Attacking full-disk encryption with Inception

When using whole-disk encryption, it's sometimes tempting to be less concerned about attacks requiring physical presence. After all, putting a laptop to sleep is quite convenient, even though attacks like "Evil Maid" or "Cold Boot" are possible. A more recent attack just adds another worry to that list.

Inception is a tool released in 2011 that uses Firewire direct memory access (DMA) to access the memory of a sleeping (or simply powered-on, but locked) system. While it is an an older tool, Inception recently got a notoriety boost from Cory Doctorow at Boing Boing, which is where I came across it. It is a rather interesting attack, and one that isn't really exploiting a bug.

In order to facilitate high-speed transfers, Firewire (aka IEEE 1394) requires the availability of a DMA mode. DMA allows the Firewire controller to directly access system memory, bypassing the CPU. While removing the potential bottleneck of the CPU does make transfers faster, it also opens up the contents of memory for any Firewire device to inspect or modify. This is the same memory that contains various things of interest, including the code to check passwords.

It is the password-checking code that Inception targets. When the incept program is run, it will patch the Linux, Windows, or Mac OS X code running on the system such that any password can be used to log in. After that, one can log in as root (or Administrator) without need for the password—the system is fully compromised. Since the patching is in memory only, though, the change disappears at the next reboot, which may make it more difficult to detect.

Inception doesn't require a Firewire interface on the targeted system, just some way to add one (e.g. PCMCIA, ExpressCard). Typically, the system will detect the Firewire device being added and helpfully install the drivers needed. The attacker's machine, which is attached to the victim over the Firewire interface, then sends commands to enable DMA mode. From there, the program looks for signatures of password authentication modules and patches any it finds.

There are, of course, other things one can do with access to the memory, including dumping its contents for use later on. The system memory may well contain information of interest, for example credentials of various sorts. Patching other parts of the operating system are possible as well, and the incept program has support for using custom signatures and patches. Inception is useful for more than just attacks, as it can be used to help analyze any running system—one that has been compromised for example.

The attack code runs on Linux or OS X systems. It requires Python 3 and libforensic1394. Unsurprisingly, there are some caveats. Targets with more than 4G of RAM may not be attacked reliably because DMA is limited to the low 4G and the code of interest might be loaded higher up. In addition, certain OS X targets may repel the attack by disabling DMA under certain circumstances (like sleeping).

One obvious mitigation for Linux is to disable the Firewire drivers for systems that aren't using them. One could, instead, disable Firewire DMA when the drivers are loaded, but if Firewire is actually being used, that will clearly impact performance. Inception serves as a nice reminder that a powered-on system is vulnerable to many "physically present" kinds of attacks—even if the disk is encrypted