[PATCH v9 1/4] drm: fix null ptr dereference in drm_master_release

[Desmond Cheong Zhi Xi]

drm_master_release can be called on a drm_file without a master, which
results in a null ptr dereference of file_priv->master->magic_map. The
three cases are:

1. Error path in drm_open_helper
  drm_open():
    drm_open_helper():
      drm_master_open():
        drm_new_set_master(); <--- returns -ENOMEM,
                                   drm_file.master not set
      drm_file_free():
        drm_master_release(); <--- NULL ptr dereference
                                   (file_priv->master->magic_map)

2. Error path in mock_drm_getfile
  mock_drm_getfile():
    anon_inode_getfile(); <--- returns error, drm_file.master not set
    drm_file_free():
      drm_master_release(); <--- NULL ptr dereference
                                 (file_priv->master->magic_map)

3. In drm_client_close, as drm_client_open doesn't set up a master

drm_file.master is set up in drm_open_helper through the call to
drm_master_open, so we mirror it with a call to drm_master_release in
drm_close_helper, and remove drm_master_release from drm_file_free to
avoid the null ptr dereference.

Fixes: 7eeaeb90a6a5 ("drm/file: Don't set master on in-kernel clients")
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
---
 drivers/gpu/drm/drm_file.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
index ed25168619fc..90b62f360da1 100644
--- a/drivers/gpu/drm/drm_file.c
+++ b/drivers/gpu/drm/drm_file.c
@@ -282,9 +282,6 @@ void drm_file_free(struct drm_file *file)
 
 	drm_legacy_ctxbitmap_flush(dev, file);
 
-	if (drm_is_primary_client(file))
-		drm_master_release(file);
-
 	if (dev->driver->postclose)
 		dev->driver->postclose(dev, file);
 
@@ -305,6 +302,9 @@ static void drm_close_helper(struct file *filp)
 	list_del(&file_priv->lhead);
 	mutex_unlock(&dev->filelist_mutex);
 
+	if (drm_is_primary_client(file_priv))
+		drm_master_release(file_priv);
+
 	drm_file_free(file_priv);
 }
 
-- 
2.25.1