Commit 94e980cc authored by Mauro Carvalho Chehab's avatar Mauro Carvalho Chehab

Documentation/module-signing.txt: convert to ReST markup

- Fix identatio for the document title;
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Signed-off-by: default avatarMauro Carvalho Chehab <mchehab@s-opensource.com>
parent aeb04e52
==============================
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.
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
Overview
========
The kernel module signing facility cryptographically signs modules during
......@@ -36,17 +35,19 @@ SHA-512 (the algorithm is selected by data in the signature).
==========================
CONFIGURING MODULE SIGNING
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
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) "Require modules to be validly signed" (CONFIG_MODULE_SIG_FORCE)
(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.
......@@ -64,35 +65,39 @@ This has a number of options available:
cannot be parsed, it will be rejected out of hand.
(2) "Automatically sign all modules" (CONFIG_MODULE_SIG_ALL)
(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:
be signed manually using::
scripts/sign-file
(3) "Which hash algorithm should modules be signed with?"
(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 "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"
=============================== ==========================================
``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) "File name or PKCS#11 URI of module signing key" (CONFIG_MODULE_SIG_KEY)
(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
``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
......@@ -102,10 +107,11 @@ This has a number of options available:
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.
means of the ``KBUILD_SIGN_PIN`` variable.
(5) "Additional X.509 keys for default system keyring" (CONFIG_SYSTEM_TRUSTED_KEYS)
(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
......@@ -116,7 +122,7 @@ packages to the kernel build processes for the tool that does the signing.
=======================
GENERATING SIGNING KEYS
Generating signing keys
=======================
Cryptographic keypairs are required to generate and check signatures. A
......@@ -126,14 +132,14 @@ 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
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:
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:
into vmlinux) using parameters in the::
certs/x509.genkey
......@@ -142,14 +148,14 @@ 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:
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:
The generated RSA key size can also be set with::
[ req ]
default_bits = 4096
......@@ -158,23 +164,23 @@ 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:
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
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
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:
in a keyring called ".system_keyring" that can be seen by::
[root@deneb ~]# cat /proc/keys
...
......@@ -184,27 +190,27 @@ in a keyring called ".system_keyring" 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.
``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:
Finally, it is possible to add additional public keys by doing::
keyctl padd asymmetric "" [.system_keyring-ID] <[key-file]
e.g.:
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
``.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
=========================
========================
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:
......@@ -214,7 +220,7 @@ the Linux kernel source tree. The script requires 4 arguments:
3. The public key filename
4. The kernel module to be signed
The following is an example to sign a kernel module:
The following is an example to sign a kernel module::
scripts/sign-file sha512 kernel-signkey.priv \
kernel-signkey.x509 module.ko
......@@ -228,11 +234,11 @@ $KBUILD_SIGN_PIN environment variable.
============================
SIGNED MODULES AND STRIPPING
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
``~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
......@@ -242,19 +248,19 @@ debug information present at the time of signing.
======================
LOADING SIGNED MODULES
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.
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
Non-valid signatures and unsigned modules
=========================================
If CONFIG_MODULE_SIG_FORCE is enabled or module.sig_enforce=1 is supplied on
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
......@@ -264,7 +270,7 @@ Any module that has an unparseable signature will be rejected.
=========================================
ADMINISTERING/PROTECTING THE PRIVATE KEY
Administering/protecting the private key
=========================================
Since the private key is used to sign modules, viruses and malware could use
......@@ -275,5 +281,5 @@ 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.
set ``CONFIG_MODVERSIONS=y`` or ensure that each configuration has a different
kernel release string by changing ``EXTRAVERSION`` or ``CONFIG_LOCALVERSION``.
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