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NAME

       xen-vtpm - Xen virtual Trusted Platform Module (vTPM) subsystem

RUBRIC

       Copyright (c) 2010-2012 United States Government, as represented by the Secretary of
       Defense.  All rights reserved.  November 12 2012 Authors: Matthew Fioravante (JHUAPL),
       Daniel De Graaf (NSA)

       This document describes the virtual Trusted Platform Module (vTPM) subsystem for Xen. The
       reader is assumed to have familiarity with building and installing Xen, Linux, and a basic
       understanding of the TPM and vTPM concepts.

INTRODUCTION

       The goal of this work is to provide a TPM functionality to a virtual guest operating
       system (a DomU).  This allows programs to interact with a TPM in a virtual system the same
       way they interact with a TPM on the physical system.  Each guest gets its own unique,
       emulated, software TPM.  However, each of the vTPM's secrets (Keys, NVRAM, etc) are
       managed by a vTPM Manager domain, which seals the secrets to the Physical TPM.  If the
       process of creating each of these domains (manager, vTPM, and guest) is trusted, the vTPM
       subsystem extends the chain of trust rooted in the hardware TPM to virtual machines in
       Xen. Each major component of vTPM is implemented as a separate domain, providing secure
       separation guaranteed by the hypervisor. The vTPM domains are implemented in mini-os to
       reduce memory and processor overhead.

       This mini-os vTPM subsystem was built on top of the previous vTPM work done by IBM and
       Intel corporation.

DESIGN OVERVIEW

       The architecture of vTPM is described below:

           +------------------+
           |    Linux DomU    | ...
           |       |  ^       |
           |       v  |       |
           |   xen-tpmfront   |
           +------------------+
                   |  ^
                   v  |
           +------------------+
           | mini-os/tpmback  |
           |       |  ^       |
           |       v  |       |
           |  vtpm-stubdom    | ...
           |       |  ^       |
           |       v  |       |
           | mini-os/tpmfront |
           +------------------+
                   |  ^
                   v  |
           +------------------+
           | mini-os/tpmback  |
           |       |  ^       |
           |       v  |       |
           | vtpmmgr-stubdom  |
           |       |  ^       |
           |       v  |       |
           | mini-os/tpm_tis  |
           +------------------+
                   |  ^
                   v  |
           +------------------+
           |   Hardware TPM   |
           +------------------+

       Linux DomU
           The Linux based guest that wants to use a vTPM. There many be more than one of these.

       xen-tpmfront.ko
           Linux kernel virtual TPM frontend driver. This driver provides vTPM access to a para-
           virtualized Linux based DomU.

       mini-os/tpmback
           Mini-os TPM backend driver. The Linux frontend driver connects to this backend driver
           to facilitate communications between the Linux DomU and its vTPM. This driver is also
           used by vtpmmgr-stubdom to communicate with vtpm-stubdom.

       vtpm-stubdom
           A mini-os stub domain that implements a vTPM. There is a one to one mapping between
           running vtpm-stubdom instances and logical vtpms on the system. The vTPM Platform
           Configuration Registers (PCRs) are all initialized to zero.

       mini-os/tpmfront
           Mini-os TPM frontend driver. The vTPM mini-os domain vtpm-stubdom uses this driver to
           communicate with vtpmmgr-stubdom. This driver could also be used separately to
           implement a mini-os domain that wishes to use a vTPM of its own.

       vtpmmgr-stubdom
           A mini-os domain that implements the vTPM manager.  There is only one vTPM manager and
           it should be running during the entire lifetime of the machine.  This domain regulates
           access to the physical TPM on the system and secures the persistent state of each
           vTPM.

       mini-os/tpm_tis
           Mini-os TPM version 1.2 TPM Interface Specification (TIS) driver. This driver used by
           vtpmmgr-stubdom to talk directly to the hardware TPM. Communication is facilitated by
           mapping hardware memory pages into vtpmmgr-stubdom.

       Hardware TPM
           The physical TPM that is soldered onto the motherboard.

INSTALLATION

   Prerequisites:
       You must have an x86 machine with a TPM on the motherboard.  The only extra software
       requirement for compiling vTPM is cmake.  You must use libxl to manage domains with vTPMs;
       'xm' is deprecated and does not support vTPMs.

   Compiling the Xen tree:
       Compile and install the Xen tree as usual; be sure that the vTPM domains are enabled when
       you run configure.

