sec(iac/aws): drop unconditioned account-wide KMS data-plane grant from deploy role

[cristim]

What

Closes LeanerCloud/cloud-commitments-platform#253.

The CI/CD deploy role granted kms:Decrypt, kms:Encrypt, kms:GenerateDataKey and kms:CreateGrant on Resource: "*" with no condition. CreateGrant is the serious half: it is a permission-granting action, held account-wide by the identity that runs every deployment.

The fix is deletion rather than a narrower scope, because the correct scope is empty. Nothing on the deploy path uses any of the four.

Why nothing needs them

Each claim was verified independently by a reviewer that did not write the change:

Claim	Evidence
The only customer managed key the modules create is the OIDC signing key	ECC_NIST_P256, SIGN_VERIFY; KMS rejects encrypt, decrypt and data-key operations on it regardless of IAM
The SNS key module is never instantiated	no environment references it, and it is behind a flag defaulting to false
RDS, the seven secrets, Lambda env and the state bucket use AWS managed keys	no kms_key_id set anywhere in the modules, environment or tfvars; the backend uses SSE-S3
ECR uses AES256	modules/registry/aws/main.tf:12
kms:DescribeKey, the one action the deploy path uses, is already granted	policy_compute.tf, unaffected by this change

The crux is that an AWS managed key does not consult the caller's identity policy. Its key policy is service-controlled and authorises through kms:CallerAccount and kms:ViaService; the service performs the operation on the caller's behalf. Only a customer managed key would depend on the identity policy, and the deploy path has none. So the removed statement was not adding permission to any working path.

kms:CreateGrant has one plausible consumer, EBS volumes on the NAT auto-scaling group. Those grants are created by the Auto Scaling service-linked role and authorised by the key's own policy, never by the caller of CreateAutoScalingGroup.

The guard test

A new test asserts these four actions are never granted without a condition. It fails with exactly four errors on the unfixed tree and passes after.

Its teeth were checked in several directions: a single re-added action fails naming only that action; the same grant in a different policy file is caught; so is one in a brand-new file, since the test globs; a scalar Action string is caught; kms:* produces nine errors. A tag-gated grant passes, which is the intended escape hatch, and the shape a future customer managed key should use is documented in the test's comment.

Known limitation, stated rather than hidden: the guard reaches policy_*.tf only. An inline aws_iam_role_policy in role.tf would be invisible to it. There are none today, and the sibling guard has the same reach.

policy_boundary.tf is deliberately excluded: it carries kms:* as a permissions ceiling for roles the deploy role creates, not as a grant to the deploy role, so skipping it hides nothing.

The policy description is deliberately left saying "KMS". It is ForceNew on aws_iam_policy, confirmed in the provider source, so editing it would replace the managed policy and its attachment rather than update it in place. A slightly stale word is the cheaper problem.

Operator runbook

Nothing changes until a privileged human re-applies ci-cd-permissions/. That module is applied manually by design, per the bootstrap-versus-runtime split.

Before applying, rule out a customer managed key the repository cannot see. Any one of these returning a CMK ARN means re-add the statement scoped to that key first:

aws s3api get-bucket-encryption --bucket <state-bucket>      # expect AES256, or aws:kms with no KMSMasterKeyID
aws secretsmanager describe-secret --secret-id <arn> --query KmsKeyId   # expect null, for each of the seven
aws rds describe-db-instances --query '...KmsKeyId'          # expect alias/aws/rds
aws lambda get-function-configuration --query KMSKeyArn      # expect null

The plan must show exactly one change: ~ aws_iam_policy.networking updated in place, policy attribute only. Any -/+ or attachment change means stop.

After applying, run the dev deploy workflow. Green means the removed actions were genuinely unused.

If a hidden customer managed key exists, the failure is loud and immediate: AccessDeniedException on kms:Decrypt at state read or secret read. Nothing is corrupted, and recovery is a revert plus re-apply, a matter of minutes.

Unrelated but worth knowing: if EBS encryption by default is ever enabled with a customer managed key in this account, the NAT auto-scaling group needs that key's policy to admit the Auto Scaling service-linked role. That is not caused by this PR, but it is the one KMS failure an operator might otherwise attribute to it.

Verification

Check	Result
go test ./terraform/...	ok, both packages
terraform fmt -check -recursive, validate	exit 0, configuration valid
gofmt -l, go vet	clean

terraform plan needs bootstrap credentials and was not run here. That is the operator's step above.

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Summary by CodeRabbit

• Security Improvements

  • Tightened deployment permissions by removing unnecessary KMS access from the networking deployment policy.
  • Deployment workflows now have reduced access to encryption and key-management operations from this policy.

• Tests

  • Added automated checks to prevent unconditioned grants of sensitive KMS data-plane actions across guarded deployment policies.
  • Added validation to ensure these checks scan policy statements correctly.

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• terraform/environments/aws/ci-cd-permissions/policy_guard_test.go

📝 Walkthrough

Walkthrough

The deploy networking policy removes unconditioned KMS permissions. A new guard test checks CI/CD policy documents for unconditioned KMS data-plane grants and validates parser coverage.

