rpc: add SetWebsocketReadLimit in Server (#32279)

Exposing the public method to setReadLimits for Websocket RPC to
prevent OOM.

Current, Geth Server is using a default 32MB max read limit (message
size) for websocket, which is prune to being attacked for OOM. Any one
can easily launch a client to send a bunch of concurrent large request
to cause the node to crash for OOM. One example of such script that can
easily crash a Geth node running websocket server is like this:

ec830979ac/poc.go

---------

Co-authored-by: Felix Lange <fjl@twurst.com>

rpc/server.go

@@ -54,6 +54,7 @@ type Server struct {
 	batchItemLimit     int
 	batchResponseLimit int
 	httpBodyLimit      int
+	wsReadLimit        int64
 }
 
 // NewServer creates a new server instance with no registered handlers.
@@ -62,6 +63,7 @@ func NewServer() *Server {
 		idgen:         randomIDGenerator(),
 		codecs:        make(map[ServerCodec]struct{}),
 		httpBodyLimit: defaultBodyLimit,
+		wsReadLimit:   wsDefaultReadLimit,
 	}
 	server.run.Store(true)
 	// Register the default service providing meta information about the RPC service such
@@ -89,6 +91,13 @@ func (s *Server) SetHTTPBodyLimit(limit int) {
 	s.httpBodyLimit = limit
 }
 
+// SetWebsocketReadLimit sets the limit for max message size for Websocket requests.
+//
+// This method should be called before processing any requests via Websocket server.
+func (s *Server) SetWebsocketReadLimit(limit int64) {
+	s.wsReadLimit = limit
+}
+
 // RegisterName creates a service for the given receiver type under the given name. When no
 // methods on the given receiver match the criteria to be either an RPC method or a
 // subscription an error is returned. Otherwise a new service is created and added to the

rpc/server_test.go

@@ -19,13 +19,18 @@ package rpc
 import (
 	"bufio"
 	"bytes"
+	"context"
+	"errors"
 	"io"
 	"net"
+	"net/http/httptest"
 	"os"
 	"path/filepath"
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/gorilla/websocket"
 )
 
 func TestServerRegisterName(t *testing.T) {
@@ -202,3 +207,86 @@ func TestServerBatchResponseSizeLimit(t *testing.T) {
 		}
 	}
 }
+
+func TestServerWebsocketReadLimit(t *testing.T) {
+	t.Parallel()
+
+	// Test different read limits
+	testCases := []struct {
+		name       string
+		readLimit  int64
+		testSize   int
+		shouldFail bool
+	}{
+		{
+			name:       "limit with small request - should succeed",
+			readLimit:  4096, // generous limit to comfortably allow JSON overhead
+			testSize:   256,  // reasonably small payload
+			shouldFail: false,
+		},
+		{
+			name:       "limit with large request - should fail",
+			readLimit:  256,  // tight limit to trigger server-side read limit
+			testSize:   1024, // payload that will exceed the limit including JSON overhead
+			shouldFail: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			// Create server and set read limits
+			srv := newTestServer()
+			srv.SetWebsocketReadLimit(tc.readLimit)
+			defer srv.Stop()
+
+			// Start HTTP server with WebSocket handler
+			httpsrv := httptest.NewServer(srv.WebsocketHandler([]string{"*"}))
+			defer httpsrv.Close()
+
+			wsURL := "ws:" + strings.TrimPrefix(httpsrv.URL, "http:")
+
+			// Connect WebSocket client
+			client, err := DialOptions(context.Background(), wsURL)
+			if err != nil {
+				t.Fatalf("can't dial: %v", err)
+			}
+			defer client.Close()
+
+			// Create large request data - this is what will be limited
+			largeString := strings.Repeat("A", tc.testSize)
+
+			// Send the large string as a parameter in the request
+			var result echoResult
+			err = client.Call(&result, "test_echo", largeString, 42, &echoArgs{S: "test"})
+
+			if tc.shouldFail {
+				// Expecting an error due to read limit exceeded
+				if err == nil {
+					t.Fatalf("expected error for request size %d with limit %d, but got none", tc.testSize, tc.readLimit)
+				}
+				// Be tolerant about the exact error surfaced by gorilla/websocket.
+				// Prefer a CloseError with code 1009, but accept ErrReadLimit or an error string containing 1009/message too big.
+				var cerr *websocket.CloseError
+				if errors.As(err, &cerr) {
+					if cerr.Code != websocket.CloseMessageTooBig {
+						t.Fatalf("unexpected websocket close code: have %d want %d (err=%v)", cerr.Code, websocket.CloseMessageTooBig, err)
+					}
+				} else if !errors.Is(err, websocket.ErrReadLimit) &&
+					!strings.Contains(strings.ToLower(err.Error()), "1009") &&
+					!strings.Contains(strings.ToLower(err.Error()), "message too big") {
+					// Not the error we expect from exceeding the message size limit.
+					t.Fatalf("unexpected error for read limit violation: %v", err)
+				}
+			} else {
+				// Expecting success
+				if err != nil {
+					t.Fatalf("unexpected error for request size %d with limit %d: %v", tc.testSize, tc.readLimit, err)
+				}
+				// Verify the response is correct - the echo should return our string
+				if result.String != largeString {
+					t.Fatalf("expected echo result to match input")
+				}
+			}
+		})
+	}
+}

rpc/websocket.go

@@ -60,7 +60,7 @@ func (s *Server) WebsocketHandler(allowedOrigins []string) http.Handler {
 			log.Debug("WebSocket upgrade failed", "err", err)
 			return
 		}
-		codec := newWebsocketCodec(conn, r.Host, r.Header, wsDefaultReadLimit)
+		codec := newWebsocketCodec(conn, r.Host, r.Header, s.wsReadLimit)
 		s.ServeCodec(codec, 0)
 	})
 }