linux-kernel - [PATCHv4 4/6] drm/omap: fix incorrect union usage

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Date:   Fri, 16 Nov 2018 00:06:43 +0100
From:   Sebastian Reichel <sebastian.reichel@...labora.com>
To:     Sebastian Reichel <sre@...nel.org>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Tony Lindgren <tony@...mide.com>, Pavel Machek <pavel@....cz>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     "H. Nikolaus Schaller" <hns@...delico.com>,
        dri-devel@...ts.freedesktop.org, linux-omap@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel@...labora.com,
        Sebastian Reichel <sebastian.reichel@...labora.com>
Subject: [PATCHv4 4/6] drm/omap: fix incorrect union usage

The DSI encoder sets dssdev->ops->dsi.set_config, which is stored at the
same offset as dssdev->ops->hdmi.set_hdmi_mode. The code in omap_encoder
only checks if dssdev->ops->hdmi.set_hdmi_mode is NULL. Due to the way
union works, it won't be NULL if dsi.set_config is set. This means
dsi_set_config will be called with config=hdmi_mode=false=NULL parameter
resulting in a NULL dereference. Also the dereference happens while
console is locked, so kernel hangs without any debug output (can be
avoided by fbmem's lockless_register_fb=1 parameter).

This fixes the issue by exiting early if the output type definitely
has no hdmi_set operations.

Fixes: 83910ad3f51fb ("drm/omap: Move most omap_dss_driver operations to omap_dss_device_ops")
Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.com>
---
 drivers/gpu/drm/omapdrm/omap_encoder.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/omapdrm/omap_encoder.c b/drivers/gpu/drm/omapdrm/omap_encoder.c
index 32bbe3a80e7d..ba0099f0644c 100644
--- a/drivers/gpu/drm/omapdrm/omap_encoder.c
+++ b/drivers/gpu/drm/omapdrm/omap_encoder.c
@@ -122,6 +122,14 @@ static void omap_encoder_mode_set(struct drm_encoder *encoder,
 
 	dssdev = omap_encoder->output;
 
+	/* The following operations access dssdev->ops->hdmi, which is a union
+	 * also used by DSI. This ensures, that the field does not have data
+	 * for DSI (or any other future output type).
+	 */
+	if (dssdev->output_type != OMAP_DISPLAY_TYPE_HDMI &&
+	    dssdev->output_type != OMAP_DISPLAY_TYPE_DVI)
+		return;
+
 	if (dssdev->ops->hdmi.set_hdmi_mode)
 		dssdev->ops->hdmi.set_hdmi_mode(dssdev, hdmi_mode);
 
-- 
2.19.1