hook and continuous image pullers' DaemonSets: configuring k8s ServiceAccount - yes or no?

[consideRatio]

Update

My take is that we should help people pass some compliance tests etc, even though it doesn't improve security in this case. There could be some edge case where use of service accounts can be relevant still, such as when PSPs were around or similar.

Background

There are two sets of machinery to pull images to k8s nodes:

• A temporary one-off machinery before helm upgrade proceeds in full using a pre-upgrade helm hook, referred to as "hook image pulling" or "hook pre-puller".
• A continiuous machinery after helm upgrade, referred to as "continous image pulling" or "continuous pre-puller"

Both involves a k8s DaemonSet to schedule a pod on each node, and have that pod startup containers referencing images to "pre-pull", which makes the k8s node's pull the images.

However, the hook image puller DaemonSet resource is paired with the hook-image-awaiter k8s Job, that in turn have misc permissions to inspect k8s DaemonSets - and by doing that can know if the pulling has completed. When pulling is completed, the pre-upgrade helm hook can be considered completed, and the helm upgrade command can proceed. This is why the hook-image-awaiter k8s Job needs a k8s ServiceAccount for itself, so it can be granted permissions to ask the k8s api-server about DaemonSet resources.

Question

• For the k8s DaemonSets resources created for the hook and continuous image puller machineries, is there a need to configure a k8s ServiceAccount by declaring a serviceAccountName?
• For the k8s DaemonSets resources created for the hook and continuous image puller machineries, is there a need to have the chart create a k8s ServiceAccount resource to use?

[samyuh]

I'm going to leave here the last comment that I left on the closed pull request :)

I really need to have every component in our deployment with a specific service account due to security compliance. We can just go with the option of setting the serviceAccountName with a value from the chart.

[consideRatio]

I've now updated the issue to raise two questions - is there a need to configure serviceAccountName or have the chart create a k8s ServiceAccount that is used, or is either fine?

Could you share some insights into why doing this could be relevant from a security perspective? I want to ensure complexity is only embraced for something a purpose that isn't addressed better in another way etc, for example by automountServiceAccountToken: false or similar.

[samyuh]

is there a need to configure serviceAccountName or have the chart create a k8s ServiceAccount that is used, or is either fine?

Either option is fine, as the end goal is to have a service account being assigned to the puller.

why doing this could be relevant from a security perspective?

We can't use default SA, since we need to explicitly set the permissions to be PCI-DSS compliant. Remember that the permissions can be either set in our side or on chart side.

[consideRatio]

Can you clarify even further, no permissions are granted these pods - but you need to explicitly grant them some additional permissions via a RoleBinding to a k8s ServiceAccount used by the pods?

I'm probing a lot now, because if this for example is for the sake of PSP deprecated in k8s 1.21 and removed in k8s 1.25 - its pointless i think.

[samyuh]

Yes, I'm going to gather more information about this and then I will reach back to you.

[consideRatio]

I found this: https://cloud.google.com/kubernetes-engine/enterprise/policy-controller/docs/how-to/using-pci-dss-v4

pci-dss-v4.0-restrict-creation-with-default-serviceaccount
Restricts the creation of resources using a default service account. Has no effect during audit.

I tried to figure out if all workloads must use one k8s serviceaccount at all, or if they could opt out entirely. It seems so far its either the default or another k8s serviceaccount, so complying with this requirement means to provide another i think.

[samyuh]

Hey @consideRatio. This is the answer I got from our security team:

This is a requirement from a security framework that we are required to be in compliance with, CIS Benchmarks. More specifically the control in question is the 4.1.5 - Ensure that default service accounts are not actively used (from the EKS CIS Benchmark v1.5.0).
A more practical explanation is that every workload has its specific "identity profile". This means that permissions for one workload are rarely the same across its entire lifecycle thus is always recommended for each workload to have a separate "identity". Not allowing the usage of the default service accounts helps in this topic.

We currently have a fork of this repository with the changes I suggested in the pull request that got closed but we would like to be using this public upstream release. Feel free to suggest other alternatives :)

[consideRatio]

My conclusion is that there isn't a need to use a specific service account etc with associated permissions as were the case when k8s PodSecurityPolicy (PSP) was around, but that there are misc benchmarks (EKS CIS Benchmark v1.5.0) and compliance checks (PCI-DSS v4.0) saying that we must avoid letting a pod use the default service account - even if it comes with automountServiceAccountToken: false.

I figure we can create dummy service accounts for this purpose then. Since one needs to be around non-stop (continuous image pulling), and the hook serviceaccount is to be temporary and provided before upgrade etc, I figure we need two separate. One for the hook-image-puller daemonset pods and one for the continuous-image-puller daemonset pods.

[consideRatio]

I think I'd like to do this change myself rater than accept a PR and review it, the helm chart logic for the image puller daemonsets is quite messy and hard to onboard to.

[samyuh]

Okay! Thank you for all your help and support!