rust_binder: reject context manager self-transaction

Rust binder resolved handle 0 to the context manager node, but it does not
reject the case where the caller owns the same node.

The C binder driver rejects transactions from the context-manager process
to handle 0 after resolving the target node. Match that behavior in Rust
Binder by rejecting handle 0 transactions when the resolved context-manager
node is owned by the calling process.

This applies to both synchronous and oneway transactions because both paths
resolve the target through Process::get_transaction_node().

Cc: stable <stable@kernel.org>
Fixes: eafedbc ("rust_binder: add Rust Binder driver")
Signed-off-by: Keshav Verma <iganschel@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260625103957.730-1-iganschel@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: iganschel-bit

drivers/android/binder/process.rs

@@ -900,7 +900,11 @@ impl Process {
     pub(crate) fn get_transaction_node(&self, handle: u32) -> BinderResult<NodeRef> {
         // When handle is zero, try to get the context manager.
         if handle == 0 {
-            Ok(self.ctx.get_manager_node(true)?)
+            let node_ref = self.ctx.get_manager_node(true)?;
+            if core::ptr::eq(self, &*node_ref.node.owner) {
+                return Err(EINVAL.into());
+            }
+            Ok(node_ref)
         } else {
             Ok(self.get_node_from_handle(handle, true)?)
         }