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> But the chances of reverse-hijacking are really REALLY slim - virtually all HDMI devices employ a retimer/redriver chip in front of the output So what? The
by myself248 4y ago
> But the chances of reverse-hijacking are really REALLY slim - virtually all HDMI devices employ a retimer/redriver chip in front of the output
So what? The retimer chip doesn't touch the i2c lines, or even if it provides some ESD protection and buffering, it doesn't alter their data content. Software running in the PC is still talking to software running in the monitor's scaler chip.
You can play with i2c-tools under linux to verify this. I've been using HDMI's i2c port to talk to the i2c EEPROMs on SFP transceivers: https://i.imgur.com/T7cY01m.png https://i.imgur.com/T7cY01m.png
Nothing's getting in my way, it's just a generic i2c interface. There are other folks abusing it even worse: https://hackaday.com/2022/04/01/making-your-own-technically-hdmi-oled-monitor/ https://hackaday.com/2022/04/01/making-your-own-technically-...
- mschuster91 4y agoThe EDID i2c eeprom is 256 bytes in size. Even if a malicious laptop overwrites the content, there is utterly no way that a piece of malware can embed itself in there that can cause an adverse reaction on a computer that attaches later on.
- myself248 4y agoDDC hasn't been plain EDID EEPROMs in years. Nearly everything is DDC-CI now, where the scaler chip _pretends_ to be an EEPROM in the initial negotiations but there's a few magic "memory locations" that change each time you "read" or "write" to them, and a full bidirectional protocol layered on top of that. This allows arbitrary sized message transport, reflashing of the monitor firmware, and all sorts of goodies. And baddies.