#!/bin/sh

# Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License"). You
# may not use this file except in compliance with the License. A copy of
# the License is located at
#
# http://aws.amazon.com/apache2.0/
#
# or in the "license" file accompanying this file. This file is
# distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF
# ANY KIND, either express or implied. See the License for the specific
# language governing permissions and limitations under the License.

# Reads and echoes EC2 Metadata to get the authorized keys blob for the user $1

set -e

# Set umask so only we can touch temp files
umask 077

IMDS="http://169.254.169.254/latest/meta-data"

# cURL wrapper to ensure we always use the desired flags
curl_cmd () {
    /usr/bin/curl -s -f -m 1 -H "${1}" "${2}"
}

# Fetch the IMDSv2 access token.  5 seconds is overall AKC timeout so we use that.
IMDS_TOKEN="$(/usr/bin/curl -s -f -m 1 -X PUT "http://169.254.169.254/latest/api/token" -H "X-aws-ec2-metadata-token-ttl-seconds: 5")"
token_exit="${?}"
if [ "${token_exit}" -ne 0 ] ; then
    /usr/bin/logger -i -p authpriv.info "EC2 Instance Connect failed to establish trust with Instance Metadata Service"
    exit 255
fi
if [ -z "${IMDS_TOKEN}" ] ; then
    # Fast fail
    /usr/bin/logger -i -p authpriv.info "EC2 Instance Connect failed to get a token to invoke Instance Metadata Service"
    exit 255
fi

IMDS_TOKEN_HEADER="X-aws-ec2-metadata-token: ${IMDS_TOKEN}"

# Verify the instance ID itself
# Note: if IMDSv1 is disabled here and IMDS_TOKEN is unset we fast-fail out right now.
instance=$(curl_cmd "${IMDS_TOKEN_HEADER}" "${IMDS}/instance-id/")
if [ -z "${instance}" ] ; then
  exit 0
fi
# Validate the instance ID is i-abcd1234 (8 or 17 char, hex)
# We have it buffered to 32 chars to futureproof any further EC2 format changes (given some other EC2 resources are already 32 char)
/bin/echo "${instance}" | /usr/bin/head -n 1 | /bin/grep -Eq "^i-[0-9a-f]{8,32}$" || exit 0
# Verify we have an EC2 uuid
if [ ! -f /sys/hypervisor/uuid ] ; then
    # Nitro, switch to DMI check
    if [ ! -f /sys/devices/virtual/dmi/id/board_asset_tag ] ; then
        # We're out of options.  This is definitely not an instance.
        /usr/bin/logger -i -p authpriv.info "EC2 Instance Connect was invoked on a non-instance and will do nothing."
        exit 0
    elif [ "$(/bin/cat /sys/devices/virtual/dmi/id/board_asset_tag)" != "${instance}" ] ; then
        # The board_asset_tag does not match the instance id.  This is not a valid instance.
        /usr/bin/logger -i -p authpriv.info "EC2 Instance Connect was invoked on a non-instance and will do nothing."
        exit 0
    fi
elif [ "$(/usr/bin/cut -c1-3 < /sys/hypervisor/uuid)" != "ec2" ] ; then
    # Leading bytes are not "ec2"
    /usr/bin/logger -i -p authpriv.info "EC2 Instance Connect was invoked on a non-instance and will do nothing."
    exit 0
fi

# At this point we're reasonably confident we're running on an EC2 instance.

OPENSSL=/usr/bin/openssl

if [ -z "${1}" ] ; then
    # No user provided, not really anything to query for.  Fail out.
    /usr/bin/logger -i -p authpriv.info "EC2 Instance Connect was invoked without a user to authorize and will do nothing."
    exit 1
fi

/usr/bin/id -u "${1}" > /dev/null 2>&1
id_exit="${?}"
if [ "${id_exit}" -ne 0 ] ; then
    # User doesn't actually exist.  Let sshd deal with it.
    exit 0
fi

