core: Loosen output value sensitivity requirement

Non-root module outputs no longer strip sensitivity marks from their
values, allowing dynamically sensitive values to propagate through the
configuration. We also remove the requirement for non-root module
outputs to be defined as sensitive if the value is marked as sensitive.

This avoids a static/dynamic clash when using shared modules that might
unknowingly receive sensitive values via input variables.

Co-authored-by: Martin Atkins <mart@degeneration.co.uk>

terraform/context_plan2_test.go

@@ -2,6 +2,7 @@ package terraform
 
 import (
 	"errors"
+	"strings"
 	"testing"
 
 	"github.com/hashicorp/terraform/addrs"
@@ -485,3 +486,32 @@ provider "test" {
 		t.Fatal(diags.Err())
 	}
 }
+
+func TestContext2Plan_invalidSensitiveModuleOutput(t *testing.T) {
+	m := testModuleInline(t, map[string]string{
+		"child/main.tf": `
+output "out" {
+  value = sensitive("xyz")
+}`,
+		"main.tf": `
+module "child" {
+  source = "./child"
+}
+
+output "root" {
+  value = module.child.out
+}`,
+	})
+
+	ctx := testContext2(t, &ContextOpts{
+		Config: m,
+	})
+
+	_, diags := ctx.Plan()
+	if !diags.HasErrors() {
+		t.Fatal("succeeded; want errors")
+	}
+	if got, want := diags.Err().Error(), "Output refers to sensitive values"; !strings.Contains(got, want) {
+		t.Fatalf("wrong error:\ngot:  %s\nwant: message containing %q", got, want)
+	}
+}

terraform/context_validate_test.go

@@ -1379,7 +1379,7 @@ resource "aws_instance" "foo" {
 	}
 }
 
-func TestContext2Validate_invalidSensitiveModuleOutput(t *testing.T) {
+func TestContext2Validate_sensitiveRootModuleOutput(t *testing.T) {
 	m := testModuleInline(t, map[string]string{
 		"child/main.tf": `
 variable "foo" {
@@ -1395,27 +1395,19 @@ module "child" {
   source = "./child"
 }
 
-resource "aws_instance" "foo" {
+output "root" {
-  foo = module.child.out
+  value = module.child.out
+  sensitive = true
 }`,
 	})
 
-	p := testProvider("aws")
 	ctx := testContext2(t, &ContextOpts{
 		Config: m,
-		Providers: map[addrs.Provider]providers.Factory{
-			addrs.NewDefaultProvider("aws"): testProviderFuncFixed(p),
-		},
 	})
 
 	diags := ctx.Validate()
-	if !diags.HasErrors() {
+	if diags.HasErrors() {
-		t.Fatal("succeeded; want errors")
+		t.Fatal(diags.Err())
-	}
-	// Should get this error:
-	// Output refers to sensitive values: Expressions used in outputs can only refer to sensitive values if the sensitive attribute is true.
-	if got, want := diags.Err().Error(), "Output refers to sensitive values"; !strings.Contains(got, want) {
-		t.Fatalf("wrong error:\ngot:  %s\nwant: message containing %q", got, want)
 	}
 }
 

terraform/evaluate.go

@@ -459,7 +459,7 @@ func (d *evaluationStateData) GetModule(addr addrs.ModuleCall, rng tfdiags.Sourc
 				continue
 			}
 
-			instance[cfg.Name] = change.After
+			instance[cfg.Name] = change.After.MarkWithPaths(changeSrc.AfterValMarks)
 
 			if change.Sensitive && !change.After.HasMark("sensitive") {
 				instance[cfg.Name] = change.After.Mark("sensitive")

terraform/node_output.go

@@ -275,18 +275,26 @@ func (n *NodeApplyableOutput) Execute(ctx EvalContext, op walkOperation) (diags
 		// depends_on expressions here too
 		diags = diags.Append(validateDependsOn(ctx, n.Config.DependsOn))
 
-		// Ensure that non-sensitive outputs don't include sensitive values
+		// For root module outputs in particular, an output value must be
+		// statically declared as sensitive in order to dynamically return
+		// a sensitive result, to help avoid accidental exposure in the state
+		// of a sensitive value that the user doesn't want to include there.
 		_, marks := val.UnmarkDeep()
 		_, hasSensitive := marks["sensitive"]
+		if n.Addr.Module.IsRoot() {
 			if !n.Config.Sensitive && hasSensitive {
 				diags = diags.Append(&hcl.Diagnostic{
 					Severity: hcl.DiagError,
 					Summary:  "Output refers to sensitive values",
-				Detail:   "Expressions used in outputs can only refer to sensitive values if the sensitive attribute is true.",
+					Detail: `To reduce the risk of accidentally exporting sensitive data that was intended to be only internal, Terraform requires that any root module output containing sensitive data be explicitly marked as sensitive, to confirm your intent.
+
+If you do intend to export this data, annotate the output value as sensitive by adding the following argument:
+    sensitive = true`,
 					Subject: n.Config.DeclRange.Ptr(),
 				})
 			}
 		}
+	}
 
 	// handling the interpolation error
 	if diags.HasErrors() {
@@ -454,7 +462,7 @@ func (n *NodeApplyableOutput) setValue(state *states.SyncState, changes *plans.C
 		sensitiveChange := sensitiveBefore || n.Config.Sensitive
 
 		// strip any marks here just to be sure we don't panic on the True comparison
-		val, _ = val.UnmarkDeep()
+		unmarkedVal, _ := val.UnmarkDeep()
 
 		action := plans.Update
 		switch {
@@ -468,7 +476,7 @@ func (n *NodeApplyableOutput) setValue(state *states.SyncState, changes *plans.C
 			action = plans.Create
 
 		case val.IsWhollyKnown() &&
-			val.Equals(before).True() &&
+			unmarkedVal.Equals(before).True() &&
 			n.Config.Sensitive == sensitiveBefore:
 			// Sensitivity must also match to be a NoOp.
 			// Theoretically marks may not match here, but sensitivity is the