debug/macho: fail on invalid dynamic symbol table command

Fail out when loading a file that contains a dynamic symbol table
command that indicates a larger number of symbols than exist in the
loaded symbol table.

Thanks to Burak Çarıkçı - Yunus Yıldırım (CT-Zer0 Crypttech) for
reporting this issue.

Fixes #48990
Fixes CVE-2021-41771

Change-Id: Ic3d6e6529241afcc959544b326b21b663262bad5
Reviewed-on: https://go-review.googlesource.com/c/go/+/355990
Reviewed-by: Julie Qiu <julie@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Katie Hockman <katie@golang.org>

src/debug/macho/file.go

@@ -345,6 +345,15 @@ func NewFile(r io.ReaderAt) (*File, error) {
 			if err := binary.Read(b, bo, &hdr); err != nil {
 				return nil, err
 			}
+			if hdr.Iundefsym > uint32(len(f.Symtab.Syms)) {
+				return nil, &FormatError{offset, fmt.Sprintf(
+					"undefined symbols index in dynamic symbol table command is greater than symbol table length (%d > %d)",
+					hdr.Iundefsym, len(f.Symtab.Syms)), nil}
+			} else if hdr.Iundefsym+hdr.Nundefsym > uint32(len(f.Symtab.Syms)) {
+				return nil, &FormatError{offset, fmt.Sprintf(
+					"number of undefined symbols after index in dynamic symbol table command is greater than symbol table length (%d > %d)",
+					hdr.Iundefsym+hdr.Nundefsym, len(f.Symtab.Syms)), nil}
+			}
 			dat := make([]byte, hdr.Nindirectsyms*4)
 			if _, err := r.ReadAt(dat, int64(hdr.Indirectsymoff)); err != nil {
 				return nil, err

src/debug/macho/file_test.go

@@ -416,3 +416,10 @@ func TestTypeString(t *testing.T) {
 		t.Errorf("got %v, want %v", TypeExec.GoString(), "macho.Exec")
 	}
 }
+
+func TestOpenBadDysymCmd(t *testing.T) {
+	_, err := openObscured("testdata/gcc-amd64-darwin-exec-with-bad-dysym.base64")
+	if err == nil {
+		t.Fatal("openObscured did not fail when opening a file with an invalid dynamic symbol table command")
+	}
+}