Re: [PATCH v2 2/2] dma-buf: Annul dmabuf->file on file release

[Christian König]

On 10/9/26 14:35, Matt Evans wrote:
> The dmabuf release path is split between file and dentry release:
> dma_buf_file_release() is called shortly before the file is freed, and
> dma_buf_release() calls an exporter's dmabuf->ops->release when the
> dentry is freed.  However, the dentry can outlive the file (for
> example, if opened with O_PATH), meaning .release might be called some
> time after the file is freed.
> 
> This presents a window, when closing a DMABUF file, in which
> dmabuf->file points to freed memory yet .release op has not yet been
> called.
> 
> For VFIO, if a buffer's .release has not been called it's considered
> still active and subject to move/cleanup.  If so, it attempts to
> get_file_active() on dmabuf->file: a file close with dentry held open
> will call this function with a stale pointer, a UAF.
> 
> To make this pattern safe, set dmabuf->file to NULL in
> dma_buf_file_release() to reflect that the associated file is now dead
> even if the DMABUF is not yet gone.  A get_file_active() ... fput()
> sequence concurrent with a file close then only executes as one of:
> 
>  - Gets the file before file_ref_put() (dmabuf->file valid)
>  - Observes dmabuf->file pointing to a file, but it's DEAD (no file)
>  - Observes dmabuf->file = NULL (no file)
> 
> Originally, drivers could assume dmabuf->file was valid until .release
> was called from fops->release.  This assumption was no longer valid
> after 4ab59c3c638c6 ("dma-buf: Move dma_buf_release() from fops to
> dentry_ops"), which moved the callback to the dentry release (by which
> point the file might have been freed).  With this commit, drivers must
> still consider that dmabuf->file could be NULL before .release.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 4ab59c3c638c6 ("dma-buf: Move dma_buf_release() from fops to dentry_ops")
> Signed-off-by: Matt Evans <matt@xxxxxxxxxx>
> ---
>  drivers/dma-buf/dma-buf.c | 21 ++++++++++++++++++---
>  include/linux/dma-buf.h   |  4 +++-
>  2 files changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 4c9add51f9ef..61485fc28b8b 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -193,11 +193,26 @@ static void dma_buf_release(struct dentry *dentry)
>  
>  static int dma_buf_file_release(struct inode *inode, struct file *file)
>  {
> -	if (!is_dma_buf_file(file))
> -		return -EINVAL;
> +	struct dma_buf *dmabuf = file->private_data;
> +
> +	if (!dmabuf)
> +		return 0;
>  
> -	__dma_buf_list_del(file->private_data);
> +	__dma_buf_list_del(dmabuf);
>  
> +	/*
> +	 * As the file is being closed, dmabuf->file is no longer
> +	 * valid and NULL replaces an otherwise stale pointer.  If a
> +	 * reference to the file isn't already held, the file can be
> +	 * closed at any time, including before dmabuf->ops->release()
> +	 * happens: dmabuf->file _must not_ be accessed directly.  An
> +	 * exporter should use get_file_active(&dmabuf->file) which
> +	 * atomically tests and acquires a reference to the file (or
> +	 * returns NULL if the file has closed).  The barrier ensures
> +	 * get_file_active() observes the store before file_free()
> +	 * frees the memory.
> +	 */
> +	smp_store_mb(dmabuf->file, NULL);
>  	return 0;
>  }
>  
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index d15b2b31d3c9..9f3c92c71c29 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -303,7 +303,9 @@ struct dma_buf {
>  	 * @file:
>  	 *
>  	 * File pointer used for sharing buffers across, and for refcounting.
> -	 * See dma_buf_get() and dma_buf_put().
> +	 * See dma_buf_get() and dma_buf_put().  Exporters can use
> +	 * get_file_active(&dma_buf->file) to safely acquire a reference;

Please add something like "* when the exporter holds a common lock with the ->release() callback".

Apart from that looks good to me.

Regards,
Christian.

> +	 * the file could otherwise be closed at any time.
>  	 */
>  	struct file *file;
>