Filter out unexpected object creation / destruction

well, actually most programs... so let's add it to the default / example
config.

Also change the Wayland socket name to wayland-10 to avoid conflicts.

config.toml

@@ -1,7 +1,7 @@
 [socket]
 # Which socket to listen on? If relative,
 # defaults to being relative to $XDG_RUNTIME_DIR
-listen = "wayland-2"
+listen = "wayland-10"
 # Which Wayland socket to use as upstream?
 # If missing, defaults to $WAYLAND_DISPLAY
 # upstream = "wayland-1"
@@ -16,6 +16,7 @@ allowed_globals = [
     "wl_compositor",
     "wl_shm",
     "wl_data_device_manager",
+    "wl_output", # each output is also a global
     "wl_seat",
     # Window management
     "xdg_wm_base",

src/main.rs

@@ -10,7 +10,7 @@ use std::{io, path::Path, sync::Arc};
 
 use config::Config;
 use io_util::{WlMsgReader, WlMsgWriter};
-use state::WlMitmState;
+use state::{WlMitmOutcome, WlMitmState, WlMitmVerdict};
 use tokio::net::{UnixListener, UnixStream};
 use tracing::{Instrument, Level, debug, error, info, span};
 
@@ -93,11 +93,15 @@ pub async fn handle_conn(
                     codec::DecoderOutcome::Decoded(mut wl_raw_msg) => {
                         debug!(obj_id = wl_raw_msg.obj_id, opcode = wl_raw_msg.opcode, num_fds = wl_raw_msg.fds.len(), "s2c event");
 
-                        let (num_consumed_fds, verdict) = state.on_s2c_event(&wl_raw_msg).await;
+                        let WlMitmOutcome(num_consumed_fds, verdict) = state.on_s2c_event(&wl_raw_msg).await;
                         upstream_read.return_unused_fds(&mut wl_raw_msg, num_consumed_fds);
 
-                        if verdict {
+                        match verdict {
-                            downstream_write.queue_write(wl_raw_msg);
+                            WlMitmVerdict::Allowed => {
+                                downstream_write.queue_write(wl_raw_msg);
+                            },
+                            WlMitmVerdict::Terminate => break Err(io::Error::new(io::ErrorKind::ConnectionAborted, "aborting connection")),
+                            _ => {}
                         }
                     },
                     codec::DecoderOutcome::Incomplete => continue,
@@ -109,11 +113,15 @@ pub async fn handle_conn(
                     codec::DecoderOutcome::Decoded(mut wl_raw_msg) => {
                         debug!(obj_id = wl_raw_msg.obj_id, opcode = wl_raw_msg.opcode, num_fds = wl_raw_msg.fds.len(), "c2s request");
 
-                        let (num_consumed_fds, verdict) = state.on_c2s_request(&wl_raw_msg).await;
+                        let WlMitmOutcome(num_consumed_fds, verdict) = state.on_c2s_request(&wl_raw_msg).await;
                         downstream_read.return_unused_fds(&mut wl_raw_msg, num_consumed_fds);
 
-                        if verdict {
+                        match verdict {
-                            upstream_write.queue_write(wl_raw_msg);
+                            WlMitmVerdict::Allowed => {
+                                upstream_write.queue_write(wl_raw_msg);
+                            },
+                            WlMitmVerdict::Terminate => break Err(io::Error::new(io::ErrorKind::ConnectionAborted, "aborting connection")),
+                            _ => {}
                         }
                     },
                     codec::DecoderOutcome::Incomplete => continue,

src/state.rs

@@ -12,6 +12,52 @@ use crate::{
     },
 };
 
