Re: [PATCH v8 1/7] drm: fix null ptr dereference in drm_master_release

From: Daniel Vetter
Date: Thu Aug 26 2021 - 05:53:29 EST


On Thu, Aug 26, 2021 at 10:01:16AM +0800, Desmond Cheong Zhi Xi wrote:
> drm_master_release can be called on a drm_file without a master, which
> results in a null ptr dereference of file_priv->master->magic_map. The
> three cases are:
>
> 1. Error path in drm_open_helper
> 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)
>
> 2. Error path in mock_drm_getfile
> mock_drm_getfile():
> anon_inode_getfile(); <--- returns error, drm_file.master not set
> drm_file_free():
> drm_master_release(); <--- NULL ptr dereference
> (file_priv->master->magic_map)
>
> 3. In drm_client_close, as drm_client_open doesn't set up a master
>
> drm_file.master is set up in drm_open_helper through the call to
> drm_master_open, so we mirror it with a call to drm_master_release in
> drm_close_helper, and remove drm_master_release from drm_file_free to
> avoid the null ptr dereference.
>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>

Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>

I guess we should also have a cc: stable on this one? I think this bug
existed since pretty much forever, but maybe more prominent with the
drm_client stuff added a while ago.
-Daniel

> ---
> drivers/gpu/drm/drm_file.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
> index ed25168619fc..90b62f360da1 100644
> --- a/drivers/gpu/drm/drm_file.c
> +++ b/drivers/gpu/drm/drm_file.c
> @@ -282,9 +282,6 @@ void drm_file_free(struct drm_file *file)
>
> drm_legacy_ctxbitmap_flush(dev, file);
>
> - if (drm_is_primary_client(file))
> - drm_master_release(file);
> -
> if (dev->driver->postclose)
> dev->driver->postclose(dev, file);
>
> @@ -305,6 +302,9 @@ static void drm_close_helper(struct file *filp)
> list_del(&file_priv->lhead);
> mutex_unlock(&dev->filelist_mutex);
>
> + if (drm_is_primary_client(file_priv))
> + drm_master_release(file_priv);
> +
> drm_file_free(file_priv);
> }
>
> --
> 2.25.1
>

--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch