Fix AWS IAM Roles for Service Accounts permission problem.

[serialx]

Amazon EKS supports IAM Roles for Service Accounts. It mounts tokens files to /var/run/secrets/eks.amazonaws.com/serviceaccount/token. Unfortunately, external-dns runs as 'nobody' so it cannot access this file. External DNS is then unable to make any AWS API calls to work:

time="2019-09-11T07:31:53Z" level=error msg="WebIdentityErr: unable to read file at /var/run/secrets/eks.amazonaws.com/serviceaccount/token\ncaused by: open /var/run/secrets/eks.amazonaws.com/serviceaccount/token: permission denied"

See: https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts-technical-overview.html

Below are the file permissions mounted on External DNS pod:

~ $ ls -al /var/run/secrets/eks.amazonaws.com/serviceaccount/
total 0
drwxrwxrwt    3 root     root           100 Sep 11 06:40 .
drwxr-xr-x    3 root     root            28 Sep 11 06:40 ..
drwxr-xr-x    2 root     root            60 Sep 11 06:40 ..2019_09_11_06_40_49.865776187
lrwxrwxrwx    1 root     root            31 Sep 11 06:40 ..data -> ..2019_09_11_06_40_49.865776187
lrwxrwxrwx    1 root     root            12 Sep 11 06:40 token -> ..data/token
~ $ ls -al /var/run/secrets/eks.amazonaws.com/serviceaccount/..data/token
-rw-------    1 root     root          1028 Sep 11 06:40 /var/run/secrets/eks.amazonaws.com/serviceaccount/..data/token

Pinging @taharah for review.

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[serialx]

Updated CLA account settings.

[mikkeloscar]

Interesting. Do you know how the file permissions are for the Kubernetes service account token? Because this is mounted in a similar way and works with the nobody user.

[serialx]

Kubernetes tokens seems to be o+r

/run/secrets/kubernetes.io/serviceaccount $ ls -al
total 0
drwxrwxrwt    3 root     root           140 Sep 11 07:31 .
drwxr-xr-x    3 root     root            28 Sep 11 07:31 ..
drwxr-xr-x    2 root     root           100 Sep 11 07:31 ..2019_09_11_07_31_49.116680770
lrwxrwxrwx    1 root     root            31 Sep 11 07:31 ..data -> ..2019_09_11_07_31_49.116680770
lrwxrwxrwx    1 root     root            13 Sep 11 07:31 ca.crt -> ..data/ca.crt
lrwxrwxrwx    1 root     root            16 Sep 11 07:31 namespace -> ..data/namespace
lrwxrwxrwx    1 root     root            12 Sep 11 07:31 token -> ..data/token
/run/secrets/kubernetes.io/serviceaccount $ ls -al ..data/
total 12
drwxr-xr-x    2 root     root           100 Sep 11 07:31 .
drwxrwxrwt    3 root     root           140 Sep 11 07:31 ..
-rw-r--r--    1 root     root          1025 Sep 11 07:31 ca.crt
-rw-r--r--    1 root     root            12 Sep 11 07:31 namespace
-rw-r--r--    1 root     root           875 Sep 11 07:31 token

[mikkeloscar]

Very interesting, this means containers must run as root to use this feature. I don't think this is intended and we should make at least AWS aware.

[serialx]

Comparing the two codes doesn't show any hints on why they differ in permissions:

• https://github.com/aws/amazon-eks-pod-identity-webhook/blob/2fd4becd434f92edab01cecee30ea4b12af93ab7/pkg/handler/handler.go#L157-L177
• https://github.com/kubernetes/kubernetes/blob/bad4d0ee963a71994636ebaaca52dab54820abe6/plugin/pkg/admission/serviceaccount/admission.go#L512-L555

Strange.

[serialx]

Reported to the amazon-eks-pod-identity-webhook repository.

[alfredkrohmer]

The problem is here:
https://github.com/kubernetes/kubernetes/blob/cf76868b3430be015cc1e8443a216bf863a9b9f7/pkg/volume/projected/projected.go#L357

It's hardcoded to octal 0600. 🤦‍♂️

[serialx]

Damn. But both code uses ServiceAccountToken. Maybe the code path is different?

[alfredkrohmer]

No, this one is for projected service account tokens.

[alfredkrohmer]

See kubernetes/kubernetes#82573

[linki]

I was able to read a projected service account token with nobody by using securityContext.fsGroup:

      ...
      securityContext:
        fsGroup: 65534
      volumes:
      - name: token-vol
        projected:
          sources:
          - serviceAccountToken:
              path: token

$ whoami
nobody
$ ls -la /var/run/secrets/kubernetes.io/serviceaccount/..data/
-rw-r-----    1 root     nobody        1102 Sep 11 15:33 token

[marcincuber]

Any chances to get a timeline when this is going to be released?

[Raffo]

I'd defer to @linki , but I think that if a change of configuration will allow to do that, I'd rather change the docs to mention this than change the dockerfile.

[jqmichael]

65534

https://stackoverflow.com/questions/34831861/can-i-assume-that-nobody-is-65534

You can't. nobody has had at least a few different IDs across distros and time:

Historically, the user “nobody” was assigned UID -2 by several operating systems, although other values such as 2^(15)−1 = 32,767 are also in use, such as by OpenBSD. For compatibility between 16-bit and 32-bit UIDs, many Linux distributions now set it to be 2^(16)−2 = 65,534; the Linux kernel defaults to returning this value when a 32-bit UID does not fit into the return value of the 16-bit system calls. An alternative convention assigns the last UID of the range statically allocated for system use (0-99) to nobody: 99.

[serialx]

Yeah, but the docker file already specifies 65534, so it doesn't really matter does it?

https://github.com/kubernetes-incubator/external-dns/blob/master/Dockerfile#L36

[serialx]

Can confirm workaround by using securityContext. I've changed the PR to update AWS tutorial instead of changing the Dockerfile. Should work fine with future k8s updates too.

[linki]

/lgtm
/approve