Fedora includes support for the UEFI Secure Boot feature, which means that Fedora can be installed and run on systems where UEFI Secure Boot is enabled. [1] When Secure Boot is enabled, the EFI operating system boot loaders, the Fedora kernel, and all kernel modules must be signed with a private key and authenticated with the corresponding public key. The Fedora distribution includes signed boot loaders, signed kernels, and signed kernel modules. In addition, the signed first-stage boot loader and the signed kernel include embedded Fedora public keys. These signed executable binaries and embedded keys enable Fedora to install, boot, and run with the Microsoft UEFI Secure Boot CA keys that are provided by the UEFI firmware on systems that support UEFI Secure Boot.[2]
The information provided in the following sections describes steps necessary to enable you to self-sign privately built kernel modules for use with Fedora on UEFI-based systems where Secure Boot is enabled. These sections also provide an overview of available options for getting your public key onto the target system where you want to deploy your kernel module.
Prerequisites
In order to enable signing of externally built modules, the tools listed in the following table are required to be installed on the system.
| Tool | Provided by Package | Used on | Purpose |
|---|---|---|---|
openssl | openssl | Build system | Generates public and private X.509 key pair |
sign-file | kernel-devel | Build system | C application used to sign kernel modules |
mokutil | mokutil | Target system | Optional tool used to manually enroll the public key |
keyctl | keyutils | Target system | Optional tool used to display public keys in the system key ring |
Note that 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. |
Kernel Module Authentication
In Fedora, when a kernel module is loaded, the module’s signature is checked using the public X.509 keys on the kernel’s system key ring, excluding those keys that are on the kernel’s system black list key ring.
Sources For Public Keys Used To Authenticate Kernel Modules
During boot, the kernel loads X.509 keys into the system key ring or the system black list key ring from a set of persistent key stores as shown in Sources For System Key Rings
| Source of X.509 Keys | User Ability to Add Keys | UEFI Secure Boot State | Keys Loaded During Boot |
|---|---|---|---|
Embedded in kernel | No | - |
|
UEFI Secure Boot “db” | Limited | Not enabled | No |
Enabled |
| ||
UEFI Secure Boot “dbx” | Limited | Not enabled | No |
Enabled |
| ||
Embedded in | No | Not enabled | No |
Enabled |
| ||
Machine Owner Key (MOK) list | Yes | Not enabled | No |
Enabled |
|
Note that 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 key ring and you have no ability to augment that set of keys without rebuilding the kernel. The system black list key ring 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 key ring.
To confirm if Secure Boot is enabled, enter a command as follows:
~]$ mokutil --sb-stateSecureBoot enabledIf Secure Boot is not enabled then the message Failed to read SecureBoot is displayed.
You can display information about the keys on the system key rings using the keyctl utility. The following is abbreviated example output from a Fedora system where UEFI Secure Boot is not enabled.
~]#keyctl list %:.builtin_trusted_keys1 key in keyring:265061799: ---lswrv 0 0 asymmetric: Fedora kernel signing key: ba8e2919f98f3f8e2e27541cde0d1f...
The following is abbreviated example output from a Fedora system where UEFI Secure Boot is enabled.
~]#keyctl list %:.builtin_trusted_keys 5 keys in keyring: ...asymmetric: Microsoft Windows Production PCA 2011: a92902398e16c497... ...asymmetric: Fedora kernel signing key: ba8e2919f98f3f8e2e27541cde0d... ...asymmetric: Fedora Secure Boot CA: fde32599c2d61db1bf5807335d7b20e4... ...asymmetric: Red Hat Test Certifying CA: 08a0ef5800cb02fb587c12b4032... ...asymmetric: Microsoft Corporation UEFI CA 2011: 13adbf4309bd82709c8...
The above output shows the addition of two keys from the UEFI Secure Boot “db” keys plus the Fedora Secure Boot CA which is embedded in the shim.efi boot loader.
Kernel Module Authentication Requirements
If UEFI Secure Boot is enabled or if the module.sig_enforce kernel parameter has been specified, then only signed kernel modules that are authenticated using a key on the system key ring can be successfully loaded.[3] If UEFI Secure Boot is disabled and if the module.sig_enforce kernel parameter has not been specified, then unsigned kernel modules and signed kernel modules without a public key can be successfully loaded. This is summarized in Kernel Module Authentication Requirements for Loading.
| Module Signed | Public Key Found and Signature Valid | UEFI Secure Boot State | module.sig_enforce | Module Load | Kernel 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 |
Subsequent sections will describe how to generate a public and private X.509 key pair, how to use the private key to sign a kernel module, and how to enroll the public key into a source for the system key ring.
Generating a Public and Private X.509 Key Pair
You need to generate a public and private X.509 key pair that will be used to sign a kernel module after it has been built. The corresponding public key will be used to authenticate the kernel module when it is loaded.
The openssl tool can be used to generate a key pair that satisfies the requirements for kernel module signing in Fedora. Some of the parameters for this key generation request are best specified with a configuration file; follow the example below to create your own configuration file.
