[PATCH v8 1/5] tools/certs: Add print-cert-tbs-hash.sh

Mickaël Salaün
2021-07-12T17:03:44Z

From: Mickaël Salaün <mic@linux.microsoft.com>

Add a new helper print-cert-tbs-hash.sh to generate a TBSCertificate
hash from a given certificate. This is useful to generate a blacklist
key description used to forbid loading a specific certificate in a
keyring, or to invalidate a certificate provided by a PKCS#7 file.

This kind of hash formatting is required to populate the file pointed
out by CONFIG_SYSTEM_BLACKLIST_HASH_LIST, but only the kernel code was
available to understand how to effectively create such hash.

Cc: David Howells <dhowells@redhat.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Eric Snowberg <eric.snowberg@oracle.com>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20210712170313.884724-2-mic@digikod.net
---

Changes since v5:
* Add Reviewed-by Jarkko.

Changes since v3:
* Explain in the commit message that this kind of formating is not new
but it wasn't documented.

Changes since v1:
* Fix typo.
* Use "if" block instead of "||" .
---
MAINTAINERS | 1 +
tools/certs/print-cert-tbs-hash.sh | 91 ++++++++++++++++++++++++++++++
2 files changed, 92 insertions(+)
create mode 100755 tools/certs/print-cert-tbs-hash.sh

diff --git a/MAINTAINERS b/MAINTAINERS
index a61f4f3b78a9..0b3be78d27ef 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4301,6 +4301,7 @@ F: Documentation/admin-guide/module-signing.rst
F: certs/
F: scripts/extract-cert.c
F: scripts/sign-file.c
+F: tools/certs/

CFAG12864B LCD DRIVER
M: Miguel Ojeda <ojeda@kernel.org>
diff --git a/tools/certs/print-cert-tbs-hash.sh b/tools/certs/print-cert-tbs-hash.sh
new file mode 100755
index 000000000000..c93df5387ec9
--- /dev/null
+++ b/tools/certs/print-cert-tbs-hash.sh
@@ -0,0 +1,91 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Copyright © 2020, Microsoft Corporation. All rights reserved.
+#
+# Author: Mickaël Salaün <mic@linux.microsoft.com>
+#
+# Compute and print the To Be Signed (TBS) hash of a certificate. This is used
+# as description of keys in the blacklist keyring to identify certificates.
+# This output should be redirected, without newline, in a file (hash0.txt) and
+# signed to create a PKCS#7 file (hash0.p7s). Both of these files can then be
+# loaded in the kernel with.
+#
+# Exemple on a workstation:
+# ./print-cert-tbs-hash.sh certificate-to-invalidate.pem > hash0.txt
+# openssl smime -sign -in hash0.txt -inkey builtin-private-key.pem \
+# -signer builtin-certificate.pem -certfile certificate-chain.pem \
+# -noattr -binary -outform DER -out hash0.p7s
+#
+# Exemple on a managed system:
+# keyctl padd blacklist "$(< hash0.txt)" %:.blacklist < hash0.p7s
+
+set -u -e -o pipefail
+
+CERT="${1:-}"
+BASENAME="$(basename -- "${BASH_SOURCE[0]}")"
+
+if [ $# -ne 1 ] || [ ! -f "${CERT}" ]; then
+ echo "usage: ${BASENAME} <certificate>" >&2
+ exit 1
+fi
+
+# Checks that it is indeed a certificate (PEM or DER encoded) and exclude the
+# optional PEM text header.
+if ! PEM="$(openssl x509 -inform DER -in "${CERT}" 2>/dev/null || openssl x509 -in "${CERT}")"; then
+ echo "ERROR: Failed to parse certificate" >&2
+ exit 1
+fi
+
+# TBSCertificate starts at the second entry.
+# Cf. https://tools.ietf.org/html/rfc3280#section-4.1
+#
+# Exemple of first lines printed by openssl asn1parse:
+# 0:d=0 hl=4 l= 763 cons: SEQUENCE
+# 4:d=1 hl=4 l= 483 cons: SEQUENCE
+# 8:d=2 hl=2 l= 3 cons: cont [ 0 ]
+# 10:d=3 hl=2 l= 1 prim: INTEGER :02
+# 13:d=2 hl=2 l= 20 prim: INTEGER :3CEB2CB8818D968AC00EEFE195F0DF9665328B7B
+# 35:d=2 hl=2 l= 13 cons: SEQUENCE
+# 37:d=3 hl=2 l= 9 prim: OBJECT :sha256WithRSAEncryption
+RANGE_AND_DIGEST_RE='
+2s/^\s*\([0-9]\+\):d=\s*[0-9]\+\s\+hl=\s*[0-9]\+\s\+l=\s*\([0-9]\+\)\s\+cons:\s*SEQUENCE\s*$/\1 \2/p;
+7s/^\s*[0-9]\+:d=\s*[0-9]\+\s\+hl=\s*[0-9]\+\s\+l=\s*[0-9]\+\s\+prim:\s*OBJECT\s*:\(.*\)$/\1/p;
+'
+
+RANGE_AND_DIGEST=($(echo "${PEM}" | \
+ openssl asn1parse -in - | \
+ sed -n -e "${RANGE_AND_DIGEST_RE}"))
+
+if [ "${#RANGE_AND_DIGEST[@]}" != 3 ]; then
+ echo "ERROR: Failed to parse TBSCertificate." >&2
+ exit 1
+fi
+
+OFFSET="${RANGE_AND_DIGEST[0]}"
+END="$(( OFFSET + RANGE_AND_DIGEST[1] ))"
+DIGEST="${RANGE_AND_DIGEST[2]}"
+
+# The signature hash algorithm is used by Linux to blacklist certificates.
+# Cf. crypto/asymmetric_keys/x509_cert_parser.c:x509_note_pkey_algo()
+DIGEST_MATCH=""
+while read -r DIGEST_ITEM; do
+ if [ -z "${DIGEST_ITEM}" ]; then
+ break
+ fi
+ if echo "${DIGEST}" | grep -qiF "${DIGEST_ITEM}"; then
+ DIGEST_MATCH="${DIGEST_ITEM}"
+ break
+ fi
+done < <(openssl list -digest-commands | tr ' ' '\n' | sort -ur)
+
+if [ -z "${DIGEST_MATCH}" ]; then
+ echo "ERROR: Unknown digest algorithm: ${DIGEST}" >&2
+ exit 1
+fi
+
+echo "${PEM}" | \
+ openssl x509 -in - -outform DER | \
+ dd "bs=1" "skip=${OFFSET}" "count=${END}" "status=none" | \
+ openssl dgst "-${DIGEST_MATCH}" - | \
+ awk '{printf "tbs:" $2}'
--
2.32.0