==============================
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 "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) "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.
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) "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) "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_SIG_SHA1 "Sign modules with SHA-1"
CONFIG_SIG_SHA224 "Sign modules with SHA-224"
CONFIG_SIG_SHA256 "Sign modules with SHA-256"
CONFIG_SIG_SHA384 "Sign modules with SHA-384"
CONFIG_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.
=======================
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, the kernel build will automatically generate a new
keypair using openssl if one does not exist in the files:
signing_key.priv
signing_key.x509
during the building of vmlinux (the public part of the key needs to be built
into vmlinux) using parameters in the:
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 = Magrathea
CN = Glacier signing key
emailAddress = slartibartfast@magrathea.h2g2
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 DER -out signing_key.x509 \
-keyout signing_key.priv
=========================
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 ".system_keyring" that can be seen by:
[root@deneb ~]# cat /proc/keys
...
223c7853 I------ 1 perm 1f030000 0 0 keyring .system_keyring: 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, any file
placed in the kernel source root directory or the kernel build root directory
whose name is suffixed with ".x509" will be assumed to be an X.509 public key
and will be added to the keyring.
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 "" [.system_keyring-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
.system_keyring _if_ the new key's X.509 wrapper is validly signed by a key
that is already resident in the .system_keyring 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
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.
============================
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 enforcemodulesig=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.
