Re: [ptxdist] [PATCH] kernel: add support for kernel module signing

[Michael Olbrich <m.olbrich@pengutronix.de>]

On Mon, Jul 19, 2021 at 08:30:53PM +0200, Roland Hieber wrote:
> Use the code signing role 'kernel-modules' to supply the kernel with the
> keys for kernel module signing and additional CAs for the kernel's trust
> root. 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.
> 
> We can can add CONFIG_MODULE_SIG_KEY in all cases, it is simply ignored
> unless CONFIG_MODULE_SIG is also enabled in the kernel config. However,
> current kernels don't cope well with an empty CONFIG_SYSTEM_TRUSTED_KEYS
> variable, so only add it when the singing provider actually supplies CA
> certificates.
> 
> The cs_get_* functions print undefined strings when the code signing
> provider hasn't been installed into sysroot-host yet, which is usually
> the case when kernel.make is parsed at PTXdist startup. Therefore, all
> variables that make use of need to be evaluated recursively when they
> are used ('=' instead of ':='). All other recipes using KERNEL_*
> variables already take care of this.
> 
> Signed-off-by: Roland Hieber <rhi@pengutronix.de>
> ---
> Note: this depends on "[PATCH v5] ptxd_lib_code_signing: cs_get_ca():
> improve error handling", see
> https://lore.ptxdist.org/ptxdist/20210715134224.25700-1-rhi@pengutronix.de
> ---
>  doc/daily_work.inc  | 57 +++++++++++++++++++++++++++++++++++++++++++++
>  platforms/kernel.in | 17 ++++++++++++++
>  rules/kernel.make   | 14 ++++++++---
>  3 files changed, 85 insertions(+), 3 deletions(-)
> 
> diff --git a/doc/daily_work.inc b/doc/daily_work.inc
> index 8fe7739aa0c8..2ee6d858f19e 100644
> --- a/doc/daily_work.inc
> +++ b/doc/daily_work.inc
> @@ -123,6 +123,63 @@ To rebuild the kernel:
>           package. A ``ptxdist clean kernel`` call will only delete the
>           symlinks in the build directory, but not clean the kernel compiled files.
>  
> +Kernel Module Signing
> +---------------------
> +
> +The Linux kernel can generate crypgraphic 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.
> +
> +With the :ref:`code signing infrastructure <code_signing>`, PTXdist can supply
> +the kernel's build system with the private key for module signing.
> +However, additional settings must 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>`_.
> +
> +PTXdist additionally 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-module`` role of the configured
> +  code signing provider.
> +  (The code signing provider should use :ref:`cs_set_uri` to set the URI.)
> +* ``CONFIG_SYSTEM_TRUSTED_KEYS`` ("Additional X.509 keys for default system keyring"):
> +  If the code signing provider added CA certificates to the ``kernel-module``
> +  role, PTXdist adds this option to the kernel config to specify those
> +  certificates.
> +  (The code signing provider should use :ref:`cs_append_ca_from_der`,
> +  :ref:`cs_append_ca_from_pem`, or :ref:`cs_append_ca_from_uri` with the
> +  ``kernel-module`` role.)

The documentation does not match the code. Here 'kernel-module' is used as
role for the trusted keys....

