Chapter 3. Managing kernel modules

The following sections explain what kernel modules are, how to display their information, and how to perform basic administrative tasks with kernel modules.

3.1. Introduction to kernel modules

The Red Hat Enterprise Linux kernel can be extended with optional, additional pieces of functionality, called kernel modules, without having to reboot the system. On Red Hat Enterprise Linux 8, kernel modules are extra kernel code which is built into compressed <KERNEL_MODULE_NAME>.ko.xz object files.

The most common functionality enabled by kernel modules are:

  • Device driver which adds support for new hardware
  • Support for a file system such as GFS2 or NFS
  • System calls

On modern systems, kernel modules are automatically loaded when needed. However, in some cases it is necessary to load or unload modules manually.

Like the kernel itself, the modules can take parameters that customize their behavior if needed.

Tooling is provided to inspect which modules are currently running, which modules are available to load into the kernel and which parameters a module accepts. The tooling also provides a mechanism to load and unload kernel modules into the running kernel.

3.2. Introduction to bootloader specification

The BootLoader Specification (BLS) defines a scheme and the file format to manage the bootloader configuration for each boot option in the drop-in directory without the need to manipulate the bootloader configuration files. Unlike earlier approaches, each boot entry is now represented by a separate configuration file in the drop-in directory. The drop-in directory extends its configuration without having the need to edit or regenerate the configuration files. The BLS extends this concept for the boot menu entries.

Using BLS, you can manage the bootloader menu options by adding, removing, or editing individual boot entry files in a directory. This makes the kernel installation process significantly simpler and consistent across the different architectures.

The grubby tool is a thin wrapper script around the BLS and it supports the same grubby arguments and options. It runs the dracut to create an initial ramdisk image. With this setup, the core bootloader configuration files are static and are not modified after kernel installation.

This premise is particularly relevant in Red Hat Enterprise Linux 8 because the same bootloader is not used in all architectures. GRUB2 is used in most of them such as the 64-bit ARM, but little-endian variants of IBM Power Systems with Open Power Abstraction Layer (OPAL) uses Petitboot and the IBM Z architecture uses zipl.

Additional Resources

3.3. Kernel module dependencies

Certain kernel modules sometimes depend on one or more other kernel modules. The /lib/modules/<KERNEL_VERSION>/modules.dep file contains a complete list of kernel module dependencies for the respective kernel version.

The dependency file is generated by the depmod program, which is a part of the kmod package. Many of the utilities provided by kmod take module dependencies into account when performing operations so that manual dependency-tracking is rarely necessary.

Warning

The code of kernel modules is executed in kernel-space in the unrestricted mode. Because of this, you should be mindful of what modules you are loading.

Additional resources

  • For more information about /lib/modules/<KERNEL_VERSION>/modules.dep, refer to the modules.dep(5) manual page.
  • For further details including the synopsis and options of depmod, see the depmod(8) manual page.

3.4. Listing currently loaded kernel modules

The following procedure describes how to view the currently loaded kernel modules.

Prerequisites

  • The kmod package is installed.

Procedure

  • To list all currently loaded kernel modules, execute: 

    $ lsmod

Module                  Size  Used by
fuse                  126976  3
uinput                 20480  1
xt_CHECKSUM            16384  1
ipt_MASQUERADE         16384  1
xt_conntrack           16384  1
ipt_REJECT             16384  1
nft_counter            16384  16
nf_nat_tftp            16384  0
nf_conntrack_tftp      16384  1 nf_nat_tftp
tun                    49152  1
bridge                192512  0
stp                    16384  1 bridge
llc                    16384  2 bridge,stp
nf_tables_set          32768  5
nft_fib_inet           16384  1
…​

    In the example above:

    • The first column provides the names of currently loaded modules.
    • The second column displays the amount of memory per module in kilobytes.
    • The last column shows the number, and optionally the names of modules that are dependent on a particular module.

Additional resources

  • For more information about kmod, refer to the /usr/share/doc/kmod/README file or the lsmod(8) manual page.

