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

From: Juergen Gross <jgross@suse.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: "Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>,
	"Jens Axboe" <axboe@kernel.dk>,
	xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
	linux-block@vger.kernel.org
Subject: Re: [PATCH 3/8] xen/blkfront: don't take local copy of a request from the ring page
Date: Mon, 17 May 2021 16:11:08 +0200	[thread overview]
Message-ID: <278182af-044a-9445-bbdb-fdbb65d0da7c@suse.com> (raw)
In-Reply-To: <4cbf7b7f-5f00-4aba-4d54-06aa73d1bc32@suse.com>

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On 17.05.21 16:01, Jan Beulich wrote:
> 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@suse.com>
> 
> Reviewed-by: Jan Beulich <jbeulich@suse.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?

Hmm, probably better safe than sorry. Will change it.

Juergen

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