Sign linux kernel module

Signed kernel modules

Signed kernel modules provide a mechanism for the kernel to verify the integrity of a module.

Contents

Overview

The Linux kernel distinguishes and keeps separate the verification of modules from requiring or forcing modules to verify before allowing them to be loaded. Kernel modules fall into 2 classes:

• Standard in-tree modules which come with the kernel source code. They are compiled during the normal kernel build.
• Out-of-tree modules which are not part of the kernel source distribution. They are built outside of the kernel tree, requiring the kernel headers package for each kernel they are to be built for. They can be built manually for a specific kernel and packaged, or they can be built whenever needed using DKMS.

During a standard kernel compilation, the kernel build tools create a private/public key pair and sign every in-tree module (using the private key). The public key is saved in the kernel itself. When a module is subsequently loaded, the public key can then be used to verify that the module is unchanged.

The kernel can be enabled to always verify modules and report any failures to standard logs. The choice to permit the loading and use of a module which could not be verified can be either compiled into kernel or turned on at runtime using a kernel parameter as explained below.

Summary of what needs to be done

The starting point is based on a custom kernel package as outlined in Kernel/Arch Build System. We will modify the build to sign the standard in-tree kernel modules and to provide the prerequisites for signing and verifying out-of-tree modules.

The goal is to have:

• In-tree modules signed during the standard kernel build process. The standard kernel build creates a fresh public/private key pair on each build.
• Out-of-tree modules are signed and the associated public key is compiled into the kernel. We will create a separate public/private key pair on each build.

Each kernel build needs to made aware of the key pair to be used for signing out-of-tree modules. A kernel configuration parameter is now used to make the kernel aware of additional signing keys: CONFIG_SYSTEM_TRUSTED_KEYS=»/path/to/oot-signing_keys.pem» .

Keys and signing tools will be stored in the current module build directory. Nothing needs to be done to clean this as removal is handled by the standard module cleanup. The private and public keys are both installed in /usr/lib/modules/kernel_version—build/certs-local .

Kernel configuration

CONFIG_SYSTEM_TRUSTED_KEYS will be updated automatically using the script fix_config.sh provided below. In addition, the following configuration options should be set either manually by editing the .config file, or via make menuconfig in the Linux src directory and subsequently copying the updated .config file back to the build file config .

Kernel command line

When you have confirmed that the modules are being signed and that the kernel works as it should, you can enable the following kernel parameter to require that the kernel only permits verified modules to be loaded:

Before forcing verified modules on, please confirm that the system logs do not show any module signature failures being reported.

Tools needed

kernel build package

In the directory where the kernel package is built:

This directory will provide the tools to create the keys, as well as signing kernel modules.

Put the 4 files into certs-local :

• fix_config.sh
• x509.oot.genkey
• genkeys.sh
• sign_manual.sh

The files genkeys.sh and its configuration file x509.oot.genkey are used to create key pairs.

The file fix_config.sh is run after that to provide the kernel with the key information by updating the configuration file used to build the kernel.

The script sign_manual will be used to sign out-of-tree kernel modules.

genkeys.sh will create the key pairs in a directory named by date-time.

It also creates file current_key_dir with that directory name and a soft link current to the same directory holding the current key pairs.

These files are all provided below.

DKMS support

Add 2 files to the dkms directory:

These will be installed in /etc/dkms and provide the means for DKMS to automatically sign modules using the local key. This is the recommended way to sign out-of-tree kernel modules. As explained below, once this is installed, all that is needed is for DKMS to automatically sign modules is to make a soft link for each module to the configuration file.

The link creation can easily be added to an arch package to simplify further if desired.

Modify PKGBUILD

We need to make changes to kernel build as follows:

prepare()

Add the following to the top of the prepare() function:

_package-headers()

Add the following to the bottom of the _package-headers() function:

Files required

The 6 supporting files referenced above are available for download from the github.com/gene-git/Arch-SKM repository:

Remember to ensure that the scripts are executable.

Helper scripts

The factual accuracy of this article or section is disputed.

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Kernel module signing facility¶

Overview¶

The kernel module signing facility cryptographically signs modules during installation and then checks the signature upon loading the module. This allows increased kernel security by disallowing the loading of unsigned modules or modules signed with an invalid key. Module signing increases security by making it harder to load a malicious module into the kernel. The module signature checking is done by the kernel so that it is not necessary to have trusted userspace bits.

This facility uses X.509 ITU-T standard certificates to encode the public keys involved. The signatures are not themselves encoded in any industrial standard type. The facility currently only supports the RSA public key encryption standard (though it is pluggable and permits others to be used). The possible hash algorithms that can be used are SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512 (the algorithm is selected by data in the signature).

Configuring module signing¶

The module signing facility is enabled by going to the Enable Loadable Module Support section of the kernel configuration and turning on:

This has a number of options available:

Require modules to be validly signed ( CONFIG_MODULE_SIG_FORCE )

This specifies how the kernel should deal with a module that has a signature for which the key is not known or a module that is unsigned.

If this is off (ie. “permissive”), then modules for which the key is not available and modules that are unsigned are permitted, but the kernel will be marked as being tainted, and the concerned modules will be marked as tainted, shown with the character ‘E’.

If this is on (ie. “restrictive”), only modules that have a valid signature that can be verified by a public key in the kernel’s possession will be loaded. All other modules will generate an error.

Irrespective of the setting here, if the module has a signature block that cannot be parsed, it will be rejected out of hand.

Automatically sign all modules ( CONFIG_MODULE_SIG_ALL )

If this is on then modules will be automatically signed during the modules_install phase of a build. If this is off, then the modules must be signed manually using:

Which hash algorithm should modules be signed with?

