drm/komeda: Fix an undefined behavior bug in komeda_plane_add()

In komeda_plane_add(), komeda_get_layer_fourcc_list() is assigned to
formats and used in drm_universal_plane_init().
drm_universal_plane_init() passes formats to
__drm_universal_plane_init(). __drm_universal_plane_init() further
passes formats to memcpy() as src parameter, which could lead to an
undefined behavior bug on failure of komeda_get_layer_fourcc_list().

Fix this bug by adding a check of formats.

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

Fixes: 61f1c4a8 ("drm/komeda: Attach komeda_dev to DRM-KMS")
Signed-off-by: Zhou Qingyang <zhou1615@umn.edu>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://lore.kernel.org/dri-devel/20211201033704.32054-1-zhou1615@umn.edu

drivers/gpu/drm/arm/display/komeda/komeda_plane.c

@@ -256,6 +256,10 @@ static int komeda_plane_add(struct komeda_kms_dev *kms,
 
 	formats = komeda_get_layer_fourcc_list(&mdev->fmt_tbl,
 					       layer->layer_type, &n_formats);
+	if (!formats) {
+		kfree(kplane);
+		return -ENOMEM;
+	}
 
 	err = drm_universal_plane_init(&kms->base, plane,
 			get_possible_crtcs(kms, c->pipeline),