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Date:   Thu, 26 Aug 2021 11:53:18 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Cc:     maarten.lankhorst@linux.intel.com, mripard@kernel.org,
        tzimmermann@suse.de, airlied@linux.ie, daniel@ffwll.ch,
        sumit.semwal@linaro.org, christian.koenig@amd.com,
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Subject: Re: [PATCH v8 1/7] drm: fix null ptr dereference in
 drm_master_release
Message-ID: <YSdkjvWN9RAijZJy@phenom.ffwll.local>
Mail-Followup-To: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>,
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References: <20210826020122.1488002-1-desmondcheongzx@gmail.com>
 <20210826020122.1488002-2-desmondcheongzx@gmail.com>
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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@gmail.com>

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

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