Virtual TPM Proxy Driver for Linux Containers

Authors:

Stefan Berger <stefanb@linux.vnet.ibm.com>

This document describes the virtual Trusted Platform Module (vTPM) proxy device driver for Linux containers.

Introduction

The goal of this work is to provide TPM functionality to each Linux container. This allows programs to interact with a TPM in a container the same way they interact with a TPM on the physical system. Each container gets its own unique, emulated, software TPM.

Design

To make an emulated software TPM available to each container, the container management stack needs to create a device pair consisting of a client TPM character device /dev/tpmX (with X=0,1,2…) and a ‘server side’ file descriptor. The former is moved into the container by creating a character device with the appropriate major and minor numbers while the file descriptor is passed to the TPM emulator. Software inside the container can then send TPM commands using the character device and the emulator will receive the commands via the file descriptor and use it for sending back responses.

To support this, the virtual TPM proxy driver provides a device /dev/vtpmx that is used to create device pairs using an ioctl. The ioctl takes as an input flags for configuring the device. The flags for example indicate whether TPM 1.2 or TPM 2 functionality is supported by the TPM emulator. The result of the ioctl are the file descriptor for the ‘server side’ as well as the major and minor numbers of the character device that was created. Besides that the number of the TPM character device is returned. If for example /dev/tpm10 was created, the number (dev_num) 10 is returned.

Once the device has been created, the driver will immediately try to talk to the TPM. All commands from the driver can be read from the file descriptor returned by the ioctl. The commands should be responded to immediately.

UAPI

enum vtpm_proxy_flags

flags for the proxy TPM

Constants

VTPM_PROXY_FLAG_TPM2

the proxy TPM uses TPM 2.0 protocol

struct vtpm_proxy_new_dev

parameter structure for the VTPM_PROXY_IOC_NEW_DEV ioctl

Definition

struct vtpm_proxy_new_dev {
  __u32 flags;
  __u32 tpm_num;
  __u32 fd;
  __u32 major;
  __u32 minor;
};

Members

flags

flags for the proxy TPM

tpm_num

index of the TPM device

fd

the file descriptor used by the proxy TPM

major

the major number of the TPM device

minor

the minor number of the TPM device

long vtpmx_ioc_new_dev(struct file *file, unsigned int ioctl, unsigned long arg)

handler for the VTPM_PROXY_IOC_NEW_DEV ioctl

Parameters

struct file *file

/dev/vtpmx

unsigned int ioctl

the ioctl number

unsigned long arg

pointer to the struct vtpmx_proxy_new_dev

Description

Creates an anonymous file that is used by the process acting as a TPM to communicate with the client processes. The function will also add a new TPM device through which data is proxied to this TPM acting process. The caller will be provided with a file descriptor to communicate with the clients and major and minor numbers for the TPM device.