3.5. Listing all installed kernels

The following procedure describes how to use the command line tool grubby to list the GRUB2 boot entries.

Procedure

To list the boot entries of the kernel:

  • To list the boot entries of the kernel, execute:

    # grubby --info=ALL | grep title

    The command displays the boot entries of the kernel. The kernel field shows the kernel path.

    The following procedure describes how to use grubby utility to list all installed kernels in their systems using the kernel command line.

As an example, consider listing grubby-8.40-17, from the Grub2 menu on both the BLS and non-BLS installs.

Procedure

To list all installed kernel modules:

  • Execute the following command:

    # grubby --info=ALL | grep title

    The list of all installed kernels is displayed as follows: 

    title=Red Hat Enterprise Linux (4.18.0-20.el8.x86_64) 8.0 (Ootpa)
title=Red Hat Enterprise Linux (4.18.0-19.el8.x86_64) 8.0 (Ootpa)
title=Red Hat Enterprise Linux (4.18.0-12.el8.x86_64) 8.0 (Ootpa)
title=Red Hat Enterprise Linux (4.18.0) 8.0 (Ootpa)
title=Red Hat Enterprise Linux (0-rescue-2fb13ddde2e24fde9e6a246a942caed1) 8.0 (Ootpa)

The above output displays the list of all installed kernels for grubby-8.40-17, using the Grub2 menu.

3.6. Setting a kernel as default

The following procedure describes how to set a specific kernel as default using the grubby command-line tool and GRUB2.

Procedure

Setting the kernel as default, using the grubby tool
  • Execute the following command to set the kernel as default using the grubby tool:

# grubby --set-default $kernel_path

The command uses a machine ID without the .conf suffix as an argument.

Note

The machine ID is located in the /boot/loader/entries/ directory.

Setting the kernel as default, using the id argument
  • List the boot entries using the id argument and then set an intended kernel as default: 
# grubby --info ALL | grep id
# grubby --set-default /boot/vmlinuz-<version>.<architecture>
Note

To list the boot entries using the title argument, execute the # grubby --info=ALL | grep title command.

Setting the default kernel for only the next boot
  • Execute the following command to set the default kernel for only the next reboot using the grub2-reboot command: 
# grub2-reboot <index|title|id>
Warning

Set the default kernel for only the next boot with care. Installing new kernel RPM’s, self-built kernels, and manually adding the entries to the /boot/loader/entries/ directory may change the index values.

3.7. Displaying information about kernel modules

When working with a kernel module, you may want to see further information about that module. This procedure describes how to display extra information about kernel modules.

Prerequisites

  • The kmod package is installed.

Procedure

  • To display information about any kernel module, execute: 

    $ modinfo <KERNEL_MODULE_NAME>

For example:
$ modinfo virtio_net

filename:       /lib/modules/4.18.0-94.el8.x86_64/kernel/drivers/net/virtio_net.ko.xz
license:        GPL
description:    Virtio network driver
rhelversion:    8.1
srcversion:     2E9345B281A898A91319773
alias:          virtio:d00000001v*
depends:        net_failover
intree:         Y
name:           virtio_net
vermagic:       4.18.0-94.el8.x86_64 SMP mod_unload modversions
…​
parm:           napi_weight:int
parm:           csum:bool
parm:           gso:bool
parm:           napi_tx:bool

    The modinfo command displays some detailed information about the specified kernel module. You can query information about all available modules, regardless of whether they are loaded or not. The parm entries show parameters the user is able to set for the module, and what type of value they expect.

    Note

    When entering the name of a kernel module, do not append the .ko.xz extension to the end of the name. Kernel module names do not have extensions; their corresponding files do.

Additional resources

  • For more information about the modinfo, refer to the modinfo(8) manual page.

3.8. Loading kernel modules at system runtime

The optimal way to expand the functionality of the Linux kernel is by loading kernel modules. The following procedure describes how to use the modprobe command to find and load a kernel module into the currently running kernel.

