Configure the confidence in TPM for entropy

An XCCDF Rule

Description

The TPM security chip that is available in most modern systems has a hardware RNG. It is also used to feed the entropy pool, but generally not credited entropy. Use rng_core.default_quality in the kernel command line to set the trust level on the hardware generators. The trust level defines the amount of entropy to credit. A value of 0 tells the system not to trust the hardware random number generators available, and doesn't credit any entropy to the pool. A value of 1000 assigns full confidence in the generators, and credits all the entropy it provides to the pool. Note that the value of rng_core.default_quality is global, affecting the trust on all hardware random number generators. Select the appropriate confidence by adding the argument rng_core.default_quality= to the default GRUB 2 command line for the Linux operating system. Configure the default Grub2 kernel command line to contain rng_core.default_quality= as follows:

# grub2-editenv - set "$(grub2-editenv - list | grep kernelopts) rng_core.default_quality="

Rationale

A system may struggle to initialize its entropy pool and end up starving. Crediting entropy from the hardware number generators available in the system helps fill up the entropy pool.

ID: xccdf_org.ssgproject.content_rule_grub2_rng_core_default_quality_argument
Severity: Low
References: BP28(R8)
Updated: about 1 year ago

- name: Gather the package facts
  package_facts:
    manager: auto
  tags:
  - grub2_rng_core_default_quality_argument
  - low_disruption  - low_severity
  - medium_complexity
  - reboot_required
  - restrict_strategy
- name: XCCDF Value var_rng_core_default_quality # promote to variable
  set_fact:
    var_rng_core_default_quality: !!str <xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_var_rng_core_default_quality" use="legacy"/>
  tags:
    - always

- name: Check rng_core.default_quality argument exists
  command: grep '^\s*GRUB_CMDLINE_LINUX=.*rng_core.default_quality=' /etc/default/grub
  failed_when: false
  register: argcheck
  when:
  - '"grub2-common" in ansible_facts.packages'
  - ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - grub2_rng_core_default_quality_argument
  - low_disruption
  - low_severity
  - medium_complexity
  - reboot_required
  - restrict_strategy

- name: Check rng_core.default_quality argument exists
  command: grep '^\s*GRUB_CMDLINE_LINUX=' /etc/default/grub
  failed_when: false
  register: linecheck
  when:
  - '"grub2-common" in ansible_facts.packages'
  - ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - grub2_rng_core_default_quality_argument
  - low_disruption
  - low_severity
  - medium_complexity
  - reboot_required
  - restrict_strategy

- name: Add rng_core.default_quality argument
  ansible.builtin.lineinfile:
    line: GRUB_CMDLINE_LINUX="rng_core.default_quality={{ var_rng_core_default_quality
      }} "
    state: present
    dest: /etc/default/grub
    create: true
    mode: '0644'
  when:
  - '"grub2-common" in ansible_facts.packages'
  - ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  - argcheck is not skipped and linecheck is not skipped and argcheck.rc != 0 and
    linecheck.rc != 0
  tags:
  - grub2_rng_core_default_quality_argument
  - low_disruption
  - low_severity
  - medium_complexity
  - reboot_required
  - restrict_strategy

- name: Replace existing rng_core.default_quality argument
  replace:
    path: /etc/default/grub
    regexp: rng_core.default_quality=[a-zA-Z0-9,]+
    replace: rng_core.default_quality={{ var_rng_core_default_quality }}
  when:
  - '"grub2-common" in ansible_facts.packages'
  - ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  - argcheck is not skipped and linecheck is not skipped and argcheck.rc == 0 and
    linecheck.rc == 0
  tags:
  - grub2_rng_core_default_quality_argument
  - low_disruption
  - low_severity
  - medium_complexity
  - reboot_required
  - restrict_strategy

- name: Add rng_core.default_quality argument
  replace:
    path: /etc/default/grub
    regexp: (^\s*GRUB_CMDLINE_LINUX=.*)"
    replace: \1 rng_core.default_quality={{ var_rng_core_default_quality }}"
  when:
  - '"grub2-common" in ansible_facts.packages'
  - ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  - argcheck is not skipped and linecheck is not skipped and argcheck.rc != 0 and
    linecheck.rc == 0
  tags:
  - grub2_rng_core_default_quality_argument
  - low_disruption
  - low_severity
  - medium_complexity
  - reboot_required
  - restrict_strategy

- name: Update grub defaults and the bootloader menu
  command: /usr/sbin/update-grub
  when:
  - '"grub2-common" in ansible_facts.packages'
  - ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - grub2_rng_core_default_quality_argument
  - low_disruption
  - low_severity
  - medium_complexity
  - reboot_required
  - restrict_strategy

# Remediation is applicable only in certain platforms
if dpkg-query --show --showformat='${db:Status-Status}\n' 'grub2-common' 2>/dev/null | grep -q installed && { [ ! -f /.dockerenv ] && [ ! -f /run/.containerenv ]; }; then

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



# Correct the form of default kernel command line in GRUB
if grep -q '^\s*GRUB_CMDLINE_LINUX=.*rng_core.default_quality=.*"'  '/etc/default/grub' ; then
       # modify the GRUB command-line if an rng_core.default_quality= arg already exists
       sed -i "s/\(^\s*GRUB_CMDLINE_LINUX=\".*\)rng_core.default_quality=[^[:space:]]\+\(.*\"\)/\1rng_core.default_quality=$var_rng_core_default_quality\2/"  '/etc/default/grub'
# Add to already existing GRUB_CMDLINE_LINUX parameters
elif grep -q '^\s*GRUB_CMDLINE_LINUX='  '/etc/default/grub' ; then
       # no rng_core.default_quality=arg is present, append it
       sed -i "s/\(^\s*GRUB_CMDLINE_LINUX=\".*\)\"/\1 rng_core.default_quality=$var_rng_core_default_quality\"/"  '/etc/default/grub'
# Add GRUB_CMDLINE_LINUX parameters line
else
       echo "GRUB_CMDLINE_LINUX=\"rng_core.default_quality=$var_rng_core_default_quality\"" >> '/etc/default/grub'
fi
update-grub

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

Configure the confidence in TPM for entropy

Description

Rationale

Remediation - Ansible

Remediation - Shell Script