[PATCH v3 4/9] drm: fix potential null ptr dereferences in drm_{auth, ioctl}

[Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>]

There are three areas where we dereference struct drm_master without
checking if the pointer is non-NULL.

1. drm_getmagic is called from the ioctl_handler. Since
DRM_IOCTL_GET_MAGIC has no ioctl flags, drm_getmagic is run without
any check that drm_file.master has been set.

2. Similarly, drm_getunique is called from the ioctl_handler, but
DRM_IOCTL_GET_UNIQUE has no ioctl flags. So there is no guarantee that
drm_file.master has been set.

3. drm_master_release can also be called without having a
drm_file.master set. Here is one error path:
  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)

Fix these by checking if the master pointers are NULL before use.

Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
---
 drivers/gpu/drm/drm_auth.c  | 16 ++++++++++++++--
 drivers/gpu/drm/drm_ioctl.c |  5 +++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index f9267b21556e..b7230604496b 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -95,11 +95,18 @@ EXPORT_SYMBOL(drm_is_current_master);
 int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
 {
 	struct drm_auth *auth = data;
+	struct drm_master *master;
 	int ret = 0;
 
 	mutex_lock(&dev->master_mutex);
+	master = file_priv->master;
+	if (!master) {
+		mutex_unlock(&dev->master_mutex);
+		return -EINVAL;
+	}
+
 	if (!file_priv->magic) {
-		ret = idr_alloc(&file_priv->master->magic_map, file_priv,
+		ret = idr_alloc(&master->magic_map, file_priv,
 				1, 0, GFP_KERNEL);
 		if (ret >= 0)
 			file_priv->magic = ret;
@@ -355,8 +362,12 @@ void drm_master_release(struct drm_file *file_priv)
 
 	mutex_lock(&dev->master_mutex);
 	master = file_priv->master;
+
+	if (!master)
+		goto unlock;
+
 	if (file_priv->magic)
-		idr_remove(&file_priv->master->magic_map, file_priv->magic);
+		idr_remove(&master->magic_map, file_priv->magic);
 
 	if (!drm_is_current_master_locked(file_priv))
 		goto out;
@@ -379,6 +390,7 @@ void drm_master_release(struct drm_file *file_priv)
 		drm_master_put(&file_priv->master);
 		spin_unlock(&dev->master_lookup_lock);
 	}
+unlock:
 	mutex_unlock(&dev->master_mutex);
 }
 
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 26f3a9ede8fe..4d029d3061d9 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -121,6 +121,11 @@ int drm_getunique(struct drm_device *dev, void *data,
 
 	mutex_lock(&dev->master_mutex);
 	master = file_priv->master;
+	if (!master) {
+		mutex_unlock(&dev->master_mutex);
+		return -EINVAL;
+	}
+
 	if (u->unique_len >= master->unique_len) {
 		if (copy_to_user(u->unique, master->unique, master->unique_len)) {
 			mutex_unlock(&dev->master_mutex);
-- 
2.25.1