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.

1 files changed, 16 insertions, 0 deletions

diff --git a/executor/executor_runner.h b/executor/executor_runner.h
index a3b668893..88fc785db 100644
--- a/executor/executor_runner.h
+++ b/executor/executor_runner.h
@@ -629,9 +629,24 @@ private:
 			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 @@ private:
 		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);