fix(protocol): always expect & validate block hash in requests (#10738)

Verify that block requests have a hash and that it's correct. This helps
prevent certain races and ensure that only expected data is ever
returned in response to a request.

(In Syncthing prior to 1.28.1 the block hash was omitted for encrypted
requests from trusted devices. This breaks compatibility with that
specific config on those versions.)

---------

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

lib/protocol/protocol.go

@@ -489,6 +489,12 @@ func (c *rawConnection) dispatcherLoop() (err error) {
 			if msg.Size > MaxRequestSize {
 				return newProtocolError(fmt.Errorf("request size %d exceeds maximum allowed", msg.Size), msgContext)
 			}
+			if len(msg.Hash) == 0 {
+				// Syncthing versions older than v1.28.1 omit the hash in
+				// encrypted requests from trusted devices (a rare config)
+				// and will run into this.
+				return newProtocolError(errors.New("request missing block hash"), msgContext)
+			}
 			go c.handleRequest(requestFromWire(msg))
 
 		case *bep.Response: