binder: fix possible UAF when freeing buffer

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> # 4.14 4.19
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Change-Id: I1b9c9bdc52df8ddbc5fe7c6d8308f1068265f8ae