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.TH KERNEL_LOCKDOWN 7 2020-11-01 Linux "Linux Programmer's Manual"
.SH NAME
kernel_lockdown \- kernel image access prevention feature
.SH DESCRIPTION
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.
.PP
Lockdown is typically enabled during boot and may be terminated, if configured,
by typing a special key combination on a directly attached physical keyboard.
.PP
If a prohibited or restricted feature is accessed or used, the kernel will emit
a message that looks like:
.PP
.RS
 Lockdown: X: Y is restricted, see man kernel_lockdown.7
.RE
.PP
where X indicates the process name and Y indicates what is restricted.
.PP
On an EFI-enabled x86 or arm64 machine, lockdown will be automatically enabled
if the system boots in EFI Secure Boot mode.
.PP
If the kernel is appropriately configured, lockdown may be lifted by typing
the appropriate sequence on a directly attached physical keyboard.
For x86 machines, this is
.IR SysRq+x .
.\"
.SS 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:
.PP
.RS
/dev/mem
.br
/dev/kmem
.br
/dev/kcore
.br
/dev/ioports
.br
BPF
.br
kprobes
.RE
.PP
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:
.IP \(bu 2
The use of module parameters that directly specify hardware parameters to
drivers through the kernel command line or when loading a module.
.IP \(bu
The use of direct PCI BAR access.
.IP \(bu
The use of the ioperm and iopl instructions on x86.
.IP \(bu
The use of the KD*IO console ioctls.
.IP \(bu
The use of the TIOCSSERIAL serial ioctl.
.IP \(bu
The alteration of MSR registers on x86.
.IP \(bu
The replacement of the PCMCIA CIS.
.IP \(bu
The overriding of ACPI tables.
.IP \(bu
The use of ACPI error injection.
.IP \(bu
The specification of the ACPI RDSP address.
.IP \(bu
The use of ACPI custom methods.
.PP
Certain facilities are restricted:
.IP \(bu 2
Only validly signed modules may be loaded (waived if the module file being
loaded is vouched for by IMA appraisal).
.IP \(bu
Only validly signed binaries may be kexec'd (waived if the binary image file
to be executed is vouched for by IMA appraisal).
.IP \(bu
Unencrypted hibernation/suspend to swap are disallowed as the kernel image is
saved to a medium that can then be accessed.
.IP \(bu
Use of debugfs is not permitted as this allows a whole range of actions
including direct configuration of, access to and driving of hardware.
.IP \(bu
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.
.SH VERSIONS
The Kernel Lockdown feature was added in Linux 5.4.
.SH NOTES
The Kernel Lockdown feature is enabled by CONFIG_SECURITY_LOCKDOWN_LSM.
The
.I lsm=lsm1,...,lsmN
command line parameter controls the sequence of the initialization of
Linux Security Modules.
It must contain the string
.I lockdown
to enable the Kernel Lockdown feature.
If the command line parameter is not specified,
the initialization falls back to the value of the deprecated
.I security=
command line parameter and further to the value of CONFIG_LSM.
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