doc: add EDID copying instructions
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README.md
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README.md
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@ -14,8 +14,8 @@ This is the perfect opportunity for the guest to hack the video-projector.
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Next time an employee connects to this projector, his laptop is hacked back.
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And voila, the innocent guest managed to infiltrate your company network, and can exfiltrate confidential information.
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The HDMI firewall blocks all the additional features, and only allow the equipment to receive audio and video data.
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It is based on the research of Pierre-Michel RICORDEL and José LOPES-ESTEVES from ANSSI/SDE/ST/LSF presented at the IT security conference [SSTIC 2021](https://sstic.org/2021/presentation/un_pare_feu_pour_le_hdmi/).
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The HDMI firewall blocks all additional interfaces, and only allows audio and video data transfer.
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It is based on the research of Pierre-Michel Ricordel and José Lopes Esteves from ANSSI/SDE/ST/LSF presented at the IT security conference [SSTIC 2021](https://sstic.org/2021/presentation/un_pare_feu_pour_le_hdmi/).
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usage
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=====
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@ -24,8 +24,9 @@ You first have to copy the Extended Display Identification Data (EDID) informati
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This data includes information such as the supported resolutions.
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You can read it out using the I²C-based Display Data Channel (DDC) interface.
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Write the EDID data on the EEPROM of the HDMI firewall, and break the tab using pliers to enable write protection.
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This will prevent attackers from injecting any malicious payload.
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It will prevent attackers from injecting any malicious payload.
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This only has to be done once (per monitor to protect).
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See installation for detailed instruction on copied the EDID.
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Now plug in the HDMI firewall in the monitor to be protected.
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Connect the cable going to the untrusted device on the HDMI firewall.
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@ -37,6 +38,182 @@ By default, the 5V supplied by the monitor are forwarded to the device.
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To further reduce the attack surface, you can disable this by cutting the trace between the two pads marked 5V.
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The risk is that some monitors rely on this signal to detect when a device is plugged in.
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installation
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============
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For the HDMI firewall to be used, it needs a local copy of the EDID data from the monitor to protect.
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These instructions are for Linux.
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For Windows see the instructions provided in the [original research slides](https://www.sstic.org/media/SSTIC2021/SSTIC-actes/un_pare_feu_pour_le_hdmi/SSTIC2021-Slides-un_pare_feu_pour_le_hdmi-lopes-esteves_ricordel.pdf) (untested).
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Install tools to read/write I²C devices:
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- for Debian-based distributions
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~~~
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sudo apt install i2c-tools
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~~~
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- for Arch-based distributions
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~~~
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sudo pacman -S i2c-tools
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~~~
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To make the I²C buses user accessible (under /dev/i2c-*):
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~~~
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sudo modprobe i2c-dev
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~~~
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To figure out which I²C device corresponds to your HDMI port, simply test them all.
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There might be a better way, but I don't know it.
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Disconnect everything from the HDMI port, and scan for devices on the I²C bus (0 corresponds to the bus number, at the end of /dev/i2c-0):
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~~~
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sudo i2cdetect -y 0
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~~~
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Since nothing is connected, no device should be detected:
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~~~
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0 1 2 3 4 5 6 7 8 9 a b c d e f
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00: -- -- -- -- -- -- -- --
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10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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70: -- -- -- -- -- -- -- --
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~~~
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Now connect the HDMI firewall on the device side to your HDMI port and re-scan for devices.
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If you see the following result, you found the I²C bus of the HDMI port (in this example it is bus 4).
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Else continue with the next bus.
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~~~
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0 1 2 3 4 5 6 7 8 9 a b c d e f
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00: -- -- -- -- -- -- -- --
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10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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50: 50 51 52 53 54 55 56 57 -- -- -- -- -- -- -- --
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60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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70: -- -- -- -- -- -- -- --
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~~~
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Now connect the monitor you want to copy the EDID from directly to the HDMI port.
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We will use the [EDID-RW](https://github.com/bulletmark/edid-rw) tool to make the copy.
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Install these prerequisites:
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- for Debian-based distributions
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~~~
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sudo apt-get install python3-smbus edid-decode
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~~~
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- for Arch-based distributions
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~~~
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pikaur -S python-smbus-git edid-decode-git
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~~~
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Get EDID-RW:
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~~~
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git clone https://github.com/bulletmark/edid-rw
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cd edid-rw/
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~~~
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To retrieve the EDID data from the monitor you connected:
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~~~
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sudo ./edid-rw 4 > edid.bin
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~~~
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To view the decoded EDID data (feel free to adjust it):
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~~~
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edid-decode edid.bin
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~~~
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Now connect the HDMI firewall device port to the your HDMI output.
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Ensure Write Protect is disabled (by default until the tab is broken, and no solder across the WP pads is added).
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Write the extracted EDID data to the HDMI firewall:
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~~~
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sudo ./edid-rw -w 4 < edid.bin
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~~~
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To verify of the data has been written correctly, read it back:
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~~~
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sudo ./edid-rw 4 | edid-decode
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~~~
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If the EDID information could not be decoded, the data write was unsuccessful.
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See troubleshooting for possible solutions.
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Once writing the EDID to the HDMI firewall memory succeeded, break the tab using pliers to write protect the memory.
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You can now use the HDMI firewall (only for this monitor).
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Fell free to put shrink tube around the HDMI firewall (and shrink it using hot air at 200°C, or carefully using a lighter).
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This will prevent the electronics from getting shorted when entering in contact with neighbouring metal objects.
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troubleshooting
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===============
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If the monitor does not detect the device or does not display anything (but should), try to re-enable the 5V forward (as per default) by putting solder across the 5V pads.
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In the HDMI cable there is a 5V line, with power provided by the monitor.
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In our case we use it is to power the HDMI firewall memory.
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Forwarding it to the device can be disabled by cutting the trace across the 5V pads.
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If the device does not detect the monitor (actually the HDMI firewall), or writing the EDID data to the HDMI firewall fails (better indication), try connecting the HDMI firewall with another (better quality) HDMI cable.
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Some cheap cables have a very thin 5V cable, enough to detect connected devices, but not to power the EEPROM for write operations.
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tips
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====
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xrandr
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------
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xrandr can also dump the EDID information:
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~~~
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xrandr --properties
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~~~
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sysfs also exposes the raw EDID information:
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~~~
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edid-decode /sys/devices/pci0000:00/0000:00:08.1/0000:05:00.0/drm/card0/card0-HDMI-A-1/edid
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~~~
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to access I²C devices as non-root:
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~~~
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# add UDEV rule
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cat << EOF | sudo tee /etc/udev/rules.d/20-i2c.rules
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KERNEL=="i2c-[0-9]*", GROUP="i2c"
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EOF
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# give user access to devices
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sudo groupadd i2c
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sudo gpasswd -a $USER i2c
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# reload rules
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sudo udevadm control --reload-rules
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sudo udevadm trigger
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~~~
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erase
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-----
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If you want to clear the HDMI firewall memory (for usage with another monitor):
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~~~
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for page in `seq 50 57`; do echo 0x$page; for addr in `seq 0 255`; do echo $addr; sudo i2cset -y 4 0x$page $addr 0xff; done; done
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~~~
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limitations
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===========
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