Prerequisites

  • Root permissions
  • The kmod package is installed.
  • The respective kernel module is not loaded. To ensure this is the case, list the loaded kernel modules.

Procedure

  1. Select a kernel module you want to load.

    The modules are located in the /lib/modules/$(uname -r)/kernel/<SUBSYSTEM>/ directory.

  2. Load the relevant kernel module: 

    # modprobe <MODULE_NAME>
    Note

    When entering the name of a kernel module, do not append the .ko.xz extension to the end of the name. Kernel module names do not have extensions; their corresponding files do.

  3. Optionally, verify the relevant module was loaded: 

    $ lsmod | grep <MODULE_NAME>

    If the module was loaded correctly, this command displays the relevant kernel module. For example: 

    $ lsmod | grep serio_raw
serio_raw              16384  0
Important

The changes described in this procedure will not persist after rebooting the system.

Additional resources

  • For further details about modprobe, see the modprobe(8) manual page.

3.9. Unloading kernel modules at system runtime

At times, you find that you need to unload certain kernel modules from the running kernel. The following procedure describes how to use the modprobe command to find and unload a kernel module at system runtime from the currently loaded kernel.

Prerequisites

  • Root permissions
  • The kmod package is installed.

Procedure

  1. Execute the lsmod command and select a kernel module you want to unload.

    If a kernel module has dependencies, unload those prior to unloading the kernel module. For details on identifying modules with dependencies, see Section 3.4, “Listing currently loaded kernel modules”.

  2. Unload the relevant kernel module: 

    # modprobe -r <MODULE_NAME>

    When entering the name of a kernel module, do not append the .ko.xz extension to the end of the name. Kernel module names do not have extensions; their corresponding files do.

    Warning

    Do not unload kernel modules when they are used by the running system. Doing so can lead to an unstable or non-operational system.

  3. Optionally, verify the relevant module was unloaded: 

    $ lsmod | grep <MODULE_NAME>

    If the module was unloaded successfully, this command does not display any output.

Important

After finishing this procedure, the kernel modules that are defined to be automatically loaded on boot, will not stay unloaded after rebooting the system. For information on how to counter this outcome, see Preventing kernel modules from being automatically loaded at system boot time.

Additional resources

  • For further details about modprobe, see the modprobe(8) manual page.

3.10. Loading kernel modules automatically at system boot time

The following procedure describes how to configure a kernel module so that it is loaded automatically during the boot process.

Prerequisites

  • Root permissions
  • The kmod package is installed.

Procedure

  1. Select a kernel module you want to load during the boot process.

    The modules are located in the /lib/modules/$(uname -r)/kernel/<SUBSYSTEM>/ directory.

  2. Create a configuration file for the module: 

    # echo <MODULE_NAME> > /etc/modules-load.d/<MODULE_NAME>.conf
    Note

    When entering the name of a kernel module, do not append the .ko.xz extension to the end of the name. Kernel module names do not have extensions; their corresponding files do.

  3. Optionally, after reboot, verify the relevant module was loaded: 

    $ lsmod | grep <MODULE_NAME>

    The example command above should succeed and display the relevant kernel module.

Important

The changes described in this procedure will persist after rebooting the system.

Additional resources

  • For further details about loading kernel modules during the boot process, see the modules-load.d(5) manual page.

3.11. Preventing kernel modules from being automatically loaded at system boot time

The following procedure describes how to add a kernel module to a blacklist so that it will not be automatically loaded during the boot process.

Prerequisites

  • Root permissions
  • The kmod package is installed.
  • Ensure that a blacklisted kernel module is not vital for your current system configuration.

Procedure

  1. Select a kernel module that you want to blacklist: 

    $ lsmod

Module                  Size  Used by
fuse                  126976  3
xt_CHECKSUM            16384  1
ipt_MASQUERADE         16384  1
uinput                 20480  1
xt_conntrack           16384  1
…​

    The lsmod command displays a list of modules loaded to the currently running kernel.

    • Alternatively, identify an unloaded kernel module you want to prevent from potentially loading.

