[PATCH v2] drm/gma500/psb_intel_lvds: Fix a wild pointer dereference in psb_intel_lvds_get_modes()

[Zhou Qingyang]

In psb_intel_lvds_get_modes(), the return value of drm_mode_duplicate()
is assigned to mode and used in drm_mode_probed_add().
drm_mode_probed_add() passes mode->head to list_add_tail().
list_add_tail() will further call __list_add() and there is a
dereference of mode->head in __list_add(), which could lead to a wild
pointer dereference on failure of drm_mode_duplicate().

Fix this bug by adding a check of mode.

This bug was found by a static analyzer. The analysis employs
differential checking to identify inconsistent security operations
(e.g., checks or kfrees) between two code paths and confirms that the
inconsistent operations are not recovered in the current function or
the callers, so they constitute bugs.

Note that, as a bug found by static analysis, it can be a false
positive or hard to trigger. Multiple researchers have cross-reviewed
the bug.

Builds with CONFIG_DRM_GMA500=m show no new warnings,
and our static analyzer no longer warns about this code.

Fixes: 89c78134cc54 ("gma500: Add Poulsbo support")
Signed-off-by: Zhou Qingyang <zhou1615 at umn.edu>
---
Changes in v2:
  -  Use dev_err and return 0 instead of returning error code.

 drivers/gpu/drm/gma500/psb_intel_lvds.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/gma500/psb_intel_lvds.c b/drivers/gpu/drm/gma500/psb_intel_lvds.c
index ac97e0d3c7dd..3b3aa8fba6c9 100644
--- a/drivers/gpu/drm/gma500/psb_intel_lvds.c
+++ b/drivers/gpu/drm/gma500/psb_intel_lvds.c
@@ -505,6 +505,11 @@ static int psb_intel_lvds_get_modes(struct drm_connector *connector)
 	if (mode_dev->panel_fixed_mode != NULL) {
 		struct drm_display_mode *mode =
 		    drm_mode_duplicate(dev, mode_dev->panel_fixed_mode);
+		if (!mode) {
+			dev_err(dev->dev, "drm_mode_duplicate() allocation error\n");
+			return 0;
+		}
+
 		drm_mode_probed_add(connector, mode);
 		return 1;
 	}
-- 
2.25.1