bcm2835-v4l2: Fix buffer overflow problem

#1447
port_parameter_get() failed to account for the header
(u32 id and u32 size) in the size before memcpying
the response into the response buffer, so overrunning
the provided buffer by 8 bytes.

Account for those bytes, and also a belt-and-braces
check to ensure we never copy more than *value_size
bytes into value.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>
Signed-off-by: Michael Zoran <mzoran@crowfest.net>
Tested-by: Michael Zoran <mzoran@crowfest.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vc04_services/bcm2835-camera/mmal-vchiq.c

@@ -1445,7 +1445,12 @@ static int port_parameter_get(struct vchiq_mmal_instance *instance,
 	}
 
 	ret = -rmsg->u.port_parameter_get_reply.status;
-	if (ret || (rmsg->u.port_parameter_get_reply.size > *value_size)) {
+	/* port_parameter_get_reply.size includes the header,
+	 * whilst *value_size doesn't.
+	 */
+	rmsg->u.port_parameter_get_reply.size -= (2 * sizeof(u32));
+
+	if (ret || rmsg->u.port_parameter_get_reply.size > *value_size) {
 		/* Copy only as much as we have space for
 		 * but report true size of parameter
 		 */