LXD fails to create VM with QEMU 7.2.0: "../../net/net.c:1106: net_client_init1: Assertion `nc' failed."

Host environment

• Operating system: Debian sid
• OS/kernel version: Linux tharkun 6.1.0-3-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.8-1 (2023-01-29) x86_64 GNU/Linux
• Architecture: amd64
• QEMU flavor: qemu-system-x86_64
• QEMU version: QEMU emulator version 7.2.0 (Debian 1:7.2+dfsg-2)
• LXD command line:

  lxc launch images:debian/sid debian-sid-vm --vm

Emulated/Virtualized environment

• Operating system: Debian sid
• OS/kernel version: 6.1.0-3-amd64
• Architecture: amd64

Description of problem

Beginning with QEMU 7.2.0, LXD is unable to launch virtual machines using the default network profile, which breaks the out-of-box experience if a user wishes to create a virtual machine. This worked correctly with QEMU 7.1.0.

Multiple users across different Linux distributions are reporting this issue:

• https://discuss.linuxcontainers.org/t/failed-adding-nic-netdev-monitor-is-disconnected/15946
• https://forums.gentoo.org/viewtopic-p-8774212.html
• https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1030365

gibmat@tharkun:~$ lxc launch images:debian/sid debian-sid-vm --vm
Creating debian-sid-vm
Starting debian-sid-vm                        
Error: Failed setting up device via monitor: Failed setting up device "eth0": Failed adding NIC netdev: Monitor is disconnected
Try `lxc info --show-log local:debian-sid-vm` for more info
gibmat@tharkun:~$ lxc info --show-log local:debian-sid-vm
Name: debian-sid-vm
Status: STOPPED
Type: virtual-machine
Architecture: x86_64
Created: 2023/02/10 23:47 UTC

Log:

warning: tap: open vhost char device failed: Permission denied
warning: tap: open vhost char device failed: Permission denied
qemu-system-x86_64: ../../net/net.c:1106: net_client_init1: Assertion `nc' failed.

gibmat@tharkun:~$ qemu-system-x86_64 --version
QEMU emulator version 7.2.0 (Debian 1:7.2+dfsg-2)
Copyright (c) 2003-2022 Fabrice Bellard and the QEMU Project developers
gibmat@tharkun:~$ lxc version
Client version: 5.0.2
Server version: 5.0.2

Steps to reproduce

1. Install LXD and QEMU 7.2.0
2. lxc launch images:debian/sid debian-sid-vm --vm

• This will fail as reported above

3. Downgrade to QEMU 7.1.0 (such as from https://snapshot.debian.org/package/qemu/1%3A7.1%2Bdfsg-2/)
4. lxc launch images:debian/sid debian-sid-vm --vm

• Now VM creation is successful

  gibmat@tharkun:~$ lxc launch images:debian/sid debian-sid-vm --vm
  Creating debian-sid-vm
  Starting debian-sid-vm
  gibmat@tharkun:~$ lxc list
  +---------------+---------+------+-----------------------------------------------+-----------------+-----------+
  |     NAME      |  STATE  | IPV4 |                     IPV6                      |      TYPE       | SNAPSHOTS |
  +---------------+---------+------+-----------------------------------------------+-----------------+-----------+
  | debian-sid-vm | RUNNING |      | fd42:ea61:feb4:55ef:216:3eff:feb8:2e8c (eth0) | VIRTUAL-MACHINE | 0         |
  +---------------+---------+------+-----------------------------------------------+-----------------+-----------+
  gibmat@tharkun:~$ lxc info --show-log local:debian-sid-vm
  Name: debian-sid-vm
  Status: RUNNING
  Type: virtual-machine
  Architecture: x86_64
  PID: 2502
  Created: 2023/02/11 15:08 UTC
  Last Used: 2023/02/11 15:08 UTC
  
  Resources:
    Processes: -1
    Network usage:
      eth0:
        Type: broadcast
        State: UP
        Host interface: tap5efa7582
        MAC address: 00:16:3e:b8:2e:8c
        MTU: 1500
        Bytes received: 3.13kB
        Bytes sent: 164B
        Packets received: 12
        Packets sent: 2
        IP addresses:
          inet6: fd42:ea61:feb4:55ef:216:3eff:feb8:2e8c/64 (global)
  
  Log:
  
  warning: tap: open vhost char device failed: Permission denied
  warning: tap: open vhost char device failed: Permission denied
  
  gibmat@tharkun:~$ qemu-system-x86_64 --version
  QEMU emulator version 7.1.0 (Debian 1:7.1+dfsg-2+b3)
  Copyright (c) 2003-2022 Fabrice Bellard and the QEMU Project developers