Re: [Qemu-devel] [PATCH v5 1/4] Provide support for the CUSE TPM

[Stefan Berger]

On 06/16/2016 04:05 AM, Dr. David Alan Gilbert wrote:
> * Stefan Berger (address@hidden) wrote:
> > On 06/15/2016 03:30 PM, Dr. David Alan Gilbert wrote:
> <snip>
>
> > > So what was the multi-instance vTPM proxy driver patch set about?
> > That's for containers.
> Why have the two mechanisms? Can you explain how the multi-instance
> proxy works; my brief reading when I saw your patch series seemed
> to suggest it could be used instead of CUSE for the non-container case.

The multi-instance vtpm proxy driver basically works through usage of an 
ioctl() on /dev/vtpmx that is used to spawn a new front- and backend 
pair. The front-end is a new /dev/tpm%d device that then can be moved 
into the container (mknod + device cgroup setup). The backend is an 
anonymous file descriptor that is to be passed to a TPM emulator for 
reading TPM requests coming in from that /dev/tpm%d and returning 
responses to. Since it is implemented as a kernel driver, we can hook it 
into the Linux Integrity Measurement Architecture (IMA) and have it be 
used by IMA in place of a hardware TPM driver. There's ongoing work in 
the area of namespacing support for IMA to have an independent IMA 
instance per container so that this can be used.

A TPM does not only have a data channel (/dev/tpm%d) but also a control 
channel, which is primarily implemented in its hardware interface and is 
typically not fully accessible to user space. The vtpm proxy driver 
_only_ supports the data channel through which it basically relays TPM 
commands and responses from user space to the TPM emulator. The control 
channel is provided by the software emulator through an additional TCP 
or UnixIO socket or in case of CUSE through ioctls. The control channel 
allows to reset the TPM when the container/VM is being reset or set the 
locality of a command or retrieve the state of the vTPM (for suspend) 
and set the state of the vTPM (for resume) among several other things. 
The commands for the control channel are defined here:

https://github.com/stefanberger/swtpm/blob/master/include/swtpm/tpm_ioctl.h

For a container we would require that its management stack initializes 
and resets the vTPM when the container is rebooted. (These are typically 
operations that are done through pulses on the motherboard.)

In case of QEMU we would need to have more access to the control 
channel, which includes initialization and reset of the vTPM, getting 
and setting its state for suspend/resume/migration, setting the locality 
of commands, etc., so that all low-level functionality is accessible to 
the emulator (QEMU). The proxy driver does not help with this but we 
should use the swtpm implementation that either has that CUSE interface 
with control channel (through ioctls) or provides UnixIO and TCP sockets 
for the control channel.

    Stefan

> Dave
> P.S. I've removed Jeff from the cc because I got a bounce from
> his AT&T address saying 'restricted/not authorized'
>
> >      Stefan
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK