Re: [PATCH] dma-buf: explicitely forbid exposing pages from get_user_pages()

[Mario Limonciello <mario.limonciello@amd.com>]

On 6/30/26 03:01, Christian König wrote:
> We recently had another incident where two drivers put pages they got from
> get_user_pages() into a DMA-buf and cause quite a number of problems.
> 
> Explicitely document that this is not something exporters can do.

I think it would be a good idea to send this up to 
https://github.com/masoncl/review-prompts/blob/main/kernel/subsystem/drm.md 
as well.

That's what Sashiko uses and it will help flag this in case it happens 
in future.

> 
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
>   drivers/dma-buf/dma-buf.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 71f37544a5c6..aa5af4f439c2 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -685,6 +685,14 @@ static struct file *dma_buf_getfile(size_t size, int flags)
>    *
>    * For the detailed semantics exporters are expected to implement see
>    * &dma_buf_ops.
> + *
> + * It is explicitely forbidden for exporters to expose buffers they don't "own"
> + * as DMA-buf. This includes pages acquired by get_user_pages() or other import
> + * mechanism. Not following this rule can create numerous security problems.
> + *
> + * It is also strongly discouraged to expose the same backing store through
> + * multiple DMA-bufs at the same time. This eventually creates aliasing and
> + * cache coherency problems which are extremely hard to debug and fix.
>    */
>   
>   /**