hcl2: fix crash on malformed overrides

When an override block is either not an object, or if one of its
contents are not an object, Packer would crash trying to forcefully
cast the cty.Value to a cty.Collection.

To avoid this behaviour, we add extra checks that return hcl.Diagnostics
when they fail.

hcl2template/fixtures/malformed_override.pkr.hcl

@@ -0,0 +1,4 @@
+provisioner "shell-local" {
+  inline = ["echo 'hi'"]
+  override = "hello"
+}

hcl2template/fixtures/malformed_override_innards.pkr.hcl

@@ -0,0 +1,6 @@
+provisioner "shell-local" {
+  inline = ["echo 'hi'"]
+  override = {
+    test = "hello"
+  }
+}

hcl2template/fixtures/well_formed_provisioner.pkr.hcl

@@ -0,0 +1,9 @@
+provisioner "shell-local" {
+  inline = ["echo 'hi'"]
+  override = {
+    test = {
+      "hello" = "new value"
+    }
+  }
+}
+

hcl2template/types.build.provisioners.go

@@ -104,9 +104,28 @@ func (p *Parser) decodeProvisioner(block *hcl.Block, ectx *hcl.EvalContext) (*Pr
 	}
 
 	if !b.Override.IsNull() {
+		if !b.Override.Type().IsObjectType() {
+			return nil, append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "provisioner's override block must be an HCL object",
+				Subject:  block.DefRange.Ptr(),
+			})
+		}
+
 		override := make(map[string]interface{})
 		for buildName, overrides := range b.Override.AsValueMap() {
 			buildOverrides := make(map[string]interface{})
+
+			if !overrides.Type().IsObjectType() {
+				return nil, append(diags, &hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary: fmt.Sprintf(
+						"provisioner's override.'%s' block must be an HCL object",
+						buildName),
+					Subject: block.DefRange.Ptr(),
+				})
+			}
+
 			for option, value := range overrides.AsValueMap() {
 				buildOverrides[option] = hcl2shim.ConfigValueFromHCL2(value)
 			}

hcl2template/types.build.provisioners_test.go

@@ -0,0 +1,62 @@
+package hcl2template
+
+import (
+	"testing"
+
+	"github.com/hashicorp/hcl/v2"
+)
+
+func TestPackerConfig_ParseProvisionerBlock(t *testing.T) {
+	tests := []struct {
+		name        string
+		inputFile   string
+		expectError bool
+	}{
+		{
+			"success - provisioner is valid",
+			"fixtures/well_formed_provisioner.pkr.hcl",
+			false,
+		},
+		{
+			"failure - provisioner override is malformed",
+			"fixtures/malformed_override.pkr.hcl",
+			true,
+		},
+		{
+			"failure - provisioner override.test is malformed",
+			"fixtures/malformed_override_innards.pkr.hcl",
+			true,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			cfg := PackerConfig{parser: getBasicParser()}
+			f, diags := cfg.parser.ParseHCLFile(test.inputFile)
+			if diags.HasErrors() {
+				t.Errorf("failed to parse input file %s", test.inputFile)
+				for _, d := range diags {
+					t.Errorf("%s", d)
+				}
+				return
+			}
+			provBlock := f.OutermostBlockAtPos(hcl.Pos{
+				Line:   1,
+				Column: 1,
+				Byte:   0,
+			})
+			_, diags = cfg.parser.decodeProvisioner(provBlock, nil)
+			if diags.HasErrors() == test.expectError {
+				return
+			}
+
+			if !diags.HasErrors() {
+				t.Fatalf("unexpected successful parsing of provisioner")
+			}
+
+			for _, d := range diags {
+				t.Errorf("unexpected error: %s", d)
+			}
+		})
+	}
+}