# Verify that we have active keys.  Fast-exit if we do not.
keys_status="$(/usr/bin/curl -s -m 1 -H "${IMDS_TOKEN_HEADER}" -o /dev/null -I -w %{http_code} "${IMDS}/managed-ssh-keys/active-keys/${1}/")"
if [ "${keys_status}" != "200" ]
then
    # No keys for this user.   Nothing to do.
    exit 0
fi
# We are not checking format here - that is parse_authorized_keys's job

zone=$(curl_cmd "${IMDS_TOKEN_HEADER}" "${IMDS}/placement/availability-zone/")
zone_exit="${?}"
if [ "${zone_exit}" -ne 0 ]
then
    exit "${zone_exit}"
fi
# Validate the zone is aa-bb-#c (or aa-bb-cc-#d for special partitions like AWS GovCloud)
/bin/echo "${zone}" | /usr/bin/head -n 1 | /bin/grep -Eq "^([a-z]+-){2,3}[0-9][a-z]$" || exit 255

region=$(/bin/echo "${zone}" | /bin/sed -n 's/\(\([a-z]\+-\)\+[0-9]\+\).*/\1/p')
domain=$(curl_cmd "${IMDS_TOKEN_HEADER}" "${IMDS}/services/domain/")
domain_exit="${?}"
if [ "${domain_exit}" -ne 0 ]
then
    exit "${domain_exit}"
fi
is_domain_valid=1
for valid_domain in amazonaws.com amazonaws.com.cn c2s.ic.gov sc2s.sgov.gov; do
    if [ "$domain" = "$valid_domain" ]; then
        is_domain_valid=0
        break
    fi
done
if [ $is_domain_valid -eq 1 ]; then
    /usr/bin/logger -i -p authpriv.info "EC2 Instance Connect found an invalid domain and will do nothing."
    exit 255
fi

expected_signer=$(/usr/bin/printf 'managed-ssh-signer.%s.%s' "${region}" "${domain}")

userpath=$(/bin/mktemp -d /dev/shm/eic-XXXXXXXX)
trap 'rm -rf "${userpath:?}"' EXIT

# Read the current signer cert
# This will overwrite whatever currently exists, so it will remain up-to-date
certificate=$(curl_cmd "${IMDS_TOKEN_HEADER}" "${IMDS}/managed-ssh-keys/signer-cert/")
cert_exit="${?}"
if [ "${cert_exit}" -ne 0 ] || [ -z "${certificate}" ]
then
    exit "${cert_exit}"
fi
# parse_authorized_keys will verify this

# Read the signer OCSP staples
staple_paths=$(curl_cmd "${IMDS_TOKEN_HEADER}" "${IMDS}/managed-ssh-keys/signer-ocsp/")
staple_exit="${?}"
if [ "${staple_exit}" -ne 0 ]
then
    exit "${staple_exit}"
fi

ocsp_path=$(/bin/mktemp -d "${userpath}/eic-ocsp-XXXXXXXX")
for word in $staple_paths
do
    curl_cmd "${IMDS_TOKEN_HEADER}" "${IMDS}/managed-ssh-keys/signer-ocsp/${word}" | /usr/bin/base64 -d > "${ocsp_path}/${word}"
    staple_exit="${?}"
    if [ "${staple_exit}" -ne 0 ]
    then
        exit "${staple_exit}"
    fi
    /bin/chmod 400 "${ocsp_path}/${word}" # Disable access to staple file
done
# parse_authorized_keys will verify these

# Invoke key parser (will automagically echo the results)
keys_file="${userpath}/eic-keys"
curl_cmd "${IMDS_TOKEN_HEADER}" "${IMDS}/managed-ssh-keys/active-keys/${1}/" > "${keys_file}"
DIR="$( cd "$( dirname "${0}" )" && pwd )"
ca_path=/etc/ssl/certs
if [ -z "${2}" ] ; then
    output="$("${DIR}/eic_parse_authorized_keys" -x false -p "${keys_file}" -o "${OPENSSL}" -d "${userpath}" -s "${certificate}" -i "${instance}" -c "${expected_signer}" -a "${ca_path}" -v "${ocsp_path}")"
    exitcode=$? # not quote-escaped since this must be numeric 0-255
else
    output="$("${DIR}/eic_parse_authorized_keys" -x false -p "${keys_file}" -o "${OPENSSL}" -d "${userpath}" -s "${certificate}" -i "${instance}" -c "${expected_signer}" -a "${ca_path}" -v "${ocsp_path}" -f "${2}")"
    exitcode=$? # not quote-escaped since this must be numeric 0-255
fi

/bin/echo "${output}"
exit $exitcode