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Re: Announcing RSX - non exec stack/heap module
From: Crispin Cowan <crispin () wirex com>
Date: Thu, 07 Jun 2001 12:22:40 -0700

Paul Starzetz wrote:

One don't even need code in the libc. There may also be code in regular
code 'segments' mmapped from the binary valuable for jumping into them.

True.  "libc" is just the common point of reference, because nearly all programs
link to it, so it's assured to be there.


However it is possible to develop a defense agains jumping into libc
code if the performance is not the most important thing.

It is not very hard to mmap the libc code as non-executable are into
main memory. After the regular programm code jumps into some libc
function, we can check in the gp() handler if the gp fault resulted from
jumping into the libc area by a ret (the target address should still be
on the stack) or by a regular call/jmp instruction.

That's an interesting idea, but the performance penalty will be substantial.
You will pay for (at least) two system calls per library call.  In early
StackGuard research, we experimented with hardware protection methods that
imposed 2 syscalls per function call, and the overhead was between 500% and
10,000%, which just isn't realistic for prodution use.


Of course this again
doesn't protect against function-pointer overflows but on the other hand
eliminates again 90% of the potential vulnerabilities.

But can there be a 100% protection at all?

There can be 100% protection for limited threat models.  Consumers of a
technology need to evaluate whether the threat model matches their needs.

Crispin

--
Crispin Cowan, Ph.D.
Chief Scientist, WireX Communications, Inc. http://wirex.com
Security Hardened Linux Distribution:       http://immunix.org
Available for purchase: http://wirex.com/Products/Immunix/purchase.html


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