[ptxdist] [PATCH v3 5/6] doc: dev_code_signing: rework and extend code signing section

[Bastian Krause <bst@pengutronix.de>]

Signed-off-by: Bastian Krause <bst@pengutronix.de>
Reviewed-by: Roland Hieber <rhi@pengutronix.de>
Tested-by: Ladislav Michl <ladis@linux-mips.org>
---
Changes since v2:
- adjust section "Creating Custom Code Signing Providers" to updated
  code signing provider templates
Changes since (implicit) v1:
- add hyperlink target to code signing consumers to allow references in
  a later commit
---
 doc/dev_code_signing.rst | 148 +++++++++++++++++++++++++++++++++------
 1 file changed, 125 insertions(+), 23 deletions(-)

diff --git a/doc/dev_code_signing.rst b/doc/dev_code_signing.rst
index de0087f8b..fbebb6b52 100644
--- a/doc/dev_code_signing.rst
+++ b/doc/dev_code_signing.rst
@@ -3,34 +3,136 @@
 Code Signing
 ------------
 
-This is an overview over the ptxdist signing infrastructure.
-ptxdist uses PKCS#11 internally for providing access to keys and certificates.
-Packages that wish to sign something should implement a PKCS#11 interface.
+In order to make sure an artifact was created by a known authority and was not
+altered later, digital signatures play a key role when building firmware
+images.
+This is also essential when a verified boot chain is established, e.g. via
+*High Assurance Boot* (HAB), signed FIT images, and a verified root file
+system.
+
+PTXdist uses `PKCS#11 <pkcs11-doc_>`_ internally to provide access to keys and
+certificates, therefore code signing consumers should implement a PKCS#11
+interface to make use of PTXdist's code signing infrastructure.
 
 As PKCS#11 URIs usually differ between different usecases (release vs.
-development) the URIs normally are not hardcoded in the package configuration.
-Instead, ptxdist has the idea of "roles" which are string identifiers used to
+development) the URIs are usually not hardcoded in the package configuration.
+Instead, PTXdist has the idea of **roles** which are string identifiers used to
 access a single private/public key pair and a certificate.
 
-ptxdist supports Hardware Security Modules (HSM).
-In case a HSM is not present or shall not be used SoftHSM is used internally to
-transparently provide the same API internally.
+Finally, one or several **code signing providers** supply the mapping from
+roles to the respective key material or even provide it themselves for
+development.
+
+PTXdist supports *Hardware Security Modules* (HSM).
+In case an HSM is not present or shall not be used, PTXdist can emulate it
+using `SoftHSM <softhsm_>`_, while still transparently providing the same API
+to code signing consumers.
+
+.. _pkcs11-doc: https://www.cryptsoft.com/pkcs11doc/
+.. _softhsm: https://www.opendnssec.org/softhsm/
+
+.. _code_signing_providers:
 
-For each role a PKCS#11 URI must be known by ptxdist.
-In case of a HSM the keys and certificates are stored in the HSM, but ptxdist
+Code Signing Providers
+~~~~~~~~~~~~~~~~~~~~~~
+
+For each role a PKCS#11 URI must be known by PTXdist.
+In case of an HSM the keys and certificates are stored in the HSM, but PTXdist
 needs to know the PKCS#11 URI to access the keys.
-This is done in ptxdist rule files calling cs_set_uri <role> <uri>.
-For SoftHSM the URI is generated internally by ptxdist, but instead the
-keys/certificates for each role have have to be imported.
-This is done with the cs_import_* functions below.
-
-During each invocation of ptxdist exactly one key provider is active.
-The code signing provider can be chosen with the PTXCONF_CODE_SIGNING_PROVIDER
-variable.
-A code signing provider is a package resposible for providing the role <->
-PKCS#11 URI relationships in case a HSM is used or for providing the key
+For SoftHSM the URI is generated internally by PTXdist, but instead the
+keys/certificates for each role have to be imported by the code signing
+provider into the SoftHSM.
+
+A code signing provider is a package responsible for providing the role ↔
+PKCS#11 URI relationships in case an HSM is used, or for providing the key
 material in case SoftHSM is used.
 
