Configure L1 Terminal Fault mitigations

An XCCDF Rule

Description

L1 Terminal Fault (L1TF) is a hardware vulnerability which allows unprivileged speculative access to data which is available in the Level 1 Data Cache when the page table entry isn't present. Select the appropriate mitigation by adding the argument l1tf= to the default GRUB 2 command line for the Linux operating system. To ensure that l1tf= is added as a kernel command line argument to newly installed kernels, add l1tf= to the default Grub2 command line for Linux operating systems. Modify the line within /etc/default/grub as shown below:

GRUB_CMDLINE_LINUX="... l1tf= ..."

Run the following command to update command line for already installed kernels:

# grubby --update-kernel=ALL --args="l1tf="

Since Linux Kernel 4.19 you can check the L1TF vulnerability state with the following command: cat /sys/devices/system/cpu/vulnerabilities/l1tf

warning alert: Performance Warning

Enabling L1TF mitigations may impact performance of the system.

Rationale

The L1TF vulnerability allows an attacker to bypass memory access security controls imposed by the system or hypervisor. The L1TF vulnerability allows read access to any physical memory location that is cached in the L1 Data Cache.

ID: xccdf_org.ssgproject.content_rule_grub2_l1tf_argument
Severity: High
References: BP28(R8)

CCE-90775-8
Updated: about 1 year ago

[customizations.kernel]
append = "l1tf=<xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_var_l1tf_options" use="legacy"/>"

- name: Gather the package facts
  package_facts:
    manager: auto
  tags:
  - CCE-90775-8
  - grub2_l1tf_argument  - high_severity
  - low_disruption
  - medium_complexity
  - reboot_required
  - restrict_strategy
- name: XCCDF Value var_l1tf_options # promote to variable
  set_fact:
    var_l1tf_options: !!str <xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_var_l1tf_options" use="legacy"/>
  tags:
    - always

- name: Check l1tf argument exists
  command: grep '^\s*GRUB_CMDLINE_LINUX=.*l1tf=' /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:
  - CCE-90775-8
  - grub2_l1tf_argument
  - high_severity
  - low_disruption
  - medium_complexity
  - reboot_required
  - restrict_strategy

- name: Check l1tf 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:
  - CCE-90775-8
  - grub2_l1tf_argument
  - high_severity
  - low_disruption
  - medium_complexity
  - reboot_required
  - restrict_strategy

- name: Add l1tf argument
  ansible.builtin.lineinfile:
    line: GRUB_CMDLINE_LINUX="l1tf={{ var_l1tf_options }} "
    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:
  - CCE-90775-8
  - grub2_l1tf_argument
  - high_severity
  - low_disruption
  - medium_complexity
  - reboot_required
  - restrict_strategy

- name: Replace existing l1tf argument
  replace:
    path: /etc/default/grub
    regexp: l1tf=[a-zA-Z0-9,]+
    replace: l1tf={{ var_l1tf_options }}
  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:
  - CCE-90775-8
  - grub2_l1tf_argument
  - high_severity
  - low_disruption
  - medium_complexity
  - reboot_required
  - restrict_strategy

- name: Add l1tf argument
  replace:
    path: /etc/default/grub
    regexp: (^\s*GRUB_CMDLINE_LINUX=.*)"
    replace: \1 l1tf={{ var_l1tf_options }}"
  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:
  - CCE-90775-8
  - grub2_l1tf_argument
  - high_severity
  - low_disruption
  - medium_complexity
  - reboot_required
  - restrict_strategy

- name: Update grub defaults and the bootloader menu
  command: /sbin/grubby --update-kernel=ALL --args="l1tf={{ var_l1tf_options }}"
  when:
  - '"grub2-common" in ansible_facts.packages'
  - ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - CCE-90775-8
  - grub2_l1tf_argument
  - high_severity
  - low_disruption
  - medium_complexity
  - reboot_required
  - restrict_strategy

# Remediation is applicable only in certain platforms
if rpm --quiet -q grub2-common && { [ ! -f /.dockerenv ] && [ ! -f /run/.containerenv ]; }; then

var_l1tf_options='<xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_var_l1tf_options" use="legacy"/>'



# Correct the form of default kernel command line in GRUB
if grep -q '^\s*GRUB_CMDLINE_LINUX=.*l1tf=.*"'  '/etc/default/grub' ; then
       # modify the GRUB command-line if an l1tf= arg already exists
       sed -i "s/\(^\s*GRUB_CMDLINE_LINUX=\".*\)l1tf=[^[:space:]]\+\(.*\"\)/\1l1tf=$var_l1tf_options\2/"  '/etc/default/grub'
# Add to already existing GRUB_CMDLINE_LINUX parameters
elif grep -q '^\s*GRUB_CMDLINE_LINUX='  '/etc/default/grub' ; then
       # no l1tf=arg is present, append it
       sed -i "s/\(^\s*GRUB_CMDLINE_LINUX=\".*\)\"/\1 l1tf=$var_l1tf_options\"/"  '/etc/default/grub'
# Add GRUB_CMDLINE_LINUX parameters line
else
       echo "GRUB_CMDLINE_LINUX=\"l1tf=$var_l1tf_options\"" >> '/etc/default/grub'
fi
grubby --update-kernel=ALL --args=l1tf=$var_l1tf_options

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

Configure L1 Terminal Fault mitigations

Description

Rationale

Remediation - OS Build Blueprint

Remediation - Ansible

Remediation - Shell Script