eth, les: add sanity checks for unbounded block fields (#19573)

This PR adds some hardening in the lower levels of the protocol stack, to bail early on invalid data. Primarily, attacks that this PR protects against are on the "annoyance"-level, which would otherwise write a couple of megabytes of data into the log output, which is a bit resource intensive.

les/handler.go

@@ -442,7 +442,9 @@ func (pm *ProtocolManager) handleMsg(p *peer) error {
 		if err := msg.Decode(&req); err != nil {
 			return errResp(ErrDecode, "%v: %v", msg, err)
 		}
-
+		if err := req.sanityCheck(); err != nil {
+			return err
+		}
 		update, size := req.Update.decode()
 		if p.rejectUpdate(size) {
 			return errResp(ErrRequestRejected, "")

les/protocol.go

@@ -149,6 +149,14 @@ type announceData struct {
 	Update     keyValueList
 }
 
+// sanityCheck verifies that the values are reasonable, as a DoS protection
+func (a *announceData) sanityCheck() error {
+	if tdlen := a.Td.BitLen(); tdlen > 100 {
+		return fmt.Errorf("too large block TD: bitlen %d", tdlen)
+	}
+	return nil
+}
+
 // sign adds a signature to the block announcement by the given privKey
 func (a *announceData) sign(privKey *ecdsa.PrivateKey) {
 	rlp, _ := rlp.EncodeToBytes(announceBlock{a.Hash, a.Number, a.Td})