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1.1.1.6 Ensure squashfs kernel module is not available

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

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Information

The squashfs filesystem type is a compressed read-only Linux filesystem embedded in small footprint systems. A squashfs 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 udf module:

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

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

- IF - the squashfs 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="squashfs" # 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"
}

Impact:

As Snap packages utilize squashfs as a compressed filesystem, disabling squashfs will cause Snap packages to fail.

Snap application packages of software are self-contained and work across a range of Linux distributions. This is unlike traditional Linux package management approaches, like APT or RPM, which require specifically adapted packages per Linux distribution on an application update and delay therefore application deployment from developers to their software's end-user. Snaps themselves have no dependency on any external store ("App store"), can be obtained from any source and can be therefore used for upstream software deployment.

See Also

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

Item Details

References: CSCv7|9.2

Plugin: Unix

Control ID: d9ba7c65f026df8be69af90d6a43e6da2e27d0e2e8e7fe376d8aa2cdfda85734