Minimal KMS permissions to copy a database snapshot

Question

I am trying to setup minimal permissions for doing aws rds copy-db-snapshot with a KMS encryption key:

$ aws rds copy-db-snapshot --source-db-snapshot-identifier rds-backup-share-
mysql --target-db-snapshot-identifier rds-backup-share-mysql-reencrypted --kms-key-id <kms-arn>

(Everything within <> is stripped out by me and contains valid values.)

Unfortunately I get this error:

An error occurred (KMSKeyNotAccessibleFault) when calling the CopyDBSnapshot operation: The target snapshot KMS key [<kms-arn>] does not exist, is not enabled or you do not have permissions to access it.

Currently I allow these actions:

  "Action": [
    "kms:ReEncrypt*",
    "kms:ListKeys",
    "kms:ListAliases",
    "kms:GenerateDataKey*",
    "kms:Encrypt",
    "kms:DescribeKey",
    "kms:Decrypt"
  ],

It works if I replace it with kms:*{code}, so it must be a permission issue.

I tried to figure out the correct permissions with CloudTrail, but it just contains the same unhelpful error message.

So my actual questions:

• What are the minimal KMS permissions for CopyDBSnapshot?
• Is there a generic way to figure out the required permissions? It is always a pain to waste my time by googling the required permissions.

Edit: This is the is the bottom part of the log output with --debug enabled:

2017-08-22 17:15:37,521 - MainThread - botocore.endpoint - DEBUG - Sending http request: <PreparedRequest [POST]>
2017-08-22 17:15:37,522 - MainThread - botocore.vendored.requests.packages.urllib3.connectionpool - INFO - Starting new HTTPS connection (1): rds.eu-west-1.amazonaws.com
2017-08-22 17:15:37,927 - MainThread - botocore.vendored.requests.packages.urllib3.connectionpool - DEBUG - "POST / HTTP/1.1" 400 437
2017-08-22 17:15:37,934 - MainThread - botocore.parsers - DEBUG - Response headers: {'x-amzn-requestid': 'c097fe4e-874c-11e7-a56a-9d1acedaf516', 'content-type': 'text/xml', 'content-length': '437', 'date': 'Tue, 22 Aug 2017 15:15:37 GMT', 'connection': 'close'}
2017-08-22 17:15:37,936 - MainThread - botocore.parsers - DEBUG - Response body:
b'<ErrorResponse xmlns="http://rds.amazonaws.com/doc/2014-10-31/">\n  <Error>\n    <Type>Sender</Type>\n    <Code>KMSKeyNotAccessibleFault</Code>\n    <Message>The target snapshot KMS key [<kms-arn>] does not exist, is not enabled or you do not have permissions to access it. </Message>\n  </Error>\n  <RequestId>c097fe4e-874c-11e7-a56a-9d1acedaf516</RequestId>\n</ErrorResponse>\n'
2017-08-22 17:15:37,938 - MainThread - botocore.hooks - DEBUG - Event needs-retry.rds.CopyDBSnapshot: calling handler <botocore.retryhandler.RetryHandler object at 0x7f9c7ce84860>
2017-08-22 17:15:37,939 - MainThread - botocore.retryhandler - DEBUG - No retry needed.
2017-08-22 17:15:37,952 - MainThread - awscli.clidriver - DEBUG - Exception caught in main()
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/awscli/clidriver.py", line 200, in main
    return command_table[parsed_args.command](remaining, parsed_args)
  File "/usr/lib/python3.6/site-packages/awscli/clidriver.py", line 338, in __call__
    return command_table[parsed_args.operation](remaining, parsed_globals)
  File "/usr/lib/python3.6/site-packages/awscli/clidriver.py", line 508, in __call__
    call_parameters, parsed_globals)
  File "/usr/lib/python3.6/site-packages/awscli/clidriver.py", line 627, in invoke
    client, operation_name, parameters, parsed_globals)
  File "/usr/lib/python3.6/site-packages/awscli/clidriver.py", line 639, in _make_client_call
    **parameters)
  File "/usr/lib/python3.6/site-packages/botocore/client.py", line 310, in _api_call
    return self._make_api_call(operation_name, kwargs)
  File "/usr/lib/python3.6/site-packages/botocore/client.py", line 599, in _make_api_call
    raise error_class(parsed_response, operation_name)
botocore.errorfactory.KMSKeyNotAccessibleFault: An error occurred (KMSKeyNotAccessibleFault) when calling the CopyDBSnapshot operation: The target snapshot KMS key [<kms-arn>] does not exist, is not enabled or you do not have permissions to access it. 
2017-08-22 17:15:37,955 - MainThread - awscli.clidriver - DEBUG - Exiting with rc 255

