pkg/rpcserver: pkg/flatrpc: executor: add handshake stage 0

As we figured out in #5805, syz-manager treats random incoming RPC
connections as trusted, and will crash if a non-executor client sends
an invalid packet to it.

To address this issue, we introduce another stage of handshake, which
includes a cookie exchange:
 - upon connection from an executor, the manager sends a ConnectHello RPC
   message to it, which contains a random 64-bit cookie;
 - the executor calculates a hash of that cookie and includes it into
   its ConnectRequest together with the other information;
 - before checking the validity of ConnectRequest, the manager ensures
   client sanity (passed ID didn't change, hashed cookie has the expected
   value)

We deliberately pick a random cookie instead of a magic number: if the
fuzzer somehow learns to send packets to the manager, we don't want it to
crash multiple managers on the same machine.

executor/executor_runner.h

@@ -629,9 +629,24 @@ class Runner
 			failmsg("bad restarting", "restarting=%d", restarting_);
 	}
 
+	// Implementation must match that in pkg/rpcserver/rpcserver.go.
+	uint64 HashAuthCookie(uint64 cookie)
+	{
+		const uint64_t prime1 = 73856093;
+		const uint64_t prime2 = 83492791;
+
+		return (cookie * prime1) ^ prime2;
+	}
+
 	int Handshake()
 	{
+		// Handshake stage 0: get a cookie from the manager.
+		rpc::ConnectHelloRawT conn_hello;
+		conn_.Recv(conn_hello);
+
+		// Handshake stage 1: share basic information about the client.
 		rpc::ConnectRequestRawT conn_req;
+		conn_req.cookie = HashAuthCookie(conn_hello.cookie);
 		conn_req.id = vm_index_;
 		conn_req.arch = GOARCH;
 		conn_req.git_revision = GIT_REVISION;
@@ -656,6 +671,7 @@ class Runner
 		if (conn_reply.cover)
 			max_signal_.emplace();
 
+		// Handshake stage 2: share information requested by the manager.
 		rpc::InfoRequestRawT info_req;
 		info_req.files = ReadFiles(conn_reply.files);
 

pkg/flatrpc/conn_test.go

@@ -20,7 +20,11 @@ import (
 )
 
 func TestConn(t *testing.T) {
+	connectHello := &ConnectHello{
+		Cookie: 1,
+	}
 	connectReq := &ConnectRequest{
+		Cookie:      73856093,
 		Id:          1,
 		Arch:        "arch",
 		GitRevision: "rev1",
@@ -52,6 +56,9 @@ func TestConn(t *testing.T) {
 	go func() {
 		done <- serv.Serve(context.Background(),
 			func(_ context.Context, c *Conn) error {
+				if err := Send(c, connectHello); err != nil {
+					return err
+				}
 				connectReqGot, err := Recv[*ConnectRequestRaw](c)
 				if err != nil {
 					return err
@@ -79,6 +86,12 @@ func TestConn(t *testing.T) {
 	c := dial(t, serv.Addr.String())
 	defer c.Close()
 
+	connectHelloGot, err := Recv[*ConnectHelloRaw](c)
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, connectHello, connectHelloGot)
+
 	if err := Send(c, connectReq); err != nil {
 		t.Fatal(err)
 	}
@@ -102,7 +115,11 @@ func TestConn(t *testing.T) {
 }
 
 func BenchmarkConn(b *testing.B) {
+	connectHello := &ConnectHello{
+		Cookie: 1,
+	}
 	connectReq := &ConnectRequest{
+		Cookie:      73856093,
 		Id:          1,
 		Arch:        "arch",
 		GitRevision: "rev1",
@@ -125,7 +142,11 @@ func BenchmarkConn(b *testing.B) {
 		done <- serv.Serve(context.Background(),
 			func(_ context.Context, c *Conn) error {
 				for i := 0; i < b.N; i++ {
-					_, err := Recv[*ConnectRequestRaw](c)
+					if err := Send(c, connectHello); err != nil {
+						return err
+					}
+
+					_, err = Recv[*ConnectRequestRaw](c)
 					if err != nil {
 						return err
 					}
@@ -143,10 +164,14 @@ func BenchmarkConn(b *testing.B) {
 	b.ReportAllocs()
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
+		_, err := Recv[*ConnectHelloRaw](c)
+		if err != nil {
+			b.Fatal(err)
+		}
 		if err := Send(c, connectReq); err != nil {
 			b.Fatal(err)
 		}
-		_, err := Recv[*ConnectReplyRaw](c)
+		_, err = Recv[*ConnectReplyRaw](c)
 		if err != nil {
 			b.Fatal(err)
 		}

pkg/flatrpc/flatrpc.fbs

@@ -34,8 +34,13 @@ enum Feature : uint64 (bit_flags) {
 	BinFmtMisc,
 	Swap,
 }
+
+table ConnectHelloRaw {
+	cookie			:uint64;
+}
 
 table ConnectRequestRaw {
+	cookie			:uint64;
 	id			:int64;
 	arch			:string;
 	git_revision		:string;