> +
> +  In some setups this option can be necessary:
> +  For example, when using EVM, the EVM key must be issued by a key that is
> +  trusted by the kernel.
> +  If the same key is used for EVM and module signing (see
> +  ``CONFIG_MODULE_SIG_KEY``), *and* if this key is self-signed, no additional
> +  trusted CA is necessary because the module signing key is always added to
> +  the kernel's trust root.
> +  Otherwise, the CA which issued the EVM key must be supplied additionally
> +  through this kernel config option.
> +
> +.. important::
> +
> +   Also make sure that all necessary crypto algorithms are enabled in the kernel.
> +   For example, if your module signing key is signed with an SHA256 hash,
> +   you must enable ``CONFIG_CRYPTO_SHA256`` so that the signature can be verified.
> +   Otherwise, some older kernels throw a stack trace on boot, and will not load
> +   the supplied key material.
> +
>  Discovering Runtime Dependencies
>  --------------------------------
>  
> diff --git a/platforms/kernel.in b/platforms/kernel.in
> index 68899c0f7dcc..d7bff2656fd9 100644
> --- a/platforms/kernel.in
> +++ b/platforms/kernel.in
> @@ -3,6 +3,7 @@
>  menuconfig KERNEL
>  	bool
>  	default y
> +	select CODE_SIGNING		if KERNEL_MODULES_SIGN
>  	select HOST_U_BOOT_TOOLS	if KERNEL_IMAGE_U || (KERNEL_IMAGE_SIMPLE && ARCH_MICROBLAZE)
>  	select HOST_ZSTD		if KERNEL_ZSTD
>  	select HOST_XZ			if KERNEL_XZ
> @@ -38,6 +39,22 @@ 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_MODULES_INSTALL
> +	select KERNEL_OPENSSL
> +	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-module"
> +	  role. Additionally, the CA specified in the "kernel-module" role is
> +	  added to the kernel's trust root.
> +
> +	  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 f43c1bb8de89..750a68efc6fa 100644
> --- a/rules/kernel.make
> +++ b/rules/kernel.make
> @@ -53,18 +53,24 @@ endef
>  # check for old kernel modules rules
>  KERNEL_MAKEVARS = $(call kernel/deprecated, KERNEL_MAKEVARS)
>  
> +KERNEL_SIGN_OPT	= \
> +	CONFIG_MODULE_SIG_KEY='"$(shell cs_get_uri kernel-modules)"' \
> +	$(if $(shell cs_get_ca kernel-trusted), \
> +		CONFIG_SYSTEM_TRUSTED_KEYS=$(shell cs_get_ca kernel-trusted))

... and here 'kernel-trusted' is used to import the CA.

I think the documentation should be changed. From what I understand,
trusted keys are useful for more than just module signature verification.

And I think this should only be used if PTXCONF_KERNEL_MODULES_SIGN is
enabled.

> +
>  # like kernel-opts but with different CROSS_COMPILE=
>  KERNEL_BASE_OPT		:= \
>  	$(call kernel-opts, KERNEL,$(KERNEL_CROSS_COMPILE)) \
>  	$(call remove_quotes,$(PTXCONF_KERNEL_EXTRA_MAKEVARS))
>  
>  # Intermediate option. This will be used by kernel module packages.
> -KERNEL_MODULE_OPT	:= \
> +KERNEL_MODULE_OPT	= \
>  	-C $(KERNEL_DIR) \
>  	O=$(KERNEL_BUILD_DIR) \
> +	$(KERNEL_SIGN_OPT) \

So we have the variable KERNEL_MODULE_OPT (separate from KERNEL_SHARED_OPT)
as something that is also used by out-of-tree kernel modules.

I expect, that we don't need CONFIG_SYSTEM_TRUSTED_KEYS here. But we do
need CONFIG_MODULE_SIG_KEY to sign those modules, right?
I don't mind adding both here for simplicity, but I want to make sure that
they should be added here at all and not just to KERNEL_SHARED_OPT.

Michael

>  	$(KERNEL_BASE_OPT)
>  
> -KERNEL_SHARED_OPT	:= \
> +KERNEL_SHARED_OPT	= \
>  	$(KERNEL_MODULE_OPT)
>  
>  ifndef PTXCONF_KERNEL_GCC_PLUGINS
> @@ -166,6 +172,7 @@ $(STATEDIR)/kernel.tags:
>  
>  KERNEL_MAKE_OPT		= \
>  	$(call kernel/deprecated, KERNEL_MAKE_OPT) \
> +	$(KERNEL_SIGN_OPT) \
>  	$(KERNEL_SHARED_OPT) \
>  	$(KERNEL_IMAGE) \
>  	$(call ptx/ifdef, PTXCONF_KERNEL_MODULES,modules)
> @@ -231,7 +238,8 @@ endif
>  # Install
>  # ----------------------------------------------------------------------------
>  
> -KERNEL_INSTALL_OPT := \
> +KERNEL_INSTALL_OPT = \
> +	$(KERNEL_SIGN_OPT) \
>  	$(KERNEL_BASE_OPT) \
>  	modules_install
>  
> -- 
> 2.30.2
> 
> 
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