to touch the return address by
    overflowing local buffer.
 2) The procedure introduces only one extra register -- GS, since EBP is
              very often used for the stack frame.
Of course, this two segments can be made non-executable, just in case.

 What we need to implement the idea: first, rewrite kernel to organise two
segments for every process and to place proper values into the segment
registers upon the program startup. Second, rewrite the compiler to support
the new scheme of local variables addresation. So, the changes are minimal,
in some sence.

 As I said, I hardly need your criticism, suggestions, etc. of any type.

Patrick Dolan
UNT Information Security

PGP ID: E5571154
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