This presents a choice of which hash algorithm the installation phase will sign the modules with:

CONFIG_MODULE_SIG_SHA1	Sign modules with SHA-1
CONFIG_MODULE_SIG_SHA224	Sign modules with SHA-224
CONFIG_MODULE_SIG_SHA256	Sign modules with SHA-256
CONFIG_MODULE_SIG_SHA384	Sign modules with SHA-384
CONFIG_MODULE_SIG_SHA512	Sign modules with SHA-512

The algorithm selected here will also be built into the kernel (rather than being a module) so that modules signed with that algorithm can have their signatures checked without causing a dependency loop.

File name or PKCS#11 URI of module signing key ( CONFIG_MODULE_SIG_KEY )

Setting this option to something other than its default of certs/signing_key.pem will disable the autogeneration of signing keys and allow the kernel modules to be signed with a key of your choosing. The string provided should identify a file containing both a private key and its corresponding X.509 certificate in PEM form, or — on systems where the OpenSSL ENGINE_pkcs11 is functional — a PKCS#11 URI as defined by RFC7512. In the latter case, the PKCS#11 URI should reference both a certificate and a private key.

If the PEM file containing the private key is encrypted, or if the PKCS#11 token requries a PIN, this can be provided at build time by means of the KBUILD_SIGN_PIN variable.

Additional X.509 keys for default system keyring ( CONFIG_SYSTEM_TRUSTED_KEYS )

This option can be set to the filename of a PEM-encoded file containing additional certificates which will be included in the system keyring by default.

Note that enabling module signing adds a dependency on the OpenSSL devel packages to the kernel build processes for the tool that does the signing.

Generating signing keys¶

Cryptographic keypairs are required to generate and check signatures. A private key is used to generate a signature and the corresponding public key is used to check it. The private key is only needed during the build, after which it can be deleted or stored securely. The public key gets built into the kernel so that it can be used to check the signatures as the modules are loaded.

Under normal conditions, when CONFIG_MODULE_SIG_KEY is unchanged from its default, the kernel build will automatically generate a new keypair using openssl if one does not exist in the file:

during the building of vmlinux (the public part of the key needs to be built into vmlinux) using parameters in the:

file (which is also generated if it does not already exist).

It is strongly recommended that you provide your own x509.genkey file.

Most notably, in the x509.genkey file, the req_distinguished_name section should be altered from the default:

The generated RSA key size can also be set with:

It is also possible to manually generate the key private/public files using the x509.genkey key generation configuration file in the root node of the Linux kernel sources tree and the openssl command. The following is an example to generate the public/private key files:

The full pathname for the resulting kernel_key.pem file can then be specified in the CONFIG_MODULE_SIG_KEY option, and the certificate and key therein will be used instead of an autogenerated keypair.

Public keys in the kernel¶

The kernel contains a ring of public keys that can be viewed by root. They’re in a keyring called ”.builtin_trusted_keys” that can be seen by:

Beyond the public key generated specifically for module signing, additional trusted certificates can be provided in a PEM-encoded file referenced by the CONFIG_SYSTEM_TRUSTED_KEYS configuration option.

Further, the architecture code may take public keys from a hardware store and add those in also (e.g. from the UEFI key database).

Finally, it is possible to add additional public keys by doing:

Note, however, that the kernel will only permit keys to be added to .builtin_trusted_keys if the new key’s X.509 wrapper is validly signed by a key that is already resident in the .builtin_trusted_keys at the time the key was added.

Manually signing modules¶

To manually sign a module, use the scripts/sign-file tool available in the Linux kernel source tree. The script requires 4 arguments:

1. The hash algorithm (e.g., sha256)
2. The private key filename or PKCS#11 URI
3. The public key filename
4. The kernel module to be signed

The following is an example to sign a kernel module:

The hash algorithm used does not have to match the one configured, but if it doesn’t, you should make sure that hash algorithm is either built into the kernel or can be loaded without requiring itself.

If the private key requires a passphrase or PIN, it can be provided in the $KBUILD_SIGN_PIN environment variable.

Signed modules and stripping¶

A signed module has a digital signature simply appended at the end. The string

Module signature appended

. at the end of the module’s file confirms that a signature is present but it does not confirm that the signature is valid!

Signed modules are BRITTLE as the signature is outside of the defined ELF container. Thus they MAY NOT be stripped once the signature is computed and attached. Note the entire module is the signed payload, including any and all debug information present at the time of signing.

Loading signed modules¶

Modules are loaded with insmod, modprobe, init_module() or finit_module() , exactly as for unsigned modules as no processing is done in userspace. The signature checking is all done within the kernel.

Non-valid signatures and unsigned modules¶

If CONFIG_MODULE_SIG_FORCE is enabled or module.sig_enforce=1 is supplied on the kernel command line, the kernel will only load validly signed modules for which it has a public key. Otherwise, it will also load modules that are unsigned. Any module for which the kernel has a key, but which proves to have a signature mismatch will not be permitted to load.

Any module that has an unparseable signature will be rejected.

Administering/protecting the private key¶

Since the private key is used to sign modules, viruses and malware could use the private key to sign modules and compromise the operating system. The private key must be either destroyed or moved to a secure location and not kept in the root node of the kernel source tree.

If you use the same private key to sign modules for multiple kernel configurations, you must ensure that the module version information is sufficient to prevent loading a module into a different kernel. Either set CONFIG_MODVERSIONS=y or ensure that each configuration has a different kernel release string by changing EXTRAVERSION or CONFIG_LOCALVERSION .

© Copyright The kernel development community.

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