kernel: add support for module signing

Use the code signing role 'kernel-modules' to supply the kernel with the
key for kernel module singing. This only works if kernel module signing
is enabled in the kernel config file, so write a short paragraph for the
"daily use" chapter in the docs what has to be considered when using
module signing in PTXdist.

Signed-off-by: Roland Hieber <rhi@pengutronix.de>
Message-Id: <20210723142956.31879-2-rhi@pengutronix.de>
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>

3 files changed, 57 insertions, 0 deletions

diff --git a/doc/daily_work.inc b/doc/daily_work.inc
index 4562b5ca7..37bb9bc48 100644
--- a/doc/daily_work.inc
+++ b/doc/daily_work.inc
@@ -157,6 +157,44 @@ add those certificates to the kernel trust root.
 :ref:`cs_append_ca_from_pem`, or :ref:`cs_append_ca_from_uri` with the
 ``kernel-trusted`` role to supply those certificates.)
 
+Note that the kernel also always adds the module signing key to the trust root
+(see :ref:`kernel_module_signing` below).
+If the EVM key is signed by the module signing key (or if the two keys are the
+same *and* it is self-signed), no additional trust CA is necessary.
+
+.. _kernel_module_signing:
+
+Kernel Module Signing
+~~~~~~~~~~~~~~~~~~~~~
+
+The kernel's build system can generate cryptographic signatures for all
+kernel modules during the build process.
+This can ensure that all modules loaded on the target at runtime have been
+built by a trustworthy source.
+
+If ``PTXCONF_KERNEL_MODULES_SIGN`` ("sign modules") is enabled in the
+platformconfig, PTXdist augments the kernel config with the following config
+options during the `kernel.compile` and `kernel.install` stages:
+
+* ``CONFIG_MODULE_SIG_KEY`` ("File name or PKCS#11 URI of module signing key"):
+  PTXdist supplies the URI from the ``kernel-modules`` role of the configured
+  code signing provider.
+  (The code signing provider should use :ref:`cs_set_uri` to set the URI.)
+
+However, additional settings must also be enabled in the kernel config:
+
+* ``CONFIG_MODULE_SIG=y`` ("Module signature verification"):
+  Enable this option for module signing, and to get access to its sub-options.
+* ``CONFIG_MODULE_SIG_ALL=y`` ("Automatically sign all modules"):
+  Enable this option so that the kernel's build system signs the modules during
+  PTXdist's `kernel.install` stage.
+* Additionally, ``CONFIG_MODULE_SIG_FORCE`` ("Require modules to be validly
+  signed") can be useful so that the kernel refuses loading modules with
+  invalid, untrusted, or no signature.
+
+For the full overview, refer to the `kernel's module signing documentation
+<https://www.kernel.org/doc/html/latest/admin-guide/module-signing.html>`_.
+
 Discovering Runtime Dependencies
 --------------------------------
 
diff --git a/platforms/kernel.in b/platforms/kernel.in
index 8fe47b741..ff3cc8df4 100644
--- a/platforms/kernel.in
+++ b/platforms/kernel.in
@@ -39,6 +39,21 @@ config KERNEL_MODULES_INSTALL
 	prompt "Install modules into /lib/modules"
 	depends on KERNEL_MODULES
 
+config KERNEL_MODULES_SIGN
+	bool
+	depends on KERNEL_MODULES
+	select KERNEL_CODE_SIGNING
+	select KERNEL_MODULES_INSTALL
+	prompt "sign modules"
+	help
+	  If enabled, kernel modules are signed during the install stage with
+	  the key specified by the code signing provider in the "kernel-modules"
+	  role.
+
+	  See the section "Kernel module signing" in the "Daily Work" chapter in
+	  the PTXdist manual for use cases and more infos about what needs to be
+	  enabled in the kernel config file.
+
 config KERNEL_VERSION
 	prompt "kernel version"
 	string
diff --git a/rules/kernel.make b/rules/kernel.make
index ac2745083..9caff6779 100644
--- a/rules/kernel.make
+++ b/rules/kernel.make
@@ -76,6 +76,10 @@ KERNEL_BASE_OPT		+= \
 	$(if $(shell cs_get_ca kernel-trusted), \
 		CONFIG_SYSTEM_TRUSTED_KEYS=$(shell cs_get_ca kernel-trusted))
 endif
+ifdef PTXCONF_KERNEL_MODULES_SIGN
+KERNEL_BASE_OPT		+= \
+	CONFIG_MODULE_SIG_KEY='"$(shell cs_get_uri kernel-modules)"'
+endif
 
 # Intermediate option. This will be used by kernel module packages.
 KERNEL_MODULE_OPT	= \