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, 5 insertions, 0 deletions

diff --git a/pkg/flatrpc/flatrpc.fbs b/pkg/flatrpc/flatrpc.fbs
index 6d2307d6a..58dc7b292 100644
--- a/pkg/flatrpc/flatrpc.fbs
+++ b/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;