Re: [PATCH v7 1/5] tools/certs: Add print-cert-tbs-hash.sh

[Eric Snowberg <eric.snowberg@oracle.com>]

> On Mar 12, 2021, at 10:12 AM, Mickaël Salaün <mic@digikod.net> wrote:
> 
> 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/20210312171232.2681989-2-mic@digikod.net

Tested-by: Eric Snowberg <eric.snowberg@oracle.com>

> ---
> 
> 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 00836f6452f0..773a362e807f 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -4120,6 +4120,7 @@ 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 Sandonis <miguel.ojeda.sandonis@gmail.com>
> 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}'
> -- 
> 2.30.2
>