[ptxdist] [PATCH v1 5/5] ptxd_lib_code_signing: add key whitelist checks

[Roland Hieber <rhi@pengutronix.de>]

Signed-off-by: Roland Hieber <rhi@pengutronix.de>
---
 doc/dev_code_signing.rst                  | 68 +++++++++++++++++++++++
 platforms/code-signing.in                 | 22 ++++++++
 rules/pre/030-code-signing-consumers.make |  6 ++
 scripts/lib/ptxd_lib_code_signing.sh      | 52 ++++++++++++++++-
 4 files changed, 147 insertions(+), 1 deletion(-)

diff --git a/doc/dev_code_signing.rst b/doc/dev_code_signing.rst
index 413f694980eb..aaaaae0d9107 100644
--- a/doc/dev_code_signing.rst
+++ b/doc/dev_code_signing.rst
@@ -172,3 +172,71 @@ also via an environment variable.
   (``=``, not ``:=``).
   Otherwise the variable is expanded before a code signing provider can perform
   its setup.
+
+Key Whitelisting
+~~~~~~~~~~~~~~~~
+
+In some use cases, it may be feasible to do additional checks to make sure that
+a package uses the correct key material.
+For example, suppose you have a "development" code signing provider (e.g. with a
+SoftHSM workflow) which you use to sign your development kernels,
+and a "release" provider (e.g. on a HSM) which you use to sign your release kernels,
+and a "development" and "release" variant of the bootloader which only boots
+kernels that have been signed with the respective development or release key.
+Now you might want to prevent the "release" bootloader from accidentally being
+provided with the key for the "development" kernel –
+otherwise someone could boot a development kernel on an actual production
+board, and use the additional debugging features of the kernel to do things
+with it that it was not meant to do.
+
+When the ``CODE_SIGNING_REQUIRE_WHITELIST`` kconfig symbol is enabled,
+the consumer functions :ref:`ptx/cs-get-ca` and :ref:`ptx/cs-get-uri`
+look up the triplet of package name, role name, and the pubkey's SHA256
+fingerprint in a whitelist, whose file name is determined by the
+``CODE_SIGNING_WHITELIST_FILENAME`` kconfig symbol (the default path is
+``configs/platform-<name>/code-signing-whitelist``).
+If a key or a CA is not whitelisted for the package in which it is to be used,
+the functions will exit with an error message on the terminal::
+
+   $ ptxdist -v print KERNEL_MAKE_OPT
+   ptxdist: error: SPKI whitelist record 'kernel kernel-modules
+   69C9BBB8BB4DFAE74AB21D06DFB5F2C67066373AE545453276847340822CDF04' not found in
+   distrokit/configs/platform-v7a/code-signing-whitelist
+
+   …/ptxdist/rules/kernel.make:196: *** cs-get-uri: whitelist check failed, see errors above.  Stop.
+
+
+If ``CODE_SIGNING_REQUIRE_WHITELIST`` is disabled (the default),
+all keys and CAs are provided to all packages without further checks.
+
+The format of the code signing whitelist consists of one triplet per line, in
+which the elements of the triplet are separated by whitespace.
+If a CA is to be checked, the role name is prefixed with a literal ``ca:``.
+All other unmatched lines in the file are ignored, but we suggest to use ``#``
+to start a line comment so as not to add a whitelist record accidentally.
+
+For example, here is a whitelist for use with the *devel* code provider which
+allows all provided keys to be used by their respective consumers::
+
+   # format: package-name role-name sha256-pubkey-fingerprint
+   kernel      kernel-modules 69C9BBB8BB4DFAE74AB21D06DFB5F2C67066373AE545453276847340822CDF04
+   image-rauc  update         0034F8FE5ADC3B0DFE642407275D144DE2398C68CC9A86DD6703D7151116B44E
+   image-rauc  ca:update      0034F8FE5ADC3B0DFE642407275D144DE2398C68CC9A86DD6703D7151116B44E
+   rauc        ca:update      0034F8FE5ADC3B0DFE642407275D144DE2398C68CC9A86DD6703D7151116B44E
+
+.. note:: The match is case-sensitive, and the fingerprints are expected
+   in uppercase.
+
+   If a CA consists of more than one certificate, all of their fingerprints
+   must be whitelisted.
+
+You can determine the key fingerprints by copying it from the error message,
+or with the `spki-hash`__ tool from the ``host-extract-cert`` package,
+or with openssl::
+
+   $ openssl pkey -in keyfile.pem -pubout -outform der \
+     | openssl sha256 \
+     | tr 'a-z' 'A-Z'
+   (STDIN)= 69C9BBB8BB4DFAE74AB21D06DFB5F2C67066373AE545453276847340822CDF04
+
+__ https://git.pengutronix.de/cgit/extract-cert/tree/spki-hash.c
diff --git a/platforms/code-signing.in b/platforms/code-signing.in
index 81f9ef6f3c9e..92f555c7e965 100644
--- a/platforms/code-signing.in
+++ b/platforms/code-signing.in
@@ -20,4 +20,26 @@ source "generated/code_signing_provider.in"
 