An error occurred (KMSKeyNotAccessibleFault) when calling the CopyDBSnapshot operation: The target snapshot KMS key [<kms-arn>] does not exist, is not enabled or you do not have permissions to access it.

FTR: I did a cross-post to the AWS forum: https://forums.aws.amazon.com/thread.jspa?messageID=801745

Answer 1

Now, I figured it out by trial and error. Since I don't like to do the same task more than once, I automated it (see script below).

This are the required permissions for copying a RDS snapshot:

["kms:CreateGrant","kms:DescribeKey"]

---

This is the script I used. Maybe it is useful for other people which have a similar problem. It is hacked together, so don't expect it to work out of the box.

#!/bin/bash

set -euo pipefail

unknown=(
    kms:CancelKeyDeletion
    kms:CreateAlias
    kms:CreateAlias
    kms:CreateGrant
    kms:CreateKey
    kms:Decrypt
    kms:DeleteAlias
    kms:DeleteAlias
    kms:DescribeKey
    kms:DisableKey
    kms:DisableKeyRotation
    kms:EnableKey
    kms:EnableKeyRotation
    kms:Encrypt
    kms:GenerateRandom
    kms:GenerateDataKey
    kms:GenerateDataKeyWithoutPlaintext
    kms:GetKeyPolicy
    kms:GetKeyRotationStatus
    kms:ListAliases
    kms:ListGrants
    kms:ListKeyPolicies
    kms:ListKeys
    kms:ListRetirableGrants
    kms:PutKeyPolicy
    kms:ReEncryptFrom
    kms:ReEncryptTo
    kms:RetireGrant
    kms:RevokeGrant
    kms:ScheduleKeyDeletion
    kms:UpdateAlias
    kms:UpdateAlias
    kms:UpdateKeyDescription
)
required=()

KEY_ID=86a6300d-38f9-4892-b7a1-d8f821e8438c
export AWS_DEFAULT_OUTPUT=json

function check_copy {
    permissions=$( echo -n "${required[*]} ${unknown[*]}" | jq -R -s 'split(" ")' )

    policy=$( aws kms \
        get-key-policy \
        --key-id ${KEY_ID} \
        --policy-name default \
      | jq ".Policy" -r \
      | jq ".Statement[1].Action |= ${permissions}"
    )

    aws kms \
        put-key-policy \
        --key-id ${KEY_ID} \
        --policy-name default \
        --policy "${policy}"

    aws rds \
        delete-db-snapshot \
        --db-snapshot-identifier rds-backup-share-mysql-reencrypted \
        || true

    (
        set -x
        AWS_ACCESS_KEY_ID=XXX \
        AWS_SECRET_ACCESS_KEY=XXX \
        aws rds \
            copy-db-snapshot \
            --source-db-snapshot-identifier rds-backup-share-mysql \
            --target-db-snapshot-identifier rds-backup-share-mysql-reencrypted \
            --kms-key-id alias/rds-snapshot-share \
            || return 1

    aws rds \
        wait db-snapshot-completed \
        --db-snapshot-identifier rds-backup-share-mysql-reencrypted
    ) || return 1

    return 0
}

check_copy
while [ ${#unknown[@]} -gt 0 ]
do
    removed=${unknown[0]}
    unknown=(${unknown[@]:1})

    if ! check_copy
    then
        required+=($removed)
    fi

    echo "Required permissions so far: ${required[*]}"
    echo "Unknown permissions so far: ${unknown[*]}"
done

echo -n "Minimal permissions: "
echo -n "${required[*]}" | jq -R -s -c 'split(" ")'