On 04/01/2017 04:58 PM, Vít Šesták wrote:
Hello,
I've realized that HDMI offers not only graphical/sound output, but also many
inputs. Well, some inputs are expected (listing of available output modes etc.
works AFAIK even with VGA), but others can be more or less surprising:
* audio return channel
* CEC
* ethernet (!)
* maybe even more
Let's assume I have connected an untrusted HDMI device to my laptop with
QubesOS. I am aware that screen output will be passed to untrusted device
(e.g., I don't read private e-mail on the screen, but maybe I show some public
presentation). What can happen if the device is malicious? Can it pass
compressed or otherwise complex sound input to dom0? Can it control my laptop
over CEC? Can it connect dom0 to network? Will dom0 ignore the HDMI network?
Can anything else bad happen? (Yes, the device can pass too high voltage to my
laptop, but this is not the kind of attack I can reasonably resolve.)
Maybe you assume that screen should be trusted. This is not always the case.
Let's assume we connect to our laptop variously trusted HDMI output devices,
ranging from private external screen (most trusted screen) to shared
internet-connected and DVB-connected TV with outdated crappy firmware (least
trusted). If you are interested in digital TV security, look at
https://www.bleepingcomputer.com/news/security/about-90-percent-of-smart-tvs-vulnerable-to-remote-hacking-via-rogue-tv-signals/
. As mentioned above, need of connecting laptop to an untrusted HDMI output is
pretty reasonable provided you respect the level of trust of the screen.
Regards,
Vít Šesták 'v6ak'
I think having a graphics driver that disables any auxiliary modes (on
the GPU) would be a reasonable first step in addressing the issue. It
may also be possible to disable HDMI ports in favor of simpler ones like
VGA. I'm not sure how much input DVI and Displayport allow, but I think
there's a chance that DVI is similar to VGA in this regard.
--
Chris Laprise, tas...@openmailbox.org
https://twitter.com/ttaskett
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