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Taikun OCP Guide

Table of Contents

Emulated Trusted Platform Module (vTPM)

22.0.0 (Victoria)

Starting in the 22.0.0 (Victoria) release, Nova supports adding an
emulated virtual Trusted
Platform Module
(vTPM) to guests.

Enabling vTPM

The following are required on each compute host wishing to support
the vTPM feature:

  • Currently vTPM is only supported when using the libvirt compute
    driver with a libvirt.virt_type of kvm
    or qemu.
  • A key
    manager service
    , such as barbican, must be
    configured to store secrets used to encrypt the virtual device files at
    rest.
  • The swtpm
    binary and associated libraries.
  • Set the libvirt.swtpm_enabled config option to
    True. This will enable support for both TPM version 1.2 and
    2.0.

With the above requirements satisfied, verify vTPM support by
inspecting the traits on the compute node’s resource provider:

$ COMPUTE_UUID=$(openstack resource provider list --name $HOST -f value -c uuid)
$ openstack resource provider trait list $COMPUTE_UUID | grep SECURITY_TPM
| COMPUTE_SECURITY_TPM_1_2 |
| COMPUTE_SECURITY_TPM_2_0 |

Configuring a flavor or
image

A vTPM can be requested on a server via flavor extra specs or image
metadata properties. There are two versions supported – 1.2 and 2.0 –
and two models -TPM Interface Specification (TIS) and Command-Response
Buffer (CRB). The CRB model is only supported with version 2.0.

Flavor extra_specsImage metadataDescription
hw:tpm_versionhw_tpm_versionSpecify the TPM version, 1.2 or 2.0.
Required if requesting a vTPM.
hw:tpm_modelhw_tpm_modelSpecify the TPM model, tpm-tis (the default) or
tpm-crb (only valid with version 2.0.

For example, to configure a flavor to use the TPM 2.0 with the CRB
model:

$ openstack flavor set $FLAVOR \
    --property hw:tpm_version=2.0 \
    --property hw:tpm_model=tpm-crb

Scheduling will fail if flavor and image supply conflicting values,
or if model tpm-crb is requested with version
1.2.

Upon successful boot, the server should see a TPM device such as
/dev/tpm0 which can be used in the same manner as a
hardware TPM.

Limitations

  • Only server operations performed by the server owner are supported,
    as the user’s credentials are required to unlock the virtual device
    files on the host. Thus the admin may need to decide whether to grant
    the user additional policy roles; if not, those operations are
    effectively disabled.
  • Live migration, evacuation, shelving and rescuing of servers with
    vTPMs is not currently supported.

Security

With a hardware TPM, the root of trust is a secret known only to the
TPM user. In contrast, an emulated TPM comprises a file on disk which
the libvirt daemon must be able to present to the guest. At rest, this
file is encrypted using a passphrase stored in a key manager service.
The passphrase in the key manager is associated with the credentials of
the owner of the server (the user who initially created it). The
passphrase is retrieved and used by libvirt to unlock the emulated TPM
data any time the server is booted.

Although the above mechanism uses a libvirt secret
that is both private (can’t be displayed via the libvirt
API or virsh) and ephemeral (exists only in
memory, never on disk), it is theoretically possible for a sufficiently
privileged user to retrieve the secret and/or vTPM data from memory.

A full analysis and discussion of security issues related to emulated
TPM is beyond the scope of this document.

References

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