Cannot assign KMS grant to role in AWS

Question

I have an encryption key in KMS and two roles: One KeyAdmin role which should be allowed to create a grant to the KeyUser role which should then be able to encrypt/decrypt with the key.

Here is what I am doing:

$ aws kms create-key
{
    "KeyMetadata": {
        "AWSAccountId": "1234567890",
        "KeyId": "99999999-9999-9999-9999-999999999999",
        "Arn": "arn:aws:kms:eu-north-1:1234567890:key/99999999-9999-9999-9999-999999999999",
        "CreationDate": 1583827994.922,
        "Enabled": true,
        "Description": "",
        "KeyUsage": "ENCRYPT_DECRYPT",
        "KeyState": "Enabled",
        "Origin": "AWS_KMS",
        "KeyManager": "CUSTOMER",
        "CustomerMasterKeySpec": "SYMMETRIC_DEFAULT",
        "EncryptionAlgorithms": [
            "SYMMETRIC_DEFAULT"
        ]
    }
}

$ cat /tmp/kp.json 
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "KeyAdmin",
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::1234567890:role/keyadmin-role"
            },
            "Action": "kms:CreateGrant",
            "Resource": "*"
        },
        {
            "Sid": "KMS account admin access",
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::1234567890:root"
            },
            "Action": "kms:*",
            "Resource": "*"
        }
    ]
}

$ aws kms put-key-policy --key-id 99999999-9999-9999-9999-999999999999 --policy-name default --policy file:///tmp/kp.json

$ aws --profile keyadmin-role kms create-grant --key-id 99999999-9999-9999-9999-999999999999 --grantee-principal awn:aws:iam:::1234567890/role/keyuser-role --operations Encrypt Decrypt
{
    "GrantToken": "AQpANGJiNDRhMjVhNGRmNjY0MDBjYTU2YWNlOTkyNWVjNDBkNmFlMDA1Nzc2MmEzMjFkZjk1N2Q2ODc1NzU2ZDYxMiKpAgEBAgB4S7RKJaTfZkAMpWrOmSXsQNauAFd2KjId-VfWh1dW1hIAAAEAMIH9BgkqhkiG9w0BBwagge8wgewCAQAwgeYGCSqGSIb3DQEHATAeBglghkgBZQMEAS4wEQQML6vpVnNolKHIUz9eAgEQgIG4WcppsyTfo4BBKLvV02Wz1K6LlxpNXhhEZVFlCnTzO3Lsat5LBwtlCilPxpcW5N7f8ucjfi_AiH5VYM50_nNqGF1rH5GgzZoDXn76salNvzxF9YoPP3iWiH-NQ7O695kv0svhdONpfrk8nNCBvOeQbQDj9sLCUGbOI3Di51YKKzb9TZd_hwRxWcniAnYphqQkpyYIKttwHmsftZaODzEM64Rj_hU1_bexRwzPW8E75wnjrS_vNNXCHCog5DA2gg4zsbNyECk3qsGQk8yESJzwsKT_lP6Cf8nqrps",
    "GrantId": "e43036820e33b1b372102937aac19093cc84489cf0b0a4ff94fe827fc9eaae9b"
}
$ aws kms list-grants --key-id 99999999-9999-9999-9999-999999999999
{
    "Grants": [
        {
            "KeyId": "arn:aws:kms:eu-north-1:1234567890:key/99999999-9999-9999-9999-999999999999",
            "GrantId": "e43036820e33b1b372102937aac19093cc84489cf0b0a4ff94fe827fc9eaae9b",
            "Name": "",
            "CreationDate": 1583828859.0,
            "GranteePrincipal": "awn:aws:iam:::1234567890/role/keyuser-role",
            "IssuingAccount": "arn:aws:iam::1234567890:root",
            "Operations": [
                "Decrypt",
                "Encrypt"
            ]
        }
    ]
}
$ aws  --profile keyuser-role kms encrypt --key-id 99999999-9999-9999-9999-999999999999 --plaintext "foo"

An error occurred (AccessDeniedException) when calling the Encrypt operation: User: arn:aws:sts::1234567890:assumed-role/keyuser-role/botocore-session-1583827952 is not authorized to perform: kms:Encrypt on resource: arn:aws:kms:eu-north-1:1234567890:key/99999999-9999-9999-9999-999999999999

