[PATCH v2 1/2] drm: Add a locked version of drm_is_current_master

[Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>]

While checking the master status of the DRM file in
drm_is_current_master(), the device's master mutex should be
held. Without the mutex, the pointer fpriv->master may be freed
concurrently by another process calling drm_setmaster_ioctl(). This
could lead to use-after-free errors when the pointer is subsequently
dereferenced in drm_lease_owner().

The callers of drm_is_current_master() from drm_auth.c hold the
device's master mutex, but external callers do not. Hence, we implement
drm_is_current_master_locked() to be used within drm_auth.c, and
modify drm_is_current_master() to grab the device's master mutex
before checking the master status.

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
---
 drivers/gpu/drm/drm_auth.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 232abbba3686..c6bf52c310a9 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -61,6 +61,8 @@
  * trusted clients.
  */
 
+static bool drm_is_current_master_locked(struct drm_file *fpriv);
+
 int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
 {
 	struct drm_auth *auth = data;
@@ -223,7 +225,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,
 	if (ret)
 		goto out_unlock;
 
-	if (drm_is_current_master(file_priv))
+	if (drm_is_current_master_locked(file_priv))
 		goto out_unlock;
 
 	if (dev->master) {
@@ -272,7 +274,7 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
 	if (ret)
 		goto out_unlock;
 
-	if (!drm_is_current_master(file_priv)) {
+	if (!drm_is_current_master_locked(file_priv)) {
 		ret = -EINVAL;
 		goto out_unlock;
 	}
@@ -321,7 +323,7 @@ void drm_master_release(struct drm_file *file_priv)
 	if (file_priv->magic)
 		idr_remove(&file_priv->master->magic_map, file_priv->magic);
 
-	if (!drm_is_current_master(file_priv))
+	if (!drm_is_current_master_locked(file_priv))
 		goto out;
 
 	drm_legacy_lock_master_cleanup(dev, master);
@@ -342,6 +344,13 @@ void drm_master_release(struct drm_file *file_priv)
 	mutex_unlock(&dev->master_mutex);
 }
 
+static bool drm_is_current_master_locked(struct drm_file *fpriv)
+{
+	lockdep_assert_held_once(&fpriv->master->dev->master_mutex);
+
+	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
+}
+
 /**
  * drm_is_current_master - checks whether @priv is the current master
  * @fpriv: DRM file private
@@ -354,7 +363,13 @@ void drm_master_release(struct drm_file *file_priv)
  */
 bool drm_is_current_master(struct drm_file *fpriv)
 {
-	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
+	bool ret;
+
+	mutex_lock(&fpriv->master->dev->master_mutex);
+	ret = drm_is_current_master_locked(fpriv);
+	mutex_unlock(&fpriv->master->dev->master_mutex);
+
+	return ret;
 }
 EXPORT_SYMBOL(drm_is_current_master);
 
-- 
2.25.1