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

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Date:   Fri, 9 Jul 2021 15:54:19 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Roger Pau Monné <roger.pau@...rix.com>
Cc:     xen-devel@...ts.xenproject.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Beulich <jbeulich@...e.com>
Subject: Re: [PATCH v2 2/3] xen/blkfront: don't take local copy of a request
 from the ring page

On 09.07.21 10:55, Roger Pau Monné wrote:
> 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@...e.com>
>> Reviewed-by: Jan Beulich <jbeulich@...e.com>
> 
> Acked-by: Roger Pau Monné <roger.pau@...rx.com>
> 
> Thanks!
> 
> One unrelated comment below.
> 

...

>> @@ -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.

I agree that this is a bug and should be fixed.


Juergen

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