A new fascinating Linux kernel vulnerability

Source code for a exploit of a Linux kernel vulnerability has been posted by Brad Spengler (Brad is the author of grsecurity). I have to tell you right now – this was one of the most fascinating bugs I've read about lately.

Why is it so fascinating? Because a source code audit of the vulnerable code would never find this vulnerability (well, actually, it is possible but I assure you that almost everyone would miss it). However, when you add some other variables into the game, the whole landscape changes.

While technical details about this are a bit complex, generally what's happening can be easily explained. The vulnerable code is located in the net/tun implementation. Basically, what happens here is that the developer initialized a variable (sk in the code snippet below) to a certain value that can be NULL. The developer correctly checked the value of this new variable couple of lines later and, if it is 0 (NULL), he just returns back an error. The code looks like this:

struct sock *sk = tun->sk; // initialize sk with tun->sk
…
if (!tun)
return POLLERR; // if tun is NULL return error

This code looks perfectly ok, right? Well, it is, until the compiler takes this into its hands. While optimizing the code, the compiler will see that the variable has already been assigned and will actually remove the if block (the check if tun is NULL) completely from the resulting compiled code. In other words, the compiler will introduce the vulnerability to the binary code, which didn't exist in the source code. 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. There are some other highly technical details here so you can check your favorite mailing list for details, or see a video with this exploit on YouTube at http://www.youtube.com/watch?v=UdkpJ13e6Z0. Brad was able to even bypass SELinux protections with this and LSM.

The fix for this is relatively easy, the check has to be done before assigning the value to the sk structure.
Fascinating research that again shows how security depends on every layer, and how even very expensive source code audit can result in missed vulnerabilities.