Add support for encrypted emulated virtual TPM

https://blueprints.launchpad.net/nova/+spec/add-emulated-virtual-tpm

There are a class of applications which expect to use a TPM device to store secrets. In order to run these applications in a virtual machine, it would be useful to expose a virtual TPM device within the guest. Accordingly, the suggestion is to add flavor/image properties which a) translate to placement traits for scheduling and b) cause such a device to be added to the VM by the relevant virt driver.

Problem description

Currently there is no way to create virtual machines within nova that provide a virtual TPM device to the guest.

Use Cases

Support the virtualizing of existing applications and operating systems which expect to make use of physical TPM devices. At least one hypervisor (libvirt/qemu) currently supports the creation of an emulated TPM device which is associated with a per-VM swtpm process on the host, but there is no way to tell nova to enable it.

Proposed change

In recent libvirt and qemu (and possibly other hypervisors as well) there is support for an emulated vTPM device. We propose to modify nova to make use of this capability.

This spec describes only the libvirt implementation.

XML

The desired libvirt XML arguments are something like this (source):

...
<devices>
  <tpm model='tpm-tis'>
    <backend type='emulator' version='2.0'>
      <encryption secret='6dd3e4a5-1d76-44ce-961f-f119f5aad935'/>
    </backend>
  </tpm>
</devices>
...

Prerequisites

Support for encrypted emulated TPM requires at least:

  • libvirt version 5.6.0 or greater.

  • qemu 2.11 at a minimum, though qemu 2.12 is recommended. The virt driver code should add suitable version checks (in the case of LibvirtDriver, this would include checks for both libvirt and qemu). Currently emulated TPM is only supported for x86, though this is an implementation detail rather than an architectural limitation.

  • The swtpm binary and libraries on the host.

  • Access to a castellan-compatible key manager, such as barbican, for storing the passphrase used to encrypt the virtual device’s data. (The key manager implementation’s public methods must be capable of consuming the user’s auth token from the context parameter which is part of the interface.)

  • Access to an object-store service, such as swift, for storing the file the host uses for the virtual device data during operations such as shelve.

Config

All of the following apply to the compute (not conductor/scheduler/API) configs:

  • A new config option will be introduced to act as a “master switch” enabling vTPM. This config option would apply to future drivers’ implementations as well, but since this spec and current implementation are specific to libvirt, it is in the libvirt rather than the compute group:

    [libvirt]
    vtpm_enabled = $bool (default False)
    
  • To enable move operations (anything involving rebuilding a vTPM on a new host), nova must be able to lay down the vTPM data with the correct ownership – that of the swtpm process libvirt will create – but we can’t detect a priori what that ownership will be. Thus we need a pair of config options on the compute indicating the user and group that should own vTPM data on that host:

    [libvirt]
    swtpm_user = $str (default 'tss')
    swtpm_group = $str (default 'tss')
    
  • (Existing, known) options for [key_manager].

  • New standard keystoneauth1 auth/session/adapter options for [swift] will be introduced.

Traits, Extra Specs, Image Meta

In order to support this functionality we propose to:

  • Use the existing COMPUTE_SECURITY_TPM_1_2 and COMPUTE_SECURITY_TPM_2_0 traits. These represent the two different versions of the TPM spec that are currently supported. (Note that 2.0 is not backward compatible with 1.2, so we can’t just ignore 1.2. A summary of the differences between the two versions is currently available here.) When all the Prerequisites have been met and the Config switch is on, the libvirt compute driver will set both of these traits on the compute node resource provider.

  • Support the following new flavor extra_specs and their corresponding image metadata properties (which are simply s/:/_/ of the below):

    • hw:tpm_version={1.2|2.0}. This will be:

      • translated to the corresponding required=COMPUTE_SECURITY_TPM_{1_2|2_0} in the allocation candidate request to ensure the instance lands on a host capable of vTPM at the requested version

      • used by the libvirt compute driver to inject the appropriate guest XML.

      Note

      Whereas it would be possible to specify trait:COMPUTE_SECURITY_TPM_{1_2|2_0}=required directly in the flavor extra_specs or image metadata, this would only serve to land the instance on a capable host; it would not trigger the libvirt driver to create the virtual TPM device. Therefore, to avoid confusion, this will not be documented as a possibility.

