linux-kernel - Re: [PATCH v3 2/3] xen/blkfront: don't take local copy of a request from the ring page

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Date:   Fri, 10 Sep 2021 12:36:46 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
Cc:     xen-devel@...ts.xenproject.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Roger Pau Monné <roger.pau@...rix.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 v3 2/3] xen/blkfront: don't take local copy of a request
 from the ring page

On 10.09.21 12:14, Marek Marczykowski-Górecki wrote:
> On Fri, Jul 30, 2021 at 12:38:53PM +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@...rix.com>
>> ---
>> 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 15e840287734..b7301006fb28 100644
> 
> (...)
> 
>> @@ -827,10 +832,10 @@ 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;
> 
> Is this guaranteed to not be optimized by the compiler in an unsafe way
> (like, do the operation the other way around)?

I don't think the C standard allows that. AFAIK reordering writes is
allowed only between sequence points. And each external function call is
a sequence point, making such an optimization in our case illegal.


Juergen

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