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);