IAM deny policies in GCP - Time & Space Complexity

Time Complexity: IAM deny policies

O(n)

Understanding Time Complexity

We want to understand how the time to evaluate IAM deny policies changes as we add more policies or rules.

Specifically, how does checking permissions grow when there are many deny rules?

Scenario Under Consideration

Analyze the time complexity of evaluating deny policies during an access request.

// Pseudocode for IAM deny policy evaluation
for each denyPolicy in denyPolicies:
  for each denyRule in denyPolicy.rules:
    if denyRule matches request:
      deny access
allow access if no deny rules matched

This sequence checks all deny rules in all deny policies to decide if access should be denied.

Identify Repeating Operations

Identify the API calls, resource provisioning, data transfers that repeat.

• Primary operation: Checking each deny rule against the access request.
• How many times: Once for every deny rule in all deny policies.

How Execution Grows With Input

As the number of deny rules increases, the system must check more rules one by one.

Input Size (n)	Approx. API Calls/Operations
10	10 rule checks
100	100 rule checks
1000	1000 rule checks

Pattern observation: The number of checks grows directly with the number of deny rules.

Final Time Complexity

Time Complexity: O(n)

This means the time to evaluate deny policies grows linearly with the number of deny rules.

Common Mistake

[X] Wrong: "Adding more deny policies won't affect evaluation time much because they run in parallel."

[OK] Correct: In reality, all deny rules must be checked one after another, so more rules mean more checks and longer evaluation time.

Interview Connect

Understanding how deny policies scale helps you design secure and efficient access controls in cloud environments.

Self-Check

"What if deny rules were grouped and checked using a fast lookup instead of one by one? How would the time complexity change?"

Practice

1. What is the main purpose of an IAM deny policy in Google Cloud?

easy

A. To monitor user activities without changing permissions

B. To grant additional permissions to users or groups

C. To explicitly block certain permissions even if other policies allow them

D. To automatically delete unused resources

Solution

1. Step 1: Understand IAM deny policy function and compare with other policy types

   An IAM deny policy explicitly blocks permissions, overriding any allow policies. Unlike allow policies that grant access, deny policies prevent actions even if allowed elsewhere.

2. Final Answer:

   To explicitly block certain permissions even if other policies allow them -> Option C

3. Quick Check:

   IAM deny policy blocks permissions [OK]

Hint: Deny policies block permissions regardless of other allows [OK]

Common Mistakes:

• Confusing deny policies with allow policies
• Thinking deny policies grant permissions
• Assuming deny policies only monitor actions

2. Which of the following is the correct way to specify a deny rule in an IAM policy JSON snippet?

easy

A. "denyRules": [{"deniedPermissions": ["storage.buckets.delete"]}]

B. "allowRules": [{"allowedPermissions": ["storage.buckets.delete"]}]

C. "denyRules": [{"allowedPermissions": ["storage.buckets.delete"]}]

D. "denyRules": [{"deniedPermissions": "storage.buckets.delete"}]

Solution

1. Step 1: Identify correct deny rule syntax and check each option

   Deny rules use "denyRules" with "deniedPermissions" as a list of strings. "denyRules": [{"deniedPermissions": ["storage.buckets.delete"]}] correctly uses a list for "deniedPermissions". "allowRules": [{"allowedPermissions": ["storage.buckets.delete"]}] uses allowRules, which is incorrect. "denyRules": [{"allowedPermissions": ["storage.buckets.delete"]}] mixes denyRules with allowedPermissions, which is invalid. "denyRules": [{"deniedPermissions": "storage.buckets.delete"}] uses a string instead of a list, which is invalid.

2. Final Answer:

   "denyRules": [{"deniedPermissions": ["storage.buckets.delete"]}] -> Option A

3. Quick Check:

   Deny rules need deniedPermissions as list [OK]

Hint: Deny permissions must be in a list under deniedPermissions [OK]

Common Mistakes:

• Using allowRules instead of denyRules
• Providing deniedPermissions as a string instead of list
• Mixing allowedPermissions inside denyRules

3. Given this deny policy snippet:

{"denyRules": [{"deniedPermissions": ["compute.instances.start"]}]}

What happens if a user has an allow policy for compute.instances.start but this deny policy is applied?

medium

A. The user can start instances because allow policies override deny policies

B. The deny policy has no effect unless combined with a condition

C. The user can start instances only during business hours

D. The user cannot start instances because deny policies block the permission

Solution

1. Step 1: Understand deny policy precedence and apply to the example

   Deny policies always override allow policies for the specified permissions. Even if the user has an allow for starting instances, the deny rule blocks it.

2. Final Answer:

   The user cannot start instances because deny policies block the permission -> Option D

3. Quick Check:

   Deny overrides allow for blocked permissions [OK]

Hint: Deny always beats allow for the same permission [OK]

Common Mistakes:

• Thinking allow policies override deny policies
• Assuming deny policies need conditions to work
• Believing deny policies only log actions

4. You wrote this deny policy JSON but it does not block the intended permission:

{"denyRules": [{"deniedPermissions": "storage.objects.delete"}]}

What is the error?

medium

A. denyRules should be named allowRules

B. deniedPermissions should be a list, not a string

C. storage.objects.delete is not a valid permission

D. The policy must include a condition to work

Solution

1. Step 1: Check deniedPermissions data type and identify the error in the JSON

   deniedPermissions must be a list of strings, not a single string. The policy uses a string instead of a list, so it is invalid and won't block the permission.

2. Final Answer:

   deniedPermissions should be a list, not a string -> Option B

3. Quick Check:

   deniedPermissions requires a list [OK]

Hint: Use brackets [] for deniedPermissions list [OK]

Common Mistakes:

• Using string instead of list for deniedPermissions
• Confusing denyRules with allowRules
• Thinking conditions are mandatory

5. You want to deny the permission storage.buckets.delete only for requests made after 2024-01-01T00:00:00Z. Which IAM deny policy condition should you use?

hard

A. Use a condition with request.time > '2024-01-01T00:00:00Z'

B. Use a condition with resource.name == 'storage.buckets.delete'

C. No condition is needed; deny applies to all users

D. Use a condition with request.ip != 'company-network-ip-range'

Solution

1. Step 1: Identify the condition to restrict by time and match condition syntax

   To deny only for requests after the date, use a condition checking request.time > '2024-01-01T00:00:00Z'. Use a condition with request.time > '2024-01-01T00:00:00Z' uses request.time with inequality to apply after the date, which is correct.

2. Final Answer:

   Use a condition with request.time > '2024-01-01T00:00:00Z' -> Option A

3. Quick Check:

   Condition on request.time restricts deny by time [OK]

Hint: Use request.time condition to limit deny by time [OK]

Common Mistakes:

• Using resource.name instead of request.time for time check
• Using IP condition unrelated to time
• Applying deny without condition blocks all users