mod/modfile: parse module.cue in data-only mode

This reduces the surface for denial of service attacks involving
arbitrary computations via the module.cue file.

Fixes #3138.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: If58c8d3a7b05a657543909bf9d1ebaaf4296d039

mod/modfile/modfile.go

@@ -30,12 +30,14 @@ import (
 
 	"cuelang.org/go/cue"
 	"cuelang.org/go/cue/ast"
+	"cuelang.org/go/cue/build"
 	"cuelang.org/go/cue/cuecontext"
 	"cuelang.org/go/cue/errors"
 	"cuelang.org/go/cue/format"
-	"cuelang.org/go/cue/parser"
 	"cuelang.org/go/cue/token"
 	"cuelang.org/go/internal/cueversion"
+	"cuelang.org/go/internal/encoding"
+	"cuelang.org/go/internal/filetypes"
 	"cuelang.org/go/mod/module"
 )
 
@@ -255,14 +257,14 @@ func ParseNonStrict(modfile []byte, filename string) (*File, error) {
 }
 
 func parse(modfile []byte, filename string, strict bool) (*File, error) {
-	file, err := parser.ParseFile(filename, modfile)
+	// Unfortunately we need a new context. See the note inside [moduleSchemaDo].
+	ctx := cuecontext.New()
+	file, err := parseDataOnlyCUE(ctx, modfile, filename)
 	if err != nil {
 		return nil, errors.Wrapf(err, token.NoPos, "invalid module.cue file syntax")
 	}
-	// TODO disallow non-data-mode CUE.
 
-	// Unfortunate to need a new context, but see the note inside [moduleSchemaDo].
-	v := cuecontext.New().BuildFile(file)
+	v := ctx.BuildFile(file)
 	if err := v.Validate(cue.Concrete(true)); err != nil {
 		return nil, errors.Wrapf(err, token.NoPos, "invalid module.cue file value")
 	}
@@ -389,6 +391,23 @@ func parse(modfile []byte, filename string, strict bool) (*File, error) {
 	return mf, nil
 }
 
+func parseDataOnlyCUE(ctx *cue.Context, cueData []byte, filename string) (*ast.File, error) {
+	dec := encoding.NewDecoder(ctx, &build.File{
+		Filename:       filename,
+		Encoding:       build.CUE,
+		Interpretation: build.Auto,
+		Form:           build.Data,
+		Source:         cueData,
+	}, &encoding.Config{
+		Mode:      filetypes.Export,
+		AllErrors: true,
+	})
+	if err := dec.Err(); err != nil {
+		return nil, err
+	}
+	return dec.File(), nil
+}
+
 func newCUEError(err error, filename string) error {
 	ps := errors.Positions(err)
 	for _, p := range ps {

mod/modfile/modfile_test.go

@@ -281,6 +281,38 @@ cue: lang: "xxx"
 	want: &File{
 		Module: "foo.com/bar",
 	},
+}, {
+	testName: "ReferencesNotAllowed#1",
+	parse:    Parse,
+	data: `
+module: "foo.com/bar"
+_foo: "v0.9.0"
+cue: lang: _foo
+`,
+	wantError: `invalid module.cue file syntax: references not allowed in data mode:
+    module.cue:4:12`,
+}, {
+	testName: "ReferencesNotAllowed#2",
+	parse:    Parse,
+	data: `
+module: "foo.com/bar"
+let foo = "v0.9.0"
+cue: lang: foo
+`,
+	wantError: `invalid module.cue file syntax: references not allowed in data mode:
+    module.cue:3:1
+invalid module.cue file syntax: references not allowed in data mode:
+    module.cue:4:12`,
+}, {
+	testName: "DefinitionsNotAllowed",
+	parse:    Parse,
+	data: `
+module: "foo.com/bar"
+#x: "v0.9.0"
+cue: lang: "v0.9.0"
+`,
+	wantError: `invalid module.cue file syntax: definitions not allowed in data mode:
+    module.cue:3:1`,
 }}
 
 func TestParse(t *testing.T) {