Incus vulnerable to local privilege escalation through VM screenshot path

Summary

Incus provides an API to retrieve VM screenshots, that API relies on the use of a temporary file for QEMU to write the screenshot to which is then picked up and sent to the user prior to deletion.

As Incus uses predictable paths under /tmp for this, an attacker with local access to the system can abuse this mechanism by creating their own symlinks ahead of time.

On the vast majority of Linux systems, this will result in a "Permission denied" error when requesting a screenshot. That's because the Linux kernel has a security feature designed to block such attacks, protected_symlinks.

On the rare systems with this purposefully disabled, it's then possible to trick Incus intro truncating and altering the mode and permissions of arbitrary files on the filesystem, leading to a potential denial of service or possible local privilege escalation.

Details

The incusd daemon contains a local privilege escalation (LPE) primitive in the Virtual Machine VGA screenshot handling routine. When a screenshot is requested, the daemon creates a file in the globally writable /tmp directory using a deterministic pathname derived from the instance identifier. Because this implementation uses a predictable pathname in a world-writable directory, it exposes the operation to pathname attacks. The file permissions are then restricted, and the file is passed to the QEMU screenshot routine. In the QEMU path, ownership is transferred to the unprivileged Virtual Machine UID before the QEMU Machine Protocol is invoked with the same pathname.

An attacker able to pre-place or otherwise control that pathname can redirect truncation and ownership changes to an unintended host file.

This allows attacker-chosen host files to be truncated and have ownership reassigned to the unprivileged VM UID. In practice, this can be used to destroy sensitive root-owned files and alter ownership of security-relevant host paths. Depending on the targeted path and follow-up conditions, the impact may include denial of service, corruption of credentials or configuration, persistence through modified startup or service files, and further privilege escalation on the host.

As previously mentioned, this is only possible if the kernel protection mechanism has been previously disabled. It's possible to check on its status by reading the file at /proc/sys/fs/protected_symlinks, a value of 0 is required for this attack to work.

Affected File: https://github.com/lxc/incus/blob/v6.20.0/cmd/incusd/instance_console.go

Affected Code:

func instanceConsoleGet(d *Daemon, r *http.Request) response.Response {
    [...]
    } else if inst.Type() == instancetype.VM {
        v, ok := inst.(instance.VM)
        if !ok {
            return response.SmartError(errors.New("Failed to cast inst to VM"))
        }

        var headers map[string]string
        if consoleLogType == "vga" {
            screenshotFile, err := os.Create(fmt.Sprintf("/tmp/incus_screenshot_%d", inst.ID()))
            if err != nil {
                return response.SmartError(fmt.Errorf("Couldn't create screenshot file: %w", err))
            }

            err = screenshotFile.Chmod(0o600)
            if err != nil {
                return response.SmartError(err)
            }

            ent.Cleanup = func() {
                _ = screenshotFile.Close()
                _ = os.Remove(screenshotFile.Name())
            }

            err = v.ConsoleScreenshot(screenshotFile)
            if err != nil {
                return response.SmartError(err)
            }
            [...]
    }
    [...]
}