Re: [Qemu-devel] [PATCH V10 5/5] Add a TPM Passthrough backend driver implementation

[Stefan Berger]

On 09/27/2011 01:13 PM, Michael S. Tsirkin wrote:
> On Tue, Sep 27, 2011 at 10:50:48AM -0400, Stefan Berger wrote:
> > +Since the host's firmware (BIOS/UEFI) has already initialized the TPM,
> > +the VM's firmware (BIOS/UEFI) will not be able to initialize the
> > +TPM again and may therefore not show a TPM-specific menu that would
> > +otherwise allow the user to configure the TPM.
> > +Also, if TPM ownership is released from within a VM then this will
> > +require a reboot of the host and the user will have to enter the host's
> > +firmware menu to enable and activate the TPM again.
> Rewrite:
>   Further, if TPM ownership is released from within a VM,
>   TPM gets deactivated in host.
Further, if TPM ownership is release from within a VM, the host's TPM 
gets disabled and deactivate.
>   To enable and activate the TPM again afterwards,
>   host has to be rebooted and the user is required to
>   enter the host's firmware menu.
>
> > If the TPM is left
> > +disabled and deactivated most TPM commands will fail.
> Why do we allow guest to do this then?
You cannot prevent it. This is due to how the TPM works. We cannot 
intercept the commands, either, and I don't think it makes much sense. I 
wrote this part of the docs to make people aware of these scenarios so 
they don't come as a surprise.
> Can we return an error, or ignore the release
> command? If someone really wants this unsafe behaviour
> we could make this an option, off by default.

In effect you'd have to parse every command that goes from the VM to the 
host and intercept it in the passthrough driver. I don't want to prevent 
it since it's a valid usage scenario but it has side effects (host needs 
to be rebooted). Even if we were to intercept this command then the user 
always has the possibility to send commands in an encrypted form (using 
TPM's transport 'tunnel') where one couldn't intercept this particular 
command anymore. So, my suggestion is we leave it as it is but we make 
people aware of these scenarios.

   Stefan