$ aws  --profile keyuser-role kms encrypt --key-id 99999999-9999-9999-9999-999999999999 --plaintext "foo" --grant-tokens "AQpANGJiNDRhMjVhNGRmNjY0MDBjYTU2YWNlOTkyNWVjNDBkNmFlMDA1Nzc2MmEzMjFkZjk1N2Q2ODc1NzU2ZDYxMiKpAgEBAgB4S7RKJaTfZkAMpWrOmSXsQNauAFd2KjId-VfWh1dW1hIAAAEAMIH9BgkqhkiG9w0BBwagge8wgewCAQAwgeYGCSqGSIb3DQEHATAeBglghkgBZQMEAS4wEQQML6vpVnNolKHIUz9eAgEQgIG4WcppsyTfo4BBKLvV02Wz1K6LlxpNXhhEZVFlCnTzO3Lsat5LBwtlCilPxpcW5N7f8ucjfi_AiH5VYM50_nNqGF1rH5GgzZoDXn76salNvzxF9YoPP3iWiH-NQ7O695kv0svhdONpfrk8nNCBvOeQbQDj9sLCUGbOI3Di51YKKzb9TZd_hwRxWcniAnYphqQkpyYIKttwHmsftZaODzEM64Rj_hU1_bexRwzPW8E75wnjrS_vNNXCHCog5DA2gg4zsbNyECk3qsGQk8yESJzwsKT_lP6Cf8nqrps"

An error occurred (AccessDeniedException) when calling the Encrypt operation: User: arn:aws:sts::1234567890:assumed-role/keyuser-role/botocore-session-1583827952 is not authorized to perform: kms:Encrypt on resource: arn:aws:kms:eu-north-1:1234567890:key/99999999-9999-9999-9999-999999999999

$ aws  --profile keyuser-role sts get-caller-identity
{
    "UserId": "AROA2AD3X6CJC6MODMUZP:botocore-session-1583827952",
    "Account": "1234567890",
    "Arn": "arn:aws:sts::1234567890:assumed-role/keyuser-role/botocore-session-1583827952"
}

Why is the access to key denied for the role holding the grant?

UPDATE

The roles do not have any IAM policies attached.

Just a guess. You are using `keyadmin-role` to create the grant? Does the `keyadmin-role` can use `kms:decrypt' and 'kms:encrypt'? If not, then you can't give such grants. — Marcin, Mar 10 '20 at 08:55
@Marcin Tried adding `kms:Encrypt` to the allowed actions of the *KeyAdmin* statement, still the same error. Do you have a reference in the AWS documentation for your claim? — mat, Mar 10 '20 at 09:03
"the grant only allows the grantee principal to create grants that are equally restrictive or more restrictive. " from [here](https://docs.aws.amazon.com/kms/latest/developerguide/grants.html#grant-creategrant) — Marcin, Mar 10 '20 at 09:44
That's only relevant if you got the *CreateGrant* permission via a grant yourself. In my case, it is assigned via a policy. — mat, Mar 10 '20 at 09:52

mat · Accepted Answer · 2020-03-26T11:23:44.623

1

I made a stupid mistake by passing a errornous grantee-principal flag to the create-grant operation. After replacing

awn:aws:iam:::1234567890/role/keyuser-role

with

arn:aws:iam::1234567890:role/keyuser-role

everything works as expected.

A user in the AWS forum luckily pointed out the error.

edited Mar 26 '20 at 11:23

answered Mar 26 '20 at 11:20

mat

1,645
15
36

score -1 · Answer 2 · answered Mar 10 '20 at 15:40

-1

Without seeing the policies defined for the KeyUser role, it will be hard to determine for certain, but I believe your issue is that the role may not have the required KMS actions defined for it.

The AWS docs for defining KMS access to roles describe defining the IAM role with kms:Encrypt actions on the KMS key. If the IAM role does not already have these actions defined, you will be blocked before trying to access the grants of the KMS key.

Copying from the docs, a policy like this would need to be in your KeyUser role:

{
  "Version": "2012-10-17",
  "Statement": {
    "Effect": "Allow",
    "Action": [
      "kms:Encrypt",
      "kms:Decrypt"
    ],
    "Resource": [
      "arn:aws:kms:eu-north-1:1234567890:key/99999999-9999-9999-9999-999999999999"
    ]
  }
}

answered Mar 10 '20 at 15:40

Tres' Bailey

709
7
17

I updated my question: There are no IAM policies attached to the roles mentioned. – mat Mar 11 '20 at 07:10
That answer is not relevant for my question. If I would already have access to the key via a policy, I wouldn't need the *grant* anymore. The grant is to be used instead of a policy based permission. – mat Mar 11 '20 at 07:13
1

You are correct, I had always used create-grant on granting only subsets of encryption context. After testing with an empty role, I see my assumptions were not correct. – Tres' Bailey Mar 11 '20 at 18:15

Cannot assign KMS grant to role in AWS

2 Answers2