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1.1.1.7 Ensure udf kernel module is not available

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Information

The udf filesystem type is the universal disk format used to implement ISO/IEC 13346 and ECMA-167 specifications. This is an open vendor filesystem type for data storage on a broad range of media. This filesystem type is necessary to support writing DVDs and newer optical disc formats.

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 udf kernel module is available in ANY installed kernel:

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

- IF - the udf 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="udf" # 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:

Microsoft Azure requires the usage of udf.

udf should not be disabled on systems run on Microsoft Azure.

See Also

https://workbench.cisecurity.org/benchmarks/18211

Item Details

References: CSCv7|9.2

Plugin: Unix

Control ID: 886b64f157a1991be81030f69ddefe88c2f170dd73946ae67804099aff9d9d23