+/// What to do for a message?
+pub enum WlMitmVerdict {
+    /// This message is allowed. Pass it through to the opposite end.
+    Allowed,
+    /// This message is filtered.
+    /// TODO: We should probably construct a proper error response
+    Filtered,
+    /// Terminate this entire session. Something is off.
+    Terminate,
+}
+
+impl Default for WlMitmVerdict {
+    fn default() -> Self {
+        WlMitmVerdict::Terminate
+    }
+}
+
+/// Result returned by [WlMitmState] when handling messages.
+/// It's a pair of (num_consumed_fds, verdict).
+///
+/// We need to return back unused fds from the [WlRawMsg], which
+/// is why this has to be returned from here.
+#[derive(Default)]
+pub struct WlMitmOutcome(pub usize, pub WlMitmVerdict);
+
+impl WlMitmOutcome {
+    fn set_consumed_fds(&mut self, consumed_fds: usize) {
+        self.0 = consumed_fds;
+    }
+
+    fn allowed(mut self) -> Self {
+        self.1 = WlMitmVerdict::Allowed;
+        self
+    }
+
+    fn filtered(mut self) -> Self {
+        self.1 = WlMitmVerdict::Filtered;
+        self
+    }
+
+    fn terminate(mut self) -> Self {
+        self.1 = WlMitmVerdict::Terminate;
+        self
+    }
+}
+
 pub struct WlMitmState {
     config: Arc<Config>,
     objects: WlObjects,
@@ -27,12 +73,28 @@ impl WlMitmState {
 
     /// Handle messages which register new objects with known interfaces or deletes them.
     ///
+    /// If there is an error, this function will return false and the connection shall be terminated.
+    ///
     /// Note that most _globals_ are instantiated using [WlRegistryBindRequest]. That request
     /// is not handled here.
-    fn handle_created_or_destroyed_objects(&mut self, msg: &dyn AnyWlParsedMessage<'_>) {
+    fn handle_created_or_destroyed_objects(&mut self, msg: &dyn AnyWlParsedMessage<'_>) -> bool {
         if let Some(created_objects) = msg.known_objects_created() {
             if let Some(parent_obj) = self.objects.lookup_object(msg.obj_id()) {
                 for (id, tt) in created_objects.into_iter() {
+                    if let Some(existing_obj_type) = self.objects.lookup_object(id) {
+                        debug!(
+                            parent_obj_id = msg.obj_id(),
+                            obj_type = tt.interface(),
+                            obj_id = id,
+                            existing_obj_type = existing_obj_type.interface(),
+                            "Trying to create object via message {}::{} but the object ID is already used!",
+                            parent_obj.interface(),
+                            msg.self_msg_name()
+                        );
+
+                        return false;
+                    }
+
                     debug!(
                         parent_obj_id = msg.obj_id(),
                         obj_type = tt.interface(),
@@ -44,7 +106,8 @@ impl WlMitmState {
                     self.objects.record_object(tt, id);
                 }
             } else {
-                error!("Parent object ID {} not found, ignoring", msg.obj_id());
+                error!("Parent object ID {} not found!", msg.obj_id());
+                return false;
             }
         } else if msg.is_destructor() {
             if let Some(obj_type) = self.objects.lookup_object(msg.obj_id()) {
@@ -58,25 +121,38 @@ impl WlMitmState {
 
             self.objects.remove_object(msg.obj_id());
         }
+
+        true
     }
 
     /// Returns the number of fds consumed while parsing the message as a concrete Wayland type, and a verdict
     #[tracing::instrument(skip_all)]
-    pub async fn on_c2s_request(&mut self, raw_msg: &WlRawMsg) -> (usize, bool) {
+    pub async fn on_c2s_request(&mut self, raw_msg: &WlRawMsg) -> WlMitmOutcome {
+        let mut outcome: WlMitmOutcome = Default::default();
         let msg = match crate::proto::decode_request(&self.objects, raw_msg) {
             WaylandProtocolParsingOutcome::Ok(msg) => msg,
             _ => {
+                let obj_type = self
+                    .objects
+                    .lookup_object(raw_msg.obj_id)
+                    .map(|t| t.interface());
+
                 error!(
                     obj_id = raw_msg.obj_id,
+                    obj_type = ?obj_type,
                     opcode = raw_msg.opcode,
                     num_fds = raw_msg.fds.len(),
                     "Malformed or unknown request"
                 );
-                return (0, false);
+                return outcome.terminate();
             }
         };
 
