USBIP over qrexec proxy
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USB proxy based on USBIP and qrexec

USB device passthrough using USBIP as a protocol, but qrexec as link layer. See for more details.

Technical details of USBIP

USBIP consists of two parts:

  • frontend module (vhci-hcd) - virtual USB controller
  • backend module (usbip-host) - USB device driver

Normally (with TCP as link layer), configuration is handled by usbipd and usbip tools. Those tools setup TCP connection, then pass socket FD to the kernel. For frontend it is done by writing port number, transport socket FD, busid and speed to /sys/devices/platform/vhci_hcd/attach. For backend - by attaching driver to appropriate device, then writing socket FD to /sys/bus/usb/devices/.../usbip_sockfd.

In case of qrexec, it can also provide a single (local) socket, which can be used for that purpose. One need to send SIGUSR1 to $QREXEC_AGENT_PID to switch to that mode (other wise separate sockets are used for data IN (stdin) and OUT (stdout)) - then stdin (FD 0) can be used for both directions.

Some more info is needed at frontend side (vhci-hcd):

  • port number - can be obtained by parsing /sys/devices/platform/vhci_hcd/status (status codes are defined in /usr/include/linux/usbip.h)
  • remote devid (bus and dev number) and device speed - can be obtained by the backend and transfered to the frontend script

Low level description

Internally three qrexec services are used:

  1. qubes.USBAttach - called by dom0 in frontend domain to initiate connection. Requires backend domain name and device busid on its stdin (separated by space). Service will terminate as soon as connection is established.
  2. qubes.USBDetach - similar to qubes.USBAttach but to terminate the connection. Parameters on stdin are the same.
  3. qubes.USB - actual USBIP connection, called by frontend domain to the backend domain, with desired busid as service argument.

qubes.USBAttach service calls qubes.USB in the backend, using usb-import script as local process. usb-import script is responsible for configuring vhci-hcd, which includes:

  • finding free port
  • retrieving (and validating) devid and speed from the backend
  • actually attaching the device using FD number 0 (stdin)

It also save state information (port number) for later use by qubes.USBDetach.

qubes.USB service calls usb-export script, which resolve/validate given device, bind it to the usbip-host driver and send devid and speed to the frontend (in a single line, space delimited). After that, it hands over stdin socket (FD 0) to the kernel for USBIP communication.

Supported argument formats:

  • VENDORID.PRODUCTID, where each of them is in 0xHHHH format (four hex digits)
  • BUSNUM-DEVNUM.PORT, device name as in /sys/bus/usb/devices (important: whole device, not a signle interface!)

Low level usage

First you need to setup qrexec policy to access qubes.USB+DEVID calls from frontend to backend. Then you need to call qubes.USBAttach service. Examples below.

Attach device 2-1 of domain sys-usb to domain work-usb:

echo sys-usb 2-1 | qvm-run -p -u root work-usb 'QUBESRPC qubes.USBAttach dom0'

Detach that device:

echo sys-usb 2-1 | qvm-run -p -u root work-usb 'QUBESRPC qubes.USBDetach dom0'

Alternativelly you can detach the device calling backend domain (USB VM):

echo 2-1 | qvm-run -p -u root sys-usb 'QUBESRPC qubes.USBDetach dom0'

Using python API it will be:

frontend_vm.run_service('qubes.USBAttach', input='sys-usb 2-1', user='root')
frontend_vm.run_service('qubes.USBDetach', input='sys-usb 2-1', user='root')

backend_vm.run_service('qubes.USBDetach', input='2-1', user='root')