Incus vulnerable to local privilege escalation through custom storage volumes

Impact

This affects any Incus user in an environment where an unprivileged user may have root access to a container with an attached custom storage volume that has the security.shifted property set to true as well as access to the host as an unprivileged user.

The most common case for this would be systems using incus-user with the less privileged incus group to provide unprivileged users with an isolated restricted access to Incus. Such users may be able to create a custom storage volume with the necessary property (depending on kernel and filesystem support) and can then write a setuid binary from within the container which can be executed as an unpriivleged user on the host to gain root privileges.

Patches

A patch for this issue is available here: https://github.com/lxc/incus/pull/2642

The first commit changes the permissions for any new storage pool, the second commit applies it on startup to all existing storage pools.

Workarounds

Permissions can be manually restricted until a patched version of Incus is deployed.

This is done with:

chmod 0700 /var/lib/incus/storage-pools/*/*
chmod 0711 /var/lib/incus/storage-pools/*/buckets*
chmod 0711 /var/lib/incus/storage-pools/*/container*

Those are the same permissions which will be applied by the patched Incus for both new and existing storage pools.

References

This was reported publicly on Github by here: https://github.com/lxc/incus/issues/2641