feat: add KMS encryption support for CloudTrail logs

src/kms.tf

@@ -0,0 +1,134 @@
+# KMS key policy document for CloudTrail encryption
+data "aws_iam_policy_document" "cloudtrail_kms_key_policy" {
+  count = local.enabled && var.cloudtrail_enable_kms_encryption && var.cloudtrail_create_kms_key && var.cloudtrail_kms_key_arn == null ? 1 : 0
+
+  # Enable IAM User Permissions
+  statement {
+    sid    = "Enable IAM User Permissions"
+    effect = "Allow"
+    principals {
+      type        = "AWS"
+      identifiers = ["arn:${local.aws_partition}:iam::${local.aws_account_id}:root"]
+    }
+    actions = [
+      "kms:*"
+    ]
+    resources = ["*"]
+  }
+
+  # Allow CloudTrail to encrypt logs
+  statement {
+    sid    = "Allow CloudTrail to encrypt logs"
+    effect = "Allow"
+    principals {
+      type        = "Service"
+      identifiers = ["cloudtrail.amazonaws.com"]
+    }
+    actions = [
+      "kms:GenerateDataKey*"
+    ]
+    resources = ["*"]
+    condition {
+      test     = "StringLike"
+      variable = "aws:SourceArn"
+      values   = ["arn:${local.aws_partition}:cloudtrail:*:${local.aws_account_id}:trail/${module.this.id}"]
+    }
+    condition {
+      test     = "StringEquals"
+      variable = "kms:EncryptionContext:aws:cloudtrail:arn"
+      values   = ["arn:${local.aws_partition}:cloudtrail:${var.region}:${local.aws_account_id}:trail/${module.this.id}"]
+    }
+  }
+
+  # Allow CloudTrail to describe key
+  statement {
+    sid    = "Allow CloudTrail to describe key"
+    effect = "Allow"
+    principals {
+      type        = "Service"
+      identifiers = ["cloudtrail.amazonaws.com"]
+    }
+    actions = [
+      "kms:DescribeKey"
+    ]
+    resources = ["*"]
+  }
+
+  # Allow principals in the account to decrypt log files
+  statement {
+    sid    = "Allow principals in the account to decrypt log files"
+    effect = "Allow"
+    principals {
+      type        = "AWS"
+      identifiers = ["arn:${local.aws_partition}:iam::${local.aws_account_id}:root"]
+    }
+    actions = [
+      "kms:Decrypt",
+      "kms:ReEncryptFrom"
+    ]
+    resources = ["*"]
+    condition {
+      test     = "StringEquals"
+      variable = "kms:CallerAccount"
+      values   = [local.aws_account_id]
+    }
+    condition {
+      test     = "StringLike"
+      variable = "kms:EncryptionContext:aws:cloudtrail:arn"
+      values   = ["arn:${local.aws_partition}:cloudtrail:*:${local.aws_account_id}:trail/*"]
+    }
+  }
+
+  # Allow alias creation during resource creation
+  statement {
+    sid    = "Allow alias creation during resource creation"
+    effect = "Allow"
+    principals {
+      type        = "AWS"
+      identifiers = ["arn:${local.aws_partition}:iam::${local.aws_account_id}:root"]
+    }
+    actions = [
+      "kms:CreateAlias"
+    ]
+    resources = ["*"]
+    condition {
+      test     = "StringEquals"
+      variable = "kms:CallerAccount"
+      values   = [local.aws_account_id]
+    }
+    condition {
+      test     = "StringEquals"
+      variable = "kms:ViaService"
+      values   = ["ec2.${var.region}.amazonaws.com"]
+    }
+  }
+}
+
+# KMS key for CloudTrail encryption
+resource "aws_kms_key" "cloudtrail" {
+  count = local.enabled && var.cloudtrail_enable_kms_encryption && var.cloudtrail_create_kms_key && var.cloudtrail_kms_key_arn == null ? 1 : 0
+
+  description             = "KMS key for CloudTrail log encryption - ${module.this.id}"
+  deletion_window_in_days = var.cloudtrail_kms_key_deletion_window_in_days
+  enable_key_rotation     = var.cloudtrail_kms_key_enable_rotation
+  policy                  = data.aws_iam_policy_document.cloudtrail_kms_key_policy[0].json
+
+  tags = merge(
+    module.this.tags,
+    {
+      Name       = "${module.this.id}-cloudtrail"
+      managed-by = "terraform"
+      env        = var.stage
+      service    = "datadog-logs-archive"
+      part-of    = "observability"
+    }
+  )
+}
+
+# KMS key alias for easier identification
+resource "aws_kms_alias" "cloudtrail" {
+  count = local.enabled && var.cloudtrail_enable_kms_encryption && var.cloudtrail_create_kms_key && var.cloudtrail_kms_key_arn == null ? 1 : 0
+
+  name          = "alias/${module.this.id}-cloudtrail"
+  target_key_id = aws_kms_key.cloudtrail[0].key_id
+}

