What do Rootkits and Xerxes have in common?

By: Cris Neckar and Greg Ose

Just think of your kernel as Greece…

It’s probably not the first thing you expected to hear me say but bear with me. The arms race between rootkit development and detection has been raging for years, and although there have been great advancements in detection, rootkit developers have always had the upper hand. You simply cannot defend against what you cannot predict. Recently the battle has become increasingly heated as rootkit developers dig deeper and deeper. One only need look at the Blackhat Briefings for the past few years to see the trend.

Increasingly subversive and complex rootkits threaten to over-run our kernels and turn our business critical systems into slaves. So what are we to do? Should we simply throw up our hands and resolve ourselves to a life of bondage?

To answer this question lets approach the problem from a different angle. It has become clear that we cannot simply monitor everything a rootkit developer might change. There are just too many function pointers, operating modes, and devices to allow us to watch them all. However, one commonality does exist in all kernel mode rootkits. In some way they must all be inserted into the kernel.

There are a number of ways to introduce new code into a running kernel but not nearly so many as there are places to hide once inside. I propose that, instead of building a blacklist and peeking under all the rocks where they have hidden in the past, we instead monitor the entry point. Leonidas used a similar approach when Xerxes threatened Greece, instead of fighting on even ground the Spartans guarded the narrow door through which the Persians must travel to enter Greece.

So, you say, it’s a great theory, but how do we implement it?  Assuming an attacker has compromised a system’s most privileged usermode context, for instance a process running as root, code can be introduced into the running kernel in a number of ways. The most obvious of these is to install a module or driver and be done with it. Aside from that, an attacker would need to patch kernel memory directly, this is not trivial but is still a common method of rootkit insertion. Aside from these options, the attacker would be forced to edit the kernel binary, or an existing module on disk and either cause or wait for a reboot (There is one additional threat which we will save for later). The interesting thing about all of these potential insertion points is that they are events which would almost never occur in a production server (If they do occur an administrator will certainly be aware of the reason). By monitoring these few events we would be notified of the insertion of any existing kernel mode rootkit that I am aware of, as well as any that could be written in the future (barring a further vulnerability.. more on this later).

To implement this on Linux is actually fairly trivial. To monitor for code inserted into the kernel while running we can use the tricks of rootkit developers ourselves. Namely we can hook sys_init_module (for module insertions), and the write() handlers within the kmem and mem file operations structures (for writes to /dev/(k)mem where these are still possible).

The next step is to defend against changes made to kernel components on disk. For the kernel image itself, on a system which is rarely rebooted such as a production server, it is possible to accomplish this by booting a known good kernel. For example a kernel could be booted from media which is only physically attached to the system when the kernel is initially loaded into memory. A similar approach could be used with required kernel modules, or if preferable, they could be compared on load to known good check sums.

Now that we have covered the entry points we can focus on what to do once one of these events occurs. We do not actually need to prevent the insertion as, in some cases, these events may legitimately occur (once a root compromise has occurred a re-image of the system is generally the best approach anyway). Instead we simply need to securely log the event to a location which is not controlled by the intruder. Obviously we can not log locally. Additionally we cannot log with mechanisms which are already under the attackers control (anything accessible from user mode is out). One relatively trivial solution is to simply create a syslog packet and drop it on the network stack when one of our traps triggers. This will provide us with a remote log on a system which (hopefully) has not been compromised. When implementing this you would of course want to be careful to temporarily disable any netfilter hooks (think iptables) as these could allow the intruder to block our logging packet from usermode.

Like Sparta’s defense at Thermopolae, this method also has its mountain path. I mentioned earlier that there was an additional method for introducing code into a running kernel. This would involve the existence of a vulnerability within the kernel or a loaded module which allows arbitrary memory to be written. An attacker could create an exploit which uses such a vulnerability to introduce a rootkit into the kernel. We cannot possibly predict where an exploit will occur (if we could, vulnerability research would be a lot easier 🙂 ) making it impossible (so far 😉 ) to guard against this threat using this method. Fortunately a rootkit which used this installation method would be obsolete the moment it was first detected. This means that this type of rootkit could never reach widespread usage and would most likely be used only in a very targeted attack.