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    Subject[PATCH v3 10/10] tools/certs: Add print-cert-tbs-hash.sh
    Date
    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.

    Cc: David Howells <dhowells@redhat.com>
    Cc: David Woodhouse <dwmw2@infradead.org>
    Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
    ---

    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 bda31ccbfad0..c7bad1354531 100644
    --- a/MAINTAINERS
    +++ b/MAINTAINERS
    @@ -4123,6 +4123,7 @@ F: certs/
    F: scripts/check-blacklist-hashes.awk
    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}'
    --
    2.30.0
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