rust_binder: call set_notification_done() without proc lock

Consider the following sequence of events on a death listener:
1. The remote process dies and sends a BR_DEAD_BINDER message.
2. The local process invokes the BC_CLEAR_DEATH_NOTIFICATION command.
3. The local process then invokes the BC_DEAD_BINDER_DONE.
Then, the kernel will reply to the BC_DEAD_BINDER_DONE command with a
BR_CLEAR_DEATH_NOTIFICATION_DONE reply using push_work_if_looper().

However, this can result in a deadlock if the current thread is not a
looper. This is because dead_binder_done() still holds the proc lock
during set_notification_done(), which called push_work_if_looper().
Normally, push_work_if_looper() takes the thread lock, which is fine to
take under the proc lock. But if the current thread is not a looper,
then it falls back to delivering the reply to the process work queue,
which involves taking the proc lock. Since the proc lock is already
held, this is a deadlock.

Fix this by releasing the proc lock during set_notification_done(). It
was not intentional that it was held during that function to begin with.

I don't think this ever happens in Android because BC_DEAD_BINDER_DONE
is only invoked in response to BR_DEAD_BINDER messages, and the kernel
always delivers BR_DEAD_BINDER to a looper. So there's no scenario where
Android userspace will call BC_DEAD_BINDER_DONE on a non-looper thread.

Cc: stable <stable@kernel.org>
Fixes: eafedbc7c0 ("rust_binder: add Rust Binder driver")
Reported-by: syzbot+c8287e65a57a89e7fb72@syzkaller.appspotmail.com
Tested-by: syzbot+c8287e65a57a89e7fb72@syzkaller.appspotmail.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://patch.msgid.link/20260224-binder-dead-binder-done-proc-lock-v1-1-bbe1b8a6e74a@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder/process.rs

@@ -1295,7 +1295,8 @@ impl Process {
     }
 
     pub(crate) fn dead_binder_done(&self, cookie: u64, thread: &Thread) {
-        if let Some(death) = self.inner.lock().pull_delivered_death(cookie) {
+        let death = self.inner.lock().pull_delivered_death(cookie);
+        if let Some(death) = death {
             death.set_notification_done(thread);
         }
     }