-        self.handle_created_or_destroyed_objects(&*msg);
+        outcome.set_consumed_fds(msg.num_consumed_fds());
+
+        if !self.handle_created_or_destroyed_objects(&*msg) {
+            return outcome.terminate();
+        }
 
         // The bind request doesn't create interface with a fixed type; handle it separately.
         if let Some(msg) = msg.downcast_ref::<WlRegistryBindRequest>() {
@@ -86,7 +162,7 @@ impl WlMitmState {
             // to bind to it; if it does, it's likely a malicious client!
             // So, we simply remove these messages from the stream, which will cause the Wayland server to error out.
             let Some(obj_type) = self.objects.lookup_global(msg.name) else {
-                return (0, false);
+                return outcome.terminate();
             };
 
             if obj_type.interface() != msg.id_interface_name {
@@ -96,7 +172,7 @@ impl WlMitmState {
                     msg.name,
                     obj_type.interface()
                 );
-                return (0, false);
+                return outcome.terminate();
             }
 
             info!(
@@ -144,9 +220,10 @@ impl WlMitmState {
                                         msg.self_msg_name(),
                                         status
                                     );
+                                    return outcome.filtered();
+                                } else {
+                                    return outcome.allowed();
                                 }
-
-                                return (msg.num_consumed_fds(), status.success());
                             }
                         }
 
@@ -155,7 +232,7 @@ impl WlMitmState {
                             msg.self_object_type().interface(),
                             msg.self_msg_name()
                         );
-                        return (msg.num_consumed_fds(), false);
+                        return outcome.filtered();
                     }
                     WlFilterRequestAction::Block => {
                         warn!(
@@ -164,31 +241,42 @@ impl WlMitmState {
                             msg.self_msg_name()
                         );
                         // TODO: don't just return false, build an error event
-                        return (msg.num_consumed_fds(), false);
+                        return outcome.filtered();
                     }
                 }
             }
         }
 
-        (msg.num_consumed_fds(), true)
+        outcome.allowed()
     }
 
     #[tracing::instrument(skip_all)]
-    pub async fn on_s2c_event(&mut self, raw_msg: &WlRawMsg) -> (usize, bool) {
+    pub async fn on_s2c_event(&mut self, raw_msg: &WlRawMsg) -> WlMitmOutcome {
+        let mut outcome: WlMitmOutcome = Default::default();
         let msg = match crate::proto::decode_event(&self.objects, raw_msg) {
             WaylandProtocolParsingOutcome::Ok(msg) => msg,
             _ => {
+                let obj_type = self
+                    .objects
+                    .lookup_object(raw_msg.obj_id)
+                    .map(|t| t.interface());
+
                 error!(
                     obj_id = raw_msg.obj_id,
+                    obj_type = ?obj_type,
                     opcode = raw_msg.opcode,
                     num_fds = raw_msg.fds.len(),
                     "Malformed or unknown event"
                 );
-                return (0, false);
+                return outcome.terminate();
             }
         };
 
-        self.handle_created_or_destroyed_objects(&*msg);
+        outcome.set_consumed_fds(msg.num_consumed_fds());
+
+        if !self.handle_created_or_destroyed_objects(&*msg) {
+            return outcome.terminate();
+        }
 
         if let Some(msg) = msg.downcast_ref::<WlRegistryGlobalEvent>() {
             // This event is how Wayland servers announce globals -- and they are the entrypoint to
@@ -207,7 +295,7 @@ impl WlMitmState {
                     "Unknown interface removed! If required, please include its XML when building wl-mitm!"
                 );
 
-                return (0, false);
+                return outcome.filtered();
             };
 
             // To block entire extensions, we just need to filter out their announced global objects.
@@ -216,7 +304,7 @@ impl WlMitmState {
                     interface = msg.interface,
                     "Removing interface from published globals"
                 );
-                return (0, false);
+                return outcome.filtered();
             }
 
             // Else, record the global object. These are the only ones we're ever going to allow through.
@@ -230,6 +318,6 @@ impl WlMitmState {
             self.objects.remove_object(msg.id);
         }
 
-        (msg.num_consumed_fds(), true)
+        outcome.allowed()
     }
 }