Hacking Displays Made Easy

Displays are monitors, right?  Strictly output, right?

Wrong.

DVI and HDMI both support DDC, which allows for display identification and “capability advertisement.”  In other words, the display is sending information to the computer.  HDMI also has capabilities for “content protection,” and even has an ethernet channel.

And, of course, all these capabilities provide for neat ways to create trouble …  Lots of data comes from the display, and it has to be parsed.  And any time you are parsing data, you are, in a way, following instructions from outside the machine.

(Does anyone’s display programming take care not to trust the data coming from “the display”?  Care to take a guess, based on past experience?)

The data flying back and forth has a definite format: EDID.  There are standards.  Care to guess what can happen when you mess with the EDID data?  (And there are lots of ways it can get messed up unintentionally, starting with a simple KVM switch.)

In one case, experimenting was able to shut off system logging.  In another, EDID fuzzing was able to cause instability in the kernel.

(I’ve seen one in my own machine: on Win 7 and this hardware, plugging and unplugging USBs can shut off video feed to the display.  In two cases, attempting to recover the display crashed the system, hard.)

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