drm/rockchip: dw-mipi-dsi: avoid out-of-bounds read on tx_buf

As a side-effect of this, encode the endianness explicitly rather than
casting a u16.
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170224125506.21533-7-john@metanate.com

drivers/gpu/drm/rockchip/dw-mipi-dsi.c

@@ -572,8 +572,14 @@ static int dw_mipi_dsi_gen_pkt_hdr_write(struct dw_mipi_dsi *dsi, u32 hdr_val)
 static int dw_mipi_dsi_dcs_short_write(struct dw_mipi_dsi *dsi,
 				       const struct mipi_dsi_msg *msg)
 {
-	const u16 *tx_buf = msg->tx_buf;
-	u32 val = GEN_HDATA(*tx_buf) | GEN_HTYPE(msg->type);
+	const u8 *tx_buf = msg->tx_buf;
+	u16 data = 0;
+	u32 val;
+
+	if (msg->tx_len > 0)
+		data |= tx_buf[0];
+	if (msg->tx_len > 1)
+		data |= tx_buf[1] << 8;
 
 	if (msg->tx_len > 2) {
 		dev_err(dsi->dev, "too long tx buf length %zu for short write\n",
@@ -581,6 +587,7 @@ static int dw_mipi_dsi_dcs_short_write(struct dw_mipi_dsi *dsi,
 		return -EINVAL;
 	}
 
+	val = GEN_HDATA(data) | GEN_HTYPE(msg->type);
 	return dw_mipi_dsi_gen_pkt_hdr_write(dsi, val);
 }