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Subject: Re: [PATCH 4/8] xen/blkfront: don't trust the backend response data
 blindly
To:     Jan Beulich <jbeulich@suse.com>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Jens Axboe <axboe@kernel.dk>, xen-devel@lists.xenproject.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20210513100302.22027-1-jgross@suse.com>
 <20210513100302.22027-5-jgross@suse.com>
 <315ad8b9-8a98-8d3e-f66c-ab32af2731a8@suse.com>
 <6095c4b9-a9bb-8a38-fb6c-a5483105b802@suse.com>
 <a19a13ba-a386-2808-ad85-338d47085fa6@suse.com>
 <030ef85e-b5af-f46e-c8dc-88b8d195c4e1@suse.com>
 <477f01cd-8793-705c-10f9-cf0c0cd6ed84@suse.com>
 <dca55162-ec2e-682a-824d-b657a6407249@suse.com>
 <5a9dcc69-385a-eda5-6974-cb962ae62601@suse.com>
From:   Juergen Gross <jgross@suse.com>
Message-ID: <7f606c27-173a-542d-406d-196dae784edd@suse.com>
Date:   Thu, 8 Jul 2021 08:40:59 +0200
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From: Juergen Gross <jgross@suse.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
 =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>,
 Boris Ostrovsky <boris.ostrovsky@oracle.com>,
 Stefano Stabellini <sstabellini@kernel.org>, Jens Axboe <axboe@kernel.dk>,
 xen-devel@lists.xenproject.org, linux-block@vger.kernel.org,
 linux-kernel@vger.kernel.org
Message-ID: <7f606c27-173a-542d-406d-196dae784edd@suse.com>
Subject: Re: [PATCH 4/8] xen/blkfront: don't trust the backend response data
 blindly
References: <20210513100302.22027-1-jgross@suse.com>
 <20210513100302.22027-5-jgross@suse.com>
 <315ad8b9-8a98-8d3e-f66c-ab32af2731a8@suse.com>
 <6095c4b9-a9bb-8a38-fb6c-a5483105b802@suse.com>
 <a19a13ba-a386-2808-ad85-338d47085fa6@suse.com>
 <030ef85e-b5af-f46e-c8dc-88b8d195c4e1@suse.com>
 <477f01cd-8793-705c-10f9-cf0c0cd6ed84@suse.com>
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 <5a9dcc69-385a-eda5-6974-cb962ae62601@suse.com>
In-Reply-To: <5a9dcc69-385a-eda5-6974-cb962ae62601@suse.com>

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On 08.07.21 08:37, Jan Beulich wrote:
> On 08.07.2021 07:47, Juergen Gross wrote:
>> On 17.05.21 17:33, Jan Beulich wrote:
>>> On 17.05.2021 17:22, Juergen Gross wrote:
>>>> On 17.05.21 17:12, Jan Beulich wrote:
>>>>> On 17.05.2021 16:23, Juergen Gross wrote:
>>>>>> On 17.05.21 16:11, Jan Beulich wrote:
>>>>>>> On 13.05.2021 12:02, Juergen Gross wrote:
>>>>>>>> @@ -1574,10 +1580,16 @@ static irqreturn_t blkif_interrupt(int i=
rq, void *dev_id)
>>>>>>>>      	spin_lock_irqsave(&rinfo->ring_lock, flags);
>>>>>>>>       again:
>>>>>>>>      	rp =3D rinfo->ring.sring->rsp_prod;
>>>>>>>> +	if (RING_RESPONSE_PROD_OVERFLOW(&rinfo->ring, rp)) {
>>>>>>>> +		pr_alert("%s: illegal number of responses %u\n",
>>>>>>>> +			 info->gd->disk_name, rp - rinfo->ring.rsp_cons);
>>>>>>>> +		goto err;
>>>>>>>> +	}
>>>>>>>>      	rmb(); /* Ensure we see queued responses up to 'rp'. */
>>>>>>>
>>>>>>> I think you want to insert after the barrier.
>>>>>>
>>>>>> Why? The relevant variable which is checked is "rp". The result of=
 the
>>>>>> check is in no way depending on the responses themselves. And any =
change
>>>>>> of rsp_cons is protected by ring_lock, so there is no possibility =
of
>>>>>> reading an old value here.
>>>>>
>>>>> But this is a standard double read situation: You might check a val=
ue
>>>>> and then (via a separate read) use a different one past the barrier=
=2E
>>>>
>>>> Yes and no.
>>>>
>>>> rsp_cons should never be written by the other side, and additionally=

>>>> it would be read multiple times anyway.
>>>
>>> But I'm talking about rsp_prod, as that's what rp gets loaded from.
>>
>> Oh, now I get your problem.
>>
>> But shouldn't that better be solved by using READ_ONCE() for reading r=
p
>> instead?
>=20
> Not sure - the rmb() is needed anyway aiui, and hence you could as well=

> move your code addition.

Sure.

My question was rather: does the rmb() really eliminate the possibility
of a double read introduced by the compiler? If yes, moving the code is
the correct solution.


Juergen

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