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1.1.1.1 Ensure cramfs kernel module is not available

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 inconf with install cramfs /bin/false in the /etc/modprobe.d/ directory
 - Create a file ending inconf 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

{
 a_output2=() a_output3=() l_dl="" l_mod_name="cramfs" 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 "$l_mod_chk_name" &amp;> /dev/null; then
 a_output2+=(" - unloading kernel module: \"$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/${l_mod_name/-//}" ] &amp;&amp; [ -n "$(ls -A "$l_mod_base_directory/${l_mod_name/-//}")" ]; then
 a_output3+=(" - \"$l_mod_base_directory\"")
 l_mod_chk_name="$l_mod_name"
 [[ "$l_mod_name" =~ overlay ]] &amp;&amp; l_mod_chk_name="${l_mod_name::-2}"
 [ "$l_dl" != "y" ] &amp;&amp; f_module_fix
 else
 printf '%s
' " - kernel module: \"$l_mod_name\" doesn't exist in \"$l_mod_base_directory\""
 fi
 done
 [ "${#a_output3[@]}" -gt 0 ] &amp;&amp; printf '%s
' "" " -- INFO --" " - module: \"$l_mod_name\" exists in:" "${a_output3[@]}"
 [ "${#a_output2[@]}" -gt 0 ] &amp;&amp; printf '%s
' "" "${a_output2[@]}" || printf '%s
' "" " - No changes needed"
 printf '%s
' "" " - remediation of kernel module: \"$l_mod_name\" complete" ""
}

See Also

https://workbench.cisecurity.org/benchmarks/18960

Item Details

Category: CONFIGURATION MANAGEMENT

References: 800-53|CM-6, 800-53|CM-7, CSCv7|9.2

Plugin: Unix

Control ID: 4e1ab929d63a48887fd283360f25dbcf0a07cd832bec3b5af2a5327ac0a0ee0e