fix(protocol): limit size of incoming request messages (#10629)

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/protocol/protocol.go

@@ -49,6 +49,10 @@ const (
 	// MaxBlockSize is the maximum block size allowed
 	MaxBlockSize = 16 << MiB
 
+	// MaxRequestSize is the largest amount of data that can be read in a
+	// single request
+	MaxRequestSize = 2 * MaxBlockSize
+
 	// DesiredPerFileBlocks is the number of blocks we aim for per file
 	DesiredPerFileBlocks = 2000
 
@@ -457,14 +461,6 @@ func (c *rawConnection) dispatcherLoop() (err error) {
 			}
 		}
 
-		switch msg := msg.(type) {
-		case *bep.Request:
-			err = checkFilename(msg.Name)
-		}
-		if err != nil {
-			return newProtocolError(err, msgContext)
-		}
-
 		switch msg := msg.(type) {
 		case *bep.ClusterConfig:
 			err = c.model.ClusterConfig(clusterConfigFromWire(msg))
@@ -484,6 +480,15 @@ func (c *rawConnection) dispatcherLoop() (err error) {
 			err = c.handleIndexUpdate(idxUp)
 
 		case *bep.Request:
+			if err := checkFilename(msg.Name); err != nil {
+				return newProtocolError(err, msgContext)
+			}
+			if msg.Size <= 0 {
+				return newProtocolError(fmt.Errorf("request size %d too small", msg.Size), msgContext)
+			}
+			if msg.Size > MaxRequestSize {
+				return newProtocolError(fmt.Errorf("request size %d exceeds maximum allowed", msg.Size), msgContext)
+			}
 			go c.handleRequest(requestFromWire(msg))
 
 		case *bep.Response: