- Daniel De Graaf <firstname.lastname@example.org>
- Quan Xu <email@example.com>
This document describes the operation and command line interface of
vtpmmgr-stubdom. See xen-vtpm
(7) for details on the vTPM subsystem as a
The TPM Manager has three primary functions:
- 1. Securely store the encryption keys for vTPMs
- 2. Provide a single controlled path of access to the physical TPM
- 3. Provide evidence (via TPM Quotes) of the current configuration
When combined with a platform that provides a trusted method for creating
domains, the TPM Manager provides assurance that the private keys in a vTPM
are only available in specific trusted configurations.
The manager accepts commands from the vtpm-stubdom domains via the mini-os TPM
backend driver. The vTPM manager communicates directly with hardware TPM using
the mini-os tpm_tis driver.
The TPM Manager's data is secured by using the physical TPM's seal operation,
which allows data to be bound to specific PCRs. These PCRs are populated in
the physical TPM during the boot process, either by the firmware/BIOS or by a
dynamic launch environment such as TBOOT. In order to provide assurance of the
system's security, the PCRs used to seal the TPM manager's data must contain
measurements for domains used to bootstrap the TPM Manager and vTPMs.
Because these measurements are based on hashes, they will change any time that
any component of the system is upgraded. Since it is not possible to construct
a list of all possible future good measurements, the job of approving
configurations is delegated to a third party, referred to here as the system
approval agent (SAA). The SAA is identified by its public (RSA) signature key,
which is used to sign lists of valid configurations. A single TPM manager can
support multiple SAAs via the use of vTPM groups. Each group is associated
with a single SAA; this allows the creation of a multi-tenant environment
where tenants may not all choose to trust the same SAA.
Each vTPM is bound to a vTPM group at the time of its creation. Each vTPM group
has its own AIK in the physical TPM for quotes of the hardware TPM state; when
used with a conforming Privacy CA, this allows each group on the system to
form the basis of a distinct identity.
When the TPM Manager first boots up, it will create a stub vTPM group along with
entries for any vTPMs that communicate with it. This stub group must be
provisioned with an SAA and a boot configuration in order to survive a reboot.
When a vTPM is connected to the TPM Manager using a UUID that is not recognized,
a slot will be created in group 0 for it. In the future, this auto-creation
may be restricted to specific UUIDs (such as the all-zero UUID) to enforce the
use of the TPM manager as the generator of the UUID. The first vTPM to be
connected is given administrative privileges for the TPM Manager, and should
be attached to dom0 or a control domain in order to send provisioning
Provisioning a vTPM group for the system requires the public key of the SAA and
privacy CA data used to certify the AIK (see the TPM spec for details). Once
the group is created, a signed list of boot measurements can be installed. The
initial group controls the ability to boot the system as a whole, and cannot
be deleted once provisioned.
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:
- Set the owner and SRK authdata for the TPM. If not specified, the default
is 160 zero bits (the well-known auth value). Valid values of
- Use the well known auth (default)
- Use the given 40-character ASCII hex string
- Use sha1 hash of <STR>.
- Choose the driver used for communication with the hardware TPM. Values
other than tpm_tis should only be used for testing.
The possible values of <DRIVER> are:
- Direct communication with a hardware TPM 1.2. The domain must have access
to TPM IO memory. (default)
- Use the Xen tpmfront interface to talk to another domain which provides
access to the TPM.
The following options only apply to the tpm_tis driver:
- The base address of the hardware memory pages of the TPM. The default is
0xfed40000, as defined by the TCG's PC Client spec.
- The irq of the hardware TPM if using interrupts. A value of
"probe" can be set to probe for the irq. A value of 0 disables
interrupts and uses polling (default 0).
