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

From: Stefan Berger
Subject: Re: [Qemu-devel] [PATCH v5 1/4] Provide support for the CUSE TPM
Date: Thu, 16 Jun 2016 11:04:05 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

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:

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:


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.


P.S. I've removed Jeff from the cc because I got a bounce from
his AT&T address saying 'restricted/not authorized'


Dr. David Alan Gilbert / address@hidden / Manchester, UK

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