Add Documentation/module-signing.txt file

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			KERNEL MODULE SIGNING FACILITY

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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.