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Re: Announcing RSX - non exec stack/heap module
From: Paul Starzetz <paul () starzetz de>
Date: Wed, 13 Jun 2001 12:42:38 +0200

Crispin Cowan wrote:

I presume that what you're doing here is to mark the library pages
non-executable, and then make them executable when you get a page fault due to
some code trying to make a library call.  If so, how do you distinguish between
legitmate calls into the library, and bogus calls made by a buffer overflow?

No, because there is no exec/noexec flag on x86 architecture - otherwise
the whole thing would be trivial.

I map a sort of ´zombie´ or ´ghost´ libc into the RSX address space,
marking those pages as PROT_NONE. I also map a regular copy of libc at
some other location.

There are further transition handlers in the gp() and pf() trap code
handling the segment transitions between ´normal´ code and ´libc´ code
(the mentioned ret/call/jmp emulation routines).
 
"ret-into-libc" is just a common name for the technique.  It is not technically
precise.  The general case is to change a function return address so that it
jumps to code that does "exec(sh)".  The intructions that do this don't have to
be in libc, and don't have to be in a library, they can be in the program's main
executable body.  It's just convenient for the attacker to use libc, because libc
necessarily has "exec(sh)" in it, and most programs link to libc.

Ok but I´m talking only about dynamically linked libc.

So now assume we doesn't link the libc-plt to the real libc location -

Same effect:  you deny the attacker access to the libc code body, forcing them to
look elsewhere for a fairly common sequence of a half dozen bytes in an
executable page.

Hm, I´m not convinced that this code is so common. One must look for a
sequence similar to mov value, %ebx; mov SYSCALL, %eax; int 0x80; in
order to do something dangerous after overflowing a stack buffer.
 
I think that some level of randomization in the libc location and the
plt linking code would provide a simple (but not complete) defense
against simple jump-into-system()-plt and similar attack.

Paul Starzetz.


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