linux-kernel - Re: [PATCH 4/8] xen/blkfront: don't trust the backend response data blindly

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Message-ID: <7f606c27-173a-542d-406d-196dae784edd@suse.com>
Date:   Thu, 8 Jul 2021 08:40:59 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     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>, xen-devel@...ts.xenproject.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/8] xen/blkfront: don't trust the backend response data
 blindly

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 irq, void *dev_id)
>>>>>>>>      	spin_lock_irqsave(&rinfo->ring_lock, flags);
>>>>>>>>       again:
>>>>>>>>      	rp = 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 value
>>>>> and then (via a separate read) use a different one past the barrier.
>>>>
>>>> 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 rp
>> instead?
> 
> 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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