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1.1.1.7 Ensure squashfs kernel module is not available
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 to unload and disable the squashfs kernel module. This can also be done by running the script included below.Run the following commands to unload the squashfs kernel module:
# modprobe -r squashfs 2>/dev/null
# rmmod squashfs 2>/dev/null
Perform the following to disable the squashfs kernel module:
Create a file ending inconf with install squashfs /bin/false in the /etc/modprobe.d/ directory
Example:
# printf '\n%s\n' "install squashfs /bin/false" >> squashfs.conf
Create a file ending inconf with blacklist squashfs in the /etc/modprobe.d/ directory
Example:
# printf '\n%s\n' "blacklist squashfs" >> squashfs.conf
Optional remediation script:
This script will perform the above remediation as required by the system
#!/usr/bin/env bash
{
a_output2=() a_output3=() l_dl="" l_mod_name="squashfs" l_mod_type="fs"
l_mod_path="$(readlink -f /usr/lib/modules/**/kernel/$l_mod_type || readlink -f /lib/modules/**/kernel/$l_mod_type)"
f_module_fix()
{
l_dl="y" a_showconfig=()
while IFS= read -r l_showconfig; do
a_showconfig+=("$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 "$l_mod_name" 2>/dev/null
fi
if ! grep -Pq -- '\binstall\h+'"${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\n' "install $l_mod_chk_name $(readlink -f /bin/false)" >> /etc/modprobe.d/"$l_mod_name".conf
fi
if ! grep -Pq -- '\bblacklist\h+'"${l_mod_chk_name//-/_}"'\b' <<< "${a_showconfig[*]}"; then
a_output2+=(" - denylisting kernel module: \"$l_mod_name\"")
printf '%s\n' "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}"
[ "$l_dl" != "y" ] && f_module_fix
else
printf '%s\n' " - kernel module: \"$l_mod_name\" doesn't exist in \"$l_mod_base_directory\""
fi
done
[ "${#a_output3[@]}" -gt 0 ] && printf '%s\n' "" " -- INFO --" " - module: \"$l_mod_name\" exists in:" "${a_output3[@]}"
[ "${#a_output2[@]}" -gt 0 ] && printf '%s\n' "" "${a_output2[@]}" || printf '%s\n' "" " - No changes needed"
printf '%s\n' "" " - remediation of kernel module: \"$l_mod_name\" complete" ""
}
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
Item Details
Audit Name: CIS SUSE Linux Enterprise 15 v2.0.1 L2 Server
Category: CONFIGURATION MANAGEMENT
References: 800-53|CM-6, 800-53|CM-7, CSCv7|9.2
Plugin: Unix
Control ID: 9a5922a4e7a6493e209866ff257a89986356e226ffa9f548afa469f1672bc839