In the Linux kernel, the following vulnerability has been resolved:
mailbox: qcom-cpucp: handle NULL data in send_data callback
mailbox_clear_channel() calls mbox_send_message() with NULL data to
notify the remote side that the RX channel has been cleared.
qcom_cpucp_mbox_send_data() blindly dereferenced the data pointer,
causing a NULL pointer dereference kernel panic when invoked from
this path under PREEMPT_RT.
Add an explicit NULL check and return early without writing to the
TX register, which is the correct behaviour for a channel-clear
notification.