-A package which wants to sign something or which needs access to keys has to
-select CODE_SIGNING.
-This makes sure the keys are ready when the package is being built.
+When ``PTXCONF_CODE_SIGNING`` is enabled exactly one code signing provider is
+active during each invocation of PTXdist.
+
+PTXdist comes equipped with a development code signing provider "devel"
+implemented via the package ``host-ptx-code-signing-dev``.
+It imports publicly available development keys for each role into the SoftHSM.
+
+.. important:: The ``host-ptx-code-signing-dev`` code signing provider can be
+  used to sign artifacts for development purposes, but **must not** be used for
+  production.
+
+Creating Custom Code Signing Providers
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When a set of release keys or project-specific development keys should be
+used (e.g. to achieve backward compatibility), a new code signing provider
+must be introduced.
+
+Use ``ptxdist newpackage code-signing-provider`` to generate such a new code
+signing provider.
+PTXdist distinguishes between "SoftHSM", "HSMs with OpenSC support" and "other
+HSMs".
+The generated shell script in ``local_src/<name>-code-signing/`` contains
+an examples for the selected HSM use case.
+See :ref:`code_signing_helper_functions` for an explanation of the available
+code signing helpers.
+In case of SoftHSM use cases the keys should also be placed inside
+``local_src/<name>-code-signing/``
+
+In case an HSM is used it is required to extend the ``CODE_SIGNING_ENV`` with
+additional environment variables via a pre rule in
+``$(PTXDIST_PLATFORMCONFIGDIR)/rules/pre/``.
+For SoftHSM such a pre rule exists already in upstream PTXdist in
+``rules/pre/020-code-signing-softhsm.make``.
+For HSMs with *OpenSC* support a pre rule is generated in
+``$(PTXDIST_PLATFORMCONFIGDIR)/rules/pre/020-<name>-code-signing-hsm.make``.
+For other HSMs a skeleton pre make file is generated and must be adjusted.
+
+For example, for Nitrokey HSMs which use OpenSC the generated pre rule looks
+like this:
+
+.. code-block:: make
+
+    ifdef PTXCONF_CODE_SIGNING_PROVIDER_<NAME>
+    CODE_SIGNING_ENV += \
+    	PKCS11_MODULE_PATH="${PTXDIST_SYSROOT_HOST}/lib/pkcs11/opensc-pkcs11.so"
+    endif
+
+Note that the module is built in the BSP in this case.
+This is not strictly required, it is also possible to use an otherwise
+distributed module, e.g. by the HSM manufacturer.
+For HSMs supported by OpenSC the required OpenSC selects are generated.
+If an HSM without OpenSC support is used the ``select FIXME`` should either be
+replaced with an appropriate host tool that provides the PKCS#11 module or
+removed altogether if an external PKCS#11 module is used.
+
+Switching the code signing provider is now finally possible with
+``ptxdist platformconfig``, then navigate to *Code signing* → *Code signing
+provider*.
+
+.. _code_signing_consumers:
+
+Code Signing Consumers
+~~~~~~~~~~~~~~~~~~~~~~
+
+A package has to select ``CODE_SIGNING`` if it wants to sign something, or if
+it needs access to keys and/or certificates.
+The config symbol is available in ptxconfig as well as in platformconfig.
+Selecting this symbol makes sure the keys and certificates are ready when the
+package is being built.
+
+By adding ``CODE_SIGNING_ENV`` to the package's make/conf/image environment a
+tool implementing a PKCS#11 interface can access the HSM or SoftHSM.
+The PKCS#11 URI can be retrieved via :ref:`cs_get_uri` and passed on, usually
+also via an environment variable.
+
+:ref:`cs_get_ca` can be used to install a keyring to the root file system, e.g.:
+
+.. code-block:: none
+
+    $(call install_copy, rauc, 0, 0, 0644, \
+      $(shell cs_get_ca update), \
+      /etc/rauc/ca.cert.pem)
+
+.. note:: When code signing helper functions are used in make variables (e.g.
+  for environment definitions) recursively expanded variables must be used
+  (``=``, not ``:=``).
+  Otherwise the variable is expanded before a code signing provider can perform
+  its setup.
-- 
2.27.0


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