Canonical LXD documentation improvement to make clear restricted.devices.disk=allow without restricted.devices.disk.paths also allows shift=true

Summary

### Summary

If a user has restricted access to a project that is configured with `restricted=true`, they can gain root access on the system by creating a disk device with `shift=true` and creating a setuid root executable. This is possible because the `shift` property is not restricted unless `restricted.devices.disk.paths` is set.

### Details

The following patch shows the offending code with a possible fix: ```patch --- a/lxd/device/disk.go +++ b/lxd/device/disk.go @@ -429,17 +429,19 @@ func (d *disk) validateEnvironmentSourcePath() error { if instProject.Name != api.ProjectDefaultName { // If restricted disk paths are in force, then check the disk's source is allowed, and record the // allowed parent path for later user during device start up sequence. - if shared.IsTrue(instProject.Config["restricted"]) && instProject.Config["restricted.devices.disk.paths"] != "" { - allowed, restrictedParentSourcePath := project.CheckRestrictedDevicesDiskPaths(instProject.Config, d.config["source"]) - if !allowed { - return fmt.Errorf("Disk source path %q not allowed by project for disk %q", d.config["source"], d.name) + if shared.IsTrue(instProject.Config["restricted"]) { + if instProject.Config["restricted.devices.disk.paths"] != "" { + allowed, restrictedParentSourcePath := project.CheckRestrictedDevicesDiskPaths(instProject.Config, d.config["source"]) + if !allowed { + return fmt.Errorf("Disk source path %q not allowed by project for disk %q", d.config["source"], d.name) + } + + d.restrictedParentSourcePath = shared.HostPath(restrictedParentSourcePath) }

if shared.IsTrue(d.config["shift"]) { return fmt.Errorf(`The "shift" property cannot be used with a restricted source path`) } - - d.restrictedParentSourcePath = shared.HostPath(restrictedParentSourcePath) } } ```

### PoC

```bash $ lxc project create restricted -c restricted=true -c restricted.devices.disk=allow $ lxc project switch restricted $ lxc profile device add default root disk path=/ pool=default $ lxc init ubuntu:22.04 c1 $ lxc config device add c1 d1 disk source=/ path=/mnt shift=true $ lxc start c1 # no error

$ lxc project set restricted restricted.devices.disk.paths=/ # explicitly allow mounting / $ lxc restart c1 Error: Failed to start device "d1": The "shift" property cannot be used with a restricted source path ```

Created https://github.com/canonical/lxd/issues/12606 to improve the documentation as per https://github.com/canonical/lxd/security/advisories/GHSA-x9qq-236j-gj97#advisory-comment-91918