Incorrect permissions on /dev/kvm

[davisjr]

Windows Build Number

Microsoft Windows [Version 10.0.22000.51]

WSL Version

• WSL 2
• WSL 1

Kernel Version

5.10.16

Distro Version

Ubuntu 20.04

Other Software

KVM

Repro Steps

Incorrect permissions on /dev/kvm is preventing startup of Android emulator although the user is a member of the required group.

$ emulator -avd api29
emulator: Android emulator version 30.6.5.0 (build_id 7324830) (CL:N/A)
ProbeKVM: This user doesn't have permissions to use KVM (/dev/kvm).
The KVM line in /etc/group is: [kvm:x:108:davisj]

If the current user has KVM permissions,
the KVM line in /etc/group should end with ":" followed by your username.

If we see LINE_NOT_FOUND, the kvm group may need to be created along with permissions:
    sudo groupadd -r kvm
    # Then ensure /lib/udev/rules.d/50-udev-default.rules contains something like:
    # KERNEL=="kvm", GROUP="kvm", MODE="0660"
    # and then run:
    sudo gpasswd -a $USER kvm

If we see kvm:... but no username at the end, running the following command may allow KVM access:
    sudo gpasswd -a $USER kvm

You may need to log out and back in for changes to take effect.

handleCpuAcceleration: feature check for hvf
emulator: ERROR: x86 emulation currently requires hardware acceleration!
CPU acceleration status: This user doesn't have permissions to use KVM (/dev/kvm).
The KVM line in /etc/group is: [kvm:x:108:davisj]

If the current user has KVM permissions,
the KVM line in /etc/group should end with ":" followed by your username.

If we see LINE_NOT_FOUND, the
More info on configuring VM acceleration on Linux:
https://developer.android.com/studio/run/emulator-acceleration#vm-linux
General information on acceleration: https://developer.android.com/studio/run/emulator-acceleration.

Permissions on /dev/kvm is currently (incorrect group and permissions):

$ ls -l /dev/kvm
crw------- 1 root root 10, 232 Jul  5 13:27 /dev/kvm

This should ideally be:

crw-rw---- 1 root kvm 10, 232 Jul  5 13:27 /dev/kvm

Manually changing it helps boot up the emulator. However, the settings are reverted on restart.

sudo chgrp kvm /dev/kvm
sudo chmod g+rw /dev/kvm

Looks like the correct permissions are part of udev defaults but this is not getting applied on WSL:
#4193 (comment) (#4193)

KERNEL=="kvm", GROUP="kvm", MODE="0660", OPTIONS+="static_node=kvm"

This is causing issues with React Native development on WSL where the manual step is required on every boot.
#6929

The issue is reproducible on Windows 10 21H1 builds as well.

Expected Behavior

Permissions on /dev/kvm should be as follows:

crw-rw---- 1 root kvm 10, 232 Jul  5 13:27 /dev/kvm

Actual Behavior

Current permissions on /dev/kvm:

crw------- 1 root root 10, 232 Jul  5 13:27 /dev/kvm

Diagnostic Logs

No response

[OneBlue]

That's expected, WSL2 doesn't have udev.

You can use wsl.conf to change those permissions at boot time with something like this though:

# /etc/wsl.conf
[boot]
command = /bin/bash -c 'chown root:lvm /dev/kvm && chmod 660 /dev/kvm'

[OneBlue]

Release notes for boot.command: https://docs.microsoft.com/en-us/windows/wsl/release-notes#build-21286

[BreeceW]

That's expected, WSL2 doesn't have udev.

You can use wsl.conf to change those permissions at boot time with something like this though:

# /etc/wsl.conf
[boot]
command = /bin/bash -c 'chown root:lvm /dev/kvm && chmod 660 /dev/kvm'

For anyone who comes across this, I wanted to add that this works if you change lvm to kvm, which I assume was a typo.

[gregaton]

If you still see the incorrect permissions error add your user to the kvm group and restart WSL.

sudo usermod -a -G kvm <username>

[felipemarques]

great ! works for me!

[Herbias]

That's expected, WSL2 doesn't have udev.

You can use wsl.conf to change those permissions at boot time with something like this though:

# /etc/wsl.conf
[boot]
command = /bin/bash -c 'chown root:lvm /dev/kvm && chmod 660 /dev/kvm'

Restart your WSL after. Works like a charm.

[proccedure-caze]

I cant find this folder, can anyone help me?

[smenard]

That's expected, WSL2 doesn't have udev.
You can use wsl.conf to change those permissions at boot time with something like this though:

# /etc/wsl.conf
[boot]
command = /bin/bash -c 'chown root:lvm /dev/kvm && chmod 660 /dev/kvm'

Restart your WSL after. Works like a charm.

Doesn't work for me.

I Tried

# /etc/wsl.conf
[boot]
command = /bin/bash -c 'sudo chown USER:kvm /dev/kvm && sudo chmod 660 /dev/kvm'

but it didn t work . Does someone has a hint for me ?

[Biswa96]

Are you using the WSL from Windows Store? https://apps.microsoft.com/store/detail/windows-subsystem-for-linux/9P9TQF7MRM4R?hl=en-us&gl=us

[smenard]

Are you using the WSL from Windows Store? https://apps.microsoft.com/store/detail/windows-subsystem-for-linux/9P9TQF7MRM4R?hl=en-us&gl=us

I think, is there a linux command that confirm that ?

[Biswa96]

Though not a Linux command, wsl.exe --version command from Windows would show some version details without any error or help message.

[smenard]

Here are the corresponding informations :

wsl.exe --version
WSL version: 1.1.3.0
Kernel version: 5.15.90.1
WSLg version: 1.0.49
MSRDC version: 1.2.3770
Direct3D version: 1.608.2-61064218
DXCore version: 10.0.25131.1002-220531-1700.rs-onecore-base2-hyp
Windows version: 10.0.19044.2604

[Biswa96]

This issue seem to be fixed in WSL from Windows Store if systemd is enabled.

• Without systemd:

$ ls -l /dev/kvm
crw------- 1 root root 10, 232 Mar 1 00:00 /dev/kvm

• With systemd:

$ ls -l /dev/kvm
crw-rw---- 1 root kvm 10, 232 Mar 1 00:00 /dev/kvm

So, you have to enabled systemd in wsl.conf file.

[Herbias]

It's pretty strange -- I did make it work by running the android virtual device on my machine before, but after I re-install my OS. I can't seem to make it work again.

Update

• I was able to run a virtual device by running sudo /opt/android-studio/bin/studio.sh
• I was able to run it. As @BreeceW mentioned, it was a typo.

[smenard]

This issue seem to be fixed in WSL from Windows Store if systemd is enabled.

• Without systemd:

$ ls -l /dev/kvm
crw------- 1 root root 10, 232 Mar 1 00:00 /dev/kvm

• With systemd:

$ ls -l /dev/kvm
crw-rw---- 1 root kvm 10, 232 Mar 1 00:00 /dev/kvm

So, you have to enabled systemd in wsl.conf file.

Thanks it works now.