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

[Zhou Qingyang <zhou1615@umn.edu>]

In cdv_intel_dp_get_modes(), the third 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: d112a8163f83 ("gma500/cdv: Add eDP support")
Signed-off-by: Zhou Qingyang <zhou1615@umn.edu>
---
Changes in V2:
  -  Instead of returning -ENOMEM, this patch returns 0
  -  Use DRM_DEBUG_KMS to report the failure of drm_mode_duplicate()

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

diff --git a/drivers/gpu/drm/gma500/cdv_intel_dp.c b/drivers/gpu/drm/gma500/cdv_intel_dp.c
index ba6ad1466374..bf47db488b7b 100644
--- a/drivers/gpu/drm/gma500/cdv_intel_dp.c
+++ b/drivers/gpu/drm/gma500/cdv_intel_dp.c
@@ -1773,6 +1773,11 @@ static int cdv_intel_dp_get_modes(struct drm_connector *connector)
 		if (intel_dp->panel_fixed_mode != NULL) {
 			struct drm_display_mode *mode;
 			mode = drm_mode_duplicate(dev, intel_dp->panel_fixed_mode);
+			if (!mode) {
+				DRM_DEBUG_KMS("Failure in drm_mode_duplicate()\n");
+				return 0;
+			}
+
 			drm_mode_probed_add(connector, mode);
 			return 1;
 		}
-- 
2.25.1