Detections
Analytics
1.1.1.6 Ensure overlayfs kernel module is not available
Information
overlayfs 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.The overlayfs has known CVE's: CVE-2023-32629, CVE-2023-2640, CVE-2023-0386. Disabling the overlayfs 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.
Solution
Run the following script to unload and disable the overlayfs module:- IF - the overlayfs kernel module is available in ANY installed kernel:
- Create a file ending inconf with install overlayfs /bin/false in the /etc/modprobe.d/ directory
- Create a file ending inconf with blacklist overlayfs in the /etc/modprobe.d/ directory
- Run modprobe -r overlayfs 2>/dev/null; rmmod overlayfs 2>/dev/null to remove overlayfs from the kernel
- IF - the overlayfs kernel module is not available on the system, or pre-compiled into the kernel, no remediation is necessary
#!/usr/bin/env bash
{
a_output2=() a_output3=() l_dl="" l_mod_name="overlayfs" l_mod_type="fs"
l_mod_path="$(readlink -f /lib/modules/**/kernel/$l_mod_type | sort -u)"
f_module_fix()
{
l_dl="y" a_showconfig=()
while IFS= read -r l_showconfig; do
a_showconfig+=("$l_showconfig")
done < <(modprobe --showconfig | grep -P -- 'b(install|blacklist)h+'"${l_mod_chk_name//-/_}"'b')
if lsmod | grep "$l_mod_chk_name" &> /dev/null; then
a_output2+=(" - unloading kernel module: \"$l_mod_name\"")
modprobe -r "$l_mod_chk_name" 2>/dev/null; rmmod "$l_mod_name" 2>/dev/null
fi
if ! grep -Pq -- 'binstallh+'"${l_mod_chk_name//-/_}"'h+(/usr)?/bin/(true|false)b' <<< "${a_showconfig[*]}"; then
a_output2+=(" - setting kernel module: \"$l_mod_name\" to \"$(readlink -f /bin/false)\"")
printf '%s
' "install $l_mod_chk_name $(readlink -f /bin/false)" >> /etc/modprobe.d/"$l_mod_name".conf
fi
if ! grep -Pq -- 'bblacklisth+'"${l_mod_chk_name//-/_}"'b' <<< "${a_showconfig[*]}"; then
a_output2+=(" - denylisting kernel module: \"$l_mod_name\"")
printf '%s
' "blacklist $l_mod_chk_name" >> /etc/modprobe.d/"$l_mod_name".conf
fi
}
for l_mod_base_directory in $l_mod_path; do # Check if the module exists on the system
if [ -d "$l_mod_base_directory/${l_mod_name/-//}" ] && [ -n "$(ls -A "$l_mod_base_directory/${l_mod_name/-//}")" ]; then
a_output3+=(" - \"$l_mod_base_directory\"")
l_mod_chk_name="$l_mod_name"
[[ "$l_mod_name" =~ overlay ]] && l_mod_chk_name="${l_mod_name::-2}"
[ "$l_dl" != "y" ] && f_module_fix
else
printf '%s
' " - kernel module: \"$l_mod_name\" doesn't exist in \"$l_mod_base_directory\""
fi
done
[ "${#a_output3[@]}" -gt 0 ] && printf '%s
' "" " -- INFO --" " - module: \"$l_mod_name\" exists in:" "${a_output3[@]}"
[ "${#a_output2[@]}" -gt 0 ] && printf '%s
' "" "${a_output2[@]}" || printf '%s
' "" " - No changes needed"
printf '%s
' "" " - remediation of kernel module: \"$l_mod_name\" complete" ""
}
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 overlayfs may severely disrupt containerization.
See Also
Item Details
Audit Name: CIS Ubuntu Linux 24.04 LTS v1.0.0 L2 Server
Category: CONFIGURATION MANAGEMENT
References: 800-53|CM-6, 800-53|CM-7, CSCv7|9.2
Plugin: Unix
Control ID: 1b7988f4cdb06280487d5886a45ac08c2dcfa7e4ea2b64aca5a015f5d9dcfcff