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/encryption.go

@@ -93,31 +93,25 @@ func (e encryptedModel) Request(req *Request) (RequestResponse, error) {
 	}
 	realSize := req.Size - blockOverhead
 	realOffset := req.Offset - int64(req.BlockNo*blockOverhead)
+	if realOffset < 0 {
+		panic("bug: realOffset underflow")
+	}
 
 	if req.Size < minPaddedSize {
 		return nil, errors.New("short request")
 	}
 
-	// Attempt to decrypt the block hash; it may be nil depending on what
-	// type of device the request comes from. Trusted devices with
-	// encryption enabled know the hash but don't bother to encrypt & send
-	// it to us. Untrusted devices have the hash from the encrypted index
-	// data and do send it. The model knows to only verify the hash if it
-	// actually gets one.
-
-	var realHash []byte
+	// Decrypt the block hash.
 	fileKey := e.keyGen.FileKey(realName, folderKey)
-	if len(req.Hash) > 0 {
-		var additional [8]byte
-		binary.BigEndian.PutUint64(additional[:], uint64(realOffset))
-		realHash, err = decryptDeterministic(req.Hash, fileKey, additional[:])
-		if err != nil {
-			// "Legacy", no offset additional data?
-			realHash, err = decryptDeterministic(req.Hash, fileKey, nil)
-		}
-		if err != nil {
-			return nil, fmt.Errorf("decrypting block hash: %w", err)
-		}
+	var additional [8]byte
+	binary.BigEndian.PutUint64(additional[:], uint64(realOffset))
+	realHash, err := decryptDeterministic(req.Hash, fileKey, additional[:])
+	if err != nil {
+		// "Legacy", no offset additional data?
+		realHash, err = decryptDeterministic(req.Hash, fileKey, nil)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("decrypting block hash: %w", err)
 	}
 
 	// Perform that request and grab the data.

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:

lib/protocol/protocol_test.go

@@ -560,6 +560,7 @@ func TestRequestMaxSize(t *testing.T) {
 				Id:   1,
 				Name: "valid",
 				Size: MaxRequestSize,
+				Hash: []byte{42},
 			}
 
 			res := <-c.outbox
@@ -573,6 +574,7 @@ func TestRequestMaxSize(t *testing.T) {
 				Id:   2,
 				Name: "invalid",
 				Size: int32(s),
+				Hash: []byte{42},
 			}
 
 			select {
@@ -606,6 +608,7 @@ func TestRequestZeroSize(t *testing.T) {
 		Id:   1,
 		Name: "valid",
 		Size: 0,
+		Hash: []byte{42},
 	}
 
 	select {
@@ -632,6 +635,7 @@ func TestRequestInvalidFilename(t *testing.T) {
 		Id:   1,
 		Name: "../escape",
 		Size: 1024,
+		Hash: []byte{42},
 	}
 
 	select {