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Kubernetes? You mean the container orchestration system where they forgot to add Multi-tenancy? And no namespaces are not Multi-tenancy...
https://www.vcluster.com/
Kubernetes has a lot of limitations from a multi tenancy perspective.
It's functional, but I think it's not as polished as the rest of Kubernetes which is why Kubernetes has a multi tenancy SIG that spawned the hierarchical namespace controller (https://github.com/kubernetes-sigs/hierarchical-namespaces) and virtual clusters (https://github.com/kubernetes-sigs/cluster-api-provider-nest...)
Kubernetes has a lot of limitations from a multi tenancy perspective.
It's functional, but I think it's not as polished as the rest of Kubernetes which is why Kubernetes has a multi tenancy SIG that spawned the hierarchical namespace controller (https://github.com/kubernetes-sigs/hierarchical-namespaces) and virtual clusters (https://github.com/kubernetes-sigs/cluster-api-provider-nest...)
Kubernetes has a lot of limitations from a multi tenancy perspective.
It's functional, but I think it's not as polished as the rest of Kubernetes which is why Kubernetes has a multi tenancy SIG that spawned the hierarchical namespace controller (https://github.com/kubernetes-sigs/hierarchical-namespaces) and virtual clusters (https://github.com/kubernetes-sigs/cluster-api-provider-nest...)
I'm the technical lead for IMDS. Without an IMDS, the alternative is usually hard-coding long-lived credentials, which is much worse.
We've looked a few times at different ways that IMDS could vend different credentials to different user-ids. We documented how to use local firewall rules, which you've linked to. This gives single uid restrictions, similar to filterd. We also have a BPF based tracer tool, https://github.com/aws/aws-imds-packet-analyzer which can monitor which user ids and processes are calling IMDS (and which version they are using).
Our next thought was to expose IMDS as a filesystem. That way ordinary POSIX filesystem permissions could be used to control which user ids could read which credentials, and it would even work on Windows. But our research found that security issues (in applications and libraries that customers run) that grant local file reading privileges are even more common than SSRF issues (in part this because many SSRF issues allow "file://" urls, so they become a subset).
We've looked at interfacing with the kernel keyring and the TPM 2.0 interface (and we now have Nitro TPM) ... but both are difficult to call to user space. The latter doesn't get user ID granularity, and the former is hard to coordinate with from the outside for revoking and rotating credentials.
If anyone is curious on how this can be abused, here's my "There's no place like 169.254.169.254 - (Ab)using cloud metadata URLs" slides [1] and talk [2] from 2019.
tl;dr - misconfigured reverse proxies allowed cloud metadata URL access across the bigger cloud providers.
[1] https://github.com/opsdisk/cloud_metadata_extractor/blob/mas...
[2] https://www.youtube.com/watch?v=vxReTpBCmh8