cis-ubuntu1804-v220-1-1-1-1

Ensure mounting of cramfs filesystems is disabled

Profile

Level 1 - Server / Level 1 - Workstation, Assessment: Automated

Description

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.

Rationale

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

Impact

None noted.

Audit Procedure

Command Line

Run the following script to verify the cramfs module is disabled:

#!/usr/bin/env bash
{
    l_output="" l_output2="" l_output3="" l_dl="" # Unset output variables
    l_mname="cramfs" # set module name
    l_mtype="fs" # set module type
    l_searchloc="/lib/modprobe.d/*.conf /usr/local/lib/modprobe.d/*.conf /run/modprobe.d/*.conf /etc/modprobe.d/*.conf"
    l_mpath="/lib/modules/**/kernel/$l_mtype"
    l_mpname="$(tr '-' ' ' <<< "$l_mname")"
    l_mndir="$(tr '-' '/' <<< "$l_mname")"

    module_loadable_chk()
    {
        # Check if the module is currently loadable
        l_loadable="$(modprobe -n -v "$l_mname")"
        [ "$(wc -l <<< "$l_loadable")" -gt "1" ] && l_loadable="$(grep -P -- "(^\h*install|\b$l_mname)\b" <<< "$l_loadable")"
        if grep -Pq -- '^\h*install \/bin\/(true|false)' <<< "$l_loadable"; then
            l_output="$l_output\n - module: \"$l_mname\" is not loadable: \"$l_loadable\""
        else
            l_output2="$l_output2\n - module: \"$l_mname\" is loadable: \"$l_loadable\""
        fi
    }
    module_loaded_chk()
    {
        # Check if the module is currently loaded
        if ! lsmod | grep "$l_mname" > /dev/null 2>&1; then
            l_output="$l_output\n - module: \"$l_mname\" is not loaded"
        else
            l_output2="$l_output2\n - module: \"$l_mname\" is loaded"
        fi
    }
    module_deny_chk()
    {
        # Check if the module is deny listed
        l_dl="y"
        if modprobe --showconfig | grep -Pq -- '^\h*blacklist\h+'"$l_mpname"'\b'; then
            l_output="$l_output\n - module: \"$l_mname\" is deny listed in: \"$(grep -Pls -- "^\h*blacklist\h+$l_mname\b" $l_searchloc)\""
        else
            l_output2="$l_output2\n - module: \"$l_mname\" is not deny listed"
        fi
    }
    # Check if the module exists on the system
    for l_mdir in $l_mpath; do
        if [ -d "$l_mdir/$l_mndir" ] && [ -n "$(ls -A $l_mdir/$l_mndir)" ]; then
            l_output3="$l_output3\n  - \"$l_mdir\""
            [ "$l_dl" != "y" ] && module_deny_chk
            if [ "$l_mdir" = "/lib/modules/$(uname -r)/kernel/$l_mtype" ]; then
                module_loadable_chk
                module_loaded_chk
            fi
        else
            l_output="$l_output\n - module: \"$l_mname\" doesn't exist in \"$l_mdir\""
        fi
    done
    # Report results. If no failures output in l_output2, we pass
    [ -n "$l_output3" ] && echo -e "\n\n -- INFO --\n - module: \"$l_mname\" exists in:$l_output3"
    if [ -z "$l_output2" ]; then
        echo -e "\n- Audit Result:\n  ** PASS **\n$l_output\n"
    else
        echo -e "\n- Audit Result:\n  ** FAIL **\n - Reason(s) for audit failure:\n$l_output2\n"
        [ -n "$l_output" ] && echo -e "\n- Correctly set:\n$l_output\n"
    fi
}

Expected Result

• module: "cramfs" is not loadable
• module: "cramfs" is not loaded
• module: "cramfs" is deny listed

Remediation

Command Line

Run the following script to disable the cramfs module:

#!/usr/bin/env bash
{
    l_mname="cramfs" # set module name
    l_mtype="fs" # set module type
    l_mpath="/lib/modules/**/kernel/$l_mtype"
    l_mpname="$(tr '-' ' ' <<< "$l_mname")"
    l_mndir="$(tr '-' '/' <<< "$l_mname")"

    module_loadable_fix()
    {
        # If the module is currently loadable, add "install {MODULE_NAME} /bin/false" to a file in "/etc/modprobe.d"
        l_loadable="$(modprobe -n -v "$l_mname")"
        [ "$(wc -l <<< "$l_loadable")" -gt "1" ] && l_loadable="$(grep -P -- "(^\h*install|\b$l_mname)\b" <<< "$l_loadable")"
        if ! grep -Pq -- '^\h*install \/bin\/(true|false)' <<< "$l_loadable"; then
            echo -e "\n - setting module: \"$l_mname\" to be not loadable"
            echo -e "install $l_mname /bin/false" >> /etc/modprobe.d/"$l_mpname".conf
        fi
    }
    module_loaded_fix()
    {
        # If the module is currently loaded, unload the module
        if lsmod | grep "$l_mname" > /dev/null 2>&1; then
            echo -e "\n - unloading module \"$l_mname\""
            modprobe -r "$l_mname"
        fi
    }
    module_deny_fix()
    {
        # If the module isn't deny listed, denylist the module
        if ! modprobe --showconfig | grep -Pq -- "^\h*blacklist\h+$l_mpname\b"; then
            echo -e "\n - deny listing \"$l_mname\""
            echo -e "blacklist $l_mname" >> /etc/modprobe.d/"$l_mpname".conf
        fi
    }
    # Check if the module exists on the system
    for l_mdir in $l_mpath; do
        if [ -d "$l_mdir/$l_mndir" ] && [ -n "$(ls -A $l_mdir/$l_mndir)" ]; then
            echo -e "\n - module: \"$l_mname\" exists in \"$l_mdir\"\n - checking if disabled..."
            module_deny_fix
            if [ "$l_mdir" = "/lib/modules/$(uname -r)/kernel/$l_mtype" ]; then
                module_loadable_fix
                module_loaded_fix
            fi
        else
            echo -e "\n - module: \"$l_mname\" doesn't exist in \"$l_mdir\"\n"
        fi
    done
    echo -e "\n - remediation of module: \"$l_mname\" complete\n"
}

Default Value

Not disabled by default.

References

1. NIST SP 800-53 Rev. 5: CM-7

CIS Controls

v8 - 4.8 Uninstall or Disable Unnecessary Services on Enterprise Assets and Software (IG 2, IG 3) v7 - 9.2 Ensure Only Approved Ports, Protocols and Services Are Running (IG 2, IG 3)

MITRE ATT&CK Mappings: T1005, T1005.000 (TA0005) - M1050