drm/mediatek: dp: Only trigger DRM HPD events if bridge is attached

[ Upstream commit 36b617f ]

The MediaTek DisplayPort interface bridge driver starts its interrupts
as soon as its probed. However when the interrupts trigger the bridge
might not have been attached to a DRM device. As drm_helper_hpd_irq_event()
does not check whether the passed in drm_device is valid or not, a NULL
pointer passed in results in a kernel NULL pointer dereference in it.

Check whether the bridge is attached and only trigger an HPD event if
it is.

Fixes: f70ac09 ("drm/mediatek: Add MT8195 Embedded DisplayPort driver")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Guillaume Ranquet <granquet@baylibre.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20230202045734.2773503-1-wenst@chromium.org/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/gpu/drm/mediatek/mtk_dp.c

@@ -1823,7 +1823,8 @@ static irqreturn_t mtk_dp_hpd_event_thread(int hpd, void *dev)
 	spin_unlock_irqrestore(&mtk_dp->irq_thread_lock, flags);
 
 	if (status & MTK_DP_THREAD_CABLE_STATE_CHG) {
-		drm_helper_hpd_irq_event(mtk_dp->bridge.dev);
+		if (mtk_dp->bridge.dev)
+			drm_helper_hpd_irq_event(mtk_dp->bridge.dev);
 
 		if (!mtk_dp->train_info.cable_plugged_in) {
 			mtk_dp_disable_sdp_aui(mtk_dp);