      All kernel modules are located in the /lib/modules/<KERNEL_VERSION>/kernel/<SUBSYSTEM>/ directory.

  2. Create a blacklist configuration file: 

    # vim /etc/modprobe.d/blacklist.conf

	# Blacklists <KERNEL_MODULE_1>
	blacklist <MODULE_NAME_1>
	install <MODULE_NAME_1> /bin/false

	# Blacklists <KERNEL_MODULE_2>
	blacklist <MODULE_NAME_2>
	install <MODULE_NAME_2> /bin/false

	# Blacklists <KERNEL_MODULE_n>
	blacklist <MODULE_NAME_n>
	install <MODULE_NAME_n> /bin/false
	…​

    The example shows the contents of the blacklist.conf file, edited by the vim editor. The blacklist line ensures that the relevant kernel module will not be automatically loaded during the boot process. The blacklist command, however, does not prevent the module from being loaded as a dependency for another kernel module that is not blacklisted. Therefore the install line causes the /bin/false to run instead of installing a module.

    The lines starting with a hash sign are comments to make the file more readable.

    Note

    When entering the name of a kernel module, do not append the .ko.xz extension to the end of the name. Kernel module names do not have extensions; their corresponding files do.

  3. Create a backup copy of the current initial ramdisk image before rebuilding: 

    # cp /boot/initramfs-$(uname -r).img /boot/initramfs-$(uname -r).bak.$(date +%m-%d-%H%M%S).img

    The command above creates a backup initramfs image in case the new version has an unexpected problem.

    • Alternatively, create a backup copy of other initial ramdisk image which corresponds to the kernel version for which you want to blacklist kernel modules: 

      # cp /boot/initramfs-<SOME_VERSION>.img /boot/initramfs-<SOME_VERSION>.img.bak.$(date +%m-%d-%H%M%S)

  4. Generate a new initial ramdisk image to reflect the changes: 

    # dracut -f -v

    • If you are building an initial ramdisk image for a different kernel version than you are currently booted into, specify both target initramfs and kernel version: 

      # dracut -f -v /boot/initramfs-<TARGET_VERSION>.img <CORRESPONDING_TARGET_KERNEL_VERSION>

  5. Reboot the system: 

    $ reboot
Important

The changes described in this procedure will take effect and persist after rebooting the system. Improper blacklisting of a key kernel module can result in an unstable or non-operational system.

Additional resources

  • For further details concerning the dracut utility, refer to the dracut(8) manual page.

3.12. Signing kernel modules for secure boot

You can enhance the security of your system by using signed kernel modules. The following sections describe how to self-sign privately built kernel modules for use with RHEL 8 on UEFI-based build systems where Secure Boot is enabled. These sections also provide an overview of available options for importing your public key into a target system where you want to deploy your kernel modules.

To sign and load kernel modules, you need to:

  1. Authenticate a kernel module.
  2. Generate a public and private key pair.
  3. Import the public key on the target system.
  4. Sign the kernel module with the private key.
  5. Load the signed kernel module.

If Secure Boot is enabled, the UEFI operating system boot loaders, the Red Hat Enterprise Linux kernel, and all kernel modules have to be signed with a private key and authenticated with the corresponding public key. If they are not signed and authenticated, the system will not be allowed to finish the booting process.

The RHEL 8 distribution includes:

  • Signed boot loaders
  • Signed kernels
  • Signed kernel modules

In addition, the signed first-stage boot loader and the signed kernel include embedded Red Hat public keys. These signed executable binaries and embedded keys enable RHEL 8 to install, boot, and run with the Microsoft UEFI Secure Boot Certification Authority keys that are provided by the UEFI firmware on systems that support UEFI Secure Boot. Note that not all UEFI-based systems include support for Secure Boot.

Prerequisites

To be able to sign externally built kernel modules, install the utilities listed in the following table on the build system.

