Re: [PATCH v2 2/3] xen/blkfront: don't take local copy of a request from the ring page

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Jul 08, 2021 at 02:43:44PM +0200, 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@suse.com>
> Reviewed-by: Jan Beulich <jbeulich@suse.com>

Acked-by: Roger Pau Monné <roger.pau@citrx.com>

Thanks!

One unrelated comment below.

> ---
> V2:
> - init variable to avoid potential compiler warning (Jan Beulich)
> ---
>  drivers/block/xen-blkfront.c | 25 +++++++++++++++----------
>  1 file changed, 15 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 86356014d35e..80701860870a 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -546,7 +546,7 @@ static unsigned long blkif_ring_get_request(struct blkfront_ring_info *rinfo,
>  	rinfo->shadow[id].status = REQ_WAITING;
>  	rinfo->shadow[id].associated_id = NO_ASSOCIATED_ID;
>  
> -	(*ring_req)->u.rw.id = id;
> +	rinfo->shadow[id].req.u.rw.id = id;
>  
>  	return id;
>  }
> @@ -554,11 +554,12 @@ static unsigned long blkif_ring_get_request(struct blkfront_ring_info *rinfo,
>  static int blkif_queue_discard_req(struct request *req, struct blkfront_ring_info *rinfo)
>  {
>  	struct blkfront_info *info = rinfo->dev_info;
> -	struct blkif_request *ring_req;
> +	struct blkif_request *ring_req, *final_ring_req;
>  	unsigned long id;
>  
>  	/* Fill out a communications ring structure. */
> -	id = blkif_ring_get_request(rinfo, req, &ring_req);
> +	id = blkif_ring_get_request(rinfo, req, &final_ring_req);
> +	ring_req = &rinfo->shadow[id].req;
>  
>  	ring_req->operation = BLKIF_OP_DISCARD;
>  	ring_req->u.discard.nr_sectors = blk_rq_sectors(req);
> @@ -569,8 +570,8 @@ static int blkif_queue_discard_req(struct request *req, struct blkfront_ring_inf
>  	else
>  		ring_req->u.discard.flag = 0;
>  
> -	/* Keep a private copy so we can reissue requests when recovering. */
> -	rinfo->shadow[id].req = *ring_req;
> +	/* Copy the request to the ring page. */
> +	*final_ring_req = *ring_req;
>  
>  	return 0;
>  }
> @@ -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 = NULL;
>  	unsigned long id, extra_id = NO_ASSOCIATED_ID;
>  	bool require_extra_req = false;
>  	int i;
> @@ -747,7 +749,8 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
>  	}
>  
>  	/* Fill out a communications ring structure. */
> -	id = blkif_ring_get_request(rinfo, req, &ring_req);
> +	id = blkif_ring_get_request(rinfo, req, &final_ring_req);
> +	ring_req = &rinfo->shadow[id].req;
>  
>  	num_sg = blk_rq_map_sg(req->q, req, rinfo->shadow[id].sg);
>  	num_grant = 0;
> @@ -798,7 +801,9 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
>  		ring_req->u.rw.nr_segments = num_grant;
>  		if (unlikely(require_extra_req)) {
>  			extra_id = blkif_ring_get_request(rinfo, req,
> -							  &extra_ring_req);
> +							  &final_extra_ring_req);
> +			extra_ring_req = &rinfo->shadow[extra_id].req;

I'm slightly confused about this extra request stuff because I cannot
find any check that asserts we have two empty slots on the ring before
getting here (I only see a RING_FULL check in blkif_queue_rq).

This is AFAIK only used on Arm when guest page size > 4KB.

Roger.