Linux exploit gets around security barrier

[Brian Warkoczeski]

Linux exploit gets around security barrier

by Tom Espiner

July 20, 2009

www.cnet.com

A security researcher has released zero-day code for a flaw in the Linux 
kernel, saying that it bypasses security protections in the operating 
system.

The source code for the exploit was made available last week by 
researcher Brad Spengler on the Dailydave mailing list. According to the 
researcher, the code exploits a vulnerability in Linux version 2.6.30, 
and 2.6.18, and affects both 32-bit and 64-bit versions. The 2.6.18 
kernel is used in Red Hat Enterprise Linux 5.

The exploit bypasses null pointer de-reference protection in the 
mainline kernel, which could allow an attacker to gain root control of a 
system, Spengler wrote.

It also uses arbitrary code execution to disable security features such 
as auditing, Security-Enhanced Linux (SELinux), AppArmor and Linux 
Security Module, while making the applications running outside the 
kernel believe that SELinux is still operating.

In the notes for his source code, Spengler said the exploit is 
strengthened if SELinux is applied to the operating system. SELinux is a 
set of modifications that can be applied to the kernel to harden it, by 
providing a set of security policies.
"Having SELinux enabled actually weakens system security for these kinds 
of exploits," he wrote.

Security training organization the Sans Institute called the exploit 
"fascinating." In a blog post on Friday, Sans Institute incident handler 
Bojan Zdrnja said that the exploit uses the Linux compiler to overcome 
the security features.

"The compiler will introduce the vulnerability to the binary code, which 
didn't exist in the source code," wrote Zdrnja. "This will cause the 
kernel to try to read/write data from 0x00000000, which the attacker can 
map to userland--and this finally pwns the box."

In his notes on the source code, Spengler said that a workaround would 
be for administrators to compile the kernel with 
fno-delete-null-pointer-checks.