[PATCH] drm: mxsfb: Fix crash when provided invalid DT bindings

[Marek Vasut]

The mxsfb driver will crash if the mxsfb DT node has a subnode,
but the content of the subnode is not of-graph binding with an
endpoint linking to panel. The crash was triggered by providing
old-style panel bindings to the mxsfb driver instead of the new
of-graph ones.

The problem happens in mxsfb_create_output(), which is invoked
from mxsfb_load(). The mxsfb_create_output() iterates over all
mxsfb DT subnode endpoints and tries to bind a panel on each
endpoint. If there is any problem binding the panel, that is,
mxsfb->panel == NULL, this function will return an error code,
otherwise success 0 is returned.

If the subnodes do not specify of-graph binding with an endpoint,
the iteration over endpoints in mxsfb_create_output() will have
zero cycles and the function will immediatelly return 0, but the
mxsfb->panel will remain NULL. This is propagated back into the
mxsfb_load(), which does not detect any problem and expects that
the mxsfb->panel is valid, thus calls mxsfb_panel_attach(). But
since mxsfb->panel == NULL, mxsfb_panel_attach() is called with
first argument NULL and this crashes the kernel.

This patch fixes the problem by explicitly checking for valid
mxsfb->panel at the end of the iteration in mxsfb_create_output().

Signed-off-by: Marek Vasut <marex at denx.de>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Dave Airlie <airlied at redhat.com>
Cc: Stefan Agner <stefan at agner.ch>
Cc: Breno Matheus Lima <brenomatheus at gmail.com>
---
 drivers/gpu/drm/mxsfb/mxsfb_out.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/mxsfb/mxsfb_out.c b/drivers/gpu/drm/mxsfb/mxsfb_out.c
index fa8d17399407..b8e81422d4e2 100644
--- a/drivers/gpu/drm/mxsfb/mxsfb_out.c
+++ b/drivers/gpu/drm/mxsfb/mxsfb_out.c
@@ -112,6 +112,7 @@ static int mxsfb_attach_endpoint(struct drm_device *drm,
 
 int mxsfb_create_output(struct drm_device *drm)
 {
+	struct mxsfb_drm_private *mxsfb = drm->dev_private;
 	struct device_node *ep_np = NULL;
 	struct of_endpoint ep;
 	int ret;
@@ -127,5 +128,8 @@ int mxsfb_create_output(struct drm_device *drm)
 		}
 	}
 
+	if (!mxsfb->panel)
+		return -EPROBE_DEFER;
+
 	return 0;
 }
-- 
2.11.0