Since Linux kernel version 3.7 onwards, support has been added for signed kernel modules. When enabled, the Linux kernel will only load kernel modules that are digitally signed with the proper key. This allows further hardening of the system by disallowing unsigned kernel modules, or kernel modules signed with the wrong key, to be loaded. Malicious kernel modules are a common method for loading rootkits on a Linux system.
Enabling support is a matter of toggling a few settings in the Linux kernel configuration. Unless you want to use your own keypair, this is all that has to be done to enable kernel module support. It must also be noted that a change of file structure occurred in kernel version 4.3.3. So from this version and above the certificates and a sign files used to manually sign modules has moved into: /usr/src/linux/certs/ Also the sign-file perl script is now directly executable in later kernel versions and does not require you to execute the perl command. Adjust the installation and setup methods accordingly to the kernel version that will be used.
### Configuring module signature verification
Module signature verification is a kernel feature, so has to be enabled through the Linux kernel configuration. You can find the necessary options under Enable loadable module support.
```sh
Enable module signature verification
--- Enable loadable module support
[*] Module signature verification
[*] Require modules to be validly signed
[*] Automatically sign all modules
Which hash algorithm should modules be signed with? (Sign modules with SHA-512) --->
```
The option Module signature verification (CONFIG_MODULE_SIG) enables the module signature verification in the Linux kernel. It supports two approaches on signed module support: a rather permissive one and a strict one. By default, the permissive approach is used, which means that the Linux kernel module either has to have a valid signature, or no signature. With the strict approach, a valid signature must be present. In the above example, the strict approach is used by selecting Require modules to be validly signed (CONFIG_MODULE_SIG_FORCE). Another way of enabling this strict approach is to set the kernel boot option enforcemodulesig=1.
When building the Linux kernel, the kernel modules will not be signed automatically unless you select Automatically sign all modules (CONFIG_MODULE_SIG_ALL).
Finally, we need to select the hash algorithm to use with the cryptographic signature. In the above example, we use SHA-512.
### Building the kernel with proper keys
When the Linux kernel is building with module signature verification support enabled, then you can use your own keys or have the Linux kernel build infrastructure create a set for you. If you want the Linux kernel build infrastructure to create it for you, just continue as you always do with a make and make modules_install. At the end of the build process, you will notice that signing_key.priv (/certs/signing_key.pem in kernel 4.3.3 or higher) and signing_key.x509 will be available on the root of the Linux kernel sources.
If we want to use our own keys, you can use openssl to create a key pair (private key and public key). The following command, taken from kernel/Makefile, creates such a key pair.
Note: For kernels 4.3.3 and above x509.genkey is located in:
(/certs/signing_key.pem in kernel 4.3.3 or higher)
The resulting files need to be stored as signing_key.x509 and signing_key.priv (/certs/signing_key.pem in kernel 4.3.3 or higher) in the root of the Linux kernel source tree.
The public key part will be build inside the Linux kernel. If you configured the kernel to sign modules, this signing will take place during the make modules_install part.
### Validating module signature support
Reboot with the newly configured kernel. In the output of dmesg you should be able to confirm that the proper certificate is loaded:
The string ~Module signature appended~ at the end confirms that a signature is present. Of course, it does not confirm that the signature is valid or not.
To remove the signature, we can use the strip command:
This confirms that modules without a signature are not loaded.
## Administering kernel module signatures
Once the kernel boots and we have validated that the signed kernel module support works, it is important to correctly handle the keys themselves.
### Protecting the private key
The private key, stored as signing_key.priv (/certs/signing_key.pem in kernel 4.3.3 or higher), needs to be moved to a secure location (unless you will be creating new keys for new kernels, in which case the file can be removed). Do not keep it at /usr/src/linux on production systems as malware can then easily use this key to sign the malicious kernel modules (such as rootkits) and compromise the system further.
The best way to do this is to move the key to a USB stick that you will unplug from your computer after it has been moved. That way if you update a driver such asd you graphics drivers which do not require a kernel re-compile you can insert the USB stick for that moment to sign your driver modules and unplug it after you have done so.
Other methods include technology such as a TPM yet these are not favored by many as the TPM module is burnt from manufacture with a key that the manufacturer has control over.
### Manually signing modules
If you ever need to manually sign a kernel module, you can use the scripts/sign-file script available in the Linux kernel source tree. It requires four arguments:
1. The hash algorithm to use, such as sha512.
2. The private key location.
3. The certificate (which includes the public key) location.
4. The kernel module to sign.
In this case, the key pair does not need to be named signing_file.priv and such, nor do they need to be in the root of the Linux kernel source tree location.
For Kernel 4.3.3 or higher the perl script is no more. "sign-file" is an executable and its usage is as shown bellow, Note how the perl has been removed from the beginning of the command. The location of the certificates have also moved from the root source directory /usr/src/linux to the /usr/src/linux/certs directory and now uses signing-key.pem instead of signing-key.priv.
If we create a kernel package through make tarbz2-pkg, the modules in it will be signed already so we do not need to manually sign them afterwards. The signing keys themselves are not distributed with it.
[Booting a self-signed Linux kernel](http://www.kroah.com/log/blog/2013/09/02/booting-a-self-signed-linux-kernel/) - Greg Kroah-Hartman describes how to boot a self-signed Linux kernel from EFI. As having signed kernel module support is only secure if the Linux kernel is trusted, this is an important (and related) feature to work with.
