[zstd] Add a sanity limit to decompress buffer size allocation

Fix #60
Before we were blindly trusting the data returned by ZSTD_getDecompressedSize. This mean with a specifically crafter payload, we would try to allocate a lot of memory resulting in potential DOS.
Now set a sane limit and fall back to streaming

zstd.go

@@ -28,6 +28,10 @@ var (
 	ErrEmptySlice = errors.New("Bytes slice is empty")
 )
 
+const (
+	zstdFrameHeaderSizeMax = 18 // From zstd.h. Since it's experimental API, hardcoding it
+)
+
 // CompressBound returns the worst case size needed for a destination buffer,
 // which can be used to preallocate a destination buffer or select a previously
 // allocated buffer from a pool.
@@ -46,6 +50,30 @@ func cCompressBound(srcSize int) int {
 	return int(C.ZSTD_compressBound(C.size_t(srcSize)))
 }
 
+// decompressSizeHint tries to give a hint on how much of the output buffer size we should have
+// based on zstd frame descriptors. To prevent DOS from maliciously-created payloads, limit the size
+func decompressSizeHint(src []byte) int {
+	// 1 MB or 10x input size
+	upperBound := 10 * len(src)
+	if upperBound < 1000*1000 {
+		upperBound = 1000 * 1000
+	}
+
+	hint := upperBound
+	if len(src) >= zstdFrameHeaderSizeMax {
+		hint = int(C.ZSTD_getFrameContentSize(unsafe.Pointer(&src[0]), C.size_t(len(src))))
+		if hint < 0 { // On error, just use upperBound
+			hint = upperBound
+		}
+	}
+
+	// Take the minimum of both
+	if hint > upperBound {
+		return upperBound
+	}
+	return hint
+}
+
 // Compress src into dst.  If you have a buffer to use, you can pass it to
 // prevent allocation.  If it is too small, or if nil is passed, a new buffer
 // will be allocated and returned.
@@ -113,18 +141,11 @@ func Decompress(dst, src []byte) ([]byte, error) {
 		return dst[:written], nil
 	}
 
-	if len(dst) == 0 {
-		// Attempt to use zStd to determine decompressed size (may result in error or 0)
-		size := int(C.ZSTD_getDecompressedSize(unsafe.Pointer(&src[0]), C.size_t(len(src))))
-		if err := getError(size); err != nil {
-			return nil, err
-		}
-
-		if size > 0 {
-			dst = make([]byte, size)
-		} else {
-			dst = make([]byte, len(src)*3) // starting guess
-		}
+	bound := decompressSizeHint(src)
+	if cap(dst) >= bound {
+		dst = dst[0:cap(dst)]
+	} else {
+		dst = make([]byte, bound)
 	}
 	for i := 0; i < 3; i++ { // 3 tries to allocate a bigger buffer
 		result, err := decompress(dst, src)