DC540 Defcon Group

So I managed to find a satisfactory and effective resolution to my problem with serial number 0123456789ABCDEF. If you recall, the problem is that certain consumer-grade USB devices ship with non-unique serial numbers, making it difficult to use them with USB pass-through under VirtualBox.

• Serial number on multiple devices = 0123456789ABCDEF
• VirtualBox USB filtering (permanently mapping a device to a specific VM) relies a unique field. With diverse devices, this can be the vendor/product ID. With normal devices with unique serial numbers, the serial number can be used in the filters.
• With identical product, vendor and serial numbers, you’re stuck with “Port,” which at least in CentOS/Rocky’s USB subsystem, is dynamically assigned when the device is plugged in to the host. Example: Plug in a device, it shows up on Bus 001, Device 005. Unplug it, plug it back in, and it’s now on Bus 001, Device 006.
• Allegedly, udev rules can be set to assign a specific port (or a port “nickname”) using some other unique identifier, like the actual physical port the device is connected to. Still less than ideal, since that would require adherence to a strict protocol of unplugging and plugging into the same port. Anyway, I couldn’t get that to work.

So I dug around a bit, and found that the devices DO have a unique component — the device’s MAC address. It’s visible when using lsusb in verbose mode. But VirtualBox can’t see it, and it’s not an option in the VirtualBox filtering.

So what I ended up doing was abandoning VirtualBox’s USB filtering entirely.

I created a text file mapping those MAC addresses to VMs. Then I created a script which iterates through LSUSB, and checks the MAC addresses of every USB device it finds through that mapping file. When it finds one, it needs two things to make the usb attachment happen via command line. It needs either the UUID or machine name of the VM, and it needs the UUID or “address” of the USB device. My mapping file stores the MAC address of the device (discovered via lsusb) and the UUID of the VM, which is available via vboxmanage list vms.

The UUID of the USB device can be found by comparing the Bus and Port values found in lsusb to the output of vboxmanage list usbhost. With creative use of bash and awk, I was able to iterate through the devices, determine which ones are assigned to VMs, check to see if they’re currently attached, and if not, issue the vboxmanage controlvm vm-uuid usbattach usb-uuid command to attach the device to its home VM. So in case of a host reboot or someone rejiggering ports, a single command is enough to get things back where they belong.

I realize that there are probably only three people on the planet who have run into this particular problem, but I thought the solution was an interesting exercise in information-gathering on a VirtualBox host on a Rocky Linux platform, so I felt i was worth sharing.

I generally prefer to blog when I’ve solved an obscure problem, or even a not-so-obscure problem, because I’m proud of myself for doing it, and because for everyone out there who already knows the answer, there are 20 people out there who don’t, and I like to be helpful.

Not this time. I’m blogging about it this time in the hopes that explaining it in sufficient detail might trigger an a-ha moment for me, or maybe that someone out there in the ether will find it and know, and feel that same urgent need to be helpful that I frequently feel.

So I’ll be as concise as possible here:

CentOS-based server running VirtualBox.

Multiple USB devices connected to the server, intended to be passed through to individual VMs.

These particular devices caused us to settle on VirtualBox, because it was on the only virtualization platform we found that could pass them through reliably in a usable way.

So we found one that we like as far as function set and usability. We’re happily plugging along and making our VM work happily with it. OK, let’s make another one. Spin up another VM, plug in another (identical) device.

Oh noes. The vendor, product, manufacturer name, and everything are all identical! What do we do? Those of you who’ve done this before, I know what you’re about to say. “Use the serial number in the VBox USB Filter, dumbass!” To that I say, “did you read the title of this post?”

Yeah. This particular vendor rolled out this product commercially with generic, identical serial numbers. Whatever fucktard made that decision, I curse their existence.

After a bit of futzing, we found that we could filter on “Port”, which is the port number assigned to the device, visible in lsusb and in the device in. /dev/bus/usb/001/###.

So yeah, we got on with our day and forgot about it. Oh yeah — then we got another one.

“No problem,” I said, “we know how to handle that.” So we’re working through setup, and I have the filter in place, and we pull it out and put it back in for whatever reason, and the filter’s not kicking in. I look again, and the port number has incremented!

Yeah. USB port numbers in linux, at least in this distro, are dynamic. So you can’t expect them to continue to work after a server reboot, or maybe even after a device reset. Bad news.

So a colleague sends me an article on using udev rules to find the uniqueness buried in udevadm attributes to generate a named symlink to the device. That sounds awesome.

Well, less than awesome. The only uniqueness I was able to find in the attributes was “bus” and “devpath”, which are the physical USB bus and port the device is connected to. The lovely non-unique serial number is in there glaring at me triumphantly with its useless stupid face.

So I futzed around with udev rules, trying to get that symlink to come up, and haven’t figured out quite the magic words yet. Maybe because the article is based on a ttyACM device that’s directly in /dev, and my particular situation involves a device that shows up way under /dev/bus/usb/001/### instead. In any case, I see no evidence of a symlink being created anywhere in the /dev tree, and I have no way of knowing whether it would be usable in VBox USB filtering even if it did show up. So I’m calling it a night, and I’ll put fresh eyes on it tomorrow maybe.

That’s my day, how’s youres?

I’m thinking we should meet in the backyard/basement this evening. Might be the last time in the season to enjoy a little bit of outdoor weather. I’ve refreshed the beer supply and improved the side path lighting. What say you? A little badd pizza & beer on the patio while discussing badgelife, job searches, and other relevant topics? Who’s in?