encoding/gob: change panic into error for corrupt input

decBuffer.Drop is called using data provided by the user, don't
panic if it's bogus.

Fixes #10272.

Change-Id: I913ae9c3c45cef509f2b8eb02d1efa87fbd52afa
Reviewed-on: https://go-review.googlesource.com/8496Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/gob/decode.go

@@ -688,7 +688,11 @@ func (dec *Decoder) ignoreInterface(state *decoderState) {
 		error_(dec.err)
 	}
 	// At this point, the decoder buffer contains a delimited value. Just toss it.
-	state.b.Drop(int(state.decodeUint()))
+	n := int(state.decodeUint())
+	if n < 0 || state.b.Len() < n {
+		errorf("bad interface encoding: length too large for buffer")
+	}
+	state.b.Drop(n)
 }
 
 // decodeGobDecoder decodes something implementing the GobDecoder interface.

src/encoding/gob/encoder_test.go

@@ -954,3 +954,17 @@ func TestErrorForHugeSlice(t *testing.T) {
 		t.Fatalf("decode: expected slice too big error, got %s", err.Error())
 	}
 }
+
+// Don't crash, just give error with corrupted length.
+// Issue 10270.
+func TestErrorBadDrop(t *testing.T) {
+	data := []byte{0x05, 0x10, 0x00, 0x28, 0x55, 0x7b, 0x02, 0x02, 0x7f, 0x83, 0x02}
+	d := NewDecoder(bytes.NewReader(data))
+	err := d.Decode(nil)
+	if err == nil {
+		t.Fatal("decode: no error")
+	}
+	if !strings.Contains(err.Error(), "interface encoding") {
+		t.Fatalf("decode: expected interface encoding error, got %s", err.Error())
+	}
+}