Table 3.1. Required utilities

UtilityProvided by packageUsed onPurpose

openssl

openssl

Build system

Generates public and private X.509 key pair

sign-file

kernel-devel

Build system

Executable file used to sign a kernel module with the private key

mokutil

mokutil

Target system

Optional utility used to manually enroll the public key

keyctl

keyutils

Target system

Optional utility used to display public keys in the system keyring

Note

The build system, where you build and sign your kernel module, does not need to have UEFI Secure Boot enabled and does not even need to be a UEFI-based system.

3.12.1. Authenticating kernel modules with X.509 keys

In RHEL 8, when a kernel module is loaded, the module’s signature is checked using the public X.509 keys on the kernel’s system keyring (.builtin_trusted_keys), excluding keys on the kernel’s system black-list keyring. The following sections provide an overview of sources of keys/keyrings and examples of loaded keys from different sources in the system. Also, the user can see what it takes to authenticate a kernel module.

3.12.1.1. Authentication requirements

This section explains what conditions have to be met for loading kernel modules on systems with enabled UEFI Secure Boot functionality.

If UEFI Secure Boot is enabled or if the module.sig_enforce kernel parameter has been specified, you can only load signed kernel modules that are authenticated using a key on the system keyring (.builtin_trusted_keys). In addition, the public key must not be on the system black-list keyring.

If UEFI Secure Boot is disabled and if the module.sig_enforce kernel parameter has not been specified, you can load unsigned kernel modules and signed kernel modules without a public key. This is summarized in the table below.

Table 3.2. Kernel module authentication requirements for loading

Module signedPublic key found and signature validUEFI Secure Boot statesig_enforceModule loadKernel tainted

Unsigned

-

Not enabled

Not enabled

Succeeds

Yes

Not enabled

Enabled

Fails

-

Enabled

-

Fails

-

Signed

No

Not enabled

Not enabled

Succeeds

Yes

Not enabled

Enabled

Fails

-

Enabled

-

Fails

-

Signed

Yes

Not enabled

Not enabled

Succeeds

No

Not enabled

Enabled

Succeeds

No

Enabled

-

Succeeds

No

3.12.1.2. Sources for public keys

During boot, the kernel loads X.509 keys into the system keyring (.builtin_trusted_keys) or the system black-list keyring from a set of persistent key stores as shown in the table below.

Table 3.3. Sources for system keyrings

Source of X.509 keysUser ability to add keysUEFI Secure Boot stateKeys loaded during boot

Embedded in kernel

No

-

.builtin_trusted_keys

UEFI Secure Boot "db"

Limited

Not enabled

No

Enabled

.builtin_trusted_keys

Embedded in shim.efi boot loader

No

Not enabled

No

Enabled

.builtin_trusted_keys

Machine Owner Key (MOK) list

Yes

Not enabled

No

Enabled

.builtin_trusted_keys

UEFI Secure Boot db is a signature database which stores keys (hashes) of UEFI applications, UEFI drivers, and bootloaders that can be loaded on the machine. Similarly, there exists UEFI Secure Boot dbx. This is a revoked signature database which prevents keys from being loaded.

.builtin_trusted_keys is a keyring that is built on boot and contains trusted public keys. The keys are viewable by a user with root privileges.

If the system is not UEFI-based or if UEFI Secure Boot is not enabled, then only the keys that are embedded in the kernel are loaded onto the system keyring (.builtin_trusted_keys). In that case you have no ability to augment that set of keys without rebuilding the kernel.

The system black-list keyring is a list of X.509 keys which have been revoked. If your module is signed by a key on the black-list then it will fail authentication even if your public key is in the system keyring (.builtin_trusted_keys).

You can display information about the keys on the system keyring (.builtin_trusted_keys) using the keyctl utility. The following is a shortened example output from a RHEL 8 system where UEFI Secure Boot is not enabled. 

# keyctl list %:.builtin_trusted_keys
3 keys in keyring:
...asymmetric: Red Hat Enterprise Linux Driver Update Program (key 3): bf57f3e87...
...asymmetric: Red Hat Enterprise Linux kernel signing key: 4249689eefc77e95880b...
...asymmetric: Red Hat Enterprise Linux kpatch signing key: 4d38fd864ebe18c5f0b7...

