Created attachment 1319382 [details]
[PATCH] tpm: Use /dev/null for cancel path if none was found

Description of problem:

Trusted Platform Module 2.0 (TPM2) device passthrough is not working on Fedora 26.

Version-Release number of selected component (if applicable):

libvirt-3.2.1-5

How reproducible:

Easy to reproduce

Steps to Reproduce:
1. Create a new VM on a host with a TPM2 device
2. Add a new TPM hardware to the VM using the passthrough device backend

    <tpm model='tpm-tis'>
      <backend type='passthrough'>
        <device path='/dev/tpm0'/>
      </backend>
    </tpm>

3. Try to power on the VM

Actual results:

Starting the VM fails with:

Error starting domain: internal error: No usable sysfs TPM cancel file could be found

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 89, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 125, in tmpcb
    callback(*args, **kwargs)
  File "/usr/share/virt-manager/virtManager/libvirtobject.py", line 82, in newfn
    ret = fn(self, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/domain.py", line 1489, in startup
    self._backend.create()
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 1039, in create
    if ret == -1: raise libvirtError ('virDomainCreate() failed', dom=self)
libvirtError: internal error: No usable sysfs TPM cancel file could be found

Expected results:

The VM should start and the host TPM2 be accessible from the VM guest.

Additional info:

TPM passthrough works correctly with TPM1.2, it only fails with TPM2.

The bug it's also present in upstream libvirt. Stefan Berger posted a patch to the libvirt list some time ago but it hasn't been merged yet:

https://www.spinics.net/linux/fedora/libvir/msg150107.html

I'm attaching the patch here.