    • hw:tpm_model={TIS|CRB}. Indicates the emulated model to be used. If omitted, the default is TIS (this corresponds to the libvirt default). CRB is only compatible with TPM version 2.0; if CRB is requested with version 1.2, an error will be raised from the API.

To summarize, all and only the following combinations are supported, and are mutually exclusive (none are inter-compatible):

  • Version 1.2, Model TIS

  • Version 2.0, Model TIS

  • Version 2.0, Model CRB

Note that since the TPM is emulated (a process/file on the host), the “inventory” is effectively unlimited. Thus there are no resource classes associated with this feature.

If both the flavor and the image specify a TPM trait or device model and the two values do not match, an exception will be raised from the API by the flavor/image validator.

Instance Lifecycle Operations

Descriptions below are libvirt driver-specific. However, it is left to the implementation which pieces are performed by the compute manager vs. the libvirt ComputeDriver itself.

Note

In deciding whether/how to support a given operation, we use “How does this work on baremetal” as a starting point. If we can support a VM operation without introducing inordinate complexity or user-facing weirdness, we do.

Spawn

  1. Even though swift is not required for spawn, ensure a swift endpoint is present in the service catalog (and reachable? version discovery? implementation detail) so that a future unshelve doesn’t break the instance.

  2. Nova generates a random passphrase and stores it in the configured key manager, yielding a UUID, hereinafter referred to as $secret_uuid.

  3. Nova saves the $secret_uuid in the instance’s system_metadata under key tpm_secret_uuid.

  4. Nova uses the virSecretDefineXML API to define a private (value can’t be listed), ephemeral (state is stored only in memory, never on disk) secret whose name is the instance UUID, and whose UUID is the $secret_uuid. The virSecretSetValue API is then used to set its value to the generated passphrase.

  5. Nova injects the XML into the instance’s domain. The model and version are gleaned from the flavor/image properties, and the secret is $secret_uuid.

  6. Once libvirt has created the guest, nova uses the virSecretUndefine API to delete the secret. The instance’s emulated TPM continues to function.

Note

Spawning from an image created by snapshotting a VM with a vTPM will result in a fresh, empty vTPM, even if that snapshot was created by shelve. By contrast, spawn during unshelve will restore such vTPM data.

Cold Boot

…and any other operation that starts the guest afresh. (Depending on the key manager security model, these may be restricted to the instance owner.)

  1. Pull the $secret_uuid from the tpm_secret_uuid of the instance’s system_metadata.

  2. Retrieve the passphrase associated with $secret_uuid via the configured key manager API.

Then perform steps 4-6 as described under Spawn.

Migrations and their ilk

For the libvirt implementation, the emulated TPM data is stored in /var/lib/libvirt/swtpm/<instance>. Certain lifecycle operations require that directory to be copied verbatim to the “destination”. For (cold/live) migrations, only the user that nova-compute runs as is guaranteed to be able to have SSH keys set up for passwordless access, and it’s only guaranteed to be able to copy files to the instance directory on the destination node. We therefore propose the following procedure for relevant lifecycle operations:

  • Copy the directory into the local instance directory, changing the ownership to match it.

  • Perform the move, which will automatically carry the data along.

  • Change ownership back and move the directory out to /var/lib/libvirt/swtpm/<instance> on the destination.

  • On confirm/revert, delete the directory from the source/destination, respectively. (This is done automatically by libvirt when the guest is torn down.)

  • On revert, the data directory must be restored (with proper permissions) on the source.

Since the expected ownership on the target may be different than on the source, and is (we think) impossible to detect, the admin must inform us of it via the new [libvirt]swtpm_user and [libvirt]swtpm_group Config options if different from the default of tss.

This should allow support of cold/live migration and resizes that don’t change the device.

Todo

Confirm that the above “manual” copying around is actually necessary for migration. It’s unclear from reading https://github.com/qemu/qemu/blob/6a5d22083d50c76a3fdc0bffc6658f42b3b37981/docs/specs/tpm.txt#L324-L383

Resize can potentially add a vTPM to an instance that didn’t have one before, or remove the vTPM from an instance that did have one, and those should “just work”. When resizing from one version/model to a different one the data can’t and won’t carry over (for same-host resize, we must remove the old backing file). If both old and new flavors have the same model/version, we must ensure we convey the virtual device data as described above (for same-host resize, we must preserve the existing backing file).

