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Subject: Re: [PATCH 3/8] xen/blkfront: don't take local copy of a request from
 the ring page
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>,
        =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>,
        Jens Axboe <axboe@kernel.dk>, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, linux-block@vger.kernel.org
References: <20210513100302.22027-1-jgross@suse.com>
 <20210513100302.22027-4-jgross@suse.com>
 <4cbf7b7f-5f00-4aba-4d54-06aa73d1bc32@suse.com>
From:   Juergen Gross <jgross@suse.com>
Message-ID: <278182af-044a-9445-bbdb-fdbb65d0da7c@suse.com>
Date:   Mon, 17 May 2021 16:11:08 +0200
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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>,
 =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>,
 Jens Axboe <axboe@kernel.dk>, xen-devel@lists.xenproject.org,
 linux-kernel@vger.kernel.org, linux-block@vger.kernel.org
Message-ID: <278182af-044a-9445-bbdb-fdbb65d0da7c@suse.com>
Subject: Re: [PATCH 3/8] xen/blkfront: don't take local copy of a request from
 the ring page
References: <20210513100302.22027-1-jgross@suse.com>
 <20210513100302.22027-4-jgross@suse.com>
 <4cbf7b7f-5f00-4aba-4d54-06aa73d1bc32@suse.com>
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 loca=
l
>> 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>
>=20
> Reviewed-by: Jan Beulich <jbeulich@suse.com>
> with one remark/question:
>=20
>> @@ -703,6 +704,7 @@ static int blkif_queue_rw_req(struct request *req,=20
struct blkfront_ring_info *ri
>>   {
>>   	struct blkfront_info *info =3D rinfo->dev_info;
>>   	struct blkif_request *ring_req, *extra_ring_req =3D NULL;
>> +	struct blkif_request *final_ring_req, *final_extra_ring_req;
>=20
> Without setting final_extra_ring_req to NULL just like is done for
> extra_ring_req, ...
>=20
>> @@ -840,10 +845,10 @@ static int blkif_queue_rw_req(struct request *re=
q, struct blkfront_ring_info *ri
>>   	if (setup.segments)
>>   		kunmap_atomic(setup.segments);
>>  =20
>> -	/* Keep a private copy so we can reissue requests when recovering. *=
/
>> -	rinfo->shadow[id].req =3D *ring_req;
>> +	/* Copy request(s) to the ring page. */
>> +	*final_ring_req =3D *ring_req;
>>   	if (unlikely(require_extra_req))
>> -		rinfo->shadow[extra_id].req =3D *extra_ring_req;
>> +		*final_extra_ring_req =3D *extra_ring_req;
>=20
> ... 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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