Ludovic Rousseau's blog: Share a smart card reader between a host and its guest VM(s)

Wednesday, November 16, 2022

Share a smart card reader between a host and its guest VM(s)

As I wrote in "One smart card reader accessible from many computers" it is possible to share a smart card reader between 2 or more systems.

Problem

I recently received a bug report about a problem between pcsc-lite and VirtualBox. When the smart card reader is connected to the VM guest then the kernel on the host reports errors like:

2022-11-11T14:25:01.186983-08:00 track pcscd[2474]: 00000001 eventhandler.c:336:EHStatusHandlerThread() Error communicating to: SCM Microsystems Inc. SCR 3310 [CCID Interface] (53311514247933) 00 00

2022-11-11T14:25:01.186993-08:00 track pcscd[2474]: 00000005 ccid_usb.c:1356:InterruptRead() libusb_submit_transfer failed: LIBUSB_ERROR_IO

2022-11-11T14:25:01.188050-08:00 track kernel: [ 1247.705353][ T2521] usb 1-2: usbfs: process 2521 (pcscd) did not claim interface 0 before use

2022-11-11T14:25:01.188053-08:00 track kernel: [ 1247.705386][ T2521] usb 1-2: usbfs: process 2521 (pcscd) did not claim interface 0 before use

2022-11-11T14:25:01.587034-08:00 track pcscd[2474]: 00400173 ccid_usb.c:865:WriteUSB() write failed (1/2): -1 LIBUSB_ERROR_IO

2022-11-11T14:25:01.587076-08:00 track pcscd[2474]: 00000008 ifdwrapper.c:364:IFDStatusICC() Card not transacted: 612

And after some times (in days) the host kernel crashes.

A Linux kernel crash is never a good thing. pcsc-lite may be very powerful but it can't crash the Linux kernel. Only a bug in the kernel itself can generate a crash. Here I suspect the VirtualBox Linux kernel module to do something bad.

Solution

Instead of connecting the USB smart card reader in the guest VM (and disconnecting it from the host) it is possible to share the smart card reader(s) between the host and guest with some help from pcsc-lite.

Setup

My demo setup:

Host: Debian testing system
Guest: NetBSD 9.3 running inside KVM (Kernel Virtual Machine).

I use 2 very different operating systems, GNU/Linux and NetBSD, on purpose. It is to show it is possible to mix systems.

Host

In the host, no change to the configuration. But we will redirect (inject) /run/pcscd/pcscd.comm in the virtual machine.

On the Debian host I run:

$ ssh -N -R/tmp/pcscd.comm:/run/pcscd/pcscd.comm VMNetBSD

Guest

On the NetBSD VM I use:

$ export PCSCLITE_CSOCK_NAME=/tmp/pcscd.comm

Then I can run any application using pcsc-lite and get access to the smart card(s) and reader(s) from the host. For example:

$ pcsc_scan -c

Wed Nov 16 17:26:55 2022
 Reader 0: Alcor Micro AU9540 00 00
  Event number: 0
  Card state: Card inserted, 
  ATR: 3B A7 00 40 18 80 65 A2 08 01 01 52

With a screenshot:

Limitations

pcsc-lite to pcsc-lite

As I demonstrated the solution is not limited to GNU/Linux. Any Unix system using pcsc-lite can be used. But you must use the same pcsc-lite protocol on both sides.

For example the current protocol version used by pcsc-lite 1.9.9 (current version) is 4.4. It is the same protocol version since pcsc-lite 1.8.24 released in Oct 2018.

macOS or Windows host

It should be technically possible to use Windows or macOS as the host OS. That would involve a new development. Contact me if you need something like that.