KERNEL LOCKDOWN

Section: Linux Programmer's Manual (7)
Updated: 2017-10-05
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NAME

Kernel Lockdown - Kernel image access prevention feature  

DESCRIPTION

The Kernel Lockdown feature is designed to prevent both direct and indirect access to a running kernel image, attempting to protect against unauthorised 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.

Lockdown is typically enabled during boot and may be terminated, if configured, by typing a special key combination on a directly attached physical keyboard.

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.

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 SysRq+x.  

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 builtin and custom policies in secure boot lockdown mode.


 

Index

NAME
DESCRIPTION
COVERAGE
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