linux-kernel - Re: [PATCH v2 1/2] drm: Add a locked version of drm_is_current

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Date:   Thu, 17 Jun 2021 19:03:29 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
Cc:     maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...ux.ie, daniel@...ll.ch,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        skhan@...uxfoundation.org, gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        emil.l.velikov@...il.com, Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [PATCH v2 1/2] drm: Add a locked version of drm_is_current_master

On Tue, Jun 15, 2021 at 10:36:44AM +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@...ll.ch>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
> Reviewed-by: Emil Velikov <emil.l.velikov@...il.com>
> ---
>  drivers/gpu/drm/drm_auth.c | 23 +++++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index 232abbba3686..c6bf52c310a9 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -61,6 +61,8 @@
>   * trusted clients.
>   */
>  
> +static bool drm_is_current_master_locked(struct drm_file *fpriv);

A bit a bikeshed, but we try to avoid forward declarations when they're
not needed. If you don't want to tear apart drm_is_current_master and the
_locked version then just move them together.

Can you pls do that and respin?

Otherwise looks all great.
-Daniel


> +
>  int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
>  {
>  	struct drm_auth *auth = data;
> @@ -223,7 +225,7 @@ 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 +274,7 @@ 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 +323,7 @@ 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);
> @@ -342,6 +344,13 @@ void drm_master_release(struct drm_file *file_priv)
>  	mutex_unlock(&dev->master_mutex);
>  }
>  
> +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
> @@ -354,7 +363,13 @@ void drm_master_release(struct drm_file *file_priv)
>   */
>  bool drm_is_current_master(struct drm_file *fpriv)
>  {
> -	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
> +	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);
>  
> -- 
> 2.25.1
> 

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