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

If drm_gem_handle_create() fails in vgem_gem_create(), then the
drm_vgem_gem_object is freed twice: once when the reference is dropped
by drm_gem_object_put_unlocked(), and again by __vgem_gem_destroy().

This was hit by syzkaller using fault injection.

Fix it by skipping the second free.

Reported-by: syzbot+e73f2fb5ed5a5df36d33@syzkaller.appspotmail.com
Fixes: af33a919 ("drm/vgem: Enable dmabuf import interfaces")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Laura Abbott <labbott@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190226214451.195123-1-ebiggers@kernel.orgSigned-off-by: Maxime Ripard <maxime.ripard@bootlin.com>

drivers/gpu/drm/vgem/vgem_drv.c

@@ -191,13 +191,9 @@ static struct drm_gem_object *vgem_gem_create(struct drm_device *dev,
 	ret = drm_gem_handle_create(file, &obj->base, handle);
 	drm_gem_object_put_unlocked(&obj->base);
 	if (ret)
-		goto err;
+		return ERR_PTR(ret);
 
 	return &obj->base;
-
-err:
-	__vgem_gem_destroy(obj);
-	return ERR_PTR(ret);
 }
 
 static int vgem_gem_dumb_create(struct drm_file *file, struct drm_device *dev,