Protected write & undefine operations for NV index

protected_nv_setup.sh

#!/bin/bash

#
# Copyright (C) 2021  Imran Desai
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.
#

#
# Goal: Protect the write and undefine operations of an NV Index
#
# Constraints:
# 1. NV index must be readable by all
# 2. NV index can only be written by authenticating signing authority.
# 3. NV index can only be undefined by authenticating signing authority.
# 4. TPM2_CC_Clear shouldn't be able to delete the NV index.
# 5. Platform hierarchy authorization must not be required after provisioning.
#

# set -e

cleanup() {

    rm -f session.ctx signing_key_public.pem signing_key.ctx signing_key.name \
        write.policy undefine.policy nvwrite_OR_nvundefine.policy to_sign.bin \
        signature.dat

    tpm2_flushcontext session.ctx 2>/dev/null || true
}

# trap cleanup EXIT

setup_signing_authority() {

    openssl genrsa -out signing_key_private.pem 2048

    openssl rsa -in signing_key_private.pem -outform PEM -pubout -out \
        signing_key_public.pem

    tpm2_loadexternal -G rsa -C n -u signing_key_public.pem -c signing_key.ctx \
        -n signing_key.name
}

setup_nv_index_policies() {

    tpm2_flushcontext session.ctx 2>/dev/null || true

    tpm2_startauthsession -S session.ctx

    tpm2 policysigned -S session.ctx -c signing_key.ctx -L write.policy

    tpm2_policycommandcode -S session.ctx -L undefine.policy \
        TPM2_CC_NV_UndefineSpaceSpecial

    tpm2_flushcontext session.ctx 2>/dev/null || true

    tpm2_startauthsession -S session.ctx

    tpm2_policyor -S session.ctx -L nvwrite_OR_nvundefine.policy \
        -l sha256:write.policy,undefine.policy

    tpm2_flushcontext session.ctx 2>/dev/null || true
}

satisfy_nv_write_policy() {

    tpm2_flushcontext session.ctx 2>/dev/null || true

    tpm2_startauthsession -S session.ctx --policy-session

    tpm2_policysigned -S session.ctx -c signing_key.ctx -x \
        --raw-data to_sign.bin

    openssl dgst -sha256 -sign signing_key_private.pem -out signature.dat \
        to_sign.bin

    tpm2_policysigned -S session.ctx -g sha256 -s signature.dat -f rsassa \
        -c signing_key.ctx -x

    tpm2_policyor -S session.ctx -l sha256:write.policy,undefine.policy
}

satisfy_nv_undefine_policy() {

    tpm2_flushcontext session.ctx 2>/dev/null || true

    tpm2_startauthsession -S session.ctx --policy-session

    tpm2_policysigned -S session.ctx -c signing_key.ctx -x --raw-data \
        to_sign.bin

    openssl dgst -sha256 -sign signing_key_private.pem -out signature.dat \
        to_sign.bin

    tpm2_policysigned -S session.ctx -g sha256 -s signature.dat -f rsassa \
        -c signing_key.ctx -x

    tpm2_policycommandcode -S session.ctx TPM2_CC_NV_UndefineSpaceSpecial

    tpm2_policyor -S session.ctx -l sha256:write.policy,undefine.policy
}

#
# Sample Flow
#
<<'SAMPLE-FLOW'
setup_signing_authority

setup_nv_index_policies

# Define NV index
tpm2_nvdefine -C p 0x1500019 --size 6 --policy nvwrite_OR_nvundefine.policy \
    --attributes "authread|policydelete|policywrite|platformcreate"

# Write NV index
satisfy_nv_write_policy
echo "6byte" > 1.txt
tpm2_nvwrite 0x1500019 -P "session:session.ctx" -i 1.txt

# Read NV index contents
tpm2_nvread 0x1500019 | hexdump -Cv

# Undefine NV index
satisfy_nv_undefine_policy
tpm2_nvundefine --session session.ctx 0x1500019

# Cleanup
cleanup

exit 0
SAMPLE-FLOW