Re: [PATCH] drm: logicvc: fix device_node refcount leak in logicvc_layers_init()

[manushprajwal <manushprajwal555@gmail.com>]

From: Manush Prajwal <manushprajwal555@gmail.com>

Thanks for the review. I traced this through and the UAF is real,
though it's independent of this patch, as noted:

- logicvc_layer_init() embeds drm_plane in a devm_kzalloc()'d struct
  and registers it with the unmanaged drm_universal_plane_init(), so
  the plane stays linked into drm_dev->mode_config.plane_list.
- On a later layer's init failure, logicvc_layers_init()'s error path
  calls logicvc_layer_fini() on every already-succeeded layer, which
  does list_del() + devm_kfree() immediately, but never calls
  drm_plane_cleanup() first, so the freed layer's plane is still
  linked in mode_config.plane_list.
- drmm_mode_config_init() (drivers/gpu/drm/logicvc/logicvc_drm.c)
  registers drm_mode_config_cleanup() as a drmm action tied to
  drm_dev's own refcount, which is only dropped via
  devm_drm_dev_alloc()'s devres release on the parent device, i.e.
  later than the explicit devm_kfree() above, not before it.
- So drm_mode_config_cleanup() later walks plane_list and dereferences
  the already-freed layer/plane.

The same devm_kzalloc() plus unmanaged-DRM-object pattern exists in
logicvc_crtc_init() and logicvc_interface_init(), so this looks
driver-wide rather than layer-specific.

That's a separate, pre-existing bug in the error-unwind path and out
of scope for this one-line of_node_put() fix. A real fix would need
either drm_plane_cleanup()/drm_encoder_cleanup()/drm_connector_cleanup()
calls added to the _fini() helpers before devm_kfree(), or switching
these to drmm-managed allocations. Happy to send that as a follow-up
patch if there's interest.

Signed-off-by: Manush Prajwal <manushprajwal555@gmail.com>