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Re: Unexplicable permission error when trying to read a qemu firmware file



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


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