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[libvirt] TPM support in libvirt


I'd like to start a discussion and get some feedback on support for TPM (Trusted Platform Module) management in libvirt. I have been posting patches to the Qemu and SeaBIOS mailing lists that will provide the ground work for TPM support in Qemu, though I anticipate that it will take a while until the code goes into those repositories. So everything I am saying related to command lines etc. can still change. Nevertheless, the next obvious layer for support is libvirt. Well, and I do have patches...

First some background: the TPM is a crypto device built into many modern laptops and desktops. It's a chip connected to the motherboard and has firmware for running the TPM specifications. I would say it has a full processor on its own to run this firmware. It also has some very limited space for persistent storage to permanently store for example the owner's password (that he can establish with the TPM), store private keys and it also has some area for NVRAM spaces where the user can put any data into it he wants to.

So now, on the qemu command line the TPM support (currently) looks as follows:

... -tpm ?


Supported TPM types (choose only one):
builtin   Qemu's built-in TPM; requires 63kb of block storage

I use this for reading the size of the necessary (QCoW2) blockstorage from it, 63kb.

To start a VM with TPM support:

... -tpm type=builtin,path=/tmp/tpmstate.bin

The above indicates the type of TPM to use. Currently there is only one type available, which is the 'builtin' one. It also provides a path to persistent storage, here /tmp/tpmstate.bin. The persistent storage file is actually a Qcow2 file so we can properly support snapshotting of the image and the TPM and go back to different states.

Obviously we are now adding another 'disk' to the VM, so that the TPM can store its own persistent data. Now a user has to make sure to always keep his image file and this TPM 'disk' together, particularly once he stores vital keys in the TPM.

So now for modeling this in libvirt I thought of this XML here for a user-provided Qcow2 image:

<tpm type='built-in'>
<storage file='/tmp/tpmstate.bin'/>

In this case the user would create the above QCoW2 /tmp/tmpstate.bin at the appropriate size (63kb).

  This XML here is for a libvirt-created QCoW2 image:

<tpm type='built-in'>

This then causes libvirt to create a QCoW2 image of 63kb size (reads the 63kb from .. tpm ?) once the VM is about to start and produces for example this command line:

-tpm type=builtin,path=/var/lib/libvirt/tpm/a4d7cd22-da89-3094-6212-079a48a309a1.bin

The UUID then corresponds to the UUID of the VM. Since only one TPM per VM is supported, this should work just fine.

 Now to support an encrypted QCoW2 image, the following XML could be used:

<tpm type='built-in'>
<encryption format='qcow'>
<secret type='passphrase' uuid='13ea49f7-2553-7308-5168-e337ade36232'/>

Here the user has created a 'secret' and passes that secret's UUID into the XML above. So this is similar to how storage is dealt with with the difference that no path to the storage is provided and also internally no comparisons for storage paths are being made. I did also not want to force the user to create storage spaces (volumes) on his own, but let libvirt handle this implicitly for the TPM (use qemu-img to create the QCoW2). Also, multiple VMs' TPMs could share the same secret.

 Please let me know of any comments regarding this.

 I'll post the patches as RFC later on.


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