The Kernel Lockdown feature is designed to prevent both direct
and indirect access to a running kernel image, attempting to
protect against unauthorized modification of the kernel image and
to prevent access to security and cryptographic data located in
kernel memory, whilst still permitting driver modules to be
loaded.
If a prohibited or restricted feature is accessed or used, the
kernel will emit a message that looks like:
Lockdown: X: Y is restricted, see man kernel_lockdown.7
where X indicates the process name and Y indicates what is
restricted.
On an EFI-enabled x86 or arm64 machine, lockdown will be
automatically enabled if the system boots in EFI Secure Boot
mode.
Coverage
When lockdown is in effect, a number of features are disabled or
have their use restricted. This includes special device files
and kernel services that allow direct access of the kernel image:
/dev/mem
/dev/kmem
/dev/kcore
/dev/ioports
BPF
kprobes
and the ability to directly configure and control devices, so as
to prevent the use of a device to access or modify a kernel
image:
• The use of module parameters that directly specify hardware
parameters to drivers through the kernel command line or when
loading a module.
• The use of direct PCI BAR access.
• The use of the ioperm and iopl instructions on x86.
• The use of the KD*IO console ioctls.
• The use of the TIOCSSERIAL serial ioctl.
• The alteration of MSR registers on x86.
• The replacement of the PCMCIA CIS.
• The overriding of ACPI tables.
• The use of ACPI error injection.
• The specification of the ACPI RDSP address.
• The use of ACPI custom methods.
Certain facilities are restricted:
• Only validly signed modules may be loaded (waived if the module
file being loaded is vouched for by IMA appraisal).
• Only validly signed binaries may be kexec'd (waived if the
binary image file to be executed is vouched for by IMA
appraisal).
• Unencrypted hibernation/suspend to swap are disallowed as the
kernel image is saved to a medium that can then be accessed.
• Use of debugfs is not permitted as this allows a whole range of
actions including direct configuration of, access to and
driving of hardware.
• IMA requires the addition of the "secure_boot" rules to the
policy, whether or not they are specified on the command line,
for both the built-in and custom policies in secure boot
lockdown mode.
