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From: Juergen Gross <jgross@suse.com>
To: "Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>
Cc: xen-devel@lists.xenproject.org, linux-block@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	"Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>,
	"Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Jens Axboe" <axboe@kernel.dk>, "Jan Beulich" <jbeulich@suse.com>
Subject: Re: [PATCH v3 2/3] xen/blkfront: don't take local copy of a request from the ring page
Date: Fri, 10 Sep 2021 12:36:46 +0200	[thread overview]
Message-ID: <ca6e6e23-6e24-38cb-1d06-b25c83767e6d@suse.com> (raw)
In-Reply-To: <YTswC2T2cvsEw2dP@mail-itl>


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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@suse.com>
>> Reviewed-by: Jan Beulich <jbeulich@suse.com>
>> Acked-by: Roger Pau Monné <roger.pau@citrix.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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  reply	other threads:[~2021-09-10 10:36 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-30 10:38 [PATCH v3 0/3] xen: harden blkfront against malicious backends Juergen Gross
2021-07-30 10:38 ` [PATCH v3 1/3] xen/blkfront: read response from backend only once Juergen Gross
2021-08-02 14:06   ` Oleksandr Andrushchenko
2021-08-02 19:26     ` Julien Grall
2021-08-03  7:00       ` Oleksandr Andrushchenko
2021-07-30 10:38 ` [PATCH v3 2/3] xen/blkfront: don't take local copy of a request from the ring page Juergen Gross
2021-09-10 10:14   ` Marek Marczykowski-Górecki
2021-09-10 10:36     ` Juergen Gross [this message]
2021-07-30 10:38 ` [PATCH v3 3/3] xen/blkfront: don't trust the backend response data blindly Juergen Gross
2021-08-30 10:16 ` [PATCH v3 0/3] xen: harden blkfront against malicious backends Juergen Gross

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