drm: add a locked version of drm_is_current_master

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>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210620110327.4964-2-desmondcheongzx@gmail.com

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 232abbba36868b425321ae02cb8b83351f5b3c74..86d4b72e95cbd9d5bddf5d82ee90b091fd4daa30 100644 (file)
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -61,6 +61,35 @@
  * trusted clients.
  */
 
+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
+ *
+ * Checks whether @fpriv is current master on its device. This decides whether a
+ * client is allowed to run DRM_MASTER IOCTLs.
+ *
+ * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
+ * - the current master is assumed to own the non-shareable display hardware.
+ */
+bool drm_is_current_master(struct drm_file *fpriv)
+{
+       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);
+
 int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
 {
        struct drm_auth *auth = data;
@@ -223,7 +252,7 @@ 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 +301,7 @@ 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 +350,7 @@ 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);
@@ -342,22 +371,6 @@ out:
        mutex_unlock(&dev->master_mutex);
 }
 
-/**
- * drm_is_current_master - checks whether @priv is the current master
- * @fpriv: DRM file private
- *
- * Checks whether @fpriv is current master on its device. This decides whether a
- * client is allowed to run DRM_MASTER IOCTLs.
- *
- * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
- * - the current master is assumed to own the non-shareable display hardware.
- */
-bool drm_is_current_master(struct drm_file *fpriv)
-{
-       return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
-}
-EXPORT_SYMBOL(drm_is_current_master);
-
 /**
  * drm_master_get - reference a master pointer
  * @master: &struct drm_master