Shelve (offload) and Unshelve

Restoring vTPM data when unshelving a shelve-offloaded server requires the vTPM data to be persisted somewhere. We can’t put it with the image itself, as it’s data external to the instance disk. So we propose to put it in object-store (swift) and maintain reference to the swift object in the instance’s system_metadata.

The shelve operation needs to:

  1. Save the vTPM data directory to swift.

  2. Save the swift object ID and digital signature (sha256) of the directory to the instance’s system_metadata under the (new) tpm_object_id and tpm_object_sha256 keys.

  3. Create the appropriate hw_tpm_version and/or hw_tpm_model metadata properties on the image. (This is to close the gap where the vTPM on original VM was created at the behest of image, rather than flavor, properties. It ensures the proper scheduling on unshelve, and that the correct version/model is created on the target.)

The unshelve operation on a shelved (but not offloaded) instance should “just work” (except for deleting the swift object; see below). The code path for unshelving an offloaded instance needs to:

  1. Ensure we land on a host capable of the necessary vTPM version and model (we get this for free via the common scheduling code paths, because we did step 3 during shelve).

  2. Look for tpm_object_{id|sha256} and tpm_secret_uuid in the instance’s system_metadata.

  3. Download the swift object. Validate its checksum and fail if it doesn’t match.

  4. Assign ownership of the data directory according to [libvirt]swtpm_{user|group} on the host.

  5. Retrieve the secret and feed it to libvirt; and generate the appropriate domain XML (we get this for free via spawn()).

  6. Delete the object from swift, and the tpm_object_{id|sha256} from the instance system_metadata. This step must be done from both code paths (i.e. whether the shelved instance was offloaded or not).

Note

There are a couple of ways a user can still “outsmart” our checks and make horrible things happen on unshelve. For example:

  • The flavor specifies no vTPM properties.

  • The original image specified version 2.0.

  • Between shelve and unshelve, edit the snapshot to specify version 1.2.

We will happily create a v1.2 vTPM and restore the (v2.0) data into it. The VM will (probably) boot just fine, but unpredictable things will happen when the vTPM is accessed.

We can’t prevent all stupidity.

Note

As mentioned in Security impact, if shelve is performed by the admin, only the admin will be able to perform the corresponding unshelve operation. And depending on the key manager security model, if shelve is performed by the user, the admin may not be able to perform the corresponding unshelve operation.

Since the backing device data is virt driver-specific, it must be managed by the virt driver; but we want the object-store interaction to be done by compute manager. We therefore propose the following interplay between compute manager and virt driver:

The ComputeDriver.snapshot() contract currently does not specify a return value. It will be changed to allow returning a file-like with the (prepackaged) backing device data. The libvirt driver implementation will open a tar pipe and return that handle. The compute manager is responsible for reading from that handle and pushing the contents into the swift object. (Implementation detail: we only do the swift thing for snapshots during shelve, so a) the virt driver should not produce the handle except when the VM is in SHELVE[_OFFLOADED] state; and/or the compute manager should explicitly close the handle from other invocations of snapshot().)

The compute driver touchpoint for unshelving an offloaded instance is spawn(). This method will get a new kwarg which is a file-like. If not None, virt driver implementations are responsible for streaming from that handle and reversing whatever was done during snapshot() (in this case un-tar-ing). For the unshelve path for offloaded instances, the compute manager will pull down the swift object and stream it to spawn() via this kwarg.

createImage and createBackup

Because vTPM data is associated with the instance, not the image, the createImage and createBackup flows will not be changed. In particular, they will not attempt to save the vTPM backing device to swift.

This, along with the fact that fresh Spawn will not attempt to restore vTPM data (even if given an image created via shelve) also prevents “cloning” of vTPMs.

This is analogous to the baremetal case, where spawning from an image/backup on a “clean” system would get you a “clean” (or no) TPM.

Rebuild

Since the instance is staying on the same host, we have the ability to leave the existing vTPM backing file intact. This is analogous to baremetal behavior, where restoring a backup on an existing system will not touch the TPM (or any other devices) so you get whatever’s already there. However, it is also possible to lock your instance out of its vTPM by rebuilding with a different image, and/or one with different metadata. A certain amount of responsibility is placed on the user to avoid scenarios like using the TPM to create a master key and not saving that master key (in your rebuild image, or elsewhere).

