compress/lzw: reject writing bytes that don't fit into litWidth.

Fixes #11142.

Change-Id: Id772c4364c47776d6afe86b0939b9c6281e85edc
Reviewed-on: https://go-review.googlesource.com/11227Reviewed-by: Russ Cox <rsc@golang.org>

src/compress/lzw/writer.go

@@ -138,16 +138,23 @@ func (e *encoder) Write(p []byte) (n int, err error) {
 	if len(p) == 0 {
 		return 0, nil
 	}
+	if maxLit := uint8(1<<e.litWidth - 1); maxLit != 0xff {
+		for _, x := range p {
+			if x > maxLit {
+				e.err = errors.New("lzw: input byte too large for the litWidth")
+				return 0, e.err
+			}
+		}
+	}
 	n = len(p)
-	litMask := uint32(1<<e.litWidth - 1)
 	code := e.savedCode
 	if code == invalidCode {
 		// The first code sent is always a literal code.
-		code, p = uint32(p[0])&litMask, p[1:]
+		code, p = uint32(p[0]), p[1:]
 	}
 loop:
 	for _, x := range p {
-		literal := uint32(x) & litMask
+		literal := uint32(x)
 		key := code<<8 | literal
 		// If there is a hash table hit for this key then we continue the loop
 		// and do not emit a code yet.

src/compress/lzw/writer_test.go

@@ -104,6 +104,16 @@ func TestWriterReturnValues(t *testing.T) {
 	}
 }
 
+func TestSmallLitWidth(t *testing.T) {
+	w := NewWriter(ioutil.Discard, LSB, 2)
+	if _, err := w.Write([]byte{0x03}); err != nil {
+		t.Fatalf("write a byte < 1<<2: %v", err)
+	}
+	if _, err := w.Write([]byte{0x04}); err == nil {
+		t.Fatal("write a byte >= 1<<2: got nil error, want non-nil")
+	}
+}
+
 func benchmarkEncoder(b *testing.B, n int) {
 	b.StopTimer()
 	b.SetBytes(int64(n))