certs: Check that builtin blacklist hashes are valid

Add and use a check-blacklist-hashes.awk script to make sure that the
builtin blacklist hashes set with CONFIG_SYSTEM_BLACKLIST_HASH_LIST will
effectively be taken into account as blacklisted hashes.  This is useful
to debug invalid hash formats, and it make sure that previous hashes
which could have been loaded in the kernel, but silently ignored, are
now noticed and deal with by the user at kernel build time.

This also prevent stricter blacklist key description checking (provided
by following commits) to failed for builtin hashes.

Update CONFIG_SYSTEM_BLACKLIST_HASH_LIST help to explain the content of
a hash string and how to generate certificate ones.

Cc: David Howells <dhowells@redhat.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Eric Snowberg <eric.snowberg@oracle.com>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Link: https://lore.kernel.org/r/20210712170313.884724-3-mic@digikod.net
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

MAINTAINERS

@@ -4575,6 +4575,7 @@ L:	keyrings@vger.kernel.org
 S:	Maintained
 F:	Documentation/admin-guide/module-signing.rst
 F:	certs/
+F:	scripts/check-blacklist-hashes.awk
 F:	scripts/sign-file.c
 F:	tools/certs/
 

certs/.gitignore

@@ -1,4 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0-only
+/blacklist_hashes_checked
 /extract-cert
 /x509_certificate_list
 /x509_revocation_list

certs/Kconfig

@@ -104,8 +104,11 @@ config SYSTEM_BLACKLIST_HASH_LIST
 	help
 	  If set, this option should be the filename of a list of hashes in the
 	  form "<hash>", "<hash>", ... .  This will be included into a C
-	  wrapper to incorporate the list into the kernel.  Each <hash> should
+	  wrapper to incorporate the list into the kernel.  Each <hash> must be a
-	  be a string of hex digits.
+	  string starting with a prefix ("tbs" or "bin"), then a colon (":"), and
+	  finally an even number of hexadecimal lowercase characters (up to 128).
+	  Certificate hashes can be generated with
+	  tools/certs/print-cert-tbs-hash.sh .
 
 config SYSTEM_REVOCATION_LIST
 	bool "Provide system-wide ring of revocation certificates"

certs/Makefile

@@ -7,6 +7,18 @@ obj-$(CONFIG_SYSTEM_TRUSTED_KEYRING) += system_keyring.o system_certificates.o c
 obj-$(CONFIG_SYSTEM_BLACKLIST_KEYRING) += blacklist.o common.o
 obj-$(CONFIG_SYSTEM_REVOCATION_LIST) += revocation_certificates.o
 ifneq ($(CONFIG_SYSTEM_BLACKLIST_HASH_LIST),)
+quiet_cmd_check_blacklist_hashes = CHECK   $(patsubst "%",%,$(2))
+      cmd_check_blacklist_hashes = $(AWK) -f $(srctree)/scripts/check-blacklist-hashes.awk $(2); touch $@
+
+$(eval $(call config_filename,SYSTEM_BLACKLIST_HASH_LIST))
+
+$(obj)/blacklist_hashes.o: $(obj)/blacklist_hashes_checked
+
+CFLAGS_blacklist_hashes.o += -I$(srctree)
+
+targets += blacklist_hashes_checked
+$(obj)/blacklist_hashes_checked: $(SYSTEM_BLACKLIST_HASH_LIST_SRCPREFIX)$(SYSTEM_BLACKLIST_HASH_LIST_FILENAME) scripts/check-blacklist-hashes.awk FORCE
+       $(call if_changed,check_blacklist_hashes,$(SYSTEM_BLACKLIST_HASH_LIST_SRCPREFIX)$(CONFIG_SYSTEM_BLACKLIST_HASH_LIST))
 obj-$(CONFIG_SYSTEM_BLACKLIST_KEYRING) += blacklist_hashes.o
 else
 obj-$(CONFIG_SYSTEM_BLACKLIST_KEYRING) += blacklist_nohashes.o
@@ -21,7 +33,7 @@ $(obj)/system_certificates.o: $(obj)/x509_certificate_list
 $(obj)/x509_certificate_list: $(CONFIG_SYSTEM_TRUSTED_KEYS) $(obj)/extract-cert FORCE
 	$(call if_changed,extract_certs)
 
-targets += x509_certificate_list
+targets += x509_certificate_list blacklist_hashes_checked
 
 # If module signing is requested, say by allyesconfig, but a key has not been
 # supplied, then one will need to be generated to make sure the build does not

scripts/check-blacklist-hashes.awk

@@ -0,0 +1,37 @@
+#!/usr/bin/awk -f
+# SPDX-License-Identifier: GPL-2.0
+#
+# Copyright © 2020, Microsoft Corporation. All rights reserved.
+#
+# Author: Mickaël Salaün <mic@linux.microsoft.com>
+#
+# Check that a CONFIG_SYSTEM_BLACKLIST_HASH_LIST file contains a valid array of
+# hash strings.  Such string must start with a prefix ("tbs" or "bin"), then a
+# colon (":"), and finally an even number of hexadecimal lowercase characters
+# (up to 128).
+
+BEGIN {
+	RS = ","
+}
+{
+	if (!match($0, "^[ \t\n\r]*\"([^\"]*)\"[ \t\n\r]*$", part1)) {
+		print "Not a string (item " NR "):", $0;
+		exit 1;
+	}
+	if (!match(part1[1], "^(tbs|bin):(.*)$", part2)) {
+		print "Unknown prefix (item " NR "):", part1[1];
+		exit 1;
+	}
+	if (!match(part2[2], "^([0-9a-f]+)$", part3)) {
+		print "Not a lowercase hexadecimal string (item " NR "):", part2[2];
+		exit 1;
+	}
+	if (length(part3[1]) > 128) {
+		print "Hash string too long (item " NR "):", part3[1];
+		exit 1;
+	}
+	if (length(part3[1]) % 2 == 1) {
+		print "Not an even number of hexadecimal characters (item " NR "):", part3[1];
+		exit 1;
+	}
+}