binder: fix possible UAF when freeing buffer

commit a370003cc301d4361bae20c9ef615f89bf8d1e8a upstream.

There is a race between the binder driver cleaning
up a completed transaction via binder_free_transaction()
and a user calling binder_ioctl(BC_FREE_BUFFER) to
release a buffer. It doesn't matter which is first but
they need to be protected against running concurrently
which can result in a UAF.

Signed-off-by: Todd Kjos <tkjos@google.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder.c

@@ -1960,8 +1960,18 @@ static struct binder_thread *binder_get_txn_from_and_acq_inner(
 
 static void binder_free_transaction(struct binder_transaction *t)
 {
-	if (t->buffer)
-		t->buffer->transaction = NULL;
+	struct binder_proc *target_proc = t->to_proc;
+
+	if (target_proc) {
+		binder_inner_proc_lock(target_proc);
+		if (t->buffer)
+			t->buffer->transaction = NULL;
+		binder_inner_proc_unlock(target_proc);
+	}
+	/*
+	 * If the transaction has no target_proc, then
+	 * t->buffer->transaction has already been cleared.
+	 */
 	kfree(t);
 	binder_stats_deleted(BINDER_STAT_TRANSACTION);
 }
@@ -3484,10 +3494,12 @@ static int binder_thread_write(struct binder_proc *proc,
 				     buffer->debug_id,
 				     buffer->transaction ? "active" : "finished");
 
+			binder_inner_proc_lock(proc);
 			if (buffer->transaction) {
 				buffer->transaction->buffer = NULL;
 				buffer->transaction = NULL;
 			}
+			binder_inner_proc_unlock(proc);
 			if (buffer->async_transaction && buffer->target_node) {
 				struct binder_node *buf_node;
 				struct binder_work *w;