Re: [PATCH 3/8] xen/blkfront: don't take local copy of a request from the ring page

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.05.2021 12:02, Juergen Gross wrote:
> In order to avoid a malicious backend being able to influence the local
> copy of a request build the request locally first and then copy it to
> the ring page instead of doing it the other way round as today.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with one remark/question:

> @@ -703,6 +704,7 @@ static int blkif_queue_rw_req(struct request *req, struct 
> blkfront_ring_info *ri
>  {
>       struct blkfront_info *info = rinfo->dev_info;
>       struct blkif_request *ring_req, *extra_ring_req = NULL;
> +     struct blkif_request *final_ring_req, *final_extra_ring_req;

Without setting final_extra_ring_req to NULL just like is done for
extra_ring_req, ...

> @@ -840,10 +845,10 @@ static int blkif_queue_rw_req(struct request *req, 
> struct blkfront_ring_info *ri
>       if (setup.segments)
>               kunmap_atomic(setup.segments);
>  
> -     /* Keep a private copy so we can reissue requests when recovering. */
> -     rinfo->shadow[id].req = *ring_req;
> +     /* Copy request(s) to the ring page. */
> +     *final_ring_req = *ring_req;
>       if (unlikely(require_extra_req))
> -             rinfo->shadow[extra_id].req = *extra_ring_req;
> +             *final_extra_ring_req = *extra_ring_req;

... are you sure all supported compilers will recognize the
conditional use and not warn about use of a possibly uninitialized
variable?

Jan