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1.1.1.1 Ensure cramfs kernel module is not available

Warning! Audit Deprecated

This audit has been deprecated and will be removed in a future update.

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Information

The cramfs filesystem type is a compressed read-only Linux filesystem embedded in small footprint systems. A cramfs image can be used without having to first decompress the image.

Removing support for unneeded filesystem types reduces the local attack surface of the system. If this filesystem type is not needed, disable it.

Solution

Run the following script to unload and disable the cramfs module:

- IF - the cramfs kernel module is available in ANY installed kernel:

 - Create a file ending in .conf with install cramfs /bin/false in the /etc/modprobe.d/ directory
 - Create a file ending in .conf with blacklist cramfs in the /etc/modprobe.d/ directory
 - Run modprobe -r cramfs 2>/dev/null; rmmod cramfs 2>/dev/null to remove cramfs from the kernel

- IF - the cramfs kernel module is not available on the system, or pre-compiled into the kernel, no remediation is necessary

#!/usr/bin/env bash

{
 unset a_output2; l_output3="" l_dl="" # unset arrays and clear variables
 l_mod_name="cramfs" # set module name
 l_mod_type="fs" # set module type
 l_mod_path="$(readlink -f /lib/modules/**/kernel/$l_mod_type | sort -u)"
 f_module_fix()
 {
 l_dl="y" # Set to ignore duplicate checks
 a_showconfig=() # Create array with modprobe output
 while IFS= read -r l_showconfig; do
 a_showconfig+=("$l_showconfig")
 done < <(modprobe --showconfig | grep -P -- '\b(install|blacklist)\h+'"${l_mod_name//-/_}"'\b')
 if lsmod | grep "$l_mod_name" &> /dev/null; then # Check if the module is currently loaded
 a_output2+=(" - unloading kernel module: \"$l_mod_name\"")
 modprobe -r "$l_mod_name" 2>/dev/null; rmmod "$l_mod_name" 2>/dev/null
 fi
 if ! grep -Pq -- '\binstall\h+'"${l_mod_name//-/_}"'\h+\/bin\/(true|false)\b' <<< "${a_showconfig[*]}"; then
 a_output2+=(" - setting kernel module: \"$l_mod_name\" to \"/bin/false\"")
 printf '%s\n' "install $l_mod_name /bin/false" >> /etc/modprobe.d/"$l_mod_name".conf
 fi
 if ! grep -Pq -- '\bblacklist\h+'"${l_mod_name//-/_}"'\b' <<< "${a_showconfig[*]}"; then
 a_output2+=(" - denylisting kernel module: \"$l_mod_name\"")
 printf '%s\n' "blacklist $l_mod_name" >> /etc/modprobe.d/"$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
 l_output3="$l_output3\n - \"$l_mod_base_directory\""
 [[ "$l_mod_name" =~ overlay ]] && l_mod_name="${l_mod_name::-2}"
 [ "$l_dl" != "y" ] && f_module_fix
 else
 echo -e " - kernel module: \"$l_mod_name\" doesn't exist in \"$l_mod_base_directory\""
 fi
 done
 [ -n "$l_output3" ] && echo -e "\n\n -- INFO --\n - module: \"$l_mod_name\" exists in:$l_output3"
 [ "${#a_output2[@]}" -gt 0 ] && printf '%s\n' "${a_output2[@]}"
 echo -e "\n - remediation of kernel module: \"$l_mod_name\" complete\n"
}

See Also

https://workbench.cisecurity.org/benchmarks/18208

Item Details

References: CSCv7|9.2

Plugin: Unix

Control ID: cdd2899a830dd9efe0b6f3250337295c73027c95af1faeef2a25b780fab90d7c