drm/vkms: fix use-after-free when drm_gem_handle_create() fails

If drm_gem_handle_create() fails in vkms_gem_create(), then the
vkms_gem_object is freed twice: once when the reference is dropped by
drm_gem_object_put_unlocked(), and again by the extra calls to
drm_gem_object_release() and kfree().

Fix it by skipping the second release and free.

This bug was originally found in the vgem driver by syzkaller using
fault injection, but I noticed it's also present in the vkms driver.

Fixes: 559e50f ("drm/vkms: Add dumb operations")
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190226220858.214438-1-ebiggers@kernel.org
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>

drivers/gpu/drm/vkms/vkms_gem.c

@@ -111,11 +111,8 @@ struct drm_gem_object *vkms_gem_create(struct drm_device *dev,
 
 	ret = drm_gem_handle_create(file, &obj->gem, handle);
 	drm_gem_object_put_unlocked(&obj->gem);
-	if (ret) {
-		drm_gem_object_release(&obj->gem);
-		kfree(obj);
+	if (ret)
 		return ERR_PTR(ret);
-	}
 
 	return &obj->gem;
 }