   Compiling the LINUX dom0 kernel:
       Because the TPM manager uses direct access to the physical TPM, it may interfere with
       access to the TPM by dom0.  The simplest solution for this is to prevent dom0 from
       accessing the physical TPM by compiling the kernel without a driver or blacklisting the
       module.  If dom0 needs a TPM but does not need to use it during the boot process (i.e. it
       is not using IMA), a virtual TPM can be attached to dom0 after the system is booted.

       Access to the physical TPM may be required in order to manage the NVRAM or to perform
       other advanced operations where the vTPM is insufficient.  In order to prevent
       interference, the TPM Manager and dom0 should use different values for the TPM's locality;
       since Linux always uses locality 0, using locality 2 for the TPM Manager is recommended.
       If both Linux and the TPM Manager attempt to access the TPM at the same time, the TPM
       device will return a busy status; some applications will consider this a fatal error
       instead of retrying the command at a later time.  If a vTPM gets an error when loading its
       key, it will currently generate a fresh vTPM image (with a new EK, SRK, and blank NVRAM).

   Compiling the LINUX domU kernel:
       The domU kernel used by domains with vtpms must include the xen-tpmfront.ko driver. It can
       be built directly into the kernel or as a module; however, some features such as IMA
       require the TPM to be built in to the kernel.

           CONFIG_TCG_TPM=y
           CONFIG_TCG_XEN=y

VTPM MANAGER SETUP

   Manager disk image setup:
       The vTPM Manager requires a disk image to store its encrypted data. The image does not
       require a filesystem and can live anywhere on the host disk. The image is not large; the
       Xen 4.5 vtpmmgr is limited to using the first 2MB of the image but can support more than
       20,000 vTPMs.

   Manager config file:
       The vTPM Manager domain (vtpmmgr-stubdom) must be started like any other Xen virtual
       machine and requires a config file.  The manager requires a disk image for storage and
       permission to access the hardware memory pages for the TPM. The disk must be presented as
       "hda", and the TPM memory pages are passed using the iomem configuration parameter. The
       TPM TIS uses 5 pages of IO memory (one per locality) that start at physical address
       0xfed40000. By default, the TPM manager uses locality 0 (so only the page at 0xfed40 is
       needed); this can be changed on the domain's command line.  For full functionality in deep
       quotes, using locality 2 is required to manipulate PCR 20-22.

   Starting and stopping the manager:
       The vTPM manager should be started at boot; you may wish to create an init script to do
       this.  If a domain builder is used, the TPM Manager should be started by the domain
       builder to minimize the trusted computing base for the vTPM manager's secrets.

       Once initialization is complete you should see the following:

           INFO[VTPM]: Waiting for commands from vTPM's:

       The TPM Manager does not respond to shutdown requests; use the destroy command to shut it
       down.

VTPM AND LINUX PVM SETUP

   vTPM disk image setup:
       The vTPM requires a disk image to store its persistent data (RSA keys, NVRAM, etc). The
       image does not require a filesystem. The image does not need to be large; 2 Mb should be
       sufficient.

   vTPM config file:
       The vTPM domain requires a configuration file like any other domain. The vTPM requires a
       disk image for storage and a TPM frontend driver to communicate with the manager.  You are
       required to generate a uuid for this vtpm, which is specified on the "vtpm=" line that
       describes its connection to the vTPM Manager.  The uuidgen application may be used to
       generate a uuid, or one from the output of the "manage-vtpmmgr.pl vtpm-add" command may be
       used to create a vTPM belonging to a specific group.

       If you wish to clear the vTPM data you can either recreate the disk image or change the
       uuid.

   Linux Guest config file:
       The Linux guest config file needs to be modified to include the Linux tpmfront driver. Add
       the following line:

           vtpm=["backend=domu-vtpm"]

       Currently only Linux guests are supported (PV or HVM with PV drivers).

       While attaching a vTPM after a guest is booted (using xl vtpm-attach) is supported, the
       attached vTPM will not have a record of the boot of the attached guest.  Furthermore, if
       the vTPM has been freshly created, a malicious guest could then extend any values into
       PCRs, potentially forging its boot configuration.  Attaching a vTPM to a running domain
       should only be used for trusted domains or when measurements have already been sent to the
       vTPM from another source.

