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Filename : eic_harvest_hostkeys
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#!/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.

set -e

umask 077

# Fetch the IMDSv2 access token.  5 seconds is overall AKC timeout so we use that.
IMDS_TOKEN="$(/usr/bin/curl -s -f -m 1 -X PUT "http://169.254.169.254/latest/api/token" -H "X-aws-ec2-metadata-token-ttl-seconds: 5")"
if [ -z "${IMDS_TOKEN}" ] ; then
    # Fast fail
    exit 255
fi

# Verify the instance ID itself
instance=$(/usr/bin/curl -s -f -m 1 -H "X-aws-ec2-metadata-token: ${IMDS_TOKEN}" "http://169.254.169.254/latest/meta-data/instance-id/")
if [ -z "${instance}" ] ; then
  exit 255
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 255

# 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.
        exit 255
    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.
        exit 255
    fi
elif [ "$(/usr/bin/cut -c1-3 < /sys/hypervisor/uuid)" != "ec2" ] ; then
    # Leading bytes are not "ec2"
    exit 255
fi

# Calculate the SHA256 of a given string
# sha256 [string]
sha256 () {
    /bin/echo -n "${1}" | /usr/bin/sha256sum | /bin/sed 's/\s.*$//'
}

# Sign a message with a given key
# sign [key] [msg]
sign () {
    /usr/bin/printf "${2}" | /usr/bin/openssl dgst -binary -hex -sha256 -mac HMAC -macopt hexkey:"${1}" | /bin/sed 's/.* //'
}

# Derive a sigv4 signing key for the given secret
# get_sigv4_key [key] [datestamp] [region name] [service name]
getsigv4key () {
    base="$(/bin/echo -n "AWS4${1}" | /usr/bin/od -A n -t x1 | /bin/sed ':a;N;$!ba;s/[\n ]//g')"
    kdate="$(sign "${base}" "${2}")"
    kregion="$(sign "${kdate}" "${3}")"
    kservice="$(sign "${kregion}" "${4}")"
    sign "${kservice}" "aws4_request"
}

# Verify that we have instance identity credentials.  Fast-exit if we do not.
creds_status="$(/usr/bin/curl -s -f -m 1 -H "X-aws-ec2-metadata-token: ${IMDS_TOKEN}" -o /dev/null -I -w %{http_code} "http://169.254.169.254/latest/meta-data/identity-credentials/ec2/security-credentials/ec2-instance/")"
if [ "${creds_status}" != "200" ]
then
    # No keys for this user.   Nothing to do.
    exit 0
fi

#Iterates overs /etc/ssh to get the host keys
for file in /etc/ssh/*.pub; do
	/usr/bin/test -r "${file}" || continue
	key=$(/usr/bin/awk '{$1=$1};1' < "${file}")
	keys="${keys:+${keys},}\"${key}\""
done

#Temporary path to store request parameters
userpath=$(/bin/mktemp -d /dev/shm/eic-hostkey-XXXXXXXX)
trap 'rm -rf "${userpath}"' EXIT

#Get zone information
zone=$(/usr/bin/curl -s -f -m 1 -H "X-aws-ec2-metadata-token: ${IMDS_TOKEN}" "http://169.254.169.254/latest/meta-data/placement/availability-zone/")
zone_exit="${?}"
if [ "${zone_exit}" -ne 0 ]
then
    exit "${zone_exit}"
fi

# Validate the zone
/bin/echo "${zone}" | /usr/bin/head -n 1 | /bin/grep -Eq "^([a-z]+-){2,3}[0-9][a-z]$" || exit 255

# Get domain for calls
domain=$(/usr/bin/curl -s -f -m 1 -H "X-aws-ec2-metadata-token: ${IMDS_TOKEN}" "http://169.254.169.254/latest/meta-data/services/domain/")
domain_exit="${?}"
if [ "${domain_exit}" -ne 0 ]
then
    exit "${domain_exit}"
fi

