Pipelines do not validate child UIDs

Summary

Pipelines do not validate child UIDs, which means that a user that has access to create TaskRuns can create their own Tasks that the Pipelines controller will accept as the child Task.

We should add UID to PipelineRun status and validate that child Run status/results only come from Runs matching the same UID.

Details

While we store and validate the PipelineRun's (api version, kind, name, uid) in the child Run's OwnerReference, we only store (api version, kind, name) in the ChildStatusReference .

This means that if a client had access to create TaskRuns on a cluster, they could create a child TaskRun for a pipeline with the same name + owner reference, and the Pipeline controller picks it up as if it was the original TaskRun. This is problematic since it can let users modify the config of Pipelines at runtime, which violates SLSA L2 Service Generated / Non-falsifiable requirements.

I believe this is also true for TaskRuns -> Pods since it looks like we only lookup by name, though I haven't tested this.

If you have update permissions on tekton resources, you could also perform a similar bypass like this (because it's difficult to distinguish this from a Task retry). For now, I think relying on RBAC is fine and treat update as a privileged role (though we should perhaps update docs to stress this). Create is the most problematic for now. SPIFFE/SPIRE might be able to help with ensuring that only the controller can modify state long term (e.g. sign the expected UIDs?)

PoC

apiVersion: [tekton.dev/v1beta1](http://tekton.dev/v1beta1)
kind: PipelineRun
metadata:
  name: hello-pr
spec:
  pipelineSpec:
    tasks:
      - name: task1
        taskSpec:
          steps:
            - name: echo
              image: [distroless.dev/alpine-base](http://distroless.dev/alpine-base)
              script: |
                sleep 60
      - name: task2
        runAfter: [task1]
        taskSpec:
          steps:
            - name: echo
              image: [distroless.dev/alpine-base](http://distroless.dev/alpine-base)
              script: |
                echo "asdf" > $(results.foo.path)
          results:
            - name: foo
    results:
      - name: foo
        value: $(tasks.task2.results.foo)

Once this is running, grab the PR UID: