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Subject: Re: [PATCH v2 2/3] xen/blkfront: don't take local copy of a request
 from the ring page
To:     =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>
Cc:     xen-devel@lists.xenproject.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Jan Beulich <jbeulich@suse.com>
References: <20210708124345.10173-1-jgross@suse.com>
 <20210708124345.10173-3-jgross@suse.com> <YOgPCZbEF+t5DN+G@Air-de-Roger>
From:   Juergen Gross <jgross@suse.com>
Message-ID: <57f50e6f-4fc4-7318-9f7f-c5b4e0e63487@suse.com>
Date:   Fri, 9 Jul 2021 15:54:19 +0200
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From: Juergen Gross <jgross@suse.com>
To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>
Cc: xen-devel@lists.xenproject.org, linux-block@vger.kernel.org,
 linux-kernel@vger.kernel.org, Konrad Rzeszutek Wilk
 <konrad.wilk@oracle.com>, Boris Ostrovsky <boris.ostrovsky@oracle.com>,
 Stefano Stabellini <sstabellini@kernel.org>, Jens Axboe <axboe@kernel.dk>,
 Jan Beulich <jbeulich@suse.com>
Message-ID: <57f50e6f-4fc4-7318-9f7f-c5b4e0e63487@suse.com>
Subject: Re: [PATCH v2 2/3] xen/blkfront: don't take local copy of a request
 from the ring page
References: <20210708124345.10173-1-jgross@suse.com>
 <20210708124345.10173-3-jgross@suse.com> <YOgPCZbEF+t5DN+G@Air-de-Roger>
In-Reply-To: <YOgPCZbEF+t5DN+G@Air-de-Roger>

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On 09.07.21 10:55, Roger Pau Monn=C3=A9 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 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>
>> Reviewed-by: Jan Beulich <jbeulich@suse.com>
>=20
> Acked-by: Roger Pau Monn=C3=A9 <roger.pau@citrx.com>
>=20
> Thanks!
>=20
> One unrelated comment below.
>=20

=2E..

>> @@ -798,7 +801,9 @@ static int blkif_queue_rw_req(struct request *req,=
 struct blkfront_ring_info *ri
>>   		ring_req->u.rw.nr_segments =3D num_grant;
>>   		if (unlikely(require_extra_req)) {
>>   			extra_id =3D blkif_ring_get_request(rinfo, req,
>> -							  &extra_ring_req);
>> +							  &final_extra_ring_req);
>> +			extra_ring_req =3D &rinfo->shadow[extra_id].req;
>=20
> 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).
>=20
> 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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