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Date: Mon, 15 Mar 2021 10:57:35 -0600
From: Eric Snowberg <eric.snowberg@...cle.com>
To: Mickaël Salaün <mic@...ikod.net>
Cc: David Howells <dhowells@...hat.com>,
David Woodhouse <dwmw2@...radead.org>,
Jarkko Sakkinen <jarkko@...nel.org>,
"David S . Miller" <davem@...emloft.net>,
Herbert Xu <herbert@...dor.apana.org.au>,
James Morris <jmorris@...ei.org>,
Mickaël Salaün <mic@...ux.microsoft.com>,
Mimi Zohar <zohar@...ux.ibm.com>,
"Serge E . Hallyn" <serge@...lyn.com>,
Tyler Hicks <tyhicks@...ux.microsoft.com>,
keyrings@...r.kernel.org, linux-crypto@...r.kernel.org,
linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-security-module@...r.kernel.org
Subject: Re: [PATCH v7 1/5] tools/certs: Add print-cert-tbs-hash.sh
> On Mar 12, 2021, at 10:12 AM, Mickaël Salaün <mic@...ikod.net> wrote:
>
> From: Mickaël Salaün <mic@...ux.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@...hat.com>
> Cc: David Woodhouse <dwmw2@...radead.org>
> Cc: Eric Snowberg <eric.snowberg@...cle.com>
> Signed-off-by: Mickaël Salaün <mic@...ux.microsoft.com>
> Reviewed-by: Jarkko Sakkinen <jarkko@...nel.org>
> Link: https://lore.kernel.org/r/20210312171232.2681989-2-mic@digikod.net
Tested-by: Eric Snowberg <eric.snowberg@...cle.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@...il.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@...ux.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
>
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