[PATCH v4] drm/fb-helper: Avoid race with DRM userspace

[Noralf Trønnes]

drm_fb_helper_is_bound() is used to check if DRM userspace is in control.
This is done by looking at the fb on the primary plane. By the time
fb-helper gets around to committing, it's possible that the facts have
changed.

Avoid this race by holding the drm_device->master_mutex lock while
committing. When DRM userspace does its first open, it will now wait
until fb-helper is done. The helper will stay away if there's a master.

Locking rule: Always take the fb-helper lock first.

v2:
- Remove drm_fb_helper_is_bound() (Daniel Vetter)
- No need to check fb_helper->dev->master in
  drm_fb_helper_single_fb_probe(), restore_fbdev_mode() has the check.

Suggested-by: Daniel Vetter <daniel.vetter at ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/drm_auth.c     | 20 ++++++++++++++++++++
 drivers/gpu/drm/drm_internal.h |  2 ++
 2 files changed, 22 insertions(+)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 1669c42c40ed..db199807b7dc 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -368,3 +368,23 @@ void drm_master_put(struct drm_master **master)
 	*master = NULL;
 }
 EXPORT_SYMBOL(drm_master_put);
+
+/* Used by drm_client and drm_fb_helper */
+bool drm_master_internal_acquire(struct drm_device *dev)
+{
+	mutex_lock(&dev->master_mutex);
+	if (dev->master) {
+		mutex_unlock(&dev->master_mutex);
+		return false;
+	}
+
+	return true;
+}
+EXPORT_SYMBOL(drm_master_internal_acquire);
+
+/* Used by drm_client and drm_fb_helper */
+void drm_master_internal_release(struct drm_device *dev)
+{
+	mutex_unlock(&dev->master_mutex);
+}
+EXPORT_SYMBOL(drm_master_internal_release);
diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h
index d9a483a5fce0..3ee97c9998a2 100644
--- a/drivers/gpu/drm/drm_internal.h
+++ b/drivers/gpu/drm/drm_internal.h
@@ -91,6 +91,8 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
 			 struct drm_file *file_priv);
 int drm_master_open(struct drm_file *file_priv);
 void drm_master_release(struct drm_file *file_priv);
+bool drm_master_internal_acquire(struct drm_device *dev);
+void drm_master_internal_release(struct drm_device *dev);
 
 /* drm_sysfs.c */
 extern struct class *drm_class;
-- 
2.20.1