#Extract region from zone
region=$(/bin/echo "${zone}" | /bin/sed -n 's/\(\([a-z]\+-\)\+[0-9]\+\).*/\1/p')

hostkeys=$(/bin/echo "${keys:?}")

accountId=$(/usr/bin/curl -s -f -m 1 -H "X-aws-ec2-metadata-token: ${IMDS_TOKEN}" "http://169.254.169.254/latest/dynamic/instance-identity/document" | /bin/grep -oP '(?<="accountId" : ")[^"]*(?=")')
/bin/echo "${accountId}" | /usr/bin/head -n 1 | /bin/grep -Eq "^[0-9]{12}$" || exit 255
val='{"AccountID":"'${accountId}'","AvailabilityZone":"'${zone}'","HostKeys":['${hostkeys}'],"InstanceId":"'${instance}'"}'

# Pull the creds we need for the call
creds=$(/usr/bin/curl -s -f -m 1 -H "X-aws-ec2-metadata-token: ${IMDS_TOKEN}" "http://169.254.169.254/latest/meta-data/identity-credentials/ec2/security-credentials/ec2-instance/")
creds_exit="${?}"
if [ "${creds_exit}" -ne 0 ] ; then
    # We failed to load instance-identity credentials
    exit "${creds_exit}"
fi
AWS_ACCESS_KEY_ID=$(/bin/echo "${creds}" | /bin/sed -n 's/.*"AccessKeyId" : "\(.*\)",/\1/p')
AWS_SECRET_ACCESS_KEY=$(/bin/echo "${creds}" | /bin/sed -n 's/.*"SecretAccessKey" : "\(.*\)",/\1/p')
AWS_SESSION_TOKEN=$(/bin/echo "${creds}" | /bin/sed -n 's/.*"Token" : "\(.*\)",/\1/p')
unset creds

clearcreds () {
    unset AWS_SESSION_TOKEN
    unset AWS_SECRET_ACCESS_KEY
    unset AWS_ACCESS_KEY_ID
}
trap clearcreds EXIT

# Generate, sign, and send the sigv4 request
host="ec2-instance-connect.${region}.${domain}"
endpoint="https://${host}"

timestamp=$(/bin/date -u "+%Y-%m-%d %H:%M:%S")
isoTimestamp=$(/bin/date -ud "${timestamp}" "+%Y%m%dT%H%M%SZ")
isoDate=$(/bin/date -ud "${timestamp}" "+%Y%m%d")

canonicalQuery="" # We are using POST data, not a querystring
canonicalHeaders="host:${host}\nx-amz-date:${isoTimestamp}\nx-amz-security-token:${AWS_SESSION_TOKEN}\n"
signedHeaders="host;x-amz-date;x-amz-security-token"

payloadHash=$(/bin/echo -n "${val}" | /usr/bin/sha256sum | /bin/sed 's/\s.*$//')

canonicalRequest="$(/usr/bin/printf "POST\n/PutEC2HostKeys/\n%s\n${canonicalHeaders}\n${signedHeaders}\n%s" "${canonicalQuery}" "${payloadHash}")"
requestHash=$(/bin/echo -n "${canonicalRequest}" | /usr/bin/sha256sum | /bin/sed 's/\s.*$//')

# Derive the signature
credentialScope="${isoDate}/${region}/ec2-instance-connect/aws4_request"
toSign="AWS4-HMAC-SHA256\n${isoTimestamp}\n${credentialScope}\n${requestHash}"
signingKey=$(getsigv4key "${AWS_SECRET_ACCESS_KEY}" "${isoDate}" "${region}" "ec2-instance-connect")
signature=$(sign "${signingKey}" "${toSign}")

authorizationHeader="AWS4-HMAC-SHA256 Credential=${AWS_ACCESS_KEY_ID}/${credentialScope}, SignedHeaders=${signedHeaders}, Signature=${signature}"

# Attempt to publish host keys
# 5 second timeout helps avoid choking launches in private subnets (see https://github.com/aws/aws-ec2-instance-connect-config/issues/8)
/usr/bin/curl -sS -m 5 -X POST -H "Content-Encoding: amz-1.0" -H "Authorization: ${authorizationHeader}" -H "Content-Type: application/json" -H "x-amz-content-sha256: ${payloadHash}" -H "x-amz-date: ${isoTimestamp}" -H "x-amz-security-token: ${AWS_SESSION_TOKEN}" -H "x-amz-target: com.amazon.aws.sshaccessproxyservice.AWSEC2InstanceConnectService.PutEC2HostKeys" -d "${val}" "${endpoint}/PutEC2HostKeys/"

unset AWS_SESSION_TOKEN
unset AWS_SECRET_ACCESS_KEY
unset AWS_ACCESS_KEY_ID