 endchoice
 
+config CODE_SIGNING_REQUIRE_WHITELISTED_KEYS
+	bool
+	prompt "require whitelisted keys"
+	help
+	  Every time a key from the code provider is used, check if the consumer
+	  is allowed to use it.
+
+	  Code signing consumers can depend on this option if they want to force
+	  the key whitelist check.
+
+if CODE_SIGNING_REQUIRE_WHITELISTED_KEYS
+
+config CODE_SIGNING_WHITELIST_FILENAME
+	string
+	prompt "whitelist file name"
+	default "code-signing-whitelist"
+	help
+	  This file name is used for the key whitelist. The path is relative to
+	  PTXDIST_PLATFORMCONFIGDIR (e.g. configs/platform-nnn/).
+
+endif # CODE_SIGNING_REQUIRE_WHITELISTED_KEYS
+
 endif
diff --git a/rules/pre/030-code-signing-consumers.make b/rules/pre/030-code-signing-consumers.make
index 72ee8e63b7b9..439db626dd14 100644
--- a/rules/pre/030-code-signing-consumers.make
+++ b/rules/pre/030-code-signing-consumers.make
@@ -27,6 +27,9 @@ ptx/cs-get-uri = \
 			$(call ptx/cs-consumer-env, $(1))\
 				cs_get_uri '$(strip $(2))'\
 		)\
+		$(if $(filter-out 0,$(.SHELLSTATUS)),\
+			$(error cs-get-uri: whitelist check failed, see errors above)\
+		)\
 	)
 
 #
@@ -38,4 +41,7 @@ ptx/cs-get-ca = \
 			$(call ptx/cs-consumer-env, $(1))\
 				cs_get_ca '$(strip $(2))'\
 		)\
+		$(if $(filter-out 0,$(.SHELLSTATUS)),\
+			$(error cs-get-ca: whitelist check failed, see errors above)\
+		)\
 	)
diff --git a/scripts/lib/ptxd_lib_code_signing.sh b/scripts/lib/ptxd_lib_code_signing.sh
index 24730d3cf742..332bab22e7c8 100644
--- a/scripts/lib/ptxd_lib_code_signing.sh
+++ b/scripts/lib/ptxd_lib_code_signing.sh
@@ -1,6 +1,7 @@
 #!/bin/bash
 #
 # Copyright (C) 2019 Sascha Hauer <s.hauer@pengutronix.de>
+# Copyright (C) 2020 Jan Luebbe <j.luebbe@pengutronix.de>
 # Copyright (C) 2021 Roland Hieber, Pengutronix <rhi@pengutronix.de>
 #
 # For further information about the PTXdist project and license conditions
@@ -70,6 +71,7 @@ cs_init_variables() {
     sysroot="$(ptxd_get_ptxconf PTXCONF_SYSROOT_HOST)"
     keyprovider="$(ptxd_get_ptxconf PTXCONF_CODE_SIGNING_PROVIDER)"
     keydir="${sysroot}/var/lib/keys/${keyprovider}"
+    whitelist="$(ptxd_get_ptxconf PTXCONF_CODE_SIGNING_WHITELIST_FILENAME || true)"
 }
 export -f cs_init_variables
 
@@ -157,6 +159,40 @@ cs_group_get_roles() {
 }
 export -f cs_group_get_roles
 
+#
+# cs_check_whitelisted <role> <uri/pem or fingerprint prefixed with "sha256:">
+#
+# Checks if the SPKI (Subject Public Key Info) Hash is in the whitelist
+#
+cs_check_whitelisted() {
+    local role="${1:-ERROR_ROLE_IS_EMPTY}"
+    local src="${2}"
+
+    if [ "$(ptxd_get_ptxconf PTXCONF_CODE_SIGNING_REQUIRE_WHITELISTED_KEYS)" != "y" ]; then
+	return
+    fi
+
+    if ! ptxd_in_path PTXDIST_PATH_PLATFORMCONFIGDIR "${whitelist}"; then
+	ptxd_bailout "${pkg_name}: SPKI hash whitelist check for ${role} (${src}) is required, but configs/<platform>/${whitelist} is missing."
+    fi
+    local whitelist_path="${ptxd_reply}"
+
+    local hash
+    if [ "${src#sha256:}" != "${src}" ]; then
+	hash="${src#sha256:}"
+    else
+	hash=$(ptxd_exec_silent_stderr spki-hash "${src}")
+    fi
+    hash=${hash:-ERROR_HASH_IS_EMPTY}
+    needle="${pkg_name}\s\+${role}\s\+${hash}"
+
+    if ! grep -q --line-regexp "${needle}" "${whitelist_path}"; then
+	echo ERROR_KEY_NOT_WHITELISTED
+	ptxd_bailout "SPKI whitelist record '${pkg_name} ${role} ${hash}' not found in $(ptxd_print_path "${whitelist_path}")"
+    fi
+}
+export -f cs_check_whitelisted
+
 #
 # cs_set_uri <role> <uri>
 #
@@ -198,7 +234,12 @@ cs_get_uri() {
 	    return
 	fi
     fi
-    cat "${keydir}/${role}/uri"
+
+    local uri=$(cat "${keydir}/${role}/uri")
+    if [ -z "${cs_no_whitelist_check}" ]; then
+	cs_check_whitelisted "${role}" "${uri}"
+    fi
+    echo "${uri}"
 }
 export -f cs_get_uri
 
@@ -321,6 +362,9 @@ cs_get_ca() {
     fi
 
     if [ -e "${ca}" ]; then
+	while read fp; do
+	    cs_check_whitelisted "ca:${role}" "sha256:${fp}"
+	done < "${keydir}/${role}/ca.fingerprints"
 	echo "${ca}"
     fi
 }
@@ -339,6 +383,9 @@ cs_append_ca_from_pem() {
     cat "${pem}" >> "${keydir}/${role}/ca.pem"
     # add new line in case ${pem} does not end with an EOL
     echo >> "${keydir}/${role}/ca.pem"
+
+    openssl x509 -in "${pem}" -inform pem -noout -pubkey | \
+	spki-hash /dev/stdin >> "${keydir}/${role}/ca.fingerprints"
 }
 export -f cs_append_ca_from_pem
 
@@ -370,7 +417,10 @@ cs_append_ca_from_uri() {
     cs_init_variables
 
     if [ -z "${uri}" ]; then
+	# cs_append_ca_from_der will check this later
+	local cs_no_whitelist_check=1
 	uri=$(cs_get_uri "${role}")
+	unset cs_no_whitelist_check
     fi
 
     ptxd_exec extract-cert "${uri}" "${tmpdir}/ca.der" &&
-- 
2.30.2


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