Changes

KMS permission hardening

Layer / File(s)	Summary
Remove and guard KMS actions terraform/environments/aws/ci-cd-permissions/policy_networking.tf, terraform/environments/aws/ci-cd-permissions/policy_guard_test.go	The networking policy removes the unconditioned KMS statement. The guard test checks listed KMS data-plane actions across guarded policies and policy_iam.tf, while validating parsed action counts and scanned-statement coverage.

Priority: ➖ Normal — Impact reflects medium issue severity.

Estimated code review effort: 2 (Simple) | ~10 minutes

Severity of issue fixed: Medium

Merge Risk: 🟡 Moderate · up to dfbac

The current broad KMS grant is removed, but the new safeguard can miss grants hidden behind ineffective conditions or non-literal action expressions. These gaps should be fixed before merge so future policy changes cannot silently restore account-wide KMS access.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes removal of the unconditioned account-wide KMS data-plane grant from the deploy role.
Linked Issues check	✅ Passed	The changes address issue #1969 by removing the unconditioned KMS data-plane permissions, retaining kms:DescribeKey, and adding a regression guard against future unconditioned grants.
Out of Scope Changes check	✅ Passed	The policy removal and guard test are directly related to issue #1969 and the stated security objective. No unrelated code changes are shown.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 1 files.

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Actionable comments posted: 2

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Inline comments:
In `@terraform/environments/aws/ci-cd-permissions/policy_guard_test.go`:
- Line 1670: Update the KMS statement validation around effectIsAllowPattern and
statementConditionBody so an Allow statement requires an effective
resource-scoping condition, such as aws:ResourceTag/Project, rather than merely
any non-empty Condition block; additionally require kms:GrantIsForAWSResource
for kms:CreateGrant statements, and add fixtures covering empty and unrelated
conditions.
- Around line 1673-1675: Update
TestKMSDataPlaneActionsAreNotUnconditionallyGranted so non-literal Action
assignments such as local-backed lists cannot bypass validation: either parse
the HCL expression or explicitly reject/flag assignments that
actionAssignmentPattern cannot interpret. Ensure the KMS-action checks fail
closed when parsing is incomplete, and add a local-backed action-list fixture
covering this case.

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• terraform/environments/aws/ci-cd-permissions/policy_guard_test.go
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[cristim]

Status: all checks green on bacd4e15e. Parked for a human, not auto-merged, because it changes IAM.

Recording the review history here, since it is spread across two threads and three commits and the interesting part is not the deletion.

The change itself (dfbace67d) was reviewed independently and cleared with no findings. All five claims behind it were verified against the tree, and the crux was checked against AWS documentation rather than assumed: an AWS managed key does not consult the caller's identity policy, so the removed grant was not enabling any working path. Only a customer managed key would, and the deploy path has none.

Every subsequent finding was against the guard, not the change. That is worth stating plainly, because the guard is what will outlive this PR.

1. CodeRabbit found the guard failed open. Its parse-completeness check compared entries present against entries parsed, but both counts derive from the same literal-matching pattern, so an unreadable action list sent both to zero, the check agreed with itself, and the loop then examined nothing.
2. CodeRabbit also found a bare Condition was treated as scoping, when a condition scoping something irrelevant restricts nothing.
3. The first fix (583a013a6) reintroduced the first defect in a new place. Adversarial review reproduced three shapes still slipping through and granting kms:Decrypt with the whole package green: a one-line statement with the key mid-line, an Allow carrying NotAction, and a quoted "Action" object key. The premise behind that fix was also wrong: IAM rejects an Allow with neither Action nor NotAction, so the "grants nothing" case being preserved does not exist.
4. The same review found the substring condition check passed eight bypasses, including an inverted operator, an if-exists operator true for every untagged key, a wrong tag value, a trailing comment, and kms:GrantIsForAWSResource set to "false".
5. It also found the code comment attributed the wrong mechanism to kms:GrantIsForAWSResource. That key restricts the call path to service-initiated calls; the key governing the grantee is kms:GranteePrincipal. The effect claimed was right, the explanation was not, and it would have taught the next reader something false.

bacd4e15e closes all of it by reusing shapes already in this file rather than inventing new ones. The action branch now mirrors the sibling boundary guard and is shorter than what it replaces. The condition check pins the operator with existing helpers, no HCL evaluator.

Ten bypass shapes were reproduced failing and two legitimate shapes passing. Every statement currently in these files has a literal parseable action list, so this closed a latent hole rather than a live one, and neither tag-gated KMS statement grants a data-plane action, so neither was ever at risk from the tightening.

Before applying, run the pre-flight in the PR body that rules out a customer managed key the repository cannot see. Nothing changes until ci-cd-permissions/ is re-applied by hand, which is by design.

[cristim]

Ported to LeanerCloud/cloud-commitments-platform#4 after the monorepo split; closing here.