Disable Access to Network bpf() Syscall From Unprivileged Processes

An XCCDF Rule

Description

To prevent unprivileged processes from using the bpf() syscall the kernel.unprivileged_bpf_disabled kernel parameter must be set to 1 or 2. Writing 1 to this entry will disable unprivileged calls to bpf(); once disabled, calling bpf() without CAP_SYS_ADMIN or CAP_BPF will return -EPERM. Once set to 1, this can't be cleared from the running kernel anymore. To set the runtime status of the kernel.unprivileged_bpf_disabled kernel parameter, run the following command:

$ sudo sysctl -w kernel.unprivileged_bpf_disabled=1

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

kernel.unprivileged_bpf_disabled = 1

Writing 2 to this entry will also disable unprivileged calls to bpf(), however, an admin can still change this setting later on, if needed, by writing 0 or 1 to this entry. To set the runtime status of the kernel.unprivileged_bpf_disabled kernel parameter, run the following command:

$ sudo sysctl -w kernel.unprivileged_bpf_disabled=2

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

kernel.unprivileged_bpf_disabled = 2

Rationale

Loading and accessing the packet filters programs and maps using the bpf() syscall has the potential of revealing sensitive information about the kernel state.

ID: xccdf_org.ssgproject.content_rule_sysctl_kernel_unprivileged_bpf_disabled_accept_default
Severity: Medium
References: CCI-000366

AC-6 SC-7(10)

FMT_SMF_EXT.1

SRG-OS-000132-GPOS-00067 SRG-OS-000480-GPOS-00227
Updated: 5 days ago

- name: Gather the package facts
  package_facts:
    manager: auto
  tags:
  - NIST-800-53-AC-6
  - NIST-800-53-SC-7(10)  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_kernel_unprivileged_bpf_disabled_accept_default

- name: List /etc/sysctl.d/*.conf files
  find:
    paths:
    - /etc/sysctl.d/
    - /run/sysctl.d/
    - /usr/local/lib/sysctl.d/
    contains: ^[\s]*kernel.unprivileged_bpf_disabled.*$
    patterns: '*.conf'
    file_type: any
  register: find_sysctl_d
  when: '"kernel" in ansible_facts.packages'
  tags:
  - NIST-800-53-AC-6
  - NIST-800-53-SC-7(10)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_kernel_unprivileged_bpf_disabled_accept_default

- name: Comment out any occurrences of kernel.unprivileged_bpf_disabled from config
    files
  replace:
    path: '{{ item.path }}'
    regexp: ^[\s]*kernel.unprivileged_bpf_disabled
    replace: '#kernel.unprivileged_bpf_disabled'
  loop: '{{ find_sysctl_d.files }}'
  when: '"kernel" in ansible_facts.packages'
  tags:
  - NIST-800-53-AC-6
  - NIST-800-53-SC-7(10)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_kernel_unprivileged_bpf_disabled_accept_default
- name: XCCDF Value sysctl_kernel_unprivileged_bpf_disabled_value # promote to variable
  set_fact:
    sysctl_kernel_unprivileged_bpf_disabled_value: !!str <xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_sysctl_kernel_unprivileged_bpf_disabled_value" use="legacy"/>
  tags:
    - always

- name: Ensure sysctl kernel.unprivileged_bpf_disabled is set
  sysctl:
    name: kernel.unprivileged_bpf_disabled
    value: '{{ sysctl_kernel_unprivileged_bpf_disabled_value }}'
    sysctl_file: /etc/sysctl.conf
    state: present
    reload: true
  when: '"kernel" in ansible_facts.packages'
  tags:
  - NIST-800-53-AC-6
  - NIST-800-53-SC-7(10)
  - disable_strategy
  - low_complexity
  - medium_disruption
  - medium_severity
  - reboot_required
  - sysctl_kernel_unprivileged_bpf_disabled_accept_default

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

# Comment out any occurrences of kernel.unprivileged_bpf_disabled 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=$(grep -P '^(?!#).*[\s]*kernel.unprivileged_bpf_disabled.*$' $f | uniq )
  if ! test -z "$matching_list"; then
    while IFS= read -r entry; do
      escaped_entry=$(sed -e 's|/|\\/|g' <<< "$entry")
      # comment out "kernel.unprivileged_bpf_disabled" 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"

sysctl_kernel_unprivileged_bpf_disabled_value='<xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_sysctl_kernel_unprivileged_bpf_disabled_value" use="legacy"/>'


#
# Set runtime for kernel.unprivileged_bpf_disabled
#
/sbin/sysctl -q -n -w kernel.unprivileged_bpf_disabled="$sysctl_kernel_unprivileged_bpf_disabled_value"

#
# If kernel.unprivileged_bpf_disabled present in /etc/sysctl.conf, change value to appropriate value
#	else, add "kernel.unprivileged_bpf_disabled = value" 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' <<< "^kernel.unprivileged_bpf_disabled")

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

# 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 "^kernel.unprivileged_bpf_disabled\\>" "${SYSCONFIG_FILE}"; then
    escaped_formatted_output=$(sed -e 's|/|\\/|g' <<< "$formatted_output")
    LC_ALL=C sed -i --follow-symlinks "s/^kernel.unprivileged_bpf_disabled\\>.*/$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
    printf '%s\n' "$formatted_output" >> "${SYSCONFIG_FILE}"
fi

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

Disable Access to Network bpf() Syscall From Unprivileged Processes

Description

Rationale

Remediation - Ansible

Remediation - Shell Script