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From: Roland Hieber <rhi@pengutronix.de>
To: ptxdist@pengutronix.de
Subject: Re: [ptxdist] [PATCH] kernel: add support for kernel module signing
Date: Fri, 23 Jul 2021 12:17:36 +0200
Message-ID: <20210723101736.ul7jg7mvwurdrk34@pengutronix.de> (raw)
In-Reply-To: <20210721085453.GA735550@pengutronix.de>

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 <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....

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
> > 
> > 
> > _______________________________________________
> > ptxdist mailing list
> > ptxdist@pengutronix.de
> > To unsubscribe, send a mail with subject "unsubscribe" to ptxdist-request@pengutronix.de
> > 
> 
> -- 
> Pengutronix e.K.                           |                             |
> Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
> 31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
> Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |
> 
> _______________________________________________
> ptxdist mailing list
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> To unsubscribe, send a mail with subject "unsubscribe" to ptxdist-request@pengutronix.de
> 

-- 
Roland Hieber, Pengutronix e.K.          | r.hieber@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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  reply	other threads:[~2021-07-23 10:18 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-19 18:30 Roland Hieber
2021-07-20  9:38 ` [ptxdist] [PATCH 2/1] host-ptx-code-signing-dev: version bump 0.5.1 -> 0.6 Roland Hieber
2021-07-21  8:54 ` [ptxdist] [PATCH] kernel: add support for kernel module signing Michael Olbrich
2021-07-23 10:17   ` Roland Hieber [this message]
2021-07-23 10:39     ` Michael Olbrich
2021-07-23 14:02       ` Roland Hieber

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