That said, rebuild will cover the following scenarios:

  • If there is no existing vTPM backing data, and the rebuild image asks for a vTPM, create a fresh one, just like Spawn.

  • If there is an existing vTPM and neither the flavor nor the image asks for one, delete it.

  • If there is an existing vTPM and the flavor or image asks for one, leave the backing file alone. However, if different versions/models are requested by the old and new image in combination with the flavor, we will fail the rebuild.

Evacuate

Because the vTPM data belongs to libvirt rather than being stored in the instance disk, the vTPM is lost on evacuate, even if the instance is volume-backed. This is analogous to baremetal behavior, where the (hardware) TPM is left behind even if the rest of the state is resurrected on another system via shared storage.

(It may be possible to mitigate this by mounting /var/lib/libvirt/swtpm/ on shared storage, though libvirt’s management of that directory on guest creation/teardown may stymie such attempts. This would also bring in additional security concerns. In any case, it would be an exercise for the admin; nothing will be done in nova to support or prevent it.)

Destroy

  1. Delete the key manager secret associated with system_metadata['tpm_secret_uuid'].

  2. libvirt deletes the vTPM data directory as part of guest teardown.

  3. If system_metadata['tpm_object_id'] exists, the API side will delete the swift object it identifies. Since this metadata only exists while an instance is shelved, this should only be applicable in corner cases like:

    • If the destroy() is performed between shelve and offload.

    • Cleaning up a VM in ERROR state from a shelve, offload, or unshelve that failed (at just the right time).

    • Cleaning up a VM that is deleted while the host was down.

Limitations

This is a summary of odd or unexpected behaviors resulting from this design.

  • Except for migrations and shelve-offload, vTPM data sticks with the instance+host. In particular:

    • vTPM data is lost on Evacuate.

    • vTPM data is not carried with “reusable snapshots” (createBackup/createImage).

  • The ability of instance owners or admins to perform certain instance lifecycle operations may be limited depending on the security model used for the key manager.

  • Since secret management is done by the virt driver, deleting an instance when the compute host is down can orphan its secret. If the host comes back up, the secret will be reaped when compute invokes the virt driver’s destroy. But if the host never comes back up, it would have to be deleted manually.

Alternatives

  • Rather than using a trait, we could instead use arbitrarily large inventories of 1_2/2_0 resource classes. Unless it can be shown that there’s an actual limit we can discover, this just isn’t how we do things.

  • Instead of using specialized hw:tpm* extra_spec/image_meta properties, implicitly configure based on the placement-ese syntax (resources:COMPUTE_SECURITY_TPM_*). Rejected because we’re trying to move away from this way of doing things in general, preferring instead to support syntax specific to the feature, rather than asking the admin to understand how the feature maps to placement syntax. Also, whereas in some cases the mapping may be straightforward, in other cases additional configuration is required at the virt driver level that can’t be inferred from the placement syntax, which would require mixing and matching placement and non-placement syntax.

  • That being the case, forbid placement-ese syntax using resources[$S]:COMPUTE_SECURITY_TPM_*. Rejected mainly due to the (unnecessary) additional complexity, and because we don’t want to get in the business of assuming there’s no use case for “land me on a vTPM (in)capable host, but don’t set one up (yet)”.

  • Use physical passthrough (<backend type='passthrough'>) of a real (hardware) TPM device. This is not feasible with current TPM hardware because (among other things) changing ownership of the secrets requires a host reboot.

  • Block the operations that require object store. This is deemed nonviable, particularly since cross-cell resize uses shelve under the covers.

  • Use glance or the key manager instead of swift to store the vTPM data for those operations. NACKed because those services really aren’t intended for that purpose, and (at least glance) may block such usages in the future.

  • Save vTPM data on any snapshot operation (including createImage and createBackup). This adds complexity as well as some unintended behaviors, such as the ability to “clone” vTPMs. Users will be less surprised when their vTPM acts like a (hardware) TPM in these cases.

  • Rather than checking for swift at spawn time, add an extra spec / image prop like vtpm_I_promise_I_will_never_shelve_offload=True or vtpm_is_totally_ephemeral=True which would either error or simply not back up the vTPM, respectively, on shelve-offload.

