kernel_lockdown(7) — Linux manual page

NAME | DESCRIPTION | VERSIONS | NOTES | COLOPHON

KERNEL_LOCKDOWN(7)        Linux Programmer's Manual       KERNEL_LOCKDOWN(7)

NAME         top

       kernel_lockdown - kernel image access prevention feature

DESCRIPTION         top

       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.

       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
         built-in and custom policies in secure boot lockdown mode.

VERSIONS         top

       The Kernel Lockdown feature was added in Linux 5.4.

NOTES         top

       The Kernel Lockdown feature is enabled by
       CONFIG_SECURITY_LOCKDOWN_LSM.  The lsm=lsm1,...,lsmN command line
       parameter controls the sequence of the initialization of Linux
       Security Modules.  It must contain the string 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 security= command line parameter and further to the value
       of CONFIG_LSM.

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                            2020-11-01               KERNEL_LOCKDOWN(7)