drm: protect drm_master pointers in drm_lease.c

From Desmond Cheong Zhi Xi
34609faad0c9f9f08d4b59d25c94b78bf5710d93 in linux 5.10.y/5.10.67
56f0729a510f92151682ff6c89f69724d5595d6e in mainline linux

3 files changed, 32 insertions, 0 deletions

diff --git a/sys/dev/pci/drm/drm_auth.c b/sys/dev/pci/drm/drm_auth.c
index a711e05c700..e8ead2bd29a 100644
--- a/sys/dev/pci/drm/drm_auth.c
+++ b/sys/dev/pci/drm/drm_auth.c
@@ -382,6 +382,31 @@ struct drm_master *drm_master_get(struct drm_master *master)
 }
 EXPORT_SYMBOL(drm_master_get);
 
+/**
+ * drm_file_get_master - reference &drm_file.master of @file_priv
+ * @file_priv: DRM file private
+ *
+ * Increments the reference count of @file_priv's &drm_file.master and returns
+ * the &drm_file.master. If @file_priv has no &drm_file.master, returns NULL.
+ *
+ * Master pointers returned from this function should be unreferenced using
+ * drm_master_put().
+ */
+struct drm_master *drm_file_get_master(struct drm_file *file_priv)
+{
+	struct drm_master *master = NULL;
+
+	spin_lock(&file_priv->master_lookup_lock);
+	if (!file_priv->master)
+		goto unlock;
+	master = drm_master_get(file_priv->master);
+
+unlock:
+	spin_unlock(&file_priv->master_lookup_lock);
+	return master;
+}
+EXPORT_SYMBOL(drm_file_get_master);
+
 static void drm_master_destroy(struct kref *kref)
 {
 	struct drm_master *master = container_of(kref, struct drm_master, refcount);
diff --git a/sys/dev/pci/drm/include/drm/drm_auth.h b/sys/dev/pci/drm/include/drm/drm_auth.h
index 6bf8b2b7899..f99d3417f30 100644
--- a/sys/dev/pci/drm/include/drm/drm_auth.h
+++ b/sys/dev/pci/drm/include/drm/drm_auth.h
@@ -107,6 +107,7 @@ struct drm_master {
 };
 
 struct drm_master *drm_master_get(struct drm_master *master);
+struct drm_master *drm_file_get_master(struct drm_file *file_priv);
 void drm_master_put(struct drm_master **master);
 bool drm_is_current_master(struct drm_file *fpriv);
 
diff --git a/sys/dev/pci/drm/include/drm/drm_file.h b/sys/dev/pci/drm/include/drm/drm_file.h
index 9decb43c711..ac50dd66602 100644
--- a/sys/dev/pci/drm/include/drm/drm_file.h
+++ b/sys/dev/pci/drm/include/drm/drm_file.h
@@ -237,6 +237,12 @@ struct drm_file {
 	 * this only matches &drm_device.master if the master is the currently
 	 * active one.
 	 *
+	 * When dereferencing this pointer, either hold struct
+	 * &drm_device.master_mutex for the duration of the pointer's use, or
+	 * use drm_file_get_master() if struct &drm_device.master_mutex is not
+	 * currently held and there is no other need to hold it. This prevents
+	 * @master from being freed during use.
+	 *
 	 * See also @authentication and @is_master and the :ref:`section on
 	 * primary nodes and authentication <drm_primary_node>`.
 	 */