- Attempt to use locality <LOC> of the hardware TPM. For full
functionality of the TPM Manager, this should be set to
While the TPM Manager has the ability to check the hash of the vTPM requesting a
key, there is currently no trusted method to inform the TPM Manager of the
hash of each new domain. Because of this, the TPM Manager trusts the UUID key
in Xenstore to identify a vTPM in a trusted manner. The XSM policy may be used
to strengthen this assumption if the creation of vTPM-labeled domains is more
constrained (for example, only permitted to a domain builder service): the
only grants mapped by the TPM Manager should belong to vTPM domains, so
restricting the ability to map other domain's granted pages will prevent other
domains from directly requesting keys from the TPM Manager. The TPM Manager
uses the hash of the XSM label of the attached vTPM as the kernel hash, so
vTPMs with distinct labels may be further partitioned using vTPM groups.
A domain with direct access to the hardware TPM will be able to decrypt the TPM
Manager's disk image if the haredware TPM's PCR values are in a permitted
configuration. To protect the TPM Manager's data, the list of permitted
configurations should be chosen to include PCRs that measure the hypervisor,
domain 0, the TPM Manager, and other critical configuration such as the XSM
policy. If the TPM Manager is configured to use locality 2 as recommended, it
is safe to permit the hardware domain to access locality 0 (the default in
Linux), although concurrent use of the TPM should be avoided as it can result
in unexpected busy errors from the TPM driver. The ability to access locality
2 of the TPM should be enforced using IO memory labeling in the XSM policy;
the physical address 0xFED42xxx is always locality 2 for TPMs using the TIS
There is no direct upgrade supported from previous versions of the vtpmmgr
domain due to changes in the on-disk format and the method used to seal data.
If a vTPM domain supports migration, this feature should be used to migrate
the vTPM's data; however, the vTPM packaged with Xen does not yet support
If adding migration support to the vTPM is not desired, a simpler migration
domain usable only for local migration can be constructed. The migration
process would look like the following:
- 1. Start the old vtpmmgr
- 2. Start the vTPM migration domain
- 3. Attach the vTPM migration domain's vtpm/0 device to the old
- 4. Migration domain executes vtpmmgr_LoadHashKey on vtpm/0
- 5. Start the new vtpmmgr, possibly shutting down the old one first
- 6. Attach the vTPM migration domain's vtpm/1 device to the new
- 7. Migration domain executes vtpmmgr_SaveHashKey on vtpm/1
This requires the migration domain to be added to the list of valid vTPM kernel
hashes. In the current version of the vtpmmgr domain, this is the hash of the
XSM label, not the kernel.
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.
dd if=/dev/zero of=/home/vtpm2/vmgr bs=16M count=1
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
extra option to launch vtpmmgr-stubdom domain on TPM 2.0, and ignore it on TPM
1.x. for example:
| vTPM's secrets | ...
| |(Bind / Unbind)
- - - - - -v |- - - - - - - - TPM 2.0
| SK +
| SRK |
| TPM 2.0 Storage |
| Primary Seed |
Now the secrets for the vTPMs are only being bound to the presence of
thephysical TPM 2.0. Since using PCRs to seal the data can be an important
security feature that users of the vtpmmgr rely on. I will replace
TPM2_Bind/TPM2_Unbind with TPM2_Seal/TPM2_Unseal to provide as much security
as it did for TPM 1.2 in later series of patch.
The architecture of vTPM subsystem on TPM 2.0 is described below:
| Linux DomU | ...
| | ^ |
| v | |
| xen-tpmfront |
| mini-os/tpmback |
| | ^ |
| v | |
| vtpm-stubdom | ...
| | ^ |
| v | |
| mini-os/tpmfront |
| mini-os/tpmback |
| | ^ |
| v | |
| vtpmmgr-stubdom |
| | ^ |
| v | |
| mini-os/tpm2_tis |
| Hardware TPM 2.0 |
- Linux DomU
- The Linux based guest that wants to use a vTPM. There many be more than
one of these.
- Linux kernel virtual TPM frontend driver. This driver provides vTPM access
to a para-virtualized Linux based DomU.
- 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
- 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 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
- 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 version 2.0 TPM Interface Specification (TIS) driver. This
driver used by vtpmmgr-stubdom to talk directly to the hardware TPM 2.0.
Communication is facilitated by mapping hardware memory pages into
- Hardware TPM 2.0
- The physical TPM 2.0 that is soldered onto the motherboard.
functionality for a virtual guest operating system (a DomU) is still TPM