Kernel module signing facility
------------------------------

.. CONTENTS
..
.. - Overview.
.. - Configuring module signing.
.. - Generating signing keys.
.. - Public keys in the kernel.
.. - Manually signing modules.
.. - Signed modules and stripping.
.. - Loading signed modules.
.. - Non-valid signatures and unsigned modules.
.. - Administering/protecting the private key.


========
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
:menuselection:`Enable Loadable Module Support` section of
the kernel configuration and turning on::

        CONFIG_MODULE_SIG       "Module signature verification"

This has a number of options available:

 (1) :menuselection:`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.


 (2) :menuselection:`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::

        scripts/sign-file


 (3) :menuselection:`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``      :menuselection:`Sign modules with SHA-1`
        ``CONFIG_MODULE_SIG_SHA224``    :menuselection:`Sign modules with SHA-224`
        ``CONFIG_MODULE_SIG_SHA256``    :menuselection:`Sign modules with SHA-256`
        ``CONFIG_MODULE_SIG_SHA384``    :menuselection:`Sign modules with SHA-384`
        ``CONFIG_MODULE_SIG_SHA512``    :menuselection:`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.


 (4) :menuselection:`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 requires a PIN, this can be provided at build time by
     means of the ``KBUILD_SIGN_PIN`` variable.


 (5) :menuselection:`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::

        certs/signing_key.pem

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

        certs/x509.genkey

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::

        [ req_distinguished_name ]
        #O = Unspecified company
        CN = Build time autogenerated kernel key
        #emailAddress = unspecified.user@unspecified.company

The generated RSA key size can also be set with::

        [ req ]
        default_bits = 4096


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::

        openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
           -config x509.genkey -outform PEM -out kernel_key.pem \
           -keyout kernel_key.pem

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::

        [root@deneb ~]# cat /proc/keys
        ...
        223c7853 I------     1 perm 1f030000     0     0 keyring   .builtin_trusted_keys: 1
        302d2d52 I------     1 perm 1f010000     0     0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 []
        ...

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::

        keyctl padd asymmetric "" [.builtin_trusted_keys-ID] <[key-file]

e.g.::

        keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509

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::

        scripts/sign-file sha512 kernel-signkey.priv \
                kernel-signkey.x509 module.ko

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``.