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

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Message-ID: <4cbf7b7f-5f00-4aba-4d54-06aa73d1bc32@suse.com>
Date:   Mon, 17 May 2021 16:01:00 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Roger Pau Monné <roger.pau@...rix.com>,
        Jens Axboe <axboe@...nel.dk>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH 3/8] xen/blkfront: don't take local copy of a request from
 the ring page

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@...e.com>

Reviewed-by: Jan Beulich <jbeulich@...e.com>
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