Unexplicable permission error when trying to read a qemu firmware file

[Michal Privoznik]

On 12/29/20 6:09 PM, Emmanuel Kasper wrote:
> Hi
> 
> I want to create a CoreOS VM following the steps mentioned at
> https://docs.fedoraproject.org/en-US/fedora-coreos/getting-started,
> using a Debian 11 (bullseye) system.
> I am thus creating a VM with the following virt-install command:
> 
> virt-install --connect="qemu:///system" --name=coreos \
>      --vcpus=2 --memory=2048 \
>      --os-variant=fedora29 --import --graphics=none \
> 
> --disk="size=10,backing_store=$PWD/fedora-coreos-33.20201201.3.0-qemu.x86_64.qcow2"
> \
>      --qemu-commandline="-fw_cfg
> name=opt/com.coreos/config,file=/tmp/example.ign"

1: this ^^

> 
> which spits some warnings and this mysterious error
> 
> 2020-12-29T16:52:03.858938Z qemu-system-x86_64: warning: host doesn't
> support requested feature: MSR(48FH).vmx-exit-load-perf-global-ctrl [bit 12]
> 2020-12-29T16:52:03.858941Z qemu-system-x86_64: warning: host doesn't
> support requested feature: MSR(490H).vmx-entry-load-perf-global-ctrl
> [bit 13]

I don't know what are these, sorry.

> 
> 2020-12-29T16:52:03.864778Z qemu-system-x86_64: -fw_cfg
> name=opt/com.coreos/config,file=/tmp/example.ign: can't load
> /tmp/example.ign: Failed to open file “/tmp/example.ign”: Permission denied

But this is expected.

> 
> the file /tmp/example.ign is perfectly world readable,
> -rw-rw-rw- 1 manu manu 1130 Dec 29 17:23 /tmp/example.ign

This doesn't matter. It's SELinux what's giving you the error (as you've 
found out).

The problem here is [1]. If you bypass libvirt and put something right 
onto qemu's command line then libvirt blindly puts it there. No checks, 
no DAC/SELinux relabel, nothing. But I've implemented support for 
ignition files here:

https://gitlab.com/libvirt/libvirt/-/commit/3dda889a4426bb3555b1d8861d8314f82d8e1ef9

So what you need instead is:

   <sysinfo type='fwcfg'>
     <entry name='opt/com.coreos/config' file='/tmp/example.ign'/>
   </sysinfo>

This will make libvirt set correct labels and everything. I'm not sure 
whether virt-install supports that though, sorry.

Michal