[PATCH v3 1/2] drm: add a locked version of drm_is_current_master

[Daniel Vetter]

On Sun, Jun 20, 2021 at 07:03:26PM +0800, Desmond Cheong Zhi Xi wrote:
> 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 at ffwll.ch>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx at gmail.com>
> Reviewed-by: Emil Velikov <emil.l.velikov at gmail.com>

Merged to drm-misc-fixes, thanks for your patch.
-Daniel

> ---
>  drivers/gpu/drm/drm_auth.c | 51 ++++++++++++++++++++++++--------------
>  1 file changed, 32 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index 232abbba3686..86d4b72e95cb 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -61,6 +61,35 @@
>   * trusted clients.
>   */
>  
> +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
> + *
> + * Checks whether @fpriv is current master on its device. This decides whether a
> + * client is allowed to run DRM_MASTER IOCTLs.
> + *
> + * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
> + * - the current master is assumed to own the non-shareable display hardware.
> + */
> +bool drm_is_current_master(struct drm_file *fpriv)
> +{
> +	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);
> +
>  int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
>  {
>  	struct drm_auth *auth = data;
> @@ -223,7 +252,7 @@ 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 +301,7 @@ 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 +350,7 @@ 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);
> @@ -342,22 +371,6 @@ void drm_master_release(struct drm_file *file_priv)
>  	mutex_unlock(&dev->master_mutex);
>  }
>  
> -/**
> - * drm_is_current_master - checks whether @priv is the current master
> - * @fpriv: DRM file private
> - *
> - * Checks whether @fpriv is current master on its device. This decides whether a
> - * client is allowed to run DRM_MASTER IOCTLs.
> - *
> - * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
> - * - the current master is assumed to own the non-shareable display hardware.
> - */
> -bool drm_is_current_master(struct drm_file *fpriv)
> -{
> -	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
> -}
> -EXPORT_SYMBOL(drm_is_current_master);
> -
>  /**
>   * drm_master_get - reference a master pointer
>   * @master: &struct drm_master
> -- 
> 2.25.1
> 

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