On Wed, Jul 21, 2021 at 10:54:53AM +0200, Michael Olbrich wrote:
> 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 <r...@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-...@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....
Ah yes. I also think 'kernel-trusted' is a better name here.
> > +
> > + 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.
Yes.
> And I think this should only be used if PTXCONF_KERNEL_MODULES_SIGN is
> enabled.
Hmm. I would add the 'kernel-trusted' lines regardless of whether module
singing is enabled; it already has a check anyway and is only added if
there are any CAs, and as you said it is useful for other things besides
module signing.
> > +
> > # 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?
Yes, I think so too.
> 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.
I think it's best to add module signing and add trusted keys in two
separate patches, put CONFIG_SYSTEM_TRUSTED_KEYS into KERNEL_BASE_OPT
(with its if condition), and only have CONFIG_MODULE_SIG_KEY in
KERNEL_SIGN_OPT (behind a ifdef PTXCONF_KERNEL_MODULES_SIGN).
- Roland
> 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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> > ptxdist@pengutronix.de
> > To unsubscribe, send a mail with subject "unsubscribe" to
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> >
>
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--
Roland Hieber, Pengutronix e.K. | r.hie...@pengutronix.de |
Steuerwalder Str. 21 | https://www.pengutronix.de/ |
31137 Hildesheim, Germany | Phone: +49-5121-206917-0 |
Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
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