cyberstrikeus Ensure overlay kernel module is not available
Profile
Level 2 - Server, Level 2 - Workstation, Assessment: Automated
Description
overlay is a Linux filesystem that layers multiple filesystems to create a single unified view which allows a user to "merge" several mount points into a unified filesystem.
Rationale
The overlay has known CVE's: CVE-2023-32629, CVE-2023-2640, CVE-2023-0386. Disabling the overlay reduces the local attack surface by removing support for unnecessary filesystem types and mitigates potential risks associated with unauthorized execution of setuid files, enhancing the overall system security.
Impact
WARNING: If Container applications such as Docker, Kubernetes, Podman, Linux Containers (LXC), etc. are in use proceed with caution and consider the impact on containerized workloads, as disabling the overlay may severely disrupt containerization.
Audit Procedure
Command Line
Verify the overlay kernel module is not available on the system or has been disabled.
Run the following script to determine if the overlay kernel module is available on the system:
#!/usr/bin/env bash
{
l_mod_name="overlayfs" l_mod_type="fs"
while IFS= read -r l_mod_path; do
if [ -d "$l_mod_path/${l_mod_name}/-/\/" ] && [ -n "$(ls -A "$l_mod_path/${l_mod_name}/-/\/")" ]; then
printf '%s\n' "$l_mod_name exists in $l_mod_path"
fi
done < <(readlink -f /usr/lib/modules/**/kernel/$l_mod_type || readlink -f /lib/modules/**/kernel/$l_mod_type)
}
If nothing is returned, the overlay kernel module is not available on the system and no further audit steps are required.
Note: Some systems may include the overlay filesystem as part of the kernel opposed to being available as a kernel module. In this case, the above audit will not return anything. This is also considered a passing state.
If anything is returned, verify the overlay kernel module is not loaded and not loadable by performing the following:
Run the following command to verify the overlay kernel module is not loaded:
# lsmod | grep 'overlay'
Nothing should be returned
Run the following command to verify the overlay kernel module is not loadable:
# modprobe --showconfig | grep -P -- '\b(install|blacklist)\h+overlay\b'
Verify the output includes:
blacklist overlay
-AND-
install overlay /bin/false
-OR-
install overlay /bin/true
Expected Result
blacklist overlay
install overlay /bin/false
Remediation
Command Line
Run the following to unload and disable the overlay kernel module:
# modprobe -r overlay 2>/dev/null
# rmmod overlay 2>/dev/null
Perform the following to disable the overlay kernel module:
Create a file ending in .conf with install overlay /bin/false in the /etc/modprobe.d/ directory
Example:
# printf '\n%s\n' "install overlay /bin/false" >> overlay.conf
Create a file ending in .conf with blacklist overlay in the /etc/modprobe.d/ directory
Example:
# printf '\n%s\n' "blacklist overlay" >> overlay.conf
Default Value
Not disabled by default.
References
- NIST SP 800-53 Rev. 5: CM-7
- https://docs.kernel.org/filesystems/overlayfs.html
- https://wiki.archlinux.org/title/Overlay_filesystem
- https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=overlayfs
CIS Controls
v8 - 4.8 Uninstall or Disable Unnecessary Services on Enterprise Assets and Software (IG 2, IG 3) v7 - 9.2 Ensure Only Approved Ports, Protocols and Services Are Running (IG 2, IG 3)
MITRE ATT&CK Mappings: T1005, T1005.000 | TA0005 | M1050