~]#cat << EOF > configuration_file.config[ req ]default_bits = 4096distinguished_name = req_distinguished_nameprompt = nostring_mask = utf8onlyx509_extensions = myexts[ req_distinguished_name ]O = OrganizationCN = Organization signing keyemailAddress = E-mail address[ myexts ]basicConstraints=critical,CA:FALSEkeyUsage=digitalSignaturesubjectKeyIdentifier=hashauthorityKeyIdentifier=keyidEOF
After you have created the configuration file, you can create an X.509 public and private key pair. The public key will be written to the
public_key.derfile and the private key will be written to theprivate_key.privfile.~]#openssl req -x509 -new -nodes -utf8 -sha256 -days 36500 \ -batch -config configuration_file.config -outform DER \ -out public_key.der \ -keyout private_key.priv
Enroll your public key on all systems where you want to authenticate and load your kernel module.
Take proper care to guard the contents of your private key. In the wrong hands, the key could be used to compromise any system which has your public key. |
Enrolling Public Key on Target System
When Fedora boots on a UEFI-based system with Secure Boot enabled, all keys that are in the Secure Boot db key database, but not in the dbx database of revoked keys, are loaded onto the system keyring by the kernel. The system keyring is used to authenticate kernel modules.
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.
Executable Key Enrollment Image Adding Public Key
It is possible to add a key to an existing populated and active Secure Boot key database. This can be done by writing and providing an EFI executable enrollment image. Such an enrollment image contains a properly formed request to append a key to the Secure Boot key database. This request must include data that is properly signed by the private key that corresponds to a public key that is already in the system’s Secure Boot Key Exchange Key (KEK) database. Additionally, this EFI image must be signed by a private key that corresponds to a public key that is already in the key database.
It is also possible to write an enrollment image that runs under Fedora. However, the Fedora image must be properly signed by a private key that corresponds to a public key that is already in the KEK database.
The construction of either type of key enrollment images requires assistance from the platform vendor.
System Administrator Manually Adding Public Key to the MOK List
The Machine Owner Key (MOK) facility is a feature that is supported by Fedora and can be used to augment the UEFI Secure Boot key database. When Fedora boots on a UEFI-enabled system with Secure Boot enabled, the keys on the MOK list are also added to the system keyring 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 key database keys, but these are two separate facilities. The MOK facility is supported by shim.efi, MokManager.efi, grubx64.efi, and the Fedora mokutil utility.
The major capability provided by the MOK facility is the ability to add public keys to the MOK list without needing to have the key chain back to another key that is already in the KEK database. However, enrolling a MOK key requires manual interaction by a physically present user at the UEFI system console on each target system. Nevertheless, the MOK facility provides an excellent method for testing newly generated key pairs and testing kernel modules signed with them.
Follow these steps to add your public key to the MOK list:
Request addition of your public key to the MOK list using a Fedora userspace utility:
~]#mokutil --import my_signing_key_pub.der
You will be asked to enter and confirm a password for this MOK enrollment request.
Reboot the machine.
The pending MOK key enrollment request will be noticed by
shim.efiand it will launchMokManager.efito allow you to complete the enrollment from the UEFI console. You will need to 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 key ring on this and subsequent boots when UEFI Secure Boot is enabled.
Signing Kernel Module with the Private Key
There are no extra steps required to prepare your kernel module for signing. You build your kernel module normally. Assuming an appropriate Makefile and corresponding sources, follow these steps to build your module and sign it:
Build your
my_module.komodule the standard way:~]#make -C /usr/src/kernels/$(uname -r) M=$PWD modules
Sign your kernel module with your private key. This is done with a C application. Note that the application requires that you provide both the files that contain your private and the public key as well as the kernel module file that you want to sign.
~]#/usr/src/kernels/$(uname -r)/scripts/sign-file \ sha256 \ my_signing_key.priv \ my_signing_key_pub.der \ my_module.ko
Your kernel module is in ELF image format and this application computes and appends the signature directly to the ELF image in your my_module.ko file. The modinfo utility can be used to display information about the kernel module’s signature, if it is present. For information on using the utility, see Displaying Information About a Module.
Note that this appended signature is not contained in an ELF image section and is not a formal part of the ELF image. Therefore, tools 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.
Loading Signed Kernel Module
Once your public key is enrolled and is in the system keyring, the normal kernel module loading mechanisms will work transparently. In the following example, you will use mokutil to add your public key to the MOK list and you will manually load your kernel module with modprobe.
Optionally, you can verify that your kernel module will not load before you have enrolled your public key. First, verify what keys have been added to the system key ring on the current boot by running the keyctl list %:.builtin_trusted_keys as root. Since your public key has not been enrolled yet, it should not be displayed in the output of the command.
Request enrollment of your public key.
~]#mokutil --import my_signing_key_pub.der
Reboot, and complete the enrollment at the UEFI console.
~]#reboot
After the system reboots, verify the keys on the system key ring again.
~]#keyctl list %:.builtin_trusted_keys
You should now be able to load your kernel module successfully.
~]#modprobe -v my_moduleinsmod /lib/modules/3.17.4-302.fc21.x86_64/extra/my_module.ko~]#lsmod | grep my_modulemy_module 12425 0
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