src/main.tf

@@ -30,6 +30,14 @@ locals {
   # default datadog_logs_archive query.
   default_query = join(" OR ", concat([join(":", ["env", var.stage]), join(":", ["account", local.aws_account_id])], var.additional_query_tags))
   query         = var.query_override == null ? local.default_query : var.query_override
+
+  # CloudTrail KMS key ARN selection
+  cloudtrail_kms_key_arn = (
+    var.cloudtrail_enable_kms_encryption == false ? null :
+    var.cloudtrail_kms_key_arn != null ? var.cloudtrail_kms_key_arn :
+    var.cloudtrail_create_kms_key && local.enabled ? try(aws_kms_key.cloudtrail[0].arn, null) :
+    null
+  )
 }
 
 # We use the http data source due to lack of a data source for datadog_logs_archive_order
@@ -304,7 +312,7 @@ module "cloudtrail" {
   # dependency on the attachment of the bucket policy, leading to
   # insufficient permissions issues on cloudtrail creation if it
   # happens to be attempted prior to completion of the policy attachment.
-  depends_on = [module.cloudtrail_s3_bucket]
+  depends_on = [module.cloudtrail_s3_bucket, aws_kms_key.cloudtrail]
   source     = "cloudposse/cloudtrail/aws"
   version    = "0.24.0"
 
@@ -314,6 +322,7 @@ module "cloudtrail" {
   enabled                       = local.enabled
   enable_logging                = true
   s3_bucket_name                = module.cloudtrail_s3_bucket[0].bucket_id
+  kms_key_arn                   = local.cloudtrail_kms_key_arn
 
   event_selector = [
     {

src/outputs.tf

@@ -42,3 +42,18 @@ output "catchall_id" {
   value       = local.enabled && var.catchall_enabled ? datadog_logs_archive.catchall_archive[0].id : ""
   description = "The ID of the catchall log archive"
 }
+
+output "cloudtrail_kms_key_arn" {
+  value       = local.cloudtrail_kms_key_arn
+  description = "The ARN of the KMS key used for CloudTrail log encryption"
+}
+
+output "cloudtrail_kms_key_id" {
+  value       = local.enabled && var.cloudtrail_enable_kms_encryption && var.cloudtrail_create_kms_key && var.cloudtrail_kms_key_arn == null ? aws_kms_key.cloudtrail[0].key_id : ""
+  description = "The ID of the KMS key used for CloudTrail log encryption (only if created by this module)"
+}
+
+output "cloudtrail_kms_key_alias" {
+  value       = local.enabled && var.cloudtrail_enable_kms_encryption && var.cloudtrail_create_kms_key && var.cloudtrail_kms_key_arn == null ? aws_kms_alias.cloudtrail[0].name : ""
+  description = "The alias of the KMS key used for CloudTrail log encryption (only if created by this module)"
+}

src/variables.tf

@@ -95,4 +95,35 @@ variable "s3_force_destroy" {
   default     = false
 }
 
+variable "cloudtrail_enable_kms_encryption" {
+  type        = bool
+  description = "Enable KMS encryption for CloudTrail logs"
+  default     = true
+}
+
+variable "cloudtrail_kms_key_arn" {
+  type        = string
+  description = "ARN of an existing KMS key to use for CloudTrail log encryption. If not provided and cloudtrail_enable_kms_encryption is true, a new key will be created"
+  default     = null
+  nullable    = true
+}
+
+variable "cloudtrail_create_kms_key" {
+  type        = bool
+  description = "Create a new KMS key for CloudTrail encryption. Only used if cloudtrail_kms_key_arn is not provided and cloudtrail_enable_kms_encryption is true"
+  default     = true
+}
+
+variable "cloudtrail_kms_key_deletion_window_in_days" {
+  type        = number
+  description = "Duration in days after which the KMS key is deleted after destruction of the resource, must be between 7 and 30 days"
+  default     = 10
+}
+
+variable "cloudtrail_kms_key_enable_rotation" {
+  type        = bool
+  description = "Enable automatic rotation of the KMS key"
+  default     = true
+}
+