GCP Iam

For the GitHub Actions caller side of Workload Identity Federation (permissions: id-token: write, google-github-actions/auth setup, the OIDC token shape), see the github-actions-pipelines skill. This skill covers the GCP side: pool provider attribute conditions, service account iam.workloadIdentityUser bindings, impersonation chains.

GCP IAM Inheritance

When to invoke

Symptoms:

• permission denied: <permission> on resource <X> despite the user being "in the right group at the org level."
• iam.serviceAccounts.getAccessToken denied during Workload Identity, ESO sync, or CI/CD.
• A service account can get a resource but not list siblings (or vice versa).
• gcloud projects get-iam-policy <project> shows no binding for the user, but the user can log into the project console.
• Cross-project SA impersonation works for human users via gcloud auth login but fails from a workload.

The trap this prevents: assuming GCP IAM is additive across the hierarchy in all directions. It is — but with caveats around conditional bindings, deny policies, audit-log filtering, and the fact that some permissions only apply at specific resource levels.

The inheritance model (the part that does work)

Organization
  └── Folder(s)
        └── Project(s)
              └── Resource(s)   (Bucket, BQ Dataset, GCE Instance, etc.)

Rule: A role granted at level N applies to N and everything below it. Permissions are added — never subtracted (except by Deny policies, see below).

So roles/storage.admin granted at the folder level grants storage.admin on every project under that folder. This is the "obvious" inheritance and it does work as expected.

Why "group at org" sometimes fails to grant project access

Common reasons users report "I'm in the group but it doesn't work":

1. The group has no IAM binding at any level. Group membership alone grants nothing. The group must appear as a principal in an IAM binding somewhere in the hierarchy.

   # Find every binding that includes a group, project or higher
   gcloud projects get-iam-policy <project> \
     --flatten="bindings[].members" \
     --filter="bindings.members:group:<group-email>" \
     --format="value(bindings.role)"
   
   gcloud organizations get-iam-policy <org-id> \
     --flatten="bindings[].members" \
     --filter="bindings.members:group:<group-email>" \
     --format="value(bindings.role)"
   

2. The binding exists but uses a resource-level role. Some roles (e.g. roles/storage.objectViewer) make sense bound to a bucket, not an org. Granting them at the org grants viewer on every bucket in every project — but a deny policy somewhere downstream can block it.

3. A Deny policy at a lower level overrides the allow. Deny policies are evaluated before allows and always win. Check:

   gcloud iam policies list --attachment-point="cloudresourcemanager.googleapis.com/projects/<project-num>" --kind=denypolicies
   

4. Conditional bindings restrict scope. A binding with a condition block (CEL expression) only grants the role when the condition is true — e.g. only for resources tagged env=dev, or only between 9am–5pm.

   gcloud projects get-iam-policy <project> --format=json | jq '.bindings[] | select(.condition)'
   

5. Group nesting depth or propagation delay. Workspace group changes propagate to GCP within minutes but not instantly. Nested groups are supported but each level adds latency. After a permission change, wait 1–5 minutes before re-testing.

6. The user is in the group but using a different identity. gcloud auth list may show a service account or a different user. Confirm:

   gcloud config list account
   gcloud auth list
   

Service account impersonation — three roles, three meanings

These look similar and are routinely confused:

Diagnostic for iam.serviceAccounts.getAccessToken denied:

# Who is trying to impersonate
gcloud config list account                              # user/caller
# Is caller a tokenCreator on the target SA?
gcloud iam service-accounts get-iam-policy <target-sa> \
  --format=json | jq '.bindings[] | select(.role=="roles/iam.serviceAccountTokenCreator")'

If the caller is a K8s SA, the principal format depends on which GKE identity mode you use:

The newer Federation form lets K8s SAs hold GCP roles directly (no GSA intermediary). The older form is still supported but is the GKE-specific path; new clusters default to the Federation model.

Project-level vs org-level diagnostic flow

When debugging "permission denied":

1. What permission, on what resource? Read the error message carefully — the resource name (projects/<p>/buckets/<b>/...) tells you the level.

2. At what level should the role be bound? Some permissions only apply at certain levels. Compute access? Project-level. BQ dataset access? Dataset-level. Org-policy management? Org-level.

3. List bindings from the bottom up:

   # Most specific first — resource-level (bucket, dataset, etc.) — then up the hierarchy
   gsutil iam get gs://<bucket>                       # bucket-level
   gcloud projects get-iam-policy <project>           # project-level
   gcloud resource-manager folders get-iam-policy <folder-id>  # folder
   gcloud organizations get-iam-policy <org-id>       # org
   

4. Check for deny policies attached at any level. They override allows.

5. Check conditional bindings. A condition that evaluates false yields silent denial.

6. If using impersonation, walk the chain. Caller → tokenCreator on intermediate SA → tokenCreator on target SA. Each link must be explicit.

Quick reference — common command shapes

# Who am I, what project, what auth mode?
gcloud config list
gcloud auth list

# Resolve "what roles does <principal> have on <project>"
gcloud projects get-iam-policy <project> \
  --flatten="bindings[].members" \
  --filter="bindings.members:<user-or-sa-email>" \
  --format="value(bindings.role)"

# Same for the org (catches inherited grants)
gcloud organizations get-iam-policy <org-id> \
  --flatten="bindings[].members" \
  --filter="bindings.members:<user-or-sa-email>" \
  --format="value(bindings.role)"

# What permissions does role R actually contain?
gcloud iam roles describe <role-name>  # e.g. roles/storage.objectViewer

# Simulate: does principal P have permission X on resource R?
gcloud asset analyze-iam-policy \
  --organization=<org-id> \
  --identity="user:<email>" \
  --permissions="<permission>" \
  --resource="//<full-resource-name>"

The analyze-iam-policy command is the closest GCP has to a definitive "do I have access" answer — it walks the full inheritance, conditions, and deny policies in one call. Use it when manual inspection of multiple get-iam-policy calls is getting confusing.