Re: [PATCH] tpm: Add driver for TPM over virtio

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Mon, 2019-02-25 at 14:43 -0800, Matthew Garrett wrote:
> On Mon, Feb 25, 2019 at 2:32 PM James Bottomley
> <James.Bottomley@hansenpartnership.com> wrote:
> > On Mon, 2019-02-25 at 14:24 -0800, Matthew Garrett wrote:
> > > My understanding is that the crosvm USB code is intended to allow
> > > arbitrary USB hardware to be passed through to the guest - doing
> > > this
> > > via virtio sounds complicated (you'd need a virtio driver that
> > > covered every USB class, and how would you manage that for
> > > devices
> > > that are handled in userland at the moment),
> > 
> > I think you'd need a virtio equivalent of the host driver, say
> > xhci_virtio ... you could still use the in-kernel USB class drivers
> 
> Mm. I honestly don't know enough about the desired use case for USB
> to be able to provide meaningful input here.
> 
> > > > Effectively it bypasses the hypervisor altogether and simply
> > > > makes a direct connection to the host devices.  The TPM could
> > > > actually work in exactly the same way, except you'd have to use
> > > > the socsim IP connection (which all TSSs support) rather than a
> > > > file descriptor.
> > > 
> > > I don't really follow - how would in-kernel TPM features work
> > > then?
> > 
> > If you do it at the TSS layer, then, of course, the kernel wouldn't
> > participate.  If you used the proposed in-kernel socsim driver, I
> > suppose it could ... not that I'm advocating this, I'm saying if
> > you want to minimise hypervisor code for attack surface reduction,
> > this would be the way to do it because this solution requires no
> > in-hypervisor code at all.
> 
> You still need a transport mechanism through the hypervisor to
> communicate with the host - what would you be using in that case
> instead of virtio?

Socsim is net transported; it's sort of the TPM equivalent of NFS or
iSCSI storage for guests.

James