   Using the vTPM in the guest:
       If xen-tpmfront was compiled as a module, it must be loaded it in the guest.

           # modprobe xen-tpmfront

       After the Linux domain boots and the xen-tpmfront driver is loaded, you should see the
       following on the vtpm console:

           Info: VTPM attached to Frontend X/Y

       You can quickly test the vTPM by using the sysfs interface:

           # cat /sys/devices/vtpm-0/pubek
           # cat /sys/devices/vtpm-0/pcrs

       If you have trousers and tpm_tools installed on the guest, the tpm_version command should
       return the following:

       The version command should return the following:

           TPM 1.2 Version Info:
           Chip Version:        1.2.0.7
           Spec Level:          2
           Errata Revision:     1
           TPM Vendor ID:       ETHZ
           TPM Version:         01010000
           Manufacturer Info:   4554485a

       You should also see the command being sent to the vtpm console as well as the vtpm saving
       its state. You should see the vtpm key being encrypted and stored on the vtpmmgr console.

       You may wish to write a script to start your vtpm and guest together and to destroy the
       vtpm when the guest shuts down.

INTEGRATION WITH PV-GRUB

       The vTPM currently starts up with all PCRs set to their default values (all zeros for the
       lower 16).  This means that any decisions about the trustworthiness of the created domain
       must be made based on the environment that created the vTPM and the domU; for example, a
       system that only constructs images using a trusted configuration and guest kernel be able
       to provide guarantees about the guests and any measurements done that kernel (such as the
       IMA TCB log).  Guests wishing to use a custom kernel in such a secure environment are
       often started using the pv-grub bootloader as the kernel, which then can load the
       untrusted kernel without needing to parse an untrusted filesystem and kernel in dom0.  If
       the pv-grub stub domain succeeds in connecting to a vTPM, it will extend the hash of the
       kernel that it boots into PCR #4, and will extend the command line and initrd into PCR #5
       before booting so that a domU booted in this way can attest to its early boot state.

MORE INFORMATION

       See <xen-vtpmmgr(7)> for more details about how the manager domain works, how to use it,
       and its command line parameters.

VTPM DOMAIN OPERATION

       The vtpm-stubdom is a mini-OS domain that emulates a TPM for the guest OS to use. It is a
       small wrapper around the Berlios TPM emulator version 0.7.4.  Commands are passed from the
       linux guest via the mini-os TPM backend driver.  vTPM data is encrypted and stored via a
       disk image provided to the virtual machine. The key used to encrypt the data along with a
       hash of the vTPM's data is sent to the vTPM manager for secure storage and later
       retrieval.  The vTPM domain communicates with the manager using a mini-os tpm front/back
       device pair.

VTPM DOMAIN COMMAND LINE ARGUMENTS

       Command line arguments are passed to the domain via the 'extra' parameter in the VM config
       file. Each parameter is separated by white space. For example:

           extra="foo=bar baz"

   List of Arguments:
       loglevel=<LOG>
           Controls the amount of logging printed to the console.  The possible values for <LOG>
           are:

           •   error

           •   info (default)

           •   debug

       clear
           Start the Berlios emulator in "clear" mode. (default)

       save
           Start the Berlios emulator in "save" mode.

       deactivated
           Start the Berlios emulator in "deactivated" mode.  See the Berlios TPM emulator
           documentation for details about the startup mode. For all normal use, always use clear
           which is the default. You should not need to specify any of these.

       maintcmds=<1|0>
           Enable to disable the TPM maintenance commands.  These commands are used by tpm
           manufacturers and thus open a security hole. They are disabled by default.

       hwinitpcr=<PCRSPEC>
           Initialize the virtual Platform Configuration Registers (PCRs) with PCR values from
           the hardware TPM. Each pcr specified by <PCRSPEC> will be initialized with the value
           of that same PCR in TPM once at startup. By default all PCRs are zero initialized.
           Possible values of <PCRSPEC> are:

           •   all: copy all pcrs

           •   none: copy no pcrs (default)

           •   <N>: copy pcr n

           •   <X-Y>: copy pcrs x to y (inclusive)

           These can also be combined by comma separation, for example: "hwinitpcrs=5,12-16" will
           copy pcrs 5, 12, 13, 14, 15, and 16.

REFERENCES

       Berlios TPM Emulator: <https://github.com/PeterHuewe/tpm-emulator>