Re: [PATCH] drm/auth: Only drm_drop_master if it exists

[Dmitry Baryshkov]

On Fri, Apr 17, 2026 at 05:00:47AM +0800, Jonathan Cavitt wrote:
> It is possible that both dev->master and file_priv->master are NULL when
> passed to drm_master_release, which would result in dev being passed to
> drm_drop_master (as NULL == NULL here).  This would result in a NULL
> pointer dereference when passing dev->master to drm_master_put in
> drm_drop_master.
> 
> Only call drm_drop_master if dev->master exists.  Also, make sure the
> original calling requirement is maintained (dev->master ==
> file_priv->master).
> 
> This fixes a static analysis issue.
> 
> Signed-off-by: Jonathan Cavitt <jonathan.cavitt@xxxxxxxxx>
> Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> Cc: Maxime Ripard <mripard@xxxxxxxxxx>
> Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
> Cc: David Airlie <airlied@xxxxxxxxx>
> Cc: Simona Vetter <simona@xxxxxxxx>
> ---
>  drivers/gpu/drm/drm_auth.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index e17bb0f1f9e0..e5013b870ba0 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -347,7 +347,7 @@ void drm_master_release(struct drm_file *file_priv)
>  	if (!drm_is_current_master_locked(file_priv))
>  		goto out;
>  
> -	if (dev->master == file_priv->master)

The previous call checks that file_priv->master != NULL. So, at this
point the NULL == NULL check is not possible.

I'm sorry, but this looks like a false positive.

> +	if (dev->master && dev->master == file_priv->master)
>  		drm_drop_master(dev, file_priv);
>  out:
>  	if (drm_core_check_feature(dev, DRIVER_MODESET) && file_priv->is_master) {
> -- 
> 2.53.0
> 

-- 
With best wishes
Dmitry