[ptxdist] [PATCH v2 5/5] ptxd_lib_code_signing: add key whitelist checks

[Roland Hieber <rhi@pengutronix.de>]

Signed-off-by: Roland Hieber <rhi@pengutronix.de>
---
PATCH v2:
 - cs_check_whitelisted: make "needle"  local variable (feedback by
   Michael Olbrich)
 - cs_check_whitelisted: error out with ERROR_KEY_NOT_WHITELISTED also
   if whitelist does not exist yet (Michael Olbrich)
 - rename cs_get_uri to cs_get_uri_unchecked and let cs_get_uri wrap it
   instead of setting cs_no_whitelist_check=1 (Michael Olbrich)
 - docs: simplify introductory example (Michael Olbrich)
 - docs: add short paragraph on how to determine fingerprints of certs

PATCH v1: https://lore.ptxdist.org/ptxdist/
---
 doc/dev_code_signing.rst                  | 80 +++++++++++++++++++++++
 platforms/code-signing.in                 | 22 +++++++
 rules/pre/030-code-signing-consumers.make |  6 ++
 scripts/lib/ptxd_lib_code_signing.sh      | 71 ++++++++++++++++++--
 4 files changed, 172 insertions(+), 7 deletions(-)

diff --git a/doc/dev_code_signing.rst b/doc/dev_code_signing.rst
index 413f694980eb..28ebe055346e 100644
--- a/doc/dev_code_signing.rst
+++ b/doc/dev_code_signing.rst
@@ -172,3 +172,83 @@ 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 "release" code signing provider which you use
+to sign bootloaders for production,
+and a "development" code signing provider which you use to sign bootloaders
+with an extended feature set, e.g. to allow booting arbitrary kernels and
+userlands for debugging purposes.
+Your production boards are locked down in hardware so the ROM code only
+executes bootloaders signed with the "release" key.
+Now you don't want any bootloader with debugging features to be signed with a
+release key, otherwise someone could boot them on a locked-down production
+device, and use the additional debugging features to get extended access to the
+production device.
+In this case, key whitelisting can help to prevent signing bootloader packages
+with the wrong key.
+
+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 the 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:``,
+and the fingerprint refers to the public key of the certificate.
+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
+
+or, in the case of certificates::
+
+   $ openssl x509 -noout -in cert.pem -pubkey \
+     | openssl pkey -pubin -inform pem -pubout -outform der \
+     | openssl sha256 \
+     | tr 'a-z' 'A-Z'
+   (STDIN)= 0034F8FE5ADC3B0DFE642407275D144DE2398C68CC9A86DD6703D7151116B44E
+
+__ 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 909e8ebd6936..bca05854372d 100644
--- a/rules/pre/030-code-signing-consumers.make
+++ b/rules/pre/030-code-signing-consumers.make
@@ -28,6 +28,9 @@ $(strip \
 		$(call ptx/cs-consumer-env, $(1))\
 			cs_get_uri '$(strip $(2))'\
 	)\
+	$(if $(filter-out 0,$(.SHELLSTATUS)),\
+		$(call ptx/error, cs-get-uri: whitelist check failed – see errors above)\
+	)\
 )
 endef
 
@@ -40,5 +43,8 @@ $(strip \
 		$(call ptx/cs-consumer-env, $(1))\
 			cs_get_ca '$(strip $(2))'\
 	)\
+	$(if $(filter-out 0,$(.SHELLSTATUS)),\
+		$(call ptx/error, cs-get-ca: whitelist check failed – see errors above)\
+	)\
 )
 endef
diff --git a/scripts/lib/ptxd_lib_code_signing.sh b/scripts/lib/ptxd_lib_code_signing.sh
index 24730d3cf742..c855821bd039 100644
--- a/scripts/lib/ptxd_lib_code_signing.sh
+++ b/scripts/lib/ptxd_lib_code_signing.sh
@@ -1,6 +1,7 @@
 #!/bin/bash
 #
 # Copyright (C) 2019 Sascha Hauer <s.hauer@pengutronix.de>
+# Copyright (C) 2020 Jan Luebbe <j.luebbe@pengutronix.de>
 # Copyright (C) 2021 Roland Hieber, Pengutronix <rhi@pengutronix.de>
 #
 # For further information about the PTXdist project and license conditions
@@ -70,6 +71,7 @@ cs_init_variables() {
     sysroot="$(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 +159,42 @@ 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}"
+    cs_init_variables
+
+    if [ "$(ptxd_get_ptxconf PTXCONF_CODE_SIGNING_REQUIRE_WHITELISTED_KEYS)" != "y" ]; then
+	return
+    fi
+
+    if ! ptxd_in_path PTXDIST_PATH_PLATFORMCONFIGDIR "${whitelist}"; then
+	echo ERROR_KEY_NOT_WHITELISTED
+	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}
+    local 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>
 #
@@ -171,12 +209,8 @@ cs_set_uri() {
 }
 export -f cs_set_uri
 
-#
-# cs_get_uri <role>
-#
-# Get the uri from a role
-#
-cs_get_uri() {
+# internal helper for cs_get_uri
+cs_get_uri_unchecked() {
     if [ -z "${pkg_name}" ]; then
 	    echo ERROR_UNSUPPORTED_CS_API_CALL
 	    ptxd_bailout '$(shell cs_get_uri, <role>) is no longer supported in make files.' \
@@ -198,8 +232,24 @@ cs_get_uri() {
 	    return
 	fi
     fi
+
     cat "${keydir}/${role}/uri"
 }
+export -f cs_get_uri_unchecked
+
+#
+# cs_get_uri <role>
+#
+# Get the uri from a role
+#
+cs_get_uri() {
+    local role="${1}"
+    local uri=$(cs_get_uri_unchecked "$1")
+
+    if cs_check_whitelisted "${role}" "${uri}"; then
+	echo "${uri}"
+    fi
+}
 export -f cs_get_uri
 
 #
@@ -321,6 +371,9 @@ 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 +392,9 @@ 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 +426,8 @@ cs_append_ca_from_uri() {
     cs_init_variables
 
     if [ -z "${uri}" ]; then
-	uri=$(cs_get_uri "${role}")
+	# cs_append_ca_from_der will check this later
+	uri=$(cs_get_uri_unchecked "${role}")
     fi
 
     ptxd_exec extract-cert "${uri}" "${tmpdir}/ca.der" &&
-- 
2.30.2


_______________________________________________
ptxdist mailing list
ptxdist@pengutronix.de
To unsubscribe, send a mail with subject "unsubscribe" to ptxdist-request@pengutronix.de