Troubleshooting GCP IAM Quota Exhaustion: Why Your Compute Engine Instance is Failing with PERMISSION_DENIED During OOM
Quick Fix Summary
TL;DRIncrease IAM policy size quota via GCP Support or reduce policy bindings.
When a Compute Engine instance exhausts memory (OOM), it may trigger a restart or re-creation. If the project's IAM policy size quota is exhausted, the instance's service account cannot be validated, causing a PERMISSION_DENIED error on startup.
Diagnosis & Causes
Recovery Steps
Step 1: Verify IAM Policy Size and Quota
Check the current IAM policy size against the quota limit. This is the primary diagnostic step.
# Get the current IAM policy and check its size
gcloud projects get-iam-policy PROJECT_ID --format=json | wc -c
# Check the IAM policy size quota for your project
gcloud alpha resource-manager quotas list --project=PROJECT_ID --filter="metric:iam.policy.size" --format="value(limit, usage)"Step 2: Analyze Cloud Audit Logs for the Instance
Filter logs for the failing VM instance to confirm the PERMISSION_DENIED error correlates with IAM quota.
gcloud logging read "resource.type=gce_instance AND resource.labels.instance_id=INSTANCE_ID AND severity=ERROR" --project=PROJECT_ID --limit=10 --format="json(textPayload, timestamp)"Step 3: Identify and Reduce Redundant IAM Bindings
List all IAM bindings and look for excessive or redundant entries, especially on the project itself.
gcloud projects get-iam-policy PROJECT_ID --flatten="bindings[].members" --format="table(bindings.role, bindings.members)" | sort | uniq -c | sort -nrStep 4: Use IAM Recommendations or Prune via Terraform
Use GCP's IAM Recommender to find unused bindings, or use Terraform state to systematically remove them.
# Example Terraform command to plan removal of a specific binding (be cautious)
terraform plan -target=google_project_iam_binding.exampleStep 5: Request a Quota Increase
If policy optimization is insufficient, formally request an increase for the 'IAM policy size' quota.
# Navigate to IAM & Admin > Quotas in Cloud Console, or use:
echo "Request IAM policy size quota increase via GCP Support Console."Step 6: Restructure Policies Using Groups and Conditional Bindings
Move user bindings into Google Groups and apply policies at the group level. Use conditional IAM to replace many similar bindings.
# Example: Create a group and grant it a role
gcloud projects add-iam-policy-binding PROJECT_ID --member='group:my-developers@domain.com' --role='roles/compute.viewer'Architect's Pro Tip
"This often happens in mature projects using Infrastructure-as-Code (e.g., Terraform) where role assignments are additive over time, or after company acquisitions merging IAM policies. The 100KB-250KB quota limit is hit surprisingly fast."
Frequently Asked Questions
Why does an OOM event trigger an IAM permission error?
The OOM forces a VM restart. During the boot process, the instance metadata service must verify the attached service account's permissions. If the IAM policy is too large to process within time/memory constraints, this validation fails, resulting in PERMISSION_DENIED.
Can I check the IAM policy size quota via the console?
Yes. Go to IAM & Admin > Quotas. Filter for 'IAM policy size' metric. The limit and current usage are displayed. This is often easier than the gcloud alpha command.