The following is a shortened example output from a RHEL 8 system where UEFI Secure Boot is enabled. 

# keyctl list %:.builtin_trusted_keys
6 keys in keyring:
...asymmetric: Red Hat Enterprise Linux Driver Update Program (key 3): bf57f3e87...
...asymmetric: Red Hat Secure Boot (CA key 1): 4016841644ce3a810408050766e8f8a29...
...asymmetric: Microsoft Corporation UEFI CA 2011: 13adbf4309bd82709c8cd54f316ed...
...asymmetric: Microsoft Windows Production PCA 2011: a92902398e16c49778cd90f99e...
...asymmetric: Red Hat Enterprise Linux kernel signing key: 4249689eefc77e95880b...
...asymmetric: Red Hat Enterprise Linux kpatch signing key: 4d38fd864ebe18c5f0b7...

The above output shows the addition of two keys from the UEFI Secure Boot "db" keys as well as the Red Hat Secure Boot (CA key 1), which is embedded in the shim.efi boot loader. You can also look for the kernel console messages that identify the keys with an UEFI Secure Boot related source. These include UEFI Secure Boot db, embedded shim, and MOK list. 

# dmesg | grep 'EFI: Loaded cert'
[5.160660] EFI: Loaded cert 'Microsoft Windows Production PCA 2011: a9290239...
[5.160674] EFI: Loaded cert 'Microsoft Corporation UEFI CA 2011: 13adbf4309b...
[5.165794] EFI: Loaded cert 'Red Hat Secure Boot (CA key 1): 4016841644ce3a8...

3.12.1.3. Generating a public and private key pair

You need to generate a public and private X.509 key pair to succeed in your efforts of using kernel modules on a Secure Boot-enabled system. You will later use the private key to sign the kernel module. You will also have to add the corresponding public key to the Machine Owner Key (MOK) for Secure Boot to validate the signed module.

Some of the parameters for this key pair generation are best specified with a configuration file.

Procedure

  1. Create a configuration file with parameters for the key pair generation: 

    # cat << EOF > configuration_file.config
[ req ]
default_bits = 4096
distinguished_name = req_distinguished_name
prompt = no
string_mask = utf8only
x509_extensions = myexts

[ req_distinguished_name ]
O = Organization
CN = Organization signing key
emailAddress = E-mail address

[ myexts ]
basicConstraints=critical,CA:FALSE
keyUsage=digitalSignature
subjectKeyIdentifier=hash
authorityKeyIdentifier=keyid
EOF

  2. Create an X.509 public and private key pair as shown in the following example: 

    # openssl req -x509 -new -nodes -utf8 -sha256 -days 36500 \
-batch -config configuration_file.config -outform DER \
-out my_signing_key_pub.der \
-keyout my_signing_key.priv

    The public key will be written to the my_signing_key_pub.der file and the private key will be written to the my_signing_key.priv file.

    Important

    In RHEL 8, the validity dates of the key pair matter. The key does not expire, but the kernel module must be signed within the validity period of its signing key. For example, a key that is only valid in 2019 can be used to authenticate a kernel module signed in 2019 with that key. However, users cannot use that key to sign a kernel module in 2020.

  3. Optionally, you can review the validity dates of your public keys like in the example below: 

    # openssl x509 -inform der -text -noout -in <my_signing_key_pub.der>

Validity
            Not Before: Feb 14 16:34:37 2019 GMT
            Not After : Feb 11 16:34:37 2029 GMT
  4. Enroll your public key on all systems where you want to authenticate and load your kernel module.
Warning

Apply strong security measures and access policies to guard the contents of your private key. In the wrong hands, the key could be used to compromise any system which is authenticated by the corresponding public key.

Additional resources

3.12.2. Enrolling public key on target system

When RHEL 8 boots on a UEFI-based system with Secure Boot enabled, the kernel loads onto the system keyring (.builtin_trusted_keys) all public keys that are in the Secure Boot db key database. At the same time the kernel excludes the keys in the dbx database of revoked keys. The sections below describe different ways of importing a public key on a target system so that the system keyring (.builtin_trusted_keys) is able to use the public key to authenticate a kernel module.

3.12.2.1. Factory firmware image including public key

To facilitate authentication of your kernel module on your systems, consider requesting your system vendor to incorporate your public key into the UEFI Secure Boot key database in their factory firmware image.

3.12.2.2. Manually adding public key to the MOK list

The Machine Owner Key (MOK) facility feature can be used to expand the UEFI Secure Boot key database. When RHEL 8 boots on a UEFI-enabled system with Secure Boot enabled, the keys on the MOK list are also added to the system keyring (.builtin_trusted_keys) in addition to the keys from the key database. The MOK list keys are also stored persistently and securely in the same fashion as the Secure Boot database keys, but these are two separate facilities. The MOK facility is supported by shim.efi, MokManager.efi, grubx64.efi, and the mokutil utility.

Enrolling a MOK key requires manual interaction by a user at the UEFI system console on each target system. Nevertheless, the MOK facility provides a convenient method for testing newly generated key pairs and testing kernel modules signed with them.

Procedure

  1. Request the addition of your public key to the MOK list: 

    # mokutil --import my_signing_key_pub.der

    You will be asked to enter and confirm a password for this MOK enrollment request.

  2. Reboot the machine.

    The pending MOK key enrollment request will be noticed by shim.efi and it will launch MokManager.efi to allow you to complete the enrollment from the UEFI console.

  3. Enter the password you previously associated with this request and confirm the enrollment.

    Your public key is added to the MOK list, which is persistent.

Once a key is on the MOK list, it will be automatically propagated to the system keyring on this and subsequent boots when UEFI Secure Boot is enabled.

3.12.3. Signing kernel modules with the private key

Users are able to obtain enhanced security benefits on their systems by loading signed kernel modules if the UEFI Secure Boot mechanism is enabled. The following sections describe how to sign kernel modules with the private key.

Prerequisites

Procedure

  • Execute the sign-file utility with parameters as shown in the example below: 

    # /usr/src/kernels/$(uname -r)/scripts/sign-file \ sha256 \ my_signing_key.priv \ my_signing_key_pub.der \ my_module.ko

    sign-file computes and appends the signature directly to the ELF image in your kernel module file. The modinfo utility can be used to display information about the kernel module’s signature, if it is present.

    Note

    The appended signature is not contained in an ELF image section and is not a formal part of the ELF image. Therefore, utilities such as readelf will not be able to display the signature on your kernel module.

    Your kernel module is now ready for loading. Note that your signed kernel module is also loadable on systems where UEFI Secure Boot is disabled or on a non-UEFI system. That means you do not need to provide both a signed and unsigned version of your kernel module.

    Important

    In RHEL 8, the validity dates of the key pair matter. The key does not expire, but the kernel module must be signed within the validity period of its signing key. The sign-file utility will not warn you of this. For example, a key that is only valid in 2019 can be used to authenticate a kernel module signed in 2019 with that key. However, users cannot use that key to sign a kernel module in 2020.

Additional resources

3.12.4. Loading signed kernel modules

Once your public key is enrolled in the system keyring (.builtin_trusted_keys) and the MOK list, and after you have signed the respective kernel module with your private key, you can finally load your signed kernel module with the the modprobe command as described in the following section.

Prerequisites

Procedure

  1. Verify that your public keys are on the system keyring: 

    # keyctl list %:.builtin_trusted_keys

  2. Copy the kernel module into the /extra/ directory of the kernel you want: 

    # cp my_module.ko /lib/modules/$(uname -r)/extra/

  3. Update the modular dependency list: 

    # depmod -a

  4. Load the kernel module and verify that it was successfully loaded: 

    # modprobe -v my_module
# lsmod | grep my_module

    1. Optionally, to load the module on boot, add it to the /etc/modules-loaded.d/my_module.conf file: 

      # echo "my_module" > /etc/modules-load.d/my_module.conf

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