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>

1 files changed, 11 insertions, 2 deletions

diff --git a/drivers/gpu/drm/cirrus/cirrus_main.c b/drivers/gpu/drm/cirrus/cirrus_main.c
index 80446e2d3ab6..76bcb43e7c06 100644
--- 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);