drm/cirrus: Fix NULL pointer dereference when registering the fbdev

cirrus_modeset_init() is initializing/registering the emulated fbdev
and, since commit c61b93fe51b1 ("drm/atomic: Fix remaining places where
!funcs->best_encoder is valid"), DRM internals can access/test some of
the fields in mode_config->funcs as part of the fbdev registration
process.
Make sure dev->mode_config.funcs is properly set to avoid dereferencing
a NULL pointer.

Reported-by: Mike Marshall <hubcap@omnibond.com>
Reported-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Fixes: c61b93fe51b1 ("drm/atomic: Fix remaining places where !funcs->best_encoder is valid")
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --git a/drivers/gpu/drm/cirrus/cirrus_main.c b/drivers/gpu/drm/cirrus/cirrus_main.c
index 80446e2..76bcb43 100644 (file)
--- a/drivers/gpu/drm/cirrus/cirrus_main.c
+++ b/drivers/gpu/drm/cirrus/cirrus_main.c
@@ -185,14 +185,23 @@ int cirrus_driver_load(struct drm_device *dev, unsigned long flags)
                goto out;
        }
 
+       /*
+        * cirrus_modeset_init() is initializing/registering the emulated fbdev
+        * and DRM internals can access/test some of the fields in
+        * mode_config->funcs as part of the fbdev registration process.
+        * Make sure dev->mode_config.funcs is properly set to avoid
+        * dereferencing a NULL pointer.
+        * FIXME: mode_config.funcs assignment should probably be done in
+        * cirrus_modeset_init() (that's a common pattern seen in other DRM
+        * drivers).
+        */
+       dev->mode_config.funcs = &cirrus_mode_funcs;
        r = cirrus_modeset_init(cdev);
        if (r) {
                dev_err(&dev->pdev->dev, "Fatal error during modeset init: %d\n", r);
                goto out;
        }
 
-       dev->mode_config.funcs = (void *)&cirrus_mode_funcs;
-
        return 0;
 out:
        cirrus_driver_unload(dev);