[PATCH v3] rust_binder: clear freeze listener on node removal

[Alice Ryhl <aliceryhl@google.com>]

Generally userspace is supposed to explicitly clear freeze listeners
before they drop the refcount on the node ref to zero, but there's
nothing forcing that. Currently, in this scenario the freeze listener
remains in the freeze_listeners rbtree and in the remote node's freeze
listener list, even though the ref for which the listener is registered
is gone. This could potentially lead to a memory leak due to a refcount
cycle. Thus, remove the freeze listener in this scenario.

Cc: stable@vger.kernel.org
Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Changes in v3:
- Get rid of dependency.
- Rebase.
- Link to v2: https://lore.kernel.org/r/20260615-remove-freeze-on-remove-node-v2-1-93b31766e7a8@google.com

Changes in v2:
- Rebase on v3 of parent series.
- Link to v1: https://lore.kernel.org/r/20260609-remove-freeze-on-remove-node-v1-1-f67f3b9bfeb8@google.com
---
 drivers/android/binder/freeze.rs  | 11 +++++++++--
 drivers/android/binder/node.rs    | 10 ++++++----
 drivers/android/binder/process.rs | 12 +++++++++++-
 3 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/drivers/android/binder/freeze.rs b/drivers/android/binder/freeze.rs
index 53b60035639a..f4df14568b25 100644
--- a/drivers/android/binder/freeze.rs
+++ b/drivers/android/binder/freeze.rs
@@ -154,10 +154,17 @@ fn debug_print(&self, m: &SeqFile, prefix: &str, _tprefix: &str) -> Result<()> {
 }
 
 impl FreezeListener {
-    pub(crate) fn on_process_exit(&self, proc: &Arc<Process>) {
+    /// Called when this freeze listener is cleared abnormally.
+    ///
+    /// This occurs either because the process exited or because the process dropped its last
+    /// refcount on the node ref without explicitly removing the freeze listener first.
+    ///
+    /// The returned `KVVec` is just a value that should be dropped outside of the lock.
+    pub(crate) fn on_process_cleanup(&self, proc: &Process) -> KVVec<Arc<Process>> {
         if !self.is_clearing {
-            self.node.remove_freeze_listener(proc);
+            return self.node.remove_freeze_listener(proc);
         }
+        KVVec::new()
     }
 }
 
diff --git a/drivers/android/binder/node.rs b/drivers/android/binder/node.rs
index 69f757ff7461..c10148e9069f 100644
--- a/drivers/android/binder/node.rs
+++ b/drivers/android/binder/node.rs
@@ -682,12 +682,13 @@ pub(crate) fn add_freeze_listener(
         }
     }
 
-    pub(crate) fn remove_freeze_listener(&self, p: &Arc<Process>) {
-        let _unused_capacity;
+    pub(crate) fn remove_freeze_listener(&self, p: &Process) -> KVVec<Arc<Process>> {
         let mut guard = self.owner.inner.lock();
         let inner = self.inner.access_mut(&mut guard);
         let len = inner.freeze_list.len();
-        inner.freeze_list.retain(|proc| !Arc::ptr_eq(proc, p));
+        inner
+            .freeze_list
+            .retain(|proc| !core::ptr::eq::<Process>(&**proc, p));
         if len == inner.freeze_list.len() {
             pr_warn!(
                 "Could not remove freeze listener for {}\n",
@@ -695,8 +696,9 @@ pub(crate) fn remove_freeze_listener(&self, p: &Arc<Process>) {
             );
         }
         if inner.freeze_list.is_empty() {
-            _unused_capacity = mem::take(&mut inner.freeze_list);
+            return mem::take(&mut inner.freeze_list);
         }
+        KVVec::new()
     }
 
     pub(crate) fn freeze_list<'a>(&'a self, guard: &'a ProcessInner) -> &'a [Arc<Process>] {
diff --git a/drivers/android/binder/process.rs b/drivers/android/binder/process.rs
index ca664fda8e81..cdd1a9079726 100644
--- a/drivers/android/binder/process.rs
+++ b/drivers/android/binder/process.rs
@@ -946,6 +946,8 @@ pub(crate) fn update_ref(
 
         // To preserve original binder behaviour, we only fail requests where the manager tries to
         // increment references on itself.
+        let _to_free_freeze_listener;
+        let _to_free_freeze_listener_cleanup;
         let mut refs = self.node_refs.lock();
         if let Some(info) = refs.by_handle.get_mut(&handle) {
             if info.node_ref().update(inc, strong) {
@@ -961,6 +963,14 @@ pub(crate) fn update_ref(
                 unsafe { info.node_ref2().node.remove_node_info(info) };
 
                 let id = info.node_ref().node.global_id();
+
+                if let Some(freeze) = *info.freeze() {
+                    if let Some(fl) = refs.freeze_listeners.remove(&freeze) {
+                        _to_free_freeze_listener_cleanup = fl.on_process_cleanup(&self);
+                        _to_free_freeze_listener = fl;
+                    }
+                }
+
                 refs.by_handle.remove(&handle);
                 refs.by_node.remove(&id);
                 refs.handle_is_present.release_id(handle as usize);
@@ -1384,7 +1394,7 @@ fn deferred_release(self: Arc<Self>) {
         // Clean up freeze listeners.
         let freeze_listeners = take(&mut self.node_refs.lock().freeze_listeners);
         for listener in freeze_listeners.values() {
-            listener.on_process_exit(&self);
+            listener.on_process_cleanup(&self);
         }
         drop(freeze_listeners);
 

---
base-commit: 728e68a889bcf257b1e67298b12c360e5c3a13e0
change-id: 20260609-remove-freeze-on-remove-node-30e72ff6b46e

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>