Re: [PATCH v3 4/9] drm: fix potential null ptr dereferences in drm_{auth,ioctl}

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Aug 18, 2021 at 03:38:19PM +0800, Desmond Cheong Zhi Xi wrote:
> There are three areas where we dereference struct drm_master without
> checking if the pointer is non-NULL.
> 
> 1. drm_getmagic is called from the ioctl_handler. Since
> DRM_IOCTL_GET_MAGIC has no ioctl flags, drm_getmagic is run without
> any check that drm_file.master has been set.
> 
> 2. Similarly, drm_getunique is called from the ioctl_handler, but
> DRM_IOCTL_GET_UNIQUE has no ioctl flags. So there is no guarantee that
> drm_file.master has been set.

I think the above two are impossible, due to the refcounting rules for
struct file.

> 3. drm_master_release can also be called without having a
> drm_file.master set. Here is one error path:
>   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)
> 
> Fix these by checking if the master pointers are NULL before use.
> 
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
> ---
>  drivers/gpu/drm/drm_auth.c  | 16 ++++++++++++++--
>  drivers/gpu/drm/drm_ioctl.c |  5 +++++
>  2 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index f9267b21556e..b7230604496b 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -95,11 +95,18 @@ 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;
> +	struct drm_master *master;
>  	int ret = 0;
>  
>  	mutex_lock(&dev->master_mutex);
> +	master = file_priv->master;
> +	if (!master) {
> +		mutex_unlock(&dev->master_mutex);
> +		return -EINVAL;
> +	}
> +
>  	if (!file_priv->magic) {
> -		ret = idr_alloc(&file_priv->master->magic_map, file_priv,
> +		ret = idr_alloc(&master->magic_map, file_priv,
>  				1, 0, GFP_KERNEL);
>  		if (ret >= 0)
>  			file_priv->magic = ret;
> @@ -355,8 +362,12 @@ void drm_master_release(struct drm_file *file_priv)
>  
>  	mutex_lock(&dev->master_mutex);
>  	master = file_priv->master;
> +
> +	if (!master)
> +		goto unlock;

This is a bit convoluted, since we're in the single-threaded release path
we don't need any locking for file_priv related things. Therefore we can
pull the master check out and just directly return.

But since it's a bit surprising maybe a comment that this can happen when
drm_master_open in drm_open_helper fails?

Another option, and maybe cleaner, would be to move the drm_master_release
from drm_file_free into drm_close_helper. That would be fully symmetrical
and should also fix the bug here?
-Daniel


> +
>  	if (file_priv->magic)
> -		idr_remove(&file_priv->master->magic_map, file_priv->magic);
> +		idr_remove(&master->magic_map, file_priv->magic);
>  
>  	if (!drm_is_current_master_locked(file_priv))
>  		goto out;
> @@ -379,6 +390,7 @@ void drm_master_release(struct drm_file *file_priv)
>  		drm_master_put(&file_priv->master);
>  		spin_unlock(&dev->master_lookup_lock);
>  	}
> +unlock:
>  	mutex_unlock(&dev->master_mutex);
>  }
>  
> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
> index 26f3a9ede8fe..4d029d3061d9 100644
> --- a/drivers/gpu/drm/drm_ioctl.c
> +++ b/drivers/gpu/drm/drm_ioctl.c
> @@ -121,6 +121,11 @@ int drm_getunique(struct drm_device *dev, void *data,
>  
>  	mutex_lock(&dev->master_mutex);
>  	master = file_priv->master;
> +	if (!master) {
> +		mutex_unlock(&dev->master_mutex);
> +		return -EINVAL;
> +	}
> +
>  	if (u->unique_len >= master->unique_len) {
>  		if (copy_to_user(u->unique, master->unique, master->unique_len)) {
>  			mutex_unlock(&dev->master_mutex);
> -- 
> 2.25.1
> 

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