Enable Kernel Parameter to Enforce DAC on Hardlinks

Description

To set the runtime status of the fs.protected_hardlinks kernel parameter, run the following command:

$ sudo sysctl -w fs.protected_hardlinks=1

To make sure that the setting is persistent, add the following line to a file in the directory /etc/sysctl.d:

fs.protected_hardlinks = 1

Rationale

By enabling this kernel parameter, users can no longer create soft or hard links to files which they do not own. Disallowing such hardlinks mitigate vulnerabilities based on insecure file system accessed by privileged programs, avoiding an exploitation vector exploiting unsafe use of open() or creat().

Remediation

Shell script

The following script can be run on the host to remediate the issue.

#!/bin/bash

# Remediation is applicable only in certain platforms
if rpm --quiet -q kernel-core; then

# Comment out any occurrences of fs.protected_hardlinks from /etc/sysctl.d/*.conf files

for f in /etc/sysctl.d/*.conf /run/sysctl.d/*.conf /usr/local/lib/sysctl.d/*.conf; do


  # skip systemd-sysctl symlink (/etc/sysctl.d/99-sysctl.conf -> /etc/sysctl.conf)
  if [[ "$(readlink -f "$f")" == "/etc/sysctl.conf" ]]; then continue; fi

  matching_list=$(grep -P '^(?!#).*[\s]*fs.protected_hardlinks.*$' $f | uniq )
  if ! test -z "$matching_list"; then
    while IFS= read -r entry; do
      escaped_entry=$(sed -e 's|/|\\/|g' <<< "$entry")
      # comment out "fs.protected_hardlinks" matches to preserve user data
      sed -i --follow-symlinks "s/^${escaped_entry}$/# &/g" $f
    done <<< "$matching_list"
  fi
done

#
# Set sysctl config file which to save the desired value
#

SYSCONFIG_FILE="/etc/sysctl.conf"


#
# Set runtime for fs.protected_hardlinks
#
if ! { rpm --quiet -q kernel rpm-ostree bootc && ! rpm --quiet -q openshift-kubelet && { [ -f "/run/.containerenv" ] || [ -f "/.containerenv" ]; }; } ; then
    /sbin/sysctl -q -n -w fs.protected_hardlinks="1"
fi

#
# If fs.protected_hardlinks present in /etc/sysctl.conf, change value to "1"
#	else, add "fs.protected_hardlinks = 1" to /etc/sysctl.conf
#

# Strip any search characters in the key arg so that the key can be replaced without
# adding any search characters to the config file.
stripped_key=$(sed 's/[\^=\$,;+]*//g' <<< "^fs.protected_hardlinks")

# shellcheck disable=SC2059
printf -v formatted_output "%s = %s" "$stripped_key" "1"

# If the key exists, change it. Otherwise, add it to the config_file.
# We search for the key string followed by a word boundary (matched by \>),
# so if we search for 'setting', 'setting2' won't match.
if LC_ALL=C grep -q -m 1 -i -e "^fs.protected_hardlinks\\>" "${SYSCONFIG_FILE}"; then
    escaped_formatted_output=$(sed -e 's|/|\\/|g' <<< "$formatted_output")
    LC_ALL=C sed -i --follow-symlinks "s/^fs.protected_hardlinks\\>.*/$escaped_formatted_output/gi" "${SYSCONFIG_FILE}"
else
    if [[ -s "${SYSCONFIG_FILE}" ]] && [[ -n "$(tail -c 1 -- "${SYSCONFIG_FILE}" || true)" ]]; then
        LC_ALL=C sed -i --follow-symlinks '$a'\\ "${SYSCONFIG_FILE}"
    fi
    cce="CCE-86689-7"
    printf '# Per %s: Set %s in %s\n' "${cce}" "${formatted_output}" "${SYSCONFIG_FILE}" >> "${SYSCONFIG_FILE}"
    printf '%s\n' "$formatted_output" >> "${SYSCONFIG_FILE}"
fi

else
    >&2 echo 'Remediation is not applicable, nothing was done'
fi

Ansible playbook

The following playbook can be run with Ansible to remediate the issue.

- name: Gather the package facts
  package_facts:
    manager: auto
  tags:
  - CCE-86689-7
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_fs_protected_hardlinks

- name: Enable Kernel Parameter to Enforce DAC on Hardlinks - Set fact for sysctl
    paths
  ansible.builtin.set_fact:
    sysctl_paths:
    - /etc/sysctl.d/
    - /run/sysctl.d/
    - /usr/local/lib/sysctl.d/
  when: '"kernel-core" in ansible_facts.packages'
  tags:
  - CCE-86689-7
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_fs_protected_hardlinks

- name: Enable Kernel Parameter to Enforce DAC on Hardlinks - Find all files that
    contain fs.protected_hardlinks
  ansible.builtin.shell:
    cmd: find -L {{ sysctl_paths | join(" ") }} -type f -name '*.conf' | xargs grep
      -HP '^\s*fs.protected_hardlinks\s*=\s*.*$'
  register: find_all_values
  check_mode: false
  changed_when: false
  failed_when: false
  when: '"kernel-core" in ansible_facts.packages'
  tags:
  - CCE-86689-7
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_fs_protected_hardlinks

- name: Enable Kernel Parameter to Enforce DAC on Hardlinks - Find all files that
    set fs.protected_hardlinks to correct value
  ansible.builtin.shell:
    cmd: find -L {{ sysctl_paths | join(" ") }} -type f -name '*.conf' | xargs grep
      -HP '^\s*fs.protected_hardlinks\s*=\s*1$'
  register: find_correct_value
  check_mode: false
  changed_when: false
  failed_when: false
  when: '"kernel-core" in ansible_facts.packages'
  tags:
  - CCE-86689-7
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_fs_protected_hardlinks

- name: Enable Kernel Parameter to Enforce DAC on Hardlinks - Comment out any occurrences
    of fs.protected_hardlinks from config files
  ansible.builtin.replace:
    path: '{{ item | split(":") | first }}'
    regexp: ^[\s]*fs.protected_hardlinks
    replace: '#fs.protected_hardlinks'
  loop: '{{ find_all_values.stdout_lines }}'
  when:
  - '"kernel-core" in ansible_facts.packages'
  - find_correct_value.stdout_lines | length == 0 or find_all_values.stdout_lines
    | length > find_correct_value.stdout_lines | length
  tags:
  - CCE-86689-7
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_fs_protected_hardlinks

- name: Enable Kernel Parameter to Enforce DAC on Hardlinks - Ensure sysctl fs.protected_hardlinks
    is set to 1
  ansible.posix.sysctl:
    name: fs.protected_hardlinks
    value: '1'
    sysctl_file: /etc/sysctl.conf
    state: present
    reload: true
  when: '"kernel-core" in ansible_facts.packages'
  tags:
  - CCE-86689-7
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_fs_protected_hardlinks