Comment by userbinator | Hacker News Reader

Comment by userbinator | Hacker News Reader

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But this also has a positive side:

Ensure that FireWire drivers are present and not removed from the system

In other words, if you don't have FireWire drivers installed, then this won't do anything; another plus for not installing the drivers for those who have a system with FireWire ports but never need to use them.

My laptop has both FireWire and USB controllers disabled, the former because I never use it and the latter because I almost never use it - and when I do, I find it's not too much hassle to go into the Device Manager and enable the one for the one port I intend to use. Another positive side-effect is that the USB drivers seem to take a rather long time to initialise, so booting is much faster without them.

I don't think this kind of attack is limited to FireWire; that was just the easiest version to build. Thunderbolt can attach arbitrary PCI devices, like an FPGA that pretends to be an AHCI controller...

These interfaces have DMA, they don't need drivers to interface with the hardware.

They do need a driver to set up which DMA channel to use, etc. While these devices can go nuts once things are up and running, just plugging it in won't do anything until the computer actually acknowledges it.

So, for example, if one builds a custom Linux kernel without support for Firewire, then plugging in a firewire controller would do nothing? It couldn't access anything?

Exactly. A PCI(e) device needs to ask things of the kernel for it to initialize. Until it gets that info, it's just a motherboard warmer.

It can still read and write to memory. If it needs to "initialize" anything, it will have to commandeer the CPU by creatively rewriting memory that is in use.

This would be possible with regular PCI which is bus-based, but PCIe is a star topology with ports on the upstream switch needing to be configured into an active state for the newly plugged in device to communicate with the rest of the system. I haven't looked at the specifications but I imagine it's the same for Thunderbolt - the controller itself has to be enabled first.

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