Data model impact

The ImageMetaProps and ImageMetaPropsPayload objects need new versions adding:

  • hw_tpm_version

  • hw_tpm_model

  • tpm_object_id

  • tpm_object_sha256

REST API impact

The image/flavor validator will get new checks for consistency of properties. No new microversion is needed.

Security impact

The guest will be able to use the emulated TPM for all the security enhancing functionality that a physical TPM provides, in order to protect itself against attacks from within the guest.

The key manager and object store services are assumed to be adequately hardened against external attack. However, the deployment must consider the issue of authorized access to these services, as discussed below.

Data theft

The vTPM data file is encrypted on disk, and is therefore “safe” (within the bounds of encryption) from simple data theft.

We will use a passphrase of 384 bytes, which is the default size of an SSH key, generated from /dev/urandom. It may be desirable to make this size configurable in the future.

Compromised root

It is assumed that the root user on the compute node would be able to glean (e.g. by inspecting memory) the vTPM’s contents and/or the passphrase while it’s in flight. Beyond using private+ephemeral secrets in libvirt, no further attempt is made to guard against a compromised root user.

Object store

The object store service allows full access to an object by the admin user, regardless of who created the object. There is currently no facility for restricting admins to e.g. only deleting objects. Thus, if a shelve has been performed, the contents of the vTPM device will be available to the admin. They are encrypted, so without access to the key, we are still trusting the strength of the encryption to protect the data. However, this increases the attack surface, assuming the object store admin is different from whoever has access to the original file on the compute host.

By the same token (heh) if shelve is performed by the admin, the vTPM data object will be created and owned by the admin, and therefore only the admin will be able to unshelve that instance.

Key manager

The secret stored in the key manager is more delicate, since it can be used to decrypt the contents of the vTPM device. The barbican implementation scopes access to secrets at the project level, so the deployment must take care to limit the project to users who should all be trusted with a common set of secrets. Also note that project-scoped admins are by default allowed to access and decrypt secrets owned by any project; if the admin is not to be trusted, this should be restricted via policy.

However, castellan backends are responsible for their own authentication mechanisms. Thus, the deployment may wish to use a backend that scopes decryption to only the individual user who created the secret. (In any case it is important that admins be allowed to delete secrets so that operations such as VM deletion can be performed by admins without leaving secrets behind.)

Note that, if the admin is restricted from decrypting secrets, lifecycle operations performed by the admin cannot result in a running VM. This includes rebooting the host: even with resume_guests_state_on_host_boot set, an instance with a vTPM will not boot automatically, and will instead have to be powered on manually by its owner. Other lifecycle operations which are by default admin-only will only work when performed by the VM owner, meaning the owner must be given the appropriate policy roles to do so; otherwise these operations will be in effect disabled.

…except live migration, since the (already decrypted) running state of the vTPM is carried along to the destination. (To clarify: live migration, unlike other operations, would actually work if performed by the admin because of the above.)

Notifications impact

None

Other end user impact

None

Performance Impact

  • An additional API call to the key manager is needed during spawn (to register the passphrase), cold boot (to retrieve it), and destroy (to remove it).

  • Additional API calls to libvirt are needed during spawn and other boot-like operations to define, set the value, and undefine the vTPM’s secret in libvirt.

  • Additional API calls to the object store (swift) are needed to create (during shelve), retrieve (unshelve), and delete (unshelve/destroy) the vTPM device data object.

Other deployer impact

None

Developer impact

The various virt drivers would be able to implement the emulated vTPM as desired.

Upgrade impact

None

Implementation

Assignee(s)

Primary assignee:

efried

Other contributors:

cfriesen

Feature Liaison

efried

Work Items

  • API changes to prevalidate the flavor and image properties.

  • Scheduler changes to translate flavor/image properties to placement-isms.

  • Libvirt driver changes to

  • Testing

Dependencies

None

Testing

Unit and functional testing will be added. New fixtures for object store and key manager services will likely be necessary.

Because of the eccentricities of a) user authentication for accessing the encryption secret, and b) management of the virtual device files for some operations, CI coverage will be added for:

  • Live migration

  • Cold migration

  • Host reboot (how?)

  • Shelve (offload) and unshelve

  • Backup and rebuild

Documentation Impact

Operations Guide and End User Guide will be updated appropriately. Feature support matrix will be updated.

History

Revisions

Release Name

Description

Stein

Introduced

Train

Re-proposed

